Module 5: Global Warming - History
Module 5 Overview
You just read over a lot of information that is not controversial in the scientific community, but is controversial in some public and political discussions. For a little perspective, watch the short video below that we shot for you in Rocky Mountain National Park. Dr. Alley and others teach a class on Geology of National Parks, and we talk about earthquakes and volcanoes, and how rocks such as these, that were almost melted deep in the Earth, came to be sitting up here in the Rocky Mountain. In class, we note that earthquakes and volcanoes affect us, and they depend on convection currents deep in the Earth’s mantle, something like the currents in a pot of spaghetti cooking on the stove, and such currents help explain the rocks here. And the people say, “Great, let’s get to the earthquakes and volcanoes.”
But, with the same people, suppose we say “Climate affects us, and carbon dioxide from our fossil fuels is turning up the thermostat.” Many of them say, “How do we know that fossil fuels make carbon dioxide? How do we know carbon dioxide is rising? How do we know our fossil fuels are responsible? How do we know carbon dioxide affects climate? How do we know temperatures are rising? How do we know the rising temperatures are from the carbon dioxide? How do we…”
Now, the science of what’s convecting beneath our feet is based on hundreds, thousands of scientists working over decades, collecting and analyzing rocks and seismic records, hypothesizing and testing, arguing and agreeing. The science is not done, but it’s very good.
The science of what’s above us in the climate is actually older. Climate science is more successfully predictive, and better tested. But, because it matters more to money, we argue about climate science more.
Burning fossil fuels doesn’t make the “stuff” in them just go away; it makes carbon dioxide. In the US, we’re putting up about 20 tons per person per year. The warming effect is physics, known for over a century and really refined by the US Air Force after WWII for purposes such as sensors on heat-seeking missiles. There isn’t an alternative, there isn’t another side, there is just the reality that we are raising carbon dioxide, that is raising the planet’s thermostat, and climate affects us. Some people think we scientists are being overly dramatic, others think we’re being too conservative.
What really matters to most people is not radiation interacting with atmospheric gases, but home and food and friends. So, let's look at what climate change might mean to them.
Video: How do we Know? Rocky Mountain National Park (4:41)
Goals and Objectives
Goals
- Recognize the natural and human-driven systems and processes that produce energy and affect the environment
- Explain scientific concepts in language non-scientists can understand
Learning Objectives
The basic physics of global warming are very well understood, but by themselves don’t mean much to most people. More interesting is how the physics leads to things that do matter to people. After completing this module, student will be able to:
- Summarize how the Earth’s history confirms the warming influence of carbon dioxide
- Recognize that past climate changes have greatly affected plants and animals, usually in unpleasant ways
- Recall that future rise in CO2, and therefore surface temperature, is likely to be much worse than what we have experienced in the past 100 years
- Explain how small amounts of climate change are worse for poor people, and larger amounts are bad for everyone
- Assess what you have learned in Unit 1
Roadmap
| To Read | Materials on the course website (Module 5) | - |
|---|---|---|
| To Do | Quiz 5 Unit 1 Self-Assessment |
Due Sunday Due Sunday |
Questions?
If you prefer to use email:
If you have any questions, please email your faculty member through your campus CMS (Canvas/Moodle/myShip). We will check daily to respond. If your question is one that is relevant to the entire class, we may respond to the entire class rather than individually.
If you prefer to use the discussion forums:
If you have any questions, please post them to Help Discussion. We will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
Who Did Start the Fire?
Who did start the fire?
Nature has set fires for a very long time. Lightning is a common cause, but volcanoes, meteorites, or other natural phenomena also can start fires.
Humans also set fires, usually to cook food, to provide heat, or do other things that we want. But, rarely, humans set fires for bad reasons such as to hurt someone or to collect insurance money. This is the crime of arson. Police departments and insurance companies often have arson investigators, who must understand natural fires to be able to tell whether humans or nature were responsible when something burned down.
How does arson relate to climate change? You may know one of the many people who argue that we shouldn’t worry about human-caused climate change because nature has changed climate in the past. Some of these people seem to think that the existence of natural climate change means that we couldn’t be causing the changes going on now or that may come in the future—equivalent to arguing that a fire couldn’t be arson because nature lit fires in the past. Other people seem to think that living things survived past climate changes, so ongoing and future climate changes won’t matter—equivalent to arguing that arson might happen, but it doesn’t matter because it doesn’t really hurt anyone. But while many people make such arguments about climate change, very few people make the same arguments about arson.
Those who study the history of climate, like those who study the history of fires, generally come away with a clear understanding that both nature and humans can cause changes, and that big changes caused by nature or by humans matter a lot to people and other living things. For climate, studying the history of the Earth provides strong evidence that humans can make changes that match or exceed almost anything nature has done, with huge impacts.
Heeding Heated History
Heeding Heated History
Short version: Increasingly strong evidence shows that natural changes in carbon dioxide have been the main control on Earth's climate history and that the climate changes have greatly affected living things.
Friendlier but longer version: During the late 1700s and early 1800s, scientists were building the geologic time scale, drawing “lines” to separate history into blocks of time that could be given names. Fossils showed the species that lived at different times, and the lines were usually drawn when many species became extinct before new species evolved to take over the “jobs” left vacant by the extinctions. Those early geologists didn’t know why the species went extinct, but they knew that something big happened.
Since then, an immense amount of effort has gone into learning what happened. In one case about 65 million years ago, a giant meteorite impact killed the dinosaurs and ended the Mesozoic Era, to start the Cenozoic Era. Changing climate was responsible in other cases, and climate changes may prove to have been the main drivers in most of the big extinctions. Climate change was probably very important in how the meteorite killed the dinosaurs, too; for most of them, it didn’t fall on their heads but instead blocked the sun with dust it kicked up, causing great cooling for a few years, among many changes.
We’ll look briefly at three big changes, and then see what they say when viewed with the rest of climate history. Don’t worry about memorizing names and dates we’ve already given or the ones coming unless you’re really into that; just get the sense of the story.
Video: Continental Glaciation (3:27)
(launch image in a new window [3])
Activate Your Learning
What information is plotted on the figure above? What does this data tell us about the relationship between CO2 in the atmosphere and surface temperature over the past 400,000,000 years of Earth history?
Click for answer.
The Great Dying
The Great Dying
At the end of the Permian Period, which also is the end of the Paleozoic Era about 252 million years ago, approximately 95% of the species known from fossils went extinct. This is the same time, with very little uncertainty, as the greatest volcanic outpouring on Earth in the last 500 million years.
The rise in CO2 from the volcanic eruptions caused warming. (Volcanoes generally cause cooling over short times, such as their role in causing the Little Ice Age of a couple centuries ago, but volcanoes raise temperatures over longer times, such as their role in warming the end of the Permian.
Do you want to learn more?
Read the Enrichment titled Volcanoes Cool and Warm, without Doubletalk.
The volcanic eruptions are estimated to have raised CO2 much more slowly than humans are doing, but the volcanoes didn't run out of CO2 as rapidly as we will run out of fossil fuels, so the event back then lasted longer. Our understanding indicates that the extra warmth from the CO2 accelerated rock weathering, providing extra fertilizer reaching the ocean. This would have helped make extensive “dead zones” as parts of the ocean ran out of oxygen, aided by the lower oxygen level in the water caused by the higher temperature. Sediments from that time contain special “biomarker” molecules made by green sulfur bacteria that photosynthesize with the poisonous-to-us gas hydrogen sulfide, indicating loss of oxygen and rise of hydrogen sulfide in the ocean. New data also suggest the Earth became so hot that the few remaining large creatures could not live in the tropics immediately after the extinction, but only closer to the poles.
We do not expect the warming in our near future to produce anything nearly so bad, but fertilizer runoff from our fields and warming from our CO2 can contribute to oceanic “dead zones”. And, we cannot rule out the possibility that beginning or near the end of this century, we could make the Earth so hot that living unprotected in the tropics becomes difficult or even impossible for us and some other large creatures.
The Paleocene-Eocene Thermal Maximum (PETM)
PETM
Video: Paleoclimate Concepts (1:31)
During the Cenozoic, about 55 million years ago, an extinction event wiped out many sea-floor foraminifera, small shelly critters, at the time dividing the Paleocene and Eocene Epochs. Starting with an already-warm world, the temperature went up several degrees in roughly 10,000-20,000 years (with some uncertainty) as CO2 rose and then cooled over the next 100,000-200,000 years as CO2 fell. The Arctic was ice-free during the event. Plants and animals migrated rapidly. Many large animals became “dwarfed” during peak warmth, possibly because high temperatures cause greater trouble for larger animals. (We generate heat over the volume of our bodies and lose heat from the surface, and the ratio of surface area to volume is generally smaller in larger animals, making heat loss harder.) Insect damage to leaves spiked and patterns of rainfall and drought shifted. The ocean became more acidic, and that extra acidity was then neutralized in part by dissolving shells.
The source of the CO2 remains somewhat uncertain but most likely was volcanic eruptions linked to rifting of the North Atlantic cooking organic material including oil in rocks, amplified by the loss of carbon from soils and sea-floor methane clathrates. The event is unique over tens of millions of years in its size and speed, so may have involved a coincidence of some sort, or else more such events would have occurred.
Wherever the CO2 came from in detail, it warmed the climate as much or more than models generally calculate and had very large impacts on living things. And, the effects lasted a long time. For example, although corals did not go extinct, coral reefs disappeared as functioning ecosystems and did not come back for millions of years.
The Ice Ages
The Ice Ages
Over the last million years or so, ice has grown and shrunk on the Earth’s surface, with a main spacing of 100,000 years, and lesser wiggles at about 41,000- and 23,000-year spacing. Early geologists identified and named many of the times of large and small ice, and eventually developed tools that allow quite precise estimates of when events occurred.
Earth: The Operators' Manual
If you want to see a short animation of the orbital cycles, and how they affected the Franz Josef Glacier in New Zealand, revisit this clip for the last time (1:20 to 7:22). Dr. Alley had a lot of fun in the helicopter.
Video: CO2 and the Atmosphere (9:03)
Remarkably, astronomers had predicted the measured timings decades before they were observed because they arise from cycles in Earth’s orbit. These cycles have very little effect on the total amount of sunshine reaching the whole Earth, but they move sunshine around on the planet, with large effects (more than 10%) on the sunshine reaching a particular latitude during a particular season.
Even more remarkably, when sunshine has dropped in the far north, especially in summer, the whole world has cooled, including places getting more sunshine. And, when sunshine has risen in the far north, especially in summer, the whole world has warmed, including places getting less sunshine. The explanation is that when northern sunshine dropped, a whole lot of ice grew on the lands of the north (enough to lower sea level about 400 feet), many other things in the Earth system changed, and some of these changes caused some CO2 to move out of the air into the oceans; when ice melted in the far north, those other changes reversed and moved the CO2 from the oceans back into the air. Several processes may have contributed, including northern ice changes shifting winds that shifted ocean currents that controlled how rapidly deep ocean waters came back to the surface, bringing CO2 released by ecosystems living on the sinking organic matter from the surface.
