The Warming from the So-Far-Unavoidable Burning
The Warming from the So-Far-Unavoidable Burning azs2Short version: The Earth is warming, as shown by an interconnected web of evidence. The pattern of this warming, in space and time, matches that expected from the human-caused rise of greenhouse gases together with the other, less-important causes of climate change.
Friendlier, but longer version: We will follow the presentation of the United Nations Intergovernmental Panel on Climate Change (IPCC) here. The IPCC is the world’s effort to assess the available science. Researchers act for the public good, in the public eye, without being paid to do so, to tell policymakers and other people what is scientifically solid, speculative, or just silly by summarizing and assessing the relevant science.
If, for some reason, you don’t like the IPCC, you could check out other authoritative assessments, such as those done by the US National Academy of Sciences or the US Climate Change Science Program, or resources from the British Royal Society and others. But, for the world, the IPCC is an outstanding starting point. Dr. Alley did almost nothing for the Fifth Assessment Report or the IPCC released in 2013, but worked extensively on the Fourth Assessment Report in 2007, and contributed to the Third (2001) and Second (1995) Assessment Reports. The IPCC shared the Nobel Peace Prize after the Fourth Assessment Report.
Video: IPCC SPM1 (1:45)
History of the Most Important Greenhouse Gases (launch image in a new window)
The vertical scales on the left are concentration in the atmosphere, in either parts per billion (ppb) or parts per million (ppm). The vertical scales on the right show “radiative forcing”—you can think of this as how much brighter the sun would need to get to give as much warming as provided by the greenhouse gas. Official IPCC Caption: IPCC Figure SPM.1 Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). The corresponding radiative forcings are shown on the right-hand axes of the large panels.
DR. RICHARD ALLEY: This fascinating figure comes from the IPCC. It shows 10,000 years of history-- 10,000 years ago on your left, up to today in the big panels and then just since 1750 in the little panels in each case. And it shows it for carbon dioxide on the top, for methane in the middle, and for nitrous oxide on the bottom. These are the main greenhouse gases.
They're shown on the left in concentrations. This would be parts per million for CO2 and parts per billion for the methane and the nitrous oxide. And over on the other side, it shows radiative forcing. So this is a measure of how much the sun would have to get brighter to have as much warming affect as the greenhouse gases having. And you'll find that the radiative forcing is biggest for the CO2. That's a one up there-- one watt per square meter versus 240 from the sun-- smaller values for the other two.
These plots show ice core data from many different ice cores measured in different places by different labs and drilled in different places and so on, and then overlapping with the measurements that had been made in the atmosphere by modern instruments. You'll see because there's so much agreement among the different cores and different labs and so much agreement with the instrumental record, these are highly reliable. And what they show with very, very high confidence is that the greenhouse gas forcing, the greenhouse gases are rising. Other information shows that that rises very clearly from us.
Source: Intergovernmental Panel on Climate Change (IPCC), 2007: Summary for Policy Makers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, and H.L. Manning (eds.].
History of Carbon Dioxide
History of Carbon Dioxide azs2First, let’s start with Figure SPM-1 from the Fourth Assessment of the IPCC, showing the history of carbon dioxide and some other greenhouse gases over the last 10,000 years. Ice-core data from multiple cores and labs cover most of the history shown, and overlap with the recent instrumental record, all with very close agreement. The recent rise is unprecedented in the 10,000 years shown. Based on additional ice-core records not shown, the greenhouse-gas levels are now above anything seen in the last 800,000 years. And, data from other sources indicate that carbon dioxide has not been this high for millions of years. (Note that much further back in history, nature did cause higher CO2 levels, a topic to which we will return later.)
This image is a set of three graphs illustrating the historical concentrations of greenhouse gases and their associated radiative forcing from the year 1000 to 2005. The data is sourced from the IPCC 2007 WGI-AR4 report. Each graph represents a different greenhouse gas: carbon dioxide, methane, and nitrous oxide.
