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

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 [1])

Estimates of the history of CO₂ over the last 400 million years (MA), with today on the right. The shaded band is output from a model calculating CO₂ from inputs such as volcanic eruptions and outputs such as fossil-fuel formation; the other curves represent estimates of atmospheric CO₂ from different techniques applied to sedimentary deposits. The bars hanging down from the top show the extent of ice at low elevation (very high mountains can have ice even in times with warm temperatures at low elevations where most people live, but low-elevation ice requires cooler temperatures in most places); times with no bar had no ice at low elevation.

Click for a video transcript of "Continental Glaciation".

DR. RICHARD ALLEY: This wonderful plot is from the IPCC and other places. This is a different scale. Today is over here on your right, and the 400 over here is 400 million years-- not 400 years, 400 million years. So this is really deep time.

And what you have plotted on here are two different things. At the top, hanging down in blue is the extent of glaciers at a time. And so there were no ice on the planet, basically at sea level. And then there was a little blip of glaciers, and they went away. And then the glaciers went way down towards the equator-- not all the way by any means-- but they got more than halfway there. And then they melted away into a time with no ice near sea level. And then the glaciers have come back, and we have ice in Antarctica and Greenland today.

So there's the history, and you can think of this as a history is temperature. There was no because it was warm. There was ice because it was cold. There was no ice because it was warm. There was ice because it was cold. OK.

Shown below is the history of CO2. And what you'll notice is when there was no ice, CO2 was high, and this is estimated in various ways. But what you have here is this high CO2 back here in a no ice time. And then when CO2 got low, the ice had grown. And when CO2 went back up to being high, the ice had melted away. And when CO2 got low again, the ice had grown back. And it turns out there's actually a little dip in CO2 right here that goes with this little blip of ice.

And so what we see is a very nice relationship-- high CO2, little or no ice; low CO2, lots of ice. Furthermore, we understand from processes that you can read about in our course and elsewhere, that it is the CO2 causing the changes in ice and not, primarily, the ice causing the changes in the CO2. And this is something you just can't see from this correlation, but we get it from other sources.

Now, try to walk you through a few events in climate history. We're going to start with this one back here, The Great Dying-- a time when volcanic CO2 raises the temperature and seems to have made it so hot near the equator that large creatures couldn't live there. We then will walk you into the Paleocene-Eocene Thermal Maximum-- a time when some formerly living carbon came out of C4 methane or other sources, belched out fairly rapidly and made it warm. And we'll finish up with the Ice Ages. This is a time when features of Earth's orbit have driven temperature changes, but those temperature changes have been amplified and made global by CO2.

Credit: Dutton Institute [2]. "EARTH 104 Module 5 Continental Glaciation [3]." YouTube. November 19, 2014.

Source: From CCSP, 2009: Past Climate Variability and Change in the Arctic and at High Latitudes. A report by the U.S. Climate Change Science Program and Subcommittee on Global Change Research [Alley, R.B., J. Brigham-Grette, G.H. Miller, L. Polyak, and J.W.C. White (coordinating lead authors)]. U.S. Geological Survey, Reston, VA, 257 pp. modified from IPCC (2007)

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

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.

Trilobites (specimen from the Grand Canyon) and eurypterids (the State Fossil of New York) lived for well over 100 million years, but did not survive the great dying at the end of the Permian.

Source: Pictures from the US National Park Service [4], trilobite and eurypterid.

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

United States Geological Survey image showing the geological time scale.

In case you find yourself getting confused by the geologic names, or you just want to see whether you remember the names you learned sometime in the past, here is a fantastic image from the United States Geological Survey showing the geological time scale.