Want to learn more?
Read the Enrichment titled The History of the World.
Ice growth lowered CO2, which cooled the regions getting more sunshine; ice melting raised CO2, which warmed the regions getting more sunshine. The known physics of CO2 explain what happened, and nothing else has succeeded in doing so.
The Big Picture 2
The Big Picture 2
Let's return to the figure showing the broad histories of atmospheric CO2 (with estimates from different techniques shown by different lines plus the shaded band at the bottom), and of ice on the planet (glaciers extending farther toward the equator are shown by longer bars hanging down from the top). Clearly, CO2 and ice moved in opposite directions, with rising CO2 occurring with melting ice. The figure has been “smoothed”, and so doesn’t show the details of the shorter-lived events discussed just above.
By themselves, the correlations just discussed between CO2 and temperature do not prove that CO2 caused the warmth. But, straightforward physics shows the warming effect of CO2. And, although warming can raise CO2 over short times, as at the start of the PETM or the ends of the ice ages, over long times warming lowers CO2 by causing faster rock weathering and fossil-fuel formation. Thus, the prolonged high levels of CO2 during warm times were not caused by the hot climate; instead, such high levels were caused by faster volcanism, or thicker soils slowing access of CO2 to react with rocks, or other geological reasons.
The physics, and the lack of other plausible causes despite major efforts to find something, show that the warmth was caused by the CO2. Testing our understanding by “retrodicting” what happened—starting with the causes and simulating the effects of the climate changes—shows that our models work well. If there is a problem, the world has changed a little more in response to CO2 than expected from the models.
More than Just Temperature
More than Just Temperature
The past confirms much more about our understanding. The major events in Earth’s history were identified first by their influence on living things, including extinctions. A huge amount of additional research was required to learn that changing climate was responsible for many of those events, and perhaps for almost all of them. This long history of climatically caused extinctions supports our scientific expectation that continuing climate change risks extinctions in the future. We also expect that the CO2 we put up will continue to affect the climate for a long time, based on models and understanding that are well-confirmed by the geologic history.
The biggest of the climate changes of the past were much larger than the changes humans have caused so far. But, if we continue to burn the available fossil-fuel resource, we can cause a change that is as more-or-less as large as, and much faster than, the biggest natural events (except for the meteorite that killed the dinosaurs, which caused large changes very very rapidly).
The geologic record highlights another major issue. Science always involves uncertainty. All measurements have some “plus and minus”—Dr. Alley is within an inch of 5’7” and weighs within a few pounds of 145, for example, but he surely is not known to be exactly those measurements. And, when measurements are used to drive models that project climate changes that are used to estimate economic impacts, many sources of uncertainty are involved, and we cannot in any way be exactly certain what the future holds.
In assessing those uncertainties, though, we find evidence of an asymmetry that you probably could have figured out from common sense. In ordinary life, breaking things is almost always easier than building them. If you want to build a new house, you will need a lot of different materials and tools and know-how. But, if you want to tear down a house, you can do it with just a wrecking ball or an exploding stick of dynamite.
When we survey the history of climate, we see something similar. We don’t find evidence of Eden, a time when changing CO2 and climate had turned the whole Earth into paradise. Deserts and ice have grown and shrunk, so some times may have been “nicer” than others, with no guarantee that we now live in the best of all possible worlds. But, hazards existed at all times.
We do find evidence of occasions that were much closer to Hell, with up to 95% of the known species becoming extinct. A species might survive from just a single pregnant female or a few eggs or seeds even if all other individuals are killed, so the extinction times were very bad indeed.
If we continue to rapidly change the atmospheric concentration of CO2, we have a best estimate of likely impacts, which will be discussed further just below and in additional material later in the course. Uncertainties are real, and the future may be somewhat better than expected, or somewhat worse. But, we don’t see any reasonable chance that the changes will be much better than expected—cranking up CO2 is very, very unlikely to make Eden. And, the history of climate suggest the possibility that things will be much worse than expected—cranking up CO2 might break things we really care about.
If you drive somewhere, you face a similar situation. What you expect is very far on the "good" side of what is possible, as shown in this short piece...
Video: Possibilities (5:10)
The Projection Project
The Projection Project
Short version: With high confidence, warming from rising carbon dioxide will bring more very hot days and fewer very cold ones, more sea-level rise, stress for endangered species, plant fertilization but heat stress, more-intense peak rains but drying in many times and places, and many other impacts. Small changes will bring winners and losers, but losers will grow to far outnumber winners if we continue on our current path and cause very large changes.
Friendlier but longer version: For the next decade or two, the biggest uncertainties about future climate are linked to things we cannot know—will there be a big volcanic eruption in the next decade, or an extra El Nino or La Nina? The expected warming over a decade or two for any of the choices we are likely to make is more-or-less the same size as the cooling effects of a big volcano or La Nina. For a small number of decades after that, the biggest uncertainties are probably linked to things we don’t fully understand about the climate. Recall that the equilibrium warming from doubled CO2 is estimated to be between 1.5 and 4.5°C. The big difference between the high and low estimates might be reduced by better climate science, although the interactions among feedbacks mean that greatly reducing the uncertainty is quite difficult. But, by late in the century, the uncertainties related to volcanoes or climate sensitivity are smaller than the uncertainties related to what we humans choose to do. And remember, at least the younger students in this class are likely to live longer than that!
Because our choices are so important, climate scientists normally don’t discuss predictions, choosing instead to provide projections: “If people decide to do xxxx, then the climate will do yyyy, with an uncertainty of zzzz.” By replacing the “xxxx” with different things we might do, the science shows policymakers and other people the changes yyyy±zzzz that their decisions would cause.
Video: Past and Future CO2 Atmospheric Concentrations (1:58)
The graph just above shows the history of atmospheric CO2 over the last millennium as measured in bubbles from ice cores, including the very close agreement between ice-core and atmospheric data during the decades of overlap, and then shows various possible futures. These future “scenarios” were prepared to bracket likely paths we may follow, and provide enough curves so that one of them may prove to be fairly close. So far, we’re running near the highest of the projections, but close to the others because the different scenarios don’t diverge a lot until further in the future. None of these paths assumes that we take major efforts to reduce greenhouse-gas emissions, which could lower any of them.
Notice that in all of these scenarios the projected changes are much larger than those to date, with CO2 still rising beyond 2100. (The world does NOT end in 2100!) With notable uncertainties, fossil fuels may become rare by the time CO2 reaches the top of the chart around 1000 ppm, or may be common enough to drive CO2 more than twice that high, giving us two or three doublings from the relatively stable level of approximately 280 ppm before the industrial revolution.
We could estimate future temperature by taking the climate sensitivity of around 3°C for doubled CO2 (or between 1.5 and 4.5°C), and the two or three doublings, calculating a warming, and reducing that a bit because the warming lags the CO2 a little and the CO2 will start down before peak warming is reached. We get much more information by taking our best models, run by different groups in different ways, forcing them with the scenarios, and studying the results.
Warming to the Future
Warming to the Future
Video: Global Surface Warming (2:23)
The figure shows the past warming, which is just under 1°C or roughly 1°F, together with the future warmings for the different scenarios. The lowermost future line assumes that the atmospheric composition had been stabilized in the year 2000, with no further rise of CO2. Warming continues in that scenario because some heat is now going into the ocean, keeping the air cooler than it will be as ocean warming catches up. Note that it is already too late for us to follow that path because we have raised CO2 since 2000. Also, we are committed to some additional warming if we choose to stabilize the atmospheric concentrations at any point in any of the scenarios, again because of the slow warming of the ocean.
In all the other scenarios, if we don’t make major efforts to reduce future CO2 emissions, the future warming is projected to be quite large compared to the past warming, and the temperature is still going up as the next century starts. Also notice the uncertainty bars on the right, showing that warming may be a little less than the most-likely estimate, or a little more, or somewhat more than that.
Warming around the Globe
Warming around the Globe
Video: Projected Surface Temperature Changes (2:55)
The figures here show the projected warming, and uncertainties. The maps are the projected warming for the next decade (2020-2029, center) and the last decade in this century (2090-2099, right), for different possible emissions scenarios, with more CO2 emitted as you go down through the maps. The estimates were made with Atmosphere-Ocean General Circulation Models (AOGCMs), the big climate models of the world. And, the maps here are the averages of the projections from all of the models participating in this effort—tests in the past have shown that this average across all the models generally does better than any single model (the “wisdom of the crowd” in models).
Warming is projected to be especially slow in those places where ocean water sinks into the deep ocean, and especially fast in the Arctic. Projected warming is generally larger over land than over the ocean. Because the Earth is mostly ocean, the numbers usually given for “global warming” are closer to ocean than to land values. But, almost everyone lives on land, so the great majority of people are expected to experience above-average warming!
The panels on the left show the uncertainties in the projected warming. Notice for the 2090-2099 projections (the larger warmings, in red), that the most-likely warming tends to be towards the low end of the possible warmings. We have already seen that the most-likely impacts of a specified warming are on the low-damage side of the possible impacts, and now we see that the most-likely warming is on the low side of the possible warmings. Both of these have the same effect: the less you trust climate scientists to get the most-likely estimate correct, the more worried you probably should be about climate change, because the numbers most frequently quoted by scientists are on the optimistic side of the possibilities.
Precipitating a Change
Precipitating a Change
Video: Relative Changes in Precipitation (1:21)
This slightly complex figure shows projections of future precipitation. In general, the models project that wet places will get wetter, dry places dryer, and the dry subtropics will expand somewhat into currently wetter regions toward the poles. Evaporation is also expected to go up with warming, and many of the models find summertime drying in places we grow much of our food, so agriculture may be reduced more than you might think from looking at this, as we discuss after a quick look at sea level.
LOTS of other issues come up because climate affects so many things that we care about. A few of the larger issues include sea level rise, more floods and droughts, agriculture impacts, and impacts on people.
Sea Level Rise
Sea Level Rise
Warming causes ocean water to expand and melts mountain glaciers. (Despite a few outliers or oddities, the great majority of mountain glaciers are melting.) The big ice sheets of Greenland and Antarctica are also losing mass. With continuing warming, we expect more sea-level rise. The recent rise has been about 3 millimeters per year, or just over an inch per decade, and sea level has risen almost a foot (just under 1/3 m) over the last century or so. We expect sea-level rise to continue and probably accelerate moderately, with at least a slight chance of a large acceleration if the big ice sheets change rapidly. A foot of sea-level rise might not seem like a lot when the biggest hurricanes can have storm surges of 10 or rarely even 20 feet (3 to 6 m). But, the last foot may be the one that goes over the levee or into the subway tunnels, so even a relatively small change in sea level can have large consequences for cities and other human-built structures.