- Top Graph (Carbon Dioxide):
The graph shows carbon dioxide concentration in parts per million (ppm) on the left y-axis, ranging from 250 to 400 ppm. The x-axis represents time from the year 1000 to 2005. From around 1000 to 1800, the concentration remains relatively stable, fluctuating slightly between 250 and 280 ppm, depicted with scattered colored dots (gray, purple, green). After 1800, there is a noticeable upward trend, with the concentration rising sharply to around 380 ppm by 2005, shown with a solid red line. On the right y-axis, radiative forcing is plotted, ranging from 0 to 1 W/m². A vertical bar on the right side shows the radiative forcing in 2005, with a colored gradient (blue to red) indicating a value slightly below 1 W/m².
- Middle Graph (Methane):
The graph displays methane concentration in parts per billion (ppb) on the left y-axis, ranging from 400 to 2000 ppb. The x-axis spans from 1000 to 2005. From 1000 to 1800, methane levels are relatively stable, fluctuating between 400 and 700 ppb, shown with scattered colored dots (gray, purple, yellow, green). After 1800, the concentration increases significantly, reaching around 1750 ppb by 2005, depicted with a solid red line. The right y-axis shows radiative forcing, ranging from 0 to 0.4 W/m². A vertical bar on the right side indicates the radiative forcing in 2005, with a colored gradient (blue to red) showing a value around 0.3 W/m².
- Bottom Graph (Nitrous Oxide):
The graph illustrates nitrous oxide concentration in parts per billion (ppb) on the left y-axis, ranging from 260 to 330 ppb. The x-axis covers the years 1000 to 2005. From 1000 to 1800, nitrous oxide levels remain steady, fluctuating slightly between 260 and 280 ppb, shown with scattered colored dots (gray, purple, green, blue). After 1800, there is a gradual increase, reaching around 320 ppb by 2005, depicted with a solid red line. The right y-axis shows radiative forcing, ranging from 0 to 0.1 W/m². A vertical bar on the right side indicates the radiative forcing in 2005, with a colored gradient (blue to red) showing a value slightly below 0.1 W/m².
- Additional Details:
The x-axis at the bottom of all graphs includes a secondary time scale labeled "Time (before 2005)" ranging from 1000 to 5000 years before 2005. The graphs use a combination of scattered dots in various colors (gray, purple, yellow, green, blue) to represent historical data from different sources, and a solid red line to show the aggregated trend over time. The source of the data, "IPCC 2007 WGI-AR4," is noted in the bottom right corner.
The figure shows “radiative forcing” as well as atmospheric concentration. The Earth absorbs 240 W/m2 from the sun. The extra warming from rising CO2 is somewhat similar, although not identical, to the warming from a brighter sun, so the effect of the CO2 can be discussed in W/m2. CBy January of 2017, atmospheric CO2 was at a concentration of 405 ppm, up from 280 ppm before the industrial revolution, with the extra CO2 giving a radiative forcing of roughly 2 W/m2, equivalent to the sun getting almost 1% brighter. The contributions from methane (from rice paddies, cow guts, and other sources) and nitrous oxide (especially produced by processes in soil stimulated by nitrogen fertilizers and animal waste) are significant but smaller.
The amount of extra CO2 now in the air, and moving into the ocean to make it more acidic, closely matches the CO2 we know has been produced from fossil-fuel burning. The human source is roughly 100 times as large as the natural volcanic source, and volcanoes have not done anything bizarre recently, so cannot be blamed for the recent rise. CO2 is moving into the ocean rather than coming out, so oceans cannot be responsible for the rise.
Furthermore, the atmosphere confirms that humans are responsible, as discussed in the ETOM film clip below and the Enrichment linked below.
Want to learn more?
Read the Enrichment titles Humans are Primarily Responsible for the Rise in CO2..
