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)

Past and Future CO₂ Atmospheric Concentrations

Click for a video transcript of "Past and Future CO₂".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Future CO2 [2]." YouTube. October 6, 2012.

Source: 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.

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

Video: Global Surface Warming (2:23)

IPCC Figure SPM 5. Solid lines are multi-model global averages of surface warming (relative to 1980–1999) for the scenarios A2, A1B, and B1, shown as continuations of the 20th-century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at the year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios. The assessment of the best estimate and likely ranges in the grey bars includes the AOGCMs in the left part of the figure, as well as results from a hierarchy of independent models and observational constraints.

Click for a video transcript of "Global Surface Warming".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Warming to the Future [3]." YouTube. November 19, 2014.

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

Video: Projected Surface Temperature Changes (2:55)

IPCC Figure SPM 6. Projected surface temperature changes for the early and late 21st century relative to the period 1980–1999. The central and right panels show the AOGCM multimodel average projections for the B1 (top), A1B (middle) and A2 (bottom) SRES scenarios averaged over the decades 2020–2029 (center) and 2090–2099 (right). The left panels show corresponding uncertainties as the relative probabilities of estimated global average warming from several different AOGCM and Earth System Model of Intermediate Complexity studies for the same periods. Some studies present results only for a subset of the SRES scenarios, or for various model versions. Therefore, the difference in the number of curves shown in the left-hand panels is due only to differences in the availability of results.

Click for a video transcript of "Projected Surface Temperature Changes".

DR. RICHARD ALLEY: This is a moderately complicated plot that comes from the IPCC, and we're going to look at a few things here. This is not much CO2 in the future relative to what's possible. And so you see how much warming might occur for the global average out to 2020 to '29, that decade, and how much warming might occur out at 2090 to 2099 in degrees Celsius, which are listed along the bottom here.

And then these maps correspond here on the right to the warming from 2020 to 2029, the average of that decade, and what you expect late in the century that we're in now for this low emissions. And then here's the same thing for somewhat higher emissions of CO2. More warming. And here's one where we really keep burning like crazy. I'm just going to walk you through that when all of them show about the same thing, but we'll start down there.

First thing to notice is that these are how much warming is possible by late in the century here, and they show probabilities. The highest point is the most likely, and then there's a slight chance of having low values like this or low values like this. What I hope you notice is that the warming could be a little bit less, or it could be a little bit more. Very, very unlikely to be a lot less, but it is possible to be a lot more.

And so there's a lop-sidedness in this. And if the scientists are wrong about what's most likely, then it's more likely that we'll get more warming. More likely more warming than what people have been telling you. OK. That's important first.

Second thing. The average warming here is just over 3 degrees Celsius, which is sort of this color, which is what you'd get out here in the ocean. Most of the world is ocean. The global average is not what happens on land where we live. It's what happens primarily in the ocean.

What you will notice is that all of the colors over here tend to be darker in red colors than what's in the ocean when you go up on land. Almost everyone on the planet gets above average warming because the land warms more than the ocean, and almost everyone lives on the land rather than in the ocean. So when people tell you the global average warming they expect, in some sense, that's very optimistic because it's the low end of what's possible, and it's mostly telling you what's happening where people don't live rather than where people do live.

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Future Maps [4]." YouTube. November 19, 2014.

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

Video: Relative Changes in Precipitation (1:21)

IPCC Figure SPM 7. Relative changes in precipitation (in percent) for the period 2090–2099, relative to 1980–1999. Values are multimodel averages based on the SRES A1B scenario for December to February (left) and June to August (right). White areas are where less than 66% of the models agree in the sign of the change, and stippled areas are where more than 90% of the models agree in the sign of the change.

Click for a video transcript of "Relative Changes in Precipitation".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Future Rain [5]." YouTube. January 23, 2015.

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

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)

Sea Level Rise Regions in the southeast US that would be under water for a sea-level rise of 1 m (3.3 feet, upper left), 2 m (6.6 feet, upper right), 4 m (13.1 feet, lower left) and 8 m (26.2 feet, lower right). Many projections for late in this century include 1 m as a possible rise, and long-term the worst-case scenario is much more than 8 m.