Video: Sea Level Rise (1:17)
More Floods and Droughts
More Floods and Droughts
As noted above, there is a tendency for wet places to get wetter and dry places to get drier, with the subtropical dry zones expanding somewhat. When conditions are right to rain, warmer air holds more water (by roughly 7% per degree C or 4% per degree F), so all else equal, a warmer climate can deliver more rain in a hurry. But, evaporation speeds up with warming, too. All winter, Dr. Alley’s tomato patch is damp or frozen; in the summer, just a week or two after a downpour, he needs to water the plants again. A more summer-like world is likely to have more variability in the water cycle, with more floods and more droughts.
Video: Potential for Drought by the End of this Century (1:01)
Agricultural Impacts
Agricultural Impacts
Plants need CO2 to grow, and higher CO2 levels will give faster plant growth. But, plants need many other things, too; in experiments with extra CO2 added to natural ecosystems, an initial growth spurt lasts a few years before settling down to only slightly faster growth than before the CO2 addition because the plants need more of those other things to sustain fast growth. If CO2 is added to farm plants that also are supplied those other things, faster growth can continue, but the gain is still not huge.
Working against this fertilization effect of CO2, the projected increase in floods and droughts would make farming more difficult. Farmers have learned to handle the bugs and weeds that now annoy them, but changing climate allows new ones to invade.
Perhaps the biggest concern is heat stress on crops. At present, anomalously hot weather reduces crop growth in many agricultural regions even if the plants have enough water, fertilizer and protection from bugs and weeds. For much of the world, continuing our present path until late in this century is projected to give average summer conditions hotter than the hottest summer up to 2006 (the last data available for an influential study). Record highs are rising with average temperatures, and expected to continue doing so. Thus, unless crop breeders become highly successful at developing heat-resistant varieties, heat stress may become quite damaging if we cause large warming.
Video: Net Photosynthesis (1:19)
Note also that the tropics are the big belt around the middle of the Earth, the polar regions are the small caps on the ends, and mountain ranges taper to points at the top, so simply moving poleward or up the mountains to follow cooler conditions involves losing ground. In addition, we now grow mid-latitude crops in soil that was transported by glaciers from higher latitudes or altitudes, so moving poleward in at least some places leaves most of the soil behind. Greenlanders are doing a little farming in special places such as on raised beaches from the ice age, but much of Greenland is too rocky for good farming, as shown below. So if Greenland's ice melts, raising sea level about 7.3 m (24 feet) averaged around the globe, and flooding valuable coastal property, the land revealed beneath the former ice sheet is not likely to be a wonderfully fertile replacement.
Video: South Greenland (1:14)
Overall, the effects of the rising CO2 and the changing climate are expected to be mildly positive for farms for the near term, switching to negative and becoming increasingly worse beyond a few decades. One study found losses for US corn and soybeans of 30% to 82% by late in this century, depending on the scenario used and other factors.
Impacts on People
Impacts on People
Too hot or too cold cause problems for people. But, we have largely mastered the art of putting on coats, boots, hats and gloves, whereas personal air conditioning is not well-advanced. Thus, in too-hot places we tend to stay inside air conditioned places or be unhappy, whereas in cold places we go skiing or snowmobiling. As warming reduces the snowy season in some places, fewer automobile accidents and airports closed by blizzards will be beneficial. But, the arrival of unexpected heat can be dangerous—the highly anomalous European heat wave of 2003 is estimated to have killed 70,000 people. Adaptations such as expansion of air-conditioning tend to reduce the health impacts when heat continues.
Still, humans and other animals risk damage or death when conditions are too hot. How hot is too hot depends on humidity (we can take higher temperatures when it is drier), and on exercise level. A recent study found that, averaged across the world’s human population, heat already here reduces the ability for people to work outside in the hottest months by about 10%. If we continue to release CO2 rapidly, this is projected to rise to a 20% reduction in work by 2050, 40% by 2100 and perhaps 60% by the end of the next century. These losses are concentrated in the warmer parts of the world, where they can be very large.
Lots of Other Issues
Lots of Other Issues
Climate affects almost everything somehow, so a great number of other issues can be raised, from huge to tiny. Vines seem to like carbon dioxide, for example, so poison ivy is expected to grow well, and vines may out-compete large trees in tropical rain forests.
More broadly, almost all ecosystems will be perturbed, often in major ways. Rare and endangered species may have difficulty migrating, especially if they are persisting in a park or preserve surrounded by human-controlled landscapes, or if they are migrating up a mountain and eventually having nowhere further to climb. Acidification of the oceans, and loss of oxygen with warming, will affect marine species and those of us who eat them. Loss of wintertime cold doesn’t mean that everyone in the high latitudes is about to get malaria, but one line of defense will go away. Changes in hurricane frequency are still highly uncertain, but the strongest storms seem likely to get stronger, and so much of the damage is done by the strongest storms. Cooling towers for power plants expect enough, and cold enough, water, and may experience troubles. And on, and on.
More Big Picture
More Big Picture
Very generally, we are adapted to the climate we have. In the short term, almost any change has associated costs. If two regions with different climates simply swapped their climates, for example, both would have wrong-sized air conditioners and heaters, too many or too few snow plows and swimming pools, less-than-optimal seeds for crops, etc. All of these can be fixed, but not for free.
If changes remain small, there are likely to be winners and losers. Warming may make beach resorts happier, but ski areas less happy. Rare and endangered species, and people trying to live traditional lifestyles, may be pushed to the edge by even small changes. For most people, if you have winter that interferes with travel and other activities, air conditioners so you can work in the summer, and bulldozers to build walls against the rising sea, a little warming is not especially costly; if you lack winter, air conditioners and bulldozers, even a little warming is likely to make your life at least a little harder.
But, if the temperature continues to rise, and the big hotter-than-we-like belt around the equator expands towards the poles, life is projected to get harder for most people in most places.
There are real uncertainties, so things really may end up better than this. But recall that, because breaking is easier than building, we don’t see how raising CO2 greatly and rapidly will create Eden, but we do see at least a slight chance of huge and damaging changes.
Video: Reducing Risks of Climate Change Damage (2:59)
One way to look at the future is shown in the figure. Different things you might care about are shown by the different columns, and the risks from warming are shown by the increasingly orange-red (saturated) color going up in the columns. Another way to look at the issue is that damages are projected to go up faster than temperature; the first degree of warming is nearly free, but each degree beyond that costs more than the previous one did. The first degree has allowed us to test our models and learn that they are doing well; the next degrees really matter.
Please recognize that these projections do not include major human efforts to reduce emissions of CO2 and other greenhouse gases. And, we are certainly capable of making such reductions, or of adopting other approaches that might reduce the warming while supplying plenty of energy.
So, in the next Unit, we’ll look at some of the options. Then, we’ll return to Economics, Ethics and Policies that might address the paired issues of getting valuable energy for many people while reducing damages from climate change.
Activate Your Learning
The costs or benefits of changing climate depend on how much the climate changes. If the amount of change remains small, say 1˚C or so, who is likely to be most negatively impacted?
Click for answer.
Self-Assessment
Reminder!
After completing your Self-Assessment, don't forget to take the Module 5 Quiz. If you didn't answer the Learning Checkpoint questions, take a few minutes to complete them now. They will help you study for the quiz, and you may even see a few of those questions on the quiz!Learning Outcomes Survey
We have now come to the end of Unit 1. The purpose of this exercise is to encourage you to think a little bit about what you have learned well, and just as importantly, about what you feel you have not learned so well. Think about the learning objectives presented at the beginning of the Unit, and repeated below. What did you find difficult or challenging about the things you feel you should have learned better than you did? What do you think would have helped you learn these things better?
For each Module in Unit 1, rank the learning outcomes in order of how well you believe you have mastered them. A rank of 1 means you are most confident in your mastery of that objective. Use each rank only once — so if there are four objectives for a given module, you should mark one with a 1, one with a 2, one with a 3, and one with a 4. All items must be ranked. For each Module, indicate what was difficult about the objective you have marked at the lowest confidence level.
Module 1
- Recognize that even really smart people have failed when climate changed in the past.
- Explain how machines and trade have helped other people avoid catastrophe.
- Describe how we have burned through energy sources in the past.
- Show that people can make money and save the world at the same time.
What did you find most challenging about the objective you ranked the lowest?
Module 2
- Recall that using energy doesn’t make it go away, it is just converted into a less useful form.
- Recognize the many units of energy and power.
- Show that the amount of energy used by people around the world is much larger than the 100 watts inside most people converted from food.
- Recall that around 85% of the energy we use is derived from fossil fuels.
- Analyze energy use and production in a country other than the United States.
What did you find most challenging about the objective you ranked the lowest?
Module 3
- Recall that oil, coal and natural gas are produced naturally by well-understood processes.
- Evaluate the effects of technology, economics, and population growth on fossil fuel production using computer models.
- Demonstrate that our current consumption of fossil fuels is not sustainable by exploring future scenarios with computer models.
What did you find most challenging about the objective you ranked the lowest?
Module 4
- Recall that carbon dioxide has a well-understood and physically unavoidable warming influence on Earth’s climate.
- Recognize that positive feedbacks amplify changes, and negative feedbacks reduce them.
- Recall that multiple independent records from different places using different methods all show that both CO2 and temperature are rising.
- Explain that patterns of global warming in the past century can only be reproduced by considering both natural and human influences on climate.
- Use a model to show that global climate always finds a steady state, but certain factors may influence how long it takes to get there.
- Demonstrate that greenhouse gases are the most significant factor controlling surface temperature.
What did you find most challenging about the objective you ranked the lowest?
Module 5
- Summarize how the Earth’s history confirms the warming influence of carbon dioxide.
- Recognize that past climate changes have greatly affected plants and animals, usually in unpleasant ways.
- Recall that future rise in CO2, and therefore surface temperature, is likely to be much worse than what we have experienced in the past 100 years.
- Explain how small amounts of climate change are worse for poor people, and larger amounts are bad for everyone.
- Assess what you have learned in Unit 1.
What did you find most challenging about the objective you ranked the lowest?