Credit: U.S. Geological Survey General Interest Publication 58—Version 1.1, United States Geological Survey

Video: Paleoclimate Concepts (1:31)

The Paleocene-Eocene Thermal Maximum looks small and short in this view of 65 million years of Earth history but lasted longer than modern humans have existed on Earth, and was a major event for the creatures living on the Earth then. This is Figure 2.8, from Alley, R.B., J.J. Fitzpatrick, J. Brigham-Grette, G.H. Miller, D. Muhs, and L. Polyak, 2009: Paleoclimate concepts. In: Past Climate Variability and Change in the Arctic and at High Latitudes. A report by the U.S. Climate Change Science Program and Subcommittee on Global Change Research. U.S. Geological Survey, Reston, VA, pp. 11-30.

Click for a video transcript of "Paleoclimate Concepts".

Credit: Dutton Institute [2]. "EARTH 104 Module 5 Paleocene Eocene Thermal Maximum [5]." YouTube. November 19, 2014.

Source: Figure 2.8, from Alley, R.B., J.J. Fitzpatrick, J. Brighman-Grette, G.H. Miller, D. Muhs, and L. Polyak, 2009: Paleoclimate concepts. In: Past Climate Variability and Change in the Arctic and at High Latitudes. A report by the U.S. Climate Change Science Program and Subcommittee on Global Change Research. U.S. Geological Survey, Reston, VA, pp. 11-30.

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 CO₂fell. 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.

This foot-long Protorohippus sp. fossil from the Eocene Green River Formation of Wyoming is one of many types of horses from the western US. Larger animals overheat more easily. During the PETM, many animal species including horses that lived shortly before this one became dwarfed in response to the high heat from the extra CO₂ in the air. This horse apparently washed into a lake and was fossilized next to fish fossils.

Credit:& Protorohippus venticolum cast [6]. National Park Service (NPS) (Public Domain)

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

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)

Click here for a video transcript of "CO₂ and the Atmosphere".

DR. RICHARD ALLEY: What CO2 does was confirmed by basic research that had absolutely nothing to do with climate change.

NEWSREEL ANNOUNCER: A continuance of the upper air program will provide scientific data concerning the physics of the upper atmosphere.

DR. RICHARD ALLEY: World War II was over, but the Cold War had begun. The U.S. Air Force needed to understand the atmosphere for communications and to design heat-seeking missiles.

At certain wavelengths, carbon dioxide and water vapor block radiation. So the new missiles couldn't see very far if they used a wavelength that CO2 absorbs. Research at the Air Force Geophysics Laboratory in Hanscom, Massachusetts produced an immense database with careful measurements of atmospheric gases. Further research by others applied and extended those discoveries, clearly showing the heat-trapping influence of CO2. The Air Force hadn't set out to study global warming, they just wanted their missiles to work. But physics is physics. The atmosphere doesn't care if you're studying it for warring or warming. Adding CO2 turns up the planet's thermostat.

[helicopter sounds and music]

It works the other way as well. Remove CO2, and things cool down. These are the Southern Alps of New Zealand, and their climate history shows that the physicists really got it right. These deep, thick piles of frozen water are glaciers, slow-moving rivers of ice, sitting on land... But once, when temperatures were warmer, they were liquid water, stored in the sea. We're going to follow this one, the Franz Josef, from summit to ocean to see the real-world impact of changing levels of CO2.

It's beautiful up here on the highest snowfield, but dangers lurk beneath the surface. I've spent a lot of time on the ice. It's standard practice up here to travel in pairs, roped up for safety. The glacier is fed by something like six meters of water a year... maybe 20 meters, 60 feet of snowfall... it's a really seriously high snowfall. The snow and ice spread under their own weight and is headed downhill at something like a kilometer a year. When ice is speeding up a lot as it flows towards the coast, it can crack and open great crevasses that give you a view into the guts of the glacier. Man, this is a big one... Ten... Twenty... Thirty meters more... a hundred feet or more heading down in here, and we can see a whole lot of the structure of the glacier right here.

MAN: So, what we're going to do is just gonna sit on the edge and then walk backwards, and I'll lower you.