Earth: The Operators' Manual
Watch the short video below on how we know that the rise in CO2 is primarily from our fossil-fuel burning, filmed at the Rotorua Thermal area of New Zealand.
Video: It's Us (2:41)
DR. RICHARD ALLEY: So physics and chemistry tell us that adding carbon dioxide to the atmosphere warms things up. And Earth's climate history shows us there will be impacts, from melting ice sheets, to rising sea level. But how do we know, with equal certainty, that it's not just more natural variation, that humans are the source of the increasing CO2? When we look at a landscape like this one we know immediately that volcanoes put out all sorts of interesting things. And that includes CO2.
So how do we know that the rise of CO2 in the atmosphere that we see, comes from our burning of fossil fuels, and not from something that the volcanoes have done? Well, the first step in the problem is just bookkeeping. We measure how much CO2 comes out of the volcanoes. We measure how much CO2 comes out of our smokestacks and tail-pipes. The natural source is small. Humans are putting out 50 to a 100 times more CO2 than the natural volcanic source. We can then ask the air whether our bookkeeping is right, and the air says that it is. Volcanoes make CO2 by melting rocks to release the CO2. They don't burn and they don't use oxygen. But burning fossil fuels does use oxygen when it makes CO2. We see that the rise in CO2 goes with a fall of oxygen, which says that the rising CO2 comes from burning something.
We can then ask the carbon in the rising CO2 where it came from. Carbon comes in 3 flavors: the lightweight carbon 12, which is especially common in plants. The medium-weight carbon 13, which is a little more common in the gases coming out of volcanoes. And the heavyweight carbon 14. It's radioactive and decays almost entirely after about 50,000 years, which is why you won't find it in very old things like dinosaur bones or fossil fuels. We see a rise in carbon 12, which comes from plants. We don't see a rise of carbon 13, so the CO2 isn't coming from the volcanoes. And we don't see a rise in carbon 14, so the CO2 can't be coming from recently-living plants. And so the atmosphere says that the rising CO2 comes from burning of plants that have been dead a long time... That is fossil fuels. The CO2 is coming from our fossil fuels. It's us.
So, yes, humans are increasing the greenhouse effect, primarily by producing CO2 by burning fossil fuels, with very little uncertainty.
Natural and Anthropogenic Warming
Natural and Anthropogenic Warming azs2Video: SPM2 (2:20)
Natural and Anthropogenic Warming (launch image in a new window)
The red bars show warming influences on the recent climate, and blue bars show cooling. We have raised greenhouse gases a lot, and partially offset their warming effect by adding sun-blocking particles (“aerosols”). Official IPCC Caption: IPCC Figure SPM.2 Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing, and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness.
DR. RICHARD ALLEY: This fascinating figure is from the IPCC. There's a lot of information on here. It includes the things that are changing-- radiative forcing-- or changing the climate, how much they're doing so, including the uncertainties, whether they expect the whole globe or just part of it, and the level of scientific understanding.
If we do a lot more research-- the low, it probably will reduce the size of the uncertainties-- because we can learn more. But how much we understand is included in the uncertainty already. And it includes both the things that humans have done and the things the nature has done. And this goes from the year 1750 up to the year 2005.
The Biggie is our C02, together, with the other greenhouse gases that we put up, as well as the ozone that comes from human activities from pollution. So these all have a warming influence and they are pushing very strongly towards warming. Clearly, there's a couple of other little warming influences, especially us putting soot on top of snow. But there's also these cooling influences.
We've put up a lot of particles, aerosols that block the sun, and they make clouds last longer and make clouds more reflective. And together, those have a lot of cooling. And we've cut dark forests and replaced them by more reflective grasslands.
In addition, since 1750 the sun has brightened a little bit. Over the last 30 years or so, it's actually dimmed, but there's a little bit of that. Add all of these together and there's very clearly a warming influence. And the total warming influences is very similar in size to the CO2 that we've put up.