Scoring Information and Rubric
The self-assessment is worth a total of 25 points.
| Description | Possible Points |
|---|---|
| All options are ranked | 10 |
| Questions are answered thoughtfully and completely | 15 |
Module Summary
In Module 4, we discussed the very strong scientific evidence that our burning of fossil fuels is raising atmospheric CO2, with an unavoidable warming influence on the climate. Temperatures are in fact rising, despite the cooling effect of recent slight dimming of the sun, blocking of the sun by particles from our smokestacks, and our actions in cutting dark forests to replace them with grasslands that reflect more sunshine. The success of climate models in explaining what has happened, “retrodicting” history by starting in the past and running toward the present, and the very clear evidence that climate is doing what earlier climate scientists projected, give us high confidence that our scientific understanding is correct. And, considering how much fossil fuel remains in the ground, we have high confidence that if we continue to burn rapidly, the coming changes will be large compared to those that have happened so far.
People typically are most interested in how climate will affect them—global mean surface temperature is rarely as interesting as dinner, and whether or not dinner will be available. Looking at a great range of scholarship, small climate changes tend to cause winners and losers. Generally, poor people in hot places are hurt by a little warming, whereas wealthier people in colder places are not impacted as much and may even benefit slightly. But, as the climate changes become larger, the losers grow to far outnumber winners.
The biggest concern may be that many of our crops are already damaged by excessive heat, but by late in this century if we continue burning fossil fuels rapidly, much of the world’s cropland is likely to see average temperatures hotter than the hottest ever experienced so far. If the climate is favorable, plants grow better with more CO2 in the air, but the damages from higher temperatures are expected to grow to greatly exceed the benefits of this CO2 fertilization, made worse by increasing floods and droughts, and by invasive pests. Other impacts of climate change are also expected to hurt more than help for humans and most other species.
This is real science, so there are real uncertainties. But, this is not reassuring to most people who look carefully. The uncertainties are generally on the “bad” side—things may be a little better or a little worse, but with almost no chance of being a lot better but some chance of being a lot worse. Building almost anything requires getting many things right, but breaking can be done with a big hammer or a stick of dynamite. By analogy, adding CO2 to the air is very unlikely to create paradise, but might greatly damage many things that we care about.
In case you find this scary or depressing, please stay with us. The next Unit of the course covers the amazing resources that are available to us, with the potential to power everyone on the planet almost forever. And in the third Unit, we discuss how the use of this knowledge can make us better off, with a bigger economy, more jobs, greater national security, and a cleaner environment where we treat each other more fairly. Fossil fuels have given us another step on the ladder to a better future, and while they cannot get us to the top, other sources of energy really can.
Reminder - Complete all of the Module 5 tasks!
You have reached the end of Module 5! Double-check the Module Roadmap to make sure you have completed all of the activities listed there before you begin Module 3.
References and further reading
You can find the key results for this, and other modules in the reports of the IPCC, and of the US National Academy of Sciences, and in Dr. Alley’s book Earth: The Operators’ Manual (it isn’t free, though); a quick look by Dr. Alley indicates that most of the Wikipedia pages are pretty good, too. A few of the numbers in Modules 4 and 5 may be harder to find though and those references are given here.
- Estimates of food stress for future summers hotter than any seen up to 2006 are from Battisti, D.S. and R.L. Naylor, 2009, Historical warnings of future food insecurity with unprecedented seasonal heat, Science 323, 240-244.
- Estimated losses of US corn and soybean production with warming are from Schlenker, W. and M.J. Roberts, 2009, Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change, Proceedings of the National Academy of Sciences of the United States of America 106, 15994-15998.
- The death toll from the 2003 Europe heat wave is estimated in Robine, Jean-Marie; Siu Lan K. Cheung, Sophie Le Roy, Herman Van Oyen, Clare Griffiths, Jean-Pierre Michel, François Richard Herrmann, 2008, Death toll exceeded 70,000 in Europe during the summer of 2003, Comptes Rendus Biologies 331, 171-178.
- The reductions in work outside were estimated by Dunne, J.P., R.J. Stouffer and J.G. John, 2013, Reductions in labour capacity from heat stress under climate warming, Nature Climate Change 3, 563-566.
- IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working
Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning,
Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Note: Dr. Alley was one of the Drafting Authors for this report. - Schmidt, G.A., R. Ruedy, R.L. Miller, and A.A. Lacis, 2010: The attribution of the present-day total greenhouse effect. J. Geophys. Res., 115, D20106, doi:10.1029/2010JD014287.
- For more from the IPCC, including all of the 2013 report, see IPCC [22]. Especially look at Climate Change 2013: Physical Science [23]Basis [23] and start with Summary for Policymakers [24](SPM) [24] , focusing in on IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY. The basic information here was mostly in place in the Fourth Assessment from 2007, and really was fairly well know in the decades before that, but the science continues to get better, and the IPCC has continued to update to give you the latest and greatest.
Enrichments
Volcanoes Cool and Warm, without Doubletalk
Right after World War II, when industry powered up in peacetime and started cranking out consumer goods, emissions increased rapidly for both CO2 and particles from smokestacks. If emissions are suddenly ramped up like that, and then held constant, the number of sun-blocking particles in the air increases for a week or so. Then it stabilizes because particles are falling out of the air as rapidly as they are added. But, for a given rate of emission, the CO2 concentration of the air will rise for a few hundred thousand years, until the rate of rock weathering balances the new, raised rate of emission. (Human emissions did not remain constant, but this may help you think about things.) For industry after the war, the particles emitted in the first week had a cooling effect that was much larger than the warming effect of the CO2 emitted during that week. But, as years became decades, the particles fell down, much of the CO2 stayed up, and the warming grew to outweigh the cooling.
Volcanic eruptions have essentially the same story. Over short times, the sun-blocking cooling from particles exceeds the warming of the CO2. The volcanic particles typically get thrown into the stratosphere, above most rainfall, and so stay up a year or two rather than a week or so, but then fall out. So if extra volcanism continues long enough, the particles fall down, the CO2 builds up, and warming results. Exactly how long you have to wait for “short time” to become “long time” depends on the types of volcanoes and many other issues. In general, an increase in volcanic activity (typically involving many volcanoes, or huge volcanic provinces) will cause cooling over times of years to centuries that most economists worry about, but with warming over longer, geologic time.
The Vostok Ice Core
To help you “see” some of the material we just discussed, here are some data from an ice core at a place called Vostok in East Antarctica. It is probably not the coldest place on Earth, but it’s close. There is a Russian base there with good measuring equipment, and it observed the lowest reliably documented natural temperature ever at Earth’s surface: −89.2°C or −128.6°F. Snow accumulates very slowly there, and an ice core contains a long, accurate record of the temperature at Vostok, and of the atmospheric composition because air bubbles trapped in the ice are little samples of the old atmosphere. Several long ice-core records have been collected in Antarctica, with the longest continuous one about 800,000 years, and older ice found in other places but disturbed by ice-flow processes so that a complete, continuous record beyond 800,000 years is not yet available from ice cores. (Other sedimentary records go much further back in time, but don’t trap bubbles of old air, so estimates of older atmospheric concentrations rely on indirect indicators and are slightly less certain.)
The temperature record, from the isotopic composition of the ice, is what happened in the Vostok region, not the whole world. But, if you take records from elsewhere, and smooth them a good bit, they all look similar to Vostok; the whole world cooled and warmed together through the ice-age cycles. And as explained in the next clip, this is primarily because of changes in CO2.
As noted on the previous page, the ice ages were caused by features of Earth’s orbit. The spacing between ice ages actually was predicted decades before it was measured accurately, based on astronomical calculations from the orbits. The prediction and test are explained in the clip just below, and shown in the figure below it. The figure is from a fancy way (called a Fast Fourier Transform, or FFT) to figure out the spacing between wiggles in a curve, such as the climate record—the arrows are the predicted peaks, and you can see that the actual peaks line up beautifully.
The story is wonderfully complicated but can be made fairly simple, again as noted on the previous page. When the summer sun has dropped in the north over thousands of years, ice grew, forming vast ice sheets that have bulldozed across Scandinavia, Boston, New York and Chicago. (Antarctica is already glaciated, and it doesn’t really get cold enough to get ice onto Australia, Africa, or most of South America, so sunshine in the south isn’t so important). The ice sheets were made from water from the ocean, which dropped more than 100 m (about 400 feet). Many other changes occurred as the ice grew, and these shifted some CO2 into the ocean. Then, the whole world cooled, including places getting more sunshine. When sunshine rose in the far north, this reversed. The temperature of the whole world changed together, even though half of the world got less sunshine when the other half got more, and CO2 is the main explanation.
The last figure is then important, showing where CO2 may go this century if we don’t change our energy system.
Video: Data from the Vostok Ice Core (5:45)
Earth: The Operators' Manual
This three-minute clip visits the US National Ice Core Lab to show a little more about the changes in the CO2and the climate that occurred with the ice ages.
Video: CO2 in the Ice Core Record (3:01)
The History of the World
This is a “minitext” that was originally written by Dr. Richard Alley for Geosciences 320, Geology of Climate Change, at Penn State. It provides a short history of the world’s climate over the last 4.6 billion years. It assumes a little more background knowledge than is expected in the rest of the class, but may be useful.
Deep Time
This is a “minitext” that was originally written by Dr. Richard Alley for Geosciences 320, Geology of Climate Change, at Penn State. It provides a short history of the world’s climate over the last 4.6 billion years. It assumes a little more background knowledge than is expected in the rest of the class but may be useful.
“Deep time” is sometimes difficult to understand. The planet is 4.6 billion years old. If you substitute distance for time, and let the 100-yard length of a US football field (just under 100 m, and roughly the length of a full-sized soccer pitch) be all of the Earth’s history, start at one goal line and drive toward the other, then:
- Trilobites and other shelly critters show up after you have gone 88 yards, just 12 yards from scoring;
- The end-Permian extinction is reached after 95 yards, just 5 yards from the goal;
- The dinosaurs died out less than 1.5 yards from the goal;
- The warming from the coldest part of the last ice age started about 0.02 inches from the goal;
- Humans have been writing down our history for something like 0.005 inches, the thickness of one or two sheets of paper;
- The average college student, approximately 20 years old, has been alive for roughly 1/300th of the thickness of a sheet of paper.
Studying Earth’s history, and the physics, chemistry, biology, geology, and other “ics” and “istries” and “ologies”, provides many insights to the planet. A few of these include:
Earth shows long-term stability. The physics of radiation provide a powerful protector for the planet. Geologists generally can tell with high confidence whether sediments were deposited in liquid water. Such water-laid sediments dominate the geologic record. Furthermore, there are indications of life through most of geologic time. Together, liquid water and life show that the climate of the planet must have one or more stabilizing feedbacks (as noted below, without such feedbacks, bad things would have happened). One of these stabilizing feedbacks is easy-simple radiative balance. Because the radiation emitted by a black body is proportional to the fourth power of the absolute temperature, a 1% rise in temperature of the planet causes a 4% rise in the energy emitted to space by the planet (or a 1% drop in temperature causes a 4% drop in emitted energy — the Earth is not really a black-body, but close enough that you can work with that for now). This means that the hotter something is, the more energy you must supply to increase its temperature another degree. That is a powerful stabilizer.