Click for a video transcript of "Sea Level Rise".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Flood [3]." YouTube. November 19, 2014. Source: NOAA: GFDL [6]

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)

Projections of drought by late in the century (2090-2099). To start the video, click on the image above. Although some places are expected to become wetter (check out Siberia), most of the world, especially where the population is not concentrated, is expected to experience increases in drought.

Click for a video transcript of "Potential for Drought by the End of this Century".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Drought [7]." YouTube. January 23, 2015.

Source: Environmental Protection Agency [8]

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 CO₂is 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)

Net Photosynthesis Data from laboratories (including the single example shown here) and the field show that above some level, plant growth is greatly reduced by warmer temperatures, and this effect is understood based on how the chemistry of plants behaves. In this figure for corn (maize) from the US Department of Agriculture, “Net photosynthesis” is plant growth, and clearly decreases as temperature increases.

Click for a video transcript of "Net Photosynthesis".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Net Photosynthesis [9]." YouTube. January 23, 2015. Source: Figure modified from US Department of Agriculture [10], Learn About Our Plant Physiology Research.

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)

Glacially eroded bedrock, east Greenland. Greenland farmers are raising a few crops and pastures, as shown here in south Greenland. The weight of the ice-age ice that covered the entire island pushed the land down. As the ice age ended, the ice melted faster than the land bobbed up. The extra water flooded the coast as the ice melted away, depositing ocean sediments, especially along beaches. Then the land rose, and those sediments are exposed just above the sea. Farming is especially concentrated in those areas, as shown here. Notice, though, that almost everything else you see is hard rock that the glaciers polished clean, with little or no soil. The second picture in the video more clearly shows bedrock scratched and polished by the glacier, and not at all good for farming.

Click for a video transcript of "South Greenland".

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Greenland Farm [11]." YouTube. November 19, 2014.

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

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 CO₂rapidly, 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

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.

Poison Ivy: More vigorous poison ivy may not be the end of the world, but is among the many, many influences of changing climate.

Credit: Richard B. Alley @ Penn State is licensed by CC-BY-NC-4.0

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

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)

Reducing Risks of Climate Change Damage

Click for a video transcript.

DR. RICHARD ALLEY: This is a fascinating figure from the IPCC. This is actually from the 2001 report. So this one goes back a little farther.

This part is CO₂. And, actually, it's more CO₂ as you move towards the left here. And that gives you more warming.

So this is how much warming you get for various possible futures. So far we're tracking on the high end. But we're really not sure where we'll end up in the long term.

And each of these over here, as we release more CO₂, by late in the century, we get more warming. And so you can take the warming that you think we'll find and you can draw a line across. Maybe you think we're going to get three Celsius, because we're on the high end of the emissions. And so you draw the line across there.

Then what you see are various columns over here. So example one here is unique and threatened systems such as species extinction. If you are a rare and endangered species living in a little national park and now you need to migrate and there's cornfields in the way, even a little bit of warming is pretty bad for you.

And so you'll notice, this is going from orange up to red or into a more saturated color very, very quickly. Because even a little bit of warming causes a lot of trouble for unique and threatened systems.

If you're worried about when do we get more floods and more droughts, it doesn't take a lot of warming before you start getting to more floods and more droughts. And we'll be well into that before the end of the century. We're already seeing some of that now.

In some areas, poor people in hot places are already hurting now. Rich people in cold places-- it'll take more warming before they really get into trouble.

And because so much of the Earth's economy is in the rich people in cold places, you don't really worry about them until you get a good bit of warming. And this sort of, we killed the North Atlantic or dump the antarctic ice sheet, it takes a lot of warming until we get there.

And so in terms of the question, how much warming before we get into trouble, that bothers a lot of people, it depends what you're talking about. And for unique and threatened systems, for rare and endangered species, we're already pushing them hard, as well as for poor people in hot places.

The world's economy is not yet suffering hugely. But as the warming increases, it will tend to suffer more.

And the basic picture-- each degree of warming costs more than the previous degree and that the first degree didn't hurt a huge amount, because it's really only hurting the rare things and it's not hurting the economy. At some point, it starts to hurt the economy, too.

Credit: Dutton Institute [1]. "EARTH 104 Module 5 Embers: Reducing Risks of Climate Change Damage [12]." YouTube. January 23, 2015.

Source: IPCC, Intergovernmental Panel on Climate Change, 2001, Figure SYR 6-3.

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.

The Projection Project