Taken together, we are pushing the world in a lot of different ways. But because of these cooling influences, if you ask how much of the warming has been caused by our greenhouse gases, the answer is more than all of it. Because it is warm despite these cooling influences.
Source: IPCC, 2007: Summary for Policy Makers
Greenhouse gases are not the only things that affect climate. But, climate changes have causes; there are no magical “cycles” that somehow change the climate without letting us know why. (There are cycles that affect climate, but they have causes, such as features of Earth’s orbit, that we understand; they are NOT magical!) So, we can assess what things are affecting the climate.
More than a century ago, the Earth was a little on the cold side in what is sometimes called the “Little Ice Age” because the sun was a bit dim and volcanic eruptions were putting up dust that blocked the sun. The sun brightened early in the 20th century, contributing to warming, as shown by the little red bar extending to the right for natural solar irradiance down near the bottom of the figure. But, over the last 30 years when satellites have given us the best data, the sun seems to have dimmed just a bit. We humans have cut dark forests and replaced them with more-reflective grasslands, cooling the Earth a little, and we have put up a lot of particles to block the sun, with notable cooling influence (you can find blue bars for these, extending to the left, in the figure).
You may meet someone who agrees that the Earth is warming, but argues that much of the change is natural. This is wrong; over the last few decades, warming has occurred despite nature pushing a little toward cooling, and human particles and land-use changes pushing more strongly toward cooling. The most likely answer for how much of the warming has been caused by our greenhouse gases is “More than all of it”, because of warming despite these other cooling influences.
Video: SPM 3 (1:07)
Temperatures, Sea Level and Snow Cover (launch image in a new window)
The Earth’s surface is warming (top), sea level is rising as glaciers melt and ocean water expands from warming (middle), and springtime snow cover is shrinking as temperatures rise. Official IPCC Caption: IPCC Figure SPM.3 Observed changes in (a) global average surface temperature, (b) global average sea level from tide gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961–1990. Smoothed curves represent decadal average values, while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c).
DR. RICHARD ALLEY: This figure from the IPCC starts back in 1850 and then runs up to just pass 2000 up here on the right. And it shows indications of warming happening in the climate system. You can see on top here the thermometer record of global average temperature showing not much happening and then recent warming, very clearly.
Sea level, which is given here, rises because ocean water expands as it warms and because warming tends to melt glaciers that are holding water out of the ocean. And so we see a warming influence that shows up in the rising global sea level.
And we also look, if you go to bring time snow cover, you can see that not much was happening. And then you can see it dropping, and that's happening because of warming and the spring is melting the snow. And so these are among many indicators that are showing that yes, the climate system is warming.
Source: IPCC, 2007: Summary for Policy Makers
The temperature is going up. The figure shows a few of the indicators, but many more are known. Consider the next figure, for example.
Video: Surface Temperatures (1:38)
Decadal Land-Surface Average Temperature (launch image in a new window)
The figure shows the estimates of land surface temperature from four sources. The uncertainty in the Berkeley Earth record is also shown. Data before about 1850 are clearly quite uncertain, linked to having few thermometers back then, but more recently, the agreement among the various groups with their different techniques is very good.
DR. RICHARD ALLEY: This figure is from the Berkeley Earth Project. It was run primarily by physicists who did not start out as climate scientists-- with an interesting mix of funding from public sources. But also some of it came from private sources, including those with ties to the fossil fuel industry.
It's looking at the thermometer record of temperature, and just looking at the land. Now if you go back to 1750 up through about 1850, you could see that the uncertainties are really huge. So we're mostly going to focus since 1850.
Many groups have been estimating the temperature, including NASA-- the Goddard Institute for Space Studies, NOAA-- the National Climate Data Center, the British Group, the Hadley Centre, and the Climate Research Unit. And what you can see is those, plus the Berkeley Earth estimates up here on top. And what you'll notice is that the uncertainties in the Berkeley Earth are similar to the differences between the others, which also have their own uncertainties. But you'll see very clearly that there is a strong warming going on.