But black-body physics does not provide enough stabilization alone — the “faint young sun” paradox shows the importance of the greenhouse effect. Solar physicists are confident that the aging of the sun, as it burns hydrogen to helium, has caused the sun’s energy output to increase smoothly over time, starting from about 70% of the modern solar output at the time when the Earth formed. (Hydrogen fuses to helium, packing almost as much mass into a much smaller space in the center of the sun. This increases the sun’s gravitational pull on its outer layers, pulling the surrounding hydrogen more tightly towards the sun’s center. The fusion that powers the sun and converts hydrogen to helium requires that the hydrogen be packed tightly together, so the rising gravity makes fusion run faster, producing more energy.)
This result from solar physics yields the “faint young sun” paradox — assuming modern albedo and greenhouse effect, most of the Earth’s surface water should have been frozen for most of its history, but the available evidence shows that this did not happen. With an active hydrological cycle (as shown by the sedimentary record), hence clouds, there is no known way to lower the albedo enough to solve this problem, so the early Earth must have had a stronger greenhouse effect. (To offset solar output only 70% as large as today with the same greenhouse effect would require a perfectly black planet, not physically possible.) (The distance of the Earth from the Sun has changed a tiny bit over time, but not enough to really matter; collision with a Mars-sized body, such as the one believed to have blasted out material to form the moon, might have moved the planet a couple of percent of its distance from the sun; the meteorite that killed the dinosaurs would have moved the planet less than an inch.)
Rock-weathering stabilizes, too. Many things may have contributed to the stronger early greenhouse. A wide range of evidence indicates that the early atmosphere lacked abundant oxygen. (For example, pieces of minerals that break down rapidly in the presence of oxygen are found, not broken down, in old sedimentary deposits. The huge banded iron formations that we mine in places such as Minnesota have precipitated from ocean water long ago, but getting a whole lot of iron to the ocean in a dissolved form rather than as chunks of rust requires that the water carrying the iron lacks oxygen or rust would have formed. Also, “red beds” — rusty soils and other rusty sedimentary layers deposited above sea level — have formed commonly in “recent” geologic history but are very rare or entirely absent from the early Earth. And, there are still other indications that the early atmosphere lacked abundant free oxygen.) Carbon dioxide is a greenhouse gas, but per molecule and at concentrations vaguely similar to modern, methane is a more potent greenhouse gas than is carbon dioxide. (Raise the methane concentration a lot, and adding still more methane causes the new molecules to partly duplicate the job of existing molecules, just as for CO2 , so the importance per molecule of methane drops as the abundance rises, just as for CO2 and other greenhouse gases.) In the modern atmosphere, oxygen combines with methane over a decade or so to form carbon dioxide; for the early Earth, there may have been more methane and other reduced greenhouse gases because the oxygen wasn’t there to break them down.
The best-understood stabilizer, and the one most likely to have been important, was discovered by Penn State’s Jim Kasting and coworkers. This is the silicate weathering feedback. Volcanoes release carbon dioxide and volcanic rock, which is mostly silicate with a lot of calcium. Chemical processes (many involving biology, and generally lumped together as “rock weathering”) then recombine the carbon dioxide and rock to make dissolved materials that are washed to the ocean, turned into shell by living things (or deposited inorganically if there are no living things around to do the job, with inorganic deposition requiring somewhat higher concentrations in the water than organic deposition), deposited, then (eventually, over time scales of order 100 million years) taken down subduction zones or squeezed in obduction zones, where heating produces carbon dioxide and volcanic rock. (Metamorphic rock also may be formed, releasing carbon dioxide. For this broad-brush approach, metamorphic and volcanic rock are interchangeable.)
The formula is often oversimplified to:
which shows the volcanic rock and carbon dioxide being changed to shells (calcium carbonate is found in coral reefs, many foraminifera, clams and snails and others; silicon dioxide or silica is found especially in diatom and radiolarian shells and sponge spicules).
The transformation of these shells back to rock and carbon dioxide (draw the arrow the other way) doesn’t much care about the temperature at the surface of the Earth, but the recombination of volcanic rock and carbon dioxide goes faster in a warmer climate (almost all chemistry goes faster when it is warmer, and in this case the chemical kinetics are accelerated further by there being more rainfall on a warmer world because the reactions typically happen in water). Thus, if the temperature at the Earth’s surface increases, chemistry happens faster, removing carbon dioxide from the atmosphere and lowering the temperature back toward the original value. If the temperature falls, the removal of carbon dioxide from the air slows, the release of carbon dioxide from volcanoes continues unaffected, so the concentration of carbon dioxide in the air rises, increasing the greenhouse effect, and the planet warms back toward the original value. The time scale for this to work is something like 0.5 million years (more or less the residence time of carbon in the combined atmosphere-ocean system). This time scale may have changed over geologic history, but probably by no more than a factor of a few, not orders of magnitude.
Notice that the stabilizer of black-body radiation is almost instantaneous. The stabilizer of rock weathering takes hundreds of thousands of years to matter much. In between, we will see that amplifiers are more important.
Snowball Earth
Probably a few times, especially around 700 million years ago, the Earth seems to have come close to freezing over for a few million years at a time. Deposits of glaciers are found interbedded with marine sediments near the equator. (The Earth’s magnetic field is nearly horizontal near the equator and nearly vertical near the poles. When lava flows cool or sediments settle, the magnetization is aligned with the field and then “frozen in”. Because lava flows and sediment layers tend to be nearly horizontal, the angle between the layering and the magnetization tells the latitude when the rock formed. Near-equatorial sites of deposition for ice-related deposits have been found many times.)
We don’t think that the Earth rolled over on its side, so the planet must have been very cold. One intriguing hypothesis is that the snowball intervals represent rises in oxygen, which oxidized and thus removed chemically-reduced greenhouse gases, thus lowering the greenhouse effect (methane plus oxygen makes carbon dioxide plus water, the water rains out rapidly, and per molecule, the carbon dioxide is less effective as a greenhouse gas than the methane was, so rise of oxygen means fall of greenhouse effect—the greenhouse is still there, but just weaker!). A snowball could develop even with the rock-weathering feedback if the cooling was fast compared to 0.5 million years—a slow stabilizer can’t stop a fast cooling. Indeed, knowing what we do about the faint young sun and the slow rock-weathering feedback, we might even have predicted the occurrence of snowball-Earth events; if we make an analogy to sports, it is likely that the powerful but slow “defense” of the rock-weathering feedback would sometimes “lose” to a “fast-break” offense of climate change.
A snowball planet would have a very high albedo, and a few million years of volcanic carbon dioxide would be required to warm enough over a snowy surface to cause melting. The isotopic composition of carbon deposited during snowball events indicates that the biosphere was greatly reduced during the snowballs. (Today, plants use light carbon preferentially, so shells and the carbonate sedimentary rocks from shells end up with the heavy carbon that is left after the plants get what they want from the in-between carbon coming out of volcanoes. If the biosphere nearly stopped producing more plants, then essentially all of the carbon would be heading for carbonate rocks, probably the inorganic equivalents of shells, and so the rocks would have intermediate carbon isotopes, getting some of the light carbon that normally would go to plants, and this is observed with snowballs.)
Huge layers of odd carbonate deposited on top of the snowball layers seem to be formed from the immense amounts of carbon dioxide released during the snowball intervals. Once the snowball melted from millions of years of volcanic carbon dioxide, the warm temperatures and high carbon dioxide would have caused very rapid, extensive rock weathering, supplying immense quantities of materials to the ocean to make carbonate rocks. Thus, the snowballs show that the rock-weathering feedback works, but slowly. And, the rock-weathering feedback relies on the warming effect of carbon dioxide.
Note also that we don’t see any way that the modern Earth is heading soon for either a snowball or a Venusian runaway greenhouse, although if you look forward hundreds of millions to billions of years, a runaway Venusian greenhouse becomes likely as the sun continues to brighten. (Oddly enough, if you removed all the carbon dioxide from the air today, you probably would get a snowball. Removing the carbon dioxide would cause cooling, which would remove much of the water vapor, causing more cooling. If you removed all the water vapor, the oceans would put more up before the Earth could freeze over. So, while the water vapor contributes more of the greenhouse effect today than does carbon dioxide, the carbon dioxide is arguably the most important greenhouse gas because it controls a lot of the water vapor.) (Note also that the study of snowball-Earth events is very difficult, with only rare records of relatively short-lived but old events. The science is evolving rapidly, and some of what you read just above may be modified fairly quickly.)
Nature Has Changed Carbon Dioxide a Lot
Nature has changed carbon dioxide a lot, but slowly, and climate has responded rapidly. Younger than the snowballs, over the last half-billion years or so, we have had an atmosphere recognizably similar to the modern one in having oxygen. (You need a lot of oxygen to allow big critters, and there is a rich fossil record of big critters over the last 500 million years. Too much oxygen and everything burns rapidly, but there is a rich fossil record of unburned things.) The rate at which geology recycles shells to make carbon dioxide and sends that carbon dioxide out to make volcanic rocks can change — big belches of hot rock from deep in the mantle can occur, for example (there is carbon dioxide down there, and if a hot-spot plume head hits the surface to feed giant flood basalts, a lot of carbon dioxide can come out), and the collisions between continents that make a lot of metamorphic rocks happen only occasionally. (If North America and Asia continue moving towards each other as rapidly as your fingernails grow, another big collision may occur in a couple-hundred million years!) If there are no big mountains, soil builds up and the carbon dioxide in the air may have trouble getting all the way down to attack rocks and cause weathering. If the mountains are high, much of the soil can wash or slide off, exposing rocks to faster weathering. And, the mere accidents of geology might matter — shales at the surface don’t weather very rapidly, carbonates weather to produce carbonate shells in the ocean with no net change, but weathering of many volcanic rocks can be fast and remove carbon dioxide from the air, so the geologic accidents that control what rocks are at the surface may affect the setting of the rock-weathering “thermostat”.
And, evolution can affect how rapidly carbon is stored to make fossil fuels (which naturally release their carbon back to the atmosphere when erosion brings them to the surface and living things “eat” them.) There is a fascinating hypothesis that the great coal beds of Pennsylvania and many other places, which formed during the “Carboniferous” — the Mississippian and Pennsylvanian Periods — record the evolution of successful plants containing really hard-to-eat woody structures, and that coal formation was rapid but then slowed greatly tens of millions of years later as termites and fungi and other things evolved to break down those woody structures. When fossil fuels are being formed, carbon is being transferred from atmospheric carbon dioxide to oil or coal or natural gas in the ground, and when fossil fuels are being burned, the carbon dioxide is going back into the air. The time scale for lots of evolution to occur, or for lots of rearrangement of continents to occur, is sort of 100 million years, so it is not surprising that changes between high-carbon-dioxide and low-carbon-dioxide times have typically taken about 100 million years. There is no evidence for true cycles (no tick-tick-tick of a clock, such as we see with day-night or summer-winter), but lots of evidence that the changes in carbon dioxide occurred over the time scales one would expect given knowledge of the causes — the world does make sense.