DR. RICHARD ALLEY: Tell me when. Okay, rolling around, and down we go. Snowfall arrives in layers, each storm putting one down... Summer sun heats the snow, and makes it look a little bit different than the winter snow, and so you build up a history. In these layers, there's indications of climate, how much it snowed, what the temperature was. And all of this is being buried by more snow and the weight of that snow squeezes what's beneath it, and turns it to ice. And in doing that, it can trap bubbles. And in those bubbles are samples of old air, a record of the composition of the Earth's atmosphere, including how much CO2 was in it, a record of the temperature on the ice sheets, and how much it snowed.

As we'll see, we can open those icy bottles of ancient air, and study the history of Earth's atmosphere. This landscape also tells the story of the Ice Ages. And the forces that have shaped Earth's climate. Over the last millions of years, the brightness of the sun doesn't seem to have changed much, but the Earth's orbit and the tilt of its axis have shifted in regular patterns over tens and hundreds of thousands of years. The orbit changes shape... varying how close and far the Earth gets as it orbits the sun each year.

Over 41,000 years, the tilt of Earth's axis gets larger and smaller, shifting some of the sunshine from the equator to the poles and back. And our planet has a slight wobble, like a child's top, altering which hemisphere is most directly pointed toward the sun when Earth is closest to it. Over tens of thousands of years, these natural variations shift sunlight around on the planet, and that influences climate. More than 20,000 years ago, decreasing amounts of sunshine in the Arctic allowed great ice sheets to grow across North America and Eurasia, reaching the modern sites of New York and Chicago. Sea level fell as water was locked up on land. Changing currents let the oceans absorb CO2 from the air. That cooled the Southern Hemisphere, and unleashed the immense power of glaciers such as the Franz Josef, which advanced down this wide valley, filling it with deep, thick ice.

[helicopter noises]

DR. RICHARD ALLEY: Now we're flying over today's coastline, where giant boulders are leftovers from that last ice age.

[music]

DR. RICHARD ALLEY: A glacier is a great earth moving machine. It's a dump truck that carries rocks that fall on top of it. It's a bulldozer that pushes rocks in front of it. And it outlines itself with those rocks making a deposit that we call a moraine, that tells us where the glacier has been. We're 20 kilometers, 12 miles, from the front of the Franz Josef glacier today, but about 20,000 years ago, the ice was depositing these rocks as it flowed past us and out to sea. The rocks we can still see today confirm where the glacier once was. Now, in a computer-generated time-lapse condensing thousands of years of Earth's history... we're seeing what happened. Lower CO2, colder temperatures, more snow and ice, and the Franz Josef advanced.

Twenty thousand years ago, 30% of today's land area was covered by great ice sheets, which locked up so much water that the global sea level was almost 400 feet lower than today. Then, as Earth's orbit changed, temperatures and CO2 rose, and the glacier melted back. The orbits set the stage, but by themselves they weren't enough. We need the warming and cooling effects of rising and falling CO2 to explain the changes we know happened.

Today, atmospheric CO2 is increasing still more, temperatures are rising, and glaciers and ice sheets are melting. You can see this clearly on the lake formed by the shrinking Tasman Glacier, across the range from the Franz Josef. This is what the end of an ice age looks like. Glaciers falling apart, new lakes, new land, icebergs coming off the front of the ice. In the early 1980s, we would have been inside New Zealand's Tasman Glacier right here. Now we're passing icebergs in a new lake from a glacier that has mostly fallen apart and ends over six kilometers, four miles away.

One glacier doesn't tell us what the world is doing, but while the Tasman has been retreating, the great majority of glaciers on the planet have gotten smaller. This is the Columbia Glacier in Alaska. It's a type of glacier that makes the effects of warming easy to see. It's been retreating so fast that the Extreme Ice Survey had to reposition their time-lapse cameras to follow its motion. In Iceland, warming air temperatures have made this glacier simply melt away, leaving streams and small lakes behind. Thermometers in the air show warming, thermometers in the air far from cities, show warming.

Put your thermometer in the ground, in the ocean, look down from satellites, they show warming. The evidence is clear. The earth's climate is warming.

Credit: Earth: The Operators' Manual [7]. "CO2 & the Atmosphere. [8]" YouTube. April 9, 2012.