The different groups have used different techniques. Although, ultimately, they're all using thermometers. Whether they use them all or not this is different for the different ones. But when you have different groups with different funding, different motivations, perhaps, and some working in different places, they all give the same answer. Which is, it's getting warmer. We have very high confidence that it is warming.
Source: Berkeley Earth
The Berkeley Earth project is an interesting attempt by a group, involving a lot of physicists who were not primarily climate scientists through much of their careers, to use private as well as public funding to re-calculate the temperature record from thermometers. The Berkeley work follows efforts by NOAA and by NASA in the US, and by a British group at the Hadley Center and the University of East Anglia, and other efforts by others, to calculate global temperature changes from thermometer records. You can see clearly in the figure that over recent decades when the data are best, the different groups get the same answer despite having different funding sources and different techniques. The temperature is going up.
Furthermore, if you throw away the records from thermometers in and near the cities and just look in the country, you see warming. Thermometers in boreholes in the ground show warming. Thermometers taken aloft by balloons (radiosondes), and thermometers looking down from satellites and analyzed in different ways, show warming. So do thermometers in the ocean.
The temperature-sensitive snow and ice also show warming. You would not go searching for this effect in the coldest places; if you start off at -40 and warm by a couple of degrees, the snow and ice won’t melt yet. But, the effects of warming are seen in loss around the edges, in space and time, of seasonal snow cover, river, and lake ice, seasonally and perennially frozen ground, mountain glaciers and more. The melting of land ice and the expansion of ocean water as it warms are driving the rise in global sea level. And, the great majority of significant changes in where plants and animals live, and when they do things during the year, are in the direction of warming. So, warming is occurring, despite natural and human pushes toward cooling over recent decades.
Want to learn more? Read the Enrichment titled Global Warming Did Not Stop Recently.
We are once again taking a look at the CO2 and the Atmosphere clip. To see a little on the melting of ice, watch 7:22 - 9:04.
Earth: The Operators' Manual
Video: CO2 and the Atmosphere (9:04)
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.
[music]
ONSCREEN TEXT: Chapter 2 CO2 & The 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.
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 the summit to the 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.
[music]
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.
GUIDE: 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. Now we're flying over today's coastline, where giant boulders are leftovers from that last ice age.
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 percent 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.
For recent updates on temperature, see NASA’s Goddard Institute for Space Studies (GISTEMP).
Models Using Natural and Anthropogenic Forcings
Models Using Natural and Anthropogenic Forcings azs2Nature surely has changed the climate in the past, is contributing to climate change now, and will contribute to climate change in the future. In the figure below, models have been used to see what nature has done, compared to what humans have done. In each case, the black line shows the actual history of temperature. The blue bands, which end up below the black line recently on each plot, show the influence of changing sun and volcanoes; a band is plotted, rather than a line, to show the uncertainties in estimating the sun's and volcanic influences and turning them into temperature changes using models. The pink bands, which so nicely match the black lines showing what really happened, were calculated including the effects of natural changes plus the human causes, including both warming and cooling influences.
Video: SPM 4 (2:09)
Models Using Natural and Anthropogenic Forcings (launch image in a new window)
The history of temperature as measured by thermometers (black lines) is simulated accurately by models if they are “told” what nature and humans have done to change the climate (pink bands, which include the uncertainties), and is simulated fairly accurately early in the 20th century if the models are “told” only what nature has done to simulate the climate, but is not simulated at all accurately more recently from natural changes alone. Climate was changing mostly for natural reasons, but now is changing mostly because of humans.
Official IPCC Caption: IPCC Figure SPM.4 Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906 to 2005 (black line) plotted against the center of the decade and relative to the corresponding average for 1901–1950. Lines are dashed where spatial coverage is less than 50%. Blue shaded bands