Carbon dioxide has been the main driver of climate change on this hundred-million-year time scale. A statement such as this involves pretty much all of climatology and paleoclimatology. The general path is:
Reconstruct the history of past temperatures, which requires reading the temperature history in sediments, and knowing the time when the sediments were deposited. This can be done with considerable confidence; old crocodile-like critters on Ellesmere Island, very close to the North Pole are a pretty good indication that it wasn’t too cold there then.
Reconstruct the history of past carbon-dioxide concentration in the atmosphere, again requiring ages as well as indicators of the atmospheric composition. Before ice cores (and the oldest ice core is less than 1 million years), the indicators of carbon dioxide in the atmosphere are not as clear as we’d like, but considerable agreement from several lines of evidence allows us to tell in general when carbon dioxide was high or low, and to make some quantitative estimates. (For example, plants “prefer” the lighter carbon-12, which diffuses and reacts more rapidly, so when carbon dioxide is common, plants are especially enriched in carbon-12; when carbon dioxide is rare, plants have to use more of the carbon-13. Special cell-wall molecules in the ocean, and soil carbonates, and remains of some water plants from lakes, are used to learn the carbon-12:carbon-13 ratio and hence the carbon dioxide level. Leaves grow fewer “breathing holes” — stomata — when there is more carbon dioxide in the environment because stomata lose water while gaining carbon dioxide, so when carbon dioxide is high, plants can save water. Rising carbon dioxide shifts the ocean toward greater acidity, and this affects whether the little bit of boron in the ocean is as B(OH)3 or B(OH)4-1. The charged form substitutes more easily into carbonates, so the ratio of boron to calcium in a shell increases as the carbon dioxide drops. In addition, the charged ion of boron preferentially holds the light isotope boron-10 in comparison to the heavy boron-11. The residence time of boron in the ocean is many millions of years. Over shorter times, a drop in carbon dioxide will shift most of the boron in the ocean to the charged form, so its isotopic composition must become heavier as it comes to match the whole-ocean value, and the charged form is included in carbonates. There are other ways to get paleo-carbon-dioxide as well.)
Assess the correlation. The simple answer is that the correlation is not perfect, but is pretty darned good. There is a broad and shallow “skeptic” literature that plays with the estimates and dates to get fairly poor correlations, but the reputable sources (e.g., the IPCC Working Group I Fourth Assessment Report, chapter 6, at IPCC [29]) show a rather tight coupling.
Attribute the correlation. Does the correlation match expectation from physical understanding? And, is there any other plausible explanation for the correlation, such that the correlation is a fluke, or the correlation arises because something else is controlling both temperature and carbon dioxide? This is the hardest one, and is never complete, because there always might be a new explanation that we haven’t thought of. But, we have known for more than a century that more carbon dioxide should make it warmer, based on fundamental physics that just won’t go away. The reconstructed warmings of the past actually are just about the size expected from our understanding of the effects of carbon dioxide (if there is a problem, the world changed a bit more than we might have expected). And no plausible hypothesis has been proposed that explains what happened without including the carbon dioxide. Moving continents around on the planet, opening and closing “gateways” to affect oceanic circulation, changing land albedo with plants, and other possibilities appear to be “fine-tuning” knobs on the climate, all mattering, but not mattering enough to explain the history by themselves or combined but ignoring carbon dioxide. Whether calculated on the back of an envelope or in a full Earth-system model, these non-carbon-dioxide effects do not suffice to explain the changes reconstructed from the features of the rock record, nor do other possible causes correlate well in time with the changes that happened in the climate.
Changes in carbon dioxide and other things can matter a lot to life. The early geologists named time intervals in geologic history, and the rocks deposited during those time intervals. Name changes were chosen at key times. The end of the Mesozoic, for example, is now known to have been caused by a huge meteorite impact that killed the dinosaurs. The end of the Paleozoic killed even more living things, and seems to have been linked to carbon dioxide. The last Period of the Paleozoic Era is the Permian, and the end-Permian extinction was the biggest mass extinction. Some uncertainty remains, but the leading hypothesis now is that a “plume head”, the mushroom-shaped top of a new hot spot bringing heat and mass from deep in the mantle, produced the Siberian traps, a vast basaltic lava-flow province, the biggest known. Carbon dioxide released by this volcanism increased the Earth’s temperature. The new rocks were easily weathered, fertilizing the ocean. Sulfur released by this affected chemistry. The warming from the carbon dioxide reduced the oxygen content of the ocean, and the warming caused the surface waters to “float” more strongly, reducing the ocean circulation taking oxygen to the deep ocean. Large areas became anoxic and euxinic, producing hydrogen sulfide, which is poisonous to many, many things. Certain bacteria, called Chlorobiaceae, or green sulfur bacteria, use hydrogen sulfide instead of water in photosynthesis, and make distinctive organic molecules. These molecules are found in sediments from shallow oceans at the end of the Permian, indicating that poisonous hydrogen sulfide was widespread. (No serious science yet suggests that human carbon dioxide could cause such a disaster, but our actions can contribute to spread of “dead zones” in the ocean that are in some ways analogous. And, note that we are releasing carbon dioxide faster than we believe the volcanoes released it at that time.)
Perhaps without going all the way to poisonous hydrogen sulfide, other times have produced low-oxygen marine environments that allowed deposition of organic-rich material that would have been eaten and burned if oxygen had been higher. The sediments are often black shales, and the “fracking” for natural gas now going on is exploiting the carbon in these deposits. Warm temperatures favor such anoxic events, including the oceanic anoxic events (OAEs) of the saurian sauna of the Cretaceous Period. Note that such deposition tends to lower the carbon dioxide in the air, leading to subsequent cooling. Coal formation also will tend to lower carbon dioxide in the air and favor cooling.
Faster changes in carbon dioxide have occurred, again with higher carbon dioxide causing warming. The best-documented of these is the Paleocene-Eocene Thermal Maximum (PETM). Temperature indicators show warming over a few thousand years, with warmth persisting for 200,000 years or so. Carbon dioxide shows the same history. Isotopic indicators suggest that the carbon dioxide came from volcanic and biological sources. The rapid warming and carbon-dioxide increase came with an acidification of the ocean (carbon dioxide and water make a weak acid), and with a major extinction event for bottom-dwelling types; extinction appears to have been in response to the climate change, with no plausible way that the extinction could have somehow caused the climate change. The most-likely source of the carbon was a large amount of volcanic activity, linked to the “unzipping” of the North Atlantic, especially between Greenland and Europe, with melted rock squirting into sediments loaded with organic material (oil, coal and gas). And, the warming then seems to have released more carbon that was stored in plants, or soils, or sea-bed methane deposits. (At present, plants hold about as much carbon as does the atmosphere, soils somewhat more, and seabed methane more. Anything that caused a notable transfer of carbon from one of those other reservoirs to the atmosphere is in principle capable of explaining the event, including permafrost in Antarctica at the time. Note that the PETM is the biggest and fastest such event over very long times, so a coincidence may have been involved — if causing the PETM was easy, more PETMs would have happened over the vast span of Earth’s history.) The PETM and other abrupt events of the past point to the importance of carbon dioxide in temperature (they were far too fast for continental drift to have mattered, for example), and provide time scales for possible feedbacks in the carbon cycle (not fast enough to control the atmosphere on the time scales of decades to centuries over which human societies operate, but fast enough to matter on those time scales).
Greenhouse to Icehouse
The planet slid from greenhouse to icehouse over the last hundred million years as carbon dioxide fell. The dinosaurs lived on a high-carbon-dioxide, hot world. We have long known that the poles were ice-free in dinosaur times. Early studies indicated that the equator then was not much hotter than today, but those early studies came with the warning that the main indicator used (isotopic composition of planktonic foraminifera) was subject to alteration after deposition that might have turned an indication of “hot” into an indication of “warm”. Recent studies, using other indicators and using very careful searches for unaltered foraminifera shells, are now indicating “hot” in the tropics during dinosaur times. The work is ongoing, and a full consensus is not in, but tropical temperatures so hot that un-air-conditioned humans would have found it uncomfortable or even fatal to live on much of the planet now seem possible or even likely. Carbon dioxide remains the best explanation of the warmth, although current models, when given best-estimate carbon-dioxide loadings then, tend to simulate worlds a bit cooler than data indicate; whether this indicates shortcomings in data or models is unknown.
The planet saw widespread ice appearing at the poles about 35 million years ago, and generally carbon dioxide dropped and ice spread until recently. Details of that correlation remain unclear, with some central-estimate reconstructions indicating that some climate features are difficult to explain based on carbon-dioxide changes alone, but with the error bars including a carbon-dioxide explanation. (And the overall trend from greenhouse to icehouse is quite clearly a carbon-dioxide story. Furthermore, as more data have been collected, and better data, the mismatches between estimated carbon-dioxide level and estimated temperature have gotten smaller.)
Regionally, large and interesting changes occurred for reasons unrelated to carbon dioxide. The modern “conveyor belt” circulation in the Atlantic, for example, with surface flow directed northward from the Southern Ocean to near Norway, sinking, and return deep, does not seem to have existed more than a few million years ago when a seaway connected the Atlantic and Pacific Oceans across what is now Central America. (Now, the atmospheric transport of water vapor in the Trade Winds across Central America is not balanced by a return flow in the ocean beneath, so the Atlantic is saltier than the Pacific, and the “conveyor” circulation re-establishes the oceanic balance. With an open seaway across Panama, a much more direct route was possible. And, without the conveyor circulation, oceanic currents and coastal climates would have been quite different, although without a large globally averaged temperature change from the different currents.) In the ice-house world of the last few million years, Milankovitch cyclicity has driven ice-age cycling. The Earth’s orbit has many interesting features. These come from a few sources. First off, there are lots of planets out there, and some big ones. And all the planets run around the sun at different speeds. If you think of the solar system as a horse race, we keep passing Jupiter on the inside, and every time we pass, its gravity tugs on us a bit. The sum of all the tugs changes the Earth’s orbit a bit, giving the eccentricity changes described below. In addition, the rotation of the Earth causes the equator to bulge a bit. The planets, the sun and the moon (mostly the sun and moon) tug on this bulge, and that gives us the changes in obliquity and precession, just like a spinning top. As you might guess now, the important orbital features for this discussion are:
Obliquity
Think of an air-hockey table. Put the sun in the middle, nailed down, tie the Earth to it with a string, and hit the Earth. The Earth will zing around the sun. Put a little pin in the top of the Earth to be the North Pole. If you put the pin sticking straight up, you’re not there yet. The pin is inclined 21 degrees to 24 degrees from straight up, depending on when you look, going from 24 degrees to 21 and back over about 41,000 years. The larger the angle, the more the sun can shine on the North Pole (and on the South Pole, when the Earth is on the other side of the sun on the orbit!), and the less sun hitting the equator. This 41,000-year obliquity cycle moves some of the sun’s energy from equator to poles and back.