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

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.

Described in caption below. No ice 50 million years ago to 250 million years ago. High CO2 at that point

Click for a text description of the history of CO₂graph.

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 CO₂than expected from the models.

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)

Commuters, and citizens considering climate, face similar uncertainties—for both groups, the most-likely outcome is well on the optimistic end of the possible outcomes. Things may be a little better, or a little worse, or a lot worse, than what normally happens.

Click for a video transcript of "Possibilitiies".

DR. RICHARD ALLEY: So I'm this really lucky person. I get to ride my bicycle when I go places. And that's a great thing. But suppose you have to drive a car. You may run into problems. And you might have very few problems at really low. And you might have bad problems way over here on the right. So this is problems getting worse. And this is how likely-- this is highly likely you're really going to get this. And this is rare down here.

So what we're going to look at is, what does a commuter encounter if you go out in your car in the real world. Well, the most likely thing that happens-- and so we show way up here because it's likely. It's that you get caught in traffic. And you kill some time. And you turn on the radio, and it's just sort of boring. It's not something that you really wanted to listen to. That's really what most of us experience when we have to drive somewhere.

Be perfectly honest. It is possible that you will get to a situation that nobody's in your way. And you turn on the radio, and they're playing the Beach Boys festival. And you're just grooving as you run down the road. It's a wonderful thing, and you're having a ball. It is also possible that you get stuck in lots of traffic. You're sitting there for an hour. You turn on the radio, and they're testing the Emergency Broadcast System. And they're screaming eeeeeek out of the radio. And this is no fun at all.

But recognize that there's a slight possibility that you're sitting there stuck in traffic listening to the Emergency Broadcast System. And a drunk driver comes running over the top of you. And you know, you get-- I'm sorry. You could be seriously damaged, or you could end up dead. And that is indeed possible. It's not very likely, but it's possible.

Well, what do we do about that? We buy cars that have airbags in them, that have crumple zones. We put on our seat belts. If we have kids, we put them in a kid's seat. We take out catastrophic insurance. We pay Mothers Against Drunk Driving to try to reduce drunks. We pay engineers to make the roads safer. We put a fair chunk of our transportation budget into something that we do not expect to happen because it's so devastating if it happens.

Now, when we start talking to Congress, or to what have you, about the cost of global warming, we have a best estimate. What is the most likely thing? And when we take those problems that go with that best estimate, and you put them in an economic model, we are better off if we deal with it than if we pretend it doesn't exist.

Now, be very clear. This is science. It is not revealed truth. It is indeed possible that we will see smaller or slower changes. Absolutely correct, that could happen. It's also possibly we could see larger or faster changes. We simply do not see any way that simply adding CO2 to the air will turn the earth into the greatest place to live that could possibly be imagined. You can't make Eden with just one thing because building paradise would take getting a lot of things right.

So there's no really not much chance that we get wonderful, no problems, great benefits, just from cranking up CO2. But there's a slight chance that we actually make the tropics too hot to live in for unprotected people, that we could have dead zones belching out poison gas, that we could shut down the North Atlantic and dry out the monsoon belts, that we could dump and ice sheet in the ocean and flood the coasts in a hurry.

These are all considered to be very unlikely at this point. But we can't rule them out. And CO2 might, by itself, do that. And so if you look at the picture, yes, it could be a little better. It could be a little worse. It could be a lot worse. But we don't see any way to make it a lot better.

Now, this is an opinion. But the last times that I have sort of talked to high policymakers about this, that I've testified to Congress or what have you, my impression is that we've spent a lot of time having this argument. I present what we know best from the science. And someone says, it could be better. This is our best estimate. It could be better. This is our best estimate, this could be better. Yes, that is not both sides. Be very clear, the best scientific evidence versus don't worry is not showing you both sides. And if we scientists are wrong, it's more likely to be on the bad side than it is on the good side.

Credit: Dutton Institute [2]. "EARTH 104 Module 5 Computer [9]." YouTube. November 19, 2014.

Who Did Start the Fire?