Eccentricity
The air-hockey orbit in the previous section isn’t right; the orbit is eccentric (non-circular elliptical; think of a NASCAR track, although with a little curve even on the “straightaways”). A non-circular ellipse has two foci; think of two towers in the infield, both halfway between the straightaways, one a bit right and one a bit left when viewed from the main grandstand. The sun will sit at one of those tower positions (and the sun does not jump back and forth between the towers; it stays put). But, this is a weird NASCAR track; come back later, and the shape is changed a bit, going from almost circular to more squashed and back to almost circular over 100,000 years. (There actually is a 400,000-year modulation, so almost circular-slightly squashed-almost circular-more squashed-almost circular really squashed-almost circular-some squashed....) This change in eccentricity changes the total amount of sun reaching the planet a tiny bit; if you were in one of the towers, and the track were really squashed, the cars would spend a lot of time at the end far away from you where you had trouble seeing them, and only a little time at the near end, and if the cars are counting on being warmed by the “sun” from you, the extra time they spend far away reduces the total sun they receive. For the tiny changes in the Earth orbit, this is only a tenth of a percent or so in total sun received.
Precession
You may remember from the description of obliquity that the North Pole is inclined a bit. In addition to this angle changing, the North Pole also wobbles. Imagine putting your feet against a metal stake in the ground, grabbing the stake with your hands, leaning out until your arms are straight, and then having a friend push you in circles around the stake. Imagine a North Pole sticking up out of your head, extending your spine. The metal stake is “straight up”. If you bend your elbow and pull yourself toward the metal stake, your North Pole will point more nearly in the same direction as the metal stake, because you have changed your obliquity. But if you hold your obliquity the same (don’t bend your arm any more), and your friend pushes you around the metal stake, you are precessing.
Now, suppose you were doing this (metal stake, friend and all) on top of a NASCAR racer, with the sun in one of the towers in the infield. Your friend would have to push you really slowly, the drivers would make about 10,000 laps before you got halfway around the metal stake! But notice that you would slowly switch from being on the infield-side of the metal stake when the car was at the end of the track closest to the sun tower (summertime for your North Pole, and wintertime for your South Pole), to being on the outside of the metal stake at that closest approach and on the near side of the metal stake at the farthest distance from the sun tower. This is precession. Notice that if you are close in northern summer, you are far in northern winter, giving a big difference between seasons in the north, but that close in northern summer is close in southern winter, and far in northern winter is close in southern summer, so when the winter-summer difference is large in the north, the winter-summer difference is small in the south, and when the winter-summer difference is small in the north, it is large in the south. Also, your friend is not pushing you with perfect consistency (and, bizarrely enough, the whole track is actually turning slowly, so that the straightaways switch slowly from being mostly north- south to being mostly east-west and on around to north-south again), so that rather than making a full circle of your metal stake every 20,000 laps or so, you typically make a full circle after either 19,000 laps or 23,000 laps. Also notice that, if the orbit/NASCAR track were a perfect circle, the two towers would be exactly in the center, the distance of the car from the tower sun would never change so that this precession would not matter at all. Thus, precession matters a lot when the orbit is very eccentric, and precession matters little when the orbit is nearly round.
As scientists came to understand the Earth’s orbit and spin, calculation of the effects of these orbital features on the distribution of sunshine on the planet became possible. The most complete pre-computer treatment came from Milutin Milankovitch, so these are usually called Milankovitch cycles. Milankovitch predicted that, when ice-age cycles were understood, it would be found that the climate had varied with periods of 19,000 years, 23,000 years, 41,000 years and 100,000 years. Several decades later, when isotopic records of oceanic foraminifera were developed, these very periodicities were discovered — Milankovitch was right! And, because the different cycles affect north and south, and poles and equator, differently, it is possible to tell where the main controls reside.
The leading interpretation now is that poles are more important than equator, and north more important than south. When Canada and Eurasia received little summer sun, ice grew, and the world cooled globally; when the sun increased in the high latitudes of the north, the world warmed and the ice shrank. The changes have been large — roughly 5 C to 6 C globally averaged — and switching from the modern level of about 10% of the land under ice (Greenland and Antarctica, primarily) to about 30% of the modern land area under ice (with glaciers over Erie and the Poconos in Pennsylvania, among many other places — note that when the ice spread, sea level fell, revealing land that is now under ocean, such that the total non-ice-covered land area was about the same then as now).
Oddly enough, northern sun has been more important than southern sun, with cooling in the south during some times when sunshine was increasing there. Many people have tried to explain this odd behavior in many ways, but so far, the only successful explanations involve carbon dioxide. (The high albedo of the expanded ice contributed to the cooling, as did the sun-blocking effect of extra dust, plus shifts from trees to grasslands or tundra with higher albedo, but these together don’t explain the whole signal; the carbon dioxide, and a bit of methane and nitrous oxide change, were important.) Whenever the ice sheets have grown in the north in response to reduced sunshine there, carbon dioxide has dropped, and the carbon dioxide provides a successful explanation of the changes in the south. The path is: changing sunshine to changing things in the Earth system (temperature, ice volume, sea level, dust, etc.) to changing carbon dioxide to more changes in temperature in response, so the carbon dioxide is a positive feedback, not a cause.
The processes by which changing ice volume affects carbon dioxide are rather complex, involve many different pieces of the Earth system, and are a bit beyond our course. One, for example, is that ice-sheet growth in the north increases dust supply to the ocean (the glaciers grind up rocks, change winds, etc., increasing dust delivery, especially in the north where there is a lot of land to make dust), which fertilizes plankton that turn carbon dioxide into plant, the plankton are eaten, the eaters poop, the poop sinks, and so carbon dioxide is moved into the deep ocean and away from the atmosphere, lowering atmospheric carbon dioxide. There exist many other mechanisms — covering 20% of the land with two-mile-thick ice sheets, lowering the sea level by several hundred feet, changing winds and currents, spreading sea ice in the cold, and other things constitute large perturbations to the Earth system, and it responded in a way that amplified those changes. The most important changes probably relate to shifts in southern winds — now, the winds howl around Antarctica, moving water to their left, hence north because of the Coriolis effect on our eastward-rotating Earth, and driving upwelling that brings CO2 back from the deep ocean, but during ice-age times the winds shifted up on South America and so left more CO2 in the deep ocean, lowering the atmospheric level.
The “skeptics” of climate change are fond of pointing out that temperature change probably started slightly before carbon dioxide change, and then concluding that carbon dioxide cannot be responsible for any of the warming. This is faulty logic, but of the sort that seems sensible to people who know nothing about the subject. (Suppose you run up a big debt on your credit card, and then you end up paying lots of interest on the debt until you go bankrupt. By the skeptic logic, you went into debt before you started paying interest, so the interest cannot have contributed further to your debt because the interest payments lag the debt in time. Wrong.)
A lot of very interesting questions are not fully answered with regard to the ice ages. But, the big picture is clear. The ultimate cause is tied to Milankovitch orbital features, which change the total amount of sunshine reaching a place during a season by 10-20% or even more (although with tiny globally averaged effect). Many things happen in response to this cause, and carbon-dioxide response is especially important in the global signal. (Growing ice in Canada doesn’t directly make it much colder in Antarctica, but changing carbon dioxide does.) The changes have been large but slow. The 5 C to 6 C warming (10 F warming) from the last ice age, globally averaged, took over 10,000 years, or less than 0.1 F/century; the warming of the last century, tied especially to human activities, has been ten times faster, and the warming in the next century if we don't change our energy system is expected to be faster yet. Similarly, the carbon-dioxide changes of the ice ages were much, much slower than what humans are now doing. The ice ages provide further evidence of the warming effect of carbon dioxide, they allow us to test our models (which work pretty well), but they don’t provide any alternate explanation of recent temperature changes.
Abrupt Changes Have Punctuated Climate History
Abrupt changes have punctuated climate history. An abrupt climate change is one that occurs faster than its cause, or comes so rapidly that ecosystems or economies have trouble adapting. Abrupt climate changes can involve sudden onset of droughts, collapse of ice sheets, or other features of the climate. Studies have especially focused on the North Atlantic events that punctuated the last ice-age cycle (and, probably, earlier cycles).
In the modern world, the relatively salty Atlantic waters become dense enough in the winter to sink in the far northern Atlantic, and then flow south, while warmer surface waters flow north in replacement. Because of this, the North Atlantic Ocean does not freeze in the wintertime even at high latitudes, so the surroundings remain relatively warm all winter. While the “frozen tundra” of Lambeau Field in Green Bay becomes almost too cold for even American football at 45 N latitude in a Wisconsin winter, the Manchester United football/soccer team runs around in shorts at 53 N latitude in England through the winter. The differences in climate between England and Wisconsin arise from several processes, but it is a safe bet that if the North Atlantic Ocean froze in the winter, Manchester United would not be playing a wintertime season.
There is widespread agreement across a range of climate models, from the simplest to the most complex, that a sufficiently large freshening of the North Atlantic under modern or lower carbon-dioxide concentrations would allow wintertime freezing, changing the oceanic and atmospheric circulation. Furthermore, many models find that the climate undergoes jumps — a gradual freshening can lead to a sudden onset of freezing, which will persist through many winters and then terminate suddenly (in as little as a single year, to a few decades). (In many models, the onset of wintertime freezing occurs with loss of the conveyor-belt circulation, but the continuing Trade Winds across Panama increase saltiness in the Atlantic until the conveyor-belt turns on again.)
The data agree with the models. In the past, large floods from ice-dammed lakes, or surges of the ice sheet in Canada, or slower melting of Canadian ice, have delivered extra fresh water to the North Atlantic and led to loss of the conveyor-belt circulation, allowing wintertime freezing in the North Atlantic, and bringing widespread climate changes. These include very strong cooling in wintertime around the North Atlantic, slight cooling around most of the Northern Hemisphere, slight warming in the Southern Hemisphere (the conveyor-belt takes sun-warmed water from the South Atlantic to cool in the north-Atlantic winter, so shutting down the flow gives cooling in the north but warming in the south), a southward shift of the tropical circulation pattern, hence strong drying in the places left behind by the intertropical convergence zone (the ITCZ) and strong wetting in the places to which it moves, and general loss of rain in the monsoonal regions of Africa and Asia. Small northern glacier readvance was observed in such events during the termination of the last ice age, but with no ability to return to the ice age (glaciers mostly care about summertime temperatures, but loss of the conveyor primarily cools northern wintertime). Global-average effects of a conveyor shutdown were small-a bit more cooling in the north than warming in the south, with ice-albedo feedbacks important.
There has been much discussion of whether such an event could occur in the future. A shutdown would affect ocean currents, fisheries, etc., no matter when it occurred. If a shutdown waited too long into the future, the carbon-dioxide warming would largely block wintertime freezing, and with it the big amplifier of climate change. Model results generally show that a shutdown is more likely in a colder climate, and is more likely when a big ice sheet sits on Canada, steering winds towards Spain rather than Norway. Most models of the future agree that melting of Greenland’s ice and other processes will weaken but not shut down the conveyor-belt circulation, and the Intergovernmental Panel on Climate Change (IPCC) in 2007 assessed a <10% chance (but not zero) of an abrupt change over the next century. The movie The Day after Tomorrow surely was not accurate. (But, if your heroes are larger than life, maybe your problems must be larger than life to make an entertaining movie.)
The Holocene shows stability when carbon dioxide was not changing. After the last ice age ended (with the warming beginning about 24,000 years ago and most of the warming completed by 11,500 years ago), we entered what is called the Holocene. Temperature- wise, fluctuations have been small, except for one brief blip about 8200 years ago corresponding to the last of the outburst floods from a lake dammed by the dying ice sheet in Canada.
Not much happened to greenhouse-gas concentrations during most of the Holocene. The Holocene temperature record is well-explained through the influence of changing orbits (more midsummer sunshine in the north a few thousand years ago than more recently), volcanic eruptions (a degree or so cooling for a couple of years from a big eruption that loads the stratosphere with sun-blocking particles; a few eruptions close together can make enough cooling to matter) and solar fluctuations (reconstructed from sunspot observations, using the recent correlation between satellite-measured solar output and sunspot numbers, or reconstructed from beryllium-10 or carbon-14 using the relation between sunspots, the solar wind, and the penetration of cosmic rays that form those isotopes). Some evidence points to a role for changes in the conveyor-belt circulation, which may act to amplify the other causes by slowing slightly in colder times. Searches for influences from changes in the Earth’s magnetic field, from cosmic dust or cosmic rays, or other causes have come up empty; the paleoclimatic record continues to point to a sensible, understandable climate system. (Farther back, about 40,000 years ago, the magnetic field dropped to near zero for a millennium or so, cosmic rays streamed in to create a large spike in beryllium-10, but the climate ignored it, which argues against any serious role for cosmic rays or the magnetic field.)
The Last Century - Humanity Takes Over
Since 2007, every report from the UN IPCC has concluded that warming of the climate system is “unequivocal”. Thermometers show warming, including thermometers far from cities (so it is not just an urban-heat-island effect), thermometers in the ground, in oceans, on balloons, and looking down from satellites. Most of the world’s ice is shrinking, including in places getting more snowfall. Most of the changes in where different things live, and when they do things during a year, are moving in the direction expected with warming. Models forced with the known natural causes match changes in the late 1800s and early 1900s, but not since. Adding human forcings gives a very good match to what happened all the way along. This match includes not merely global-mean surface temperature, but also many aspects of the “fingerprint” — regional temperature changes, vertical temperature changes, oceanic temperature changes, etc. Note that the whole forcing must be included; particles from smokestacks do the volcano job of blocking the sun, but don’t stay up very long, whereas greenhouse gases warm the climate and stay up longer. (The cooling after World War II was forced by human-produced aerosols, based on available information. And the idea that scientists were warning about global cooling in the 1970s, so beloved of the “skeptics”, is a misrepresentation. Newsweek ran an article on this, and some interesting science was being done on ice-age cycling, and on cooling by particulates, and the possibility of a “nuclear winter”, but the scientific community was already primarily focused on warming at that time, and never released any consensus documents pointing to cooling. And while Newsweek may be a respected general-information source, it is NOT a respected scientific source.)
Suppose, for a moment, that you believe the sun has caused the recent warming. There is no support for this in the data; almost 30 years worth of satellite data show no trend in solar output, or a very slight drop, while temperatures on Earth were going up. But suppose you believe that the satellite data are wrong, that the sun has been getting brighter, and that the temperature changes on the planet are solar-caused. A clear prediction of this solar model is warming in the stratosphere as well as in the troposphere, as more energy is added to both. But a greenhouse-gas hypothesis points to tropospheric warming coupled to stratospheric cooling, as the greenhouse gases hold the energy closer to the surface and radiate from high elevation. So, what do the data show? Tropospheric warming-and stratospheric cooling. The “fingerprint” is human, not solar.
The future looks warmer, unless we change our behavior. Everywhere and everywhen we look, more carbon dioxide makes it warmer. This is a fundamental result of physics — there is no serious suggestion that this could be wrong, and extraordinarily strong evidence that it is right. The data agree; warmth and high carbon dioxide have gone together, the warmth is explainable through the known effects of carbon dioxide, and the warmth is not explainable if the effects of carbon dioxide are omitted. When carbon dioxide has been fairly constant, small effects from sun, volcanoes, and perhaps other things have been evident, but these have acted more as fine-tuning knobs than as coarse adjustments.
The planet’s climate is stabilized strongly by the black-body radiative feedback over very short times, and by the feedbacks involving rock weathering and carbon dioxide over very long times. Between, the feedbacks are largely amplifying.
The biggest amplifier is linked to water vapor. At higher temperature, the saturation vapor pressure is higher, and the “kinetics” (evaporation if dry air is over water) are faster. Over the ocean (which is most of the planet), relative humidity is more-or-less constant (the wind mixes dry air down from above into the wet air below, so the air holds most but not all of the water for saturation), and warmer places thus have more water vapor. Warming is increasing water vapor. And water vapor is a powerful greenhouse gas. Humans cannot change water vapor very much directly — the residence time is barely over a week, so the water vapor we put up comes down quickly — but by changing the temperature through other greenhouse gases, we can change atmospheric water vapor because there is an immense ocean out there to respond to the warming by putting more water in the air.
The ice-albedo feedback is straightforward. With warming, snow and ice melt, and that increases absorption of sunlight in the Earth system, warming the planet. Vegetative feedbacks also can matter — we may have cooled the planet a bit by replacing dark forests by more-reflective croplands — but vegetative feedbacks can’t be really huge (they are limited to land, and that particular trees-to-crops switch is limited to croplands). Clouds bring the biggest uncertainties, but the main circulation pattern of the Earth is highly stable, hence the upward and downward motions of air fairly well fixed; hence one cannot make immense changes in cloud easily.
Comparing various models indicates that, if we start from a stable climate similar to that of the Holocene, and then double carbon dioxide with no other forcings, and let water vapor, snow, cloud, etc. respond, the planet will average about 3 C warmer. The direct effect of the carbon dioxide is just over 1 C with the rest from feedbacks. The uncertainty is usually given as 1.5 C, although increasingly it appears that the lower end of that (warming from doubled carbon dioxide being less than 2 C) is more likely to be wishful thinking than science. Efforts to match the history of the last century, of the ice-age cycling, and of longer times, generally agree with the models, strongly reject lower values, but typically include a small possibility of a larger or much larger sensitivity (so things could be a little better than the central estimate, a little worse, or much worse). With enough fossil fuels still in the ground that we could quadruple atmospheric carbon dioxide, and perhaps octuple, and with each doubling of atmospheric carbon dioxide having a roughly equivalent effect on temperature, a central estimate of warming in a burn-it-all future may approach 10 C or more than 15 F; if we burn it all, and the climate is really insensitive, we may get only half of that warming, or we may get twice that much. (And, remember that the difference between the ice-age world and the recent one was about 10 F. Note that we won’t get the full equilibrium warming because the ocean takes a while to warm up, but carbon dioxide stays up long enough that we are likely to get most of the equilibrium warming.)
The “so what” part of this takes a lot more discussion, which won’t all fit here. In general, warmer temperatures are likely to bring less winter, more summer, sea-level rise, more droughts and more floods (fewer precipitation events with more water in them), drying in grain belts in summer, potential spread of tropical diseases, loss of ecosystems and species. Initially, there is not likely to be too much economic impact in the cold places where vigorous economies are driving the change, but negative impacts in the warm places where great numbers of people live. Eventually, harm is expected to spread to almost everyone almost everywhere.
Economic analysis of these issues is much cruder than physical-scientific analysis (in part because the uncertainties in the physical science are magnified in the economics). Typically, analyses show that an optimal response (considering only the economy, and not ethical issues) involves at least some investment to reduce greenhouse-gas emissions now. A complete fix is often priced at around 1% of the world economy, after a few decades of serious effort.
Of course, this is science, not revealed truth, and is subject to errors. The distribution of possible outcomes is “interesting” — things could be a little better than sketched here, or a little worse, or a lot worse. North Atlantic shutdowns, hundred-year droughts, ice-sheet collapses, and climate sensitivity of 4 or 5 C rather than 3 C for doubled carbon dioxide are clearly within the range of outcomes consistent with current knowledge, whereas no- change or tiny-change worlds are not.
A Parting Thought
A parting thought (remember that this History-of-the-World Enrichment is from Dr. Alley’s more-advanced class, and some of the policies and other issues are covered later in our class): Humans have almost always succeeded in solving problems by being smart. We have a problem with energy-supply and global-warming issues. Our understanding of the problem is very good — much better than the basis underlying many laws and budgets that are passed by our elected officials. Humans have occasionally failed spectacularly by not solving problems, by not being smart enough. These observations may have implications for wise paths forward.
A few sources:
Broecker, W.S. 2002. The Glacial World According to Wally, Third Revised Edition, Eldigio Press, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY.
Intergovernmental Panel on Climate Change Reports, IPCC [29].
National Research Council, US National Academy of Sciences Reports. In particular, see Abrupt Climate Change: Inevitable Surprises, 2002; and Climate Change Science: An Analysis of Some Key Questions, 2001, available online at The National Academies of Science, Engineering, Medicine [30].
Royer, D.L., R.A. Berner, I.P. Montanez, N.J. Tabor and D.J. Beerling. 2004. CO2 as a primary driver of Phanerozoic climate change. GSA Today, 14(3), 4-10.
Alley, R.B. The Two-Mile Time Machine. Princeton University Press. 2000.
Alley, R.B. The Biggest Control Knob [31]. 2009 Lecture, American Geophysical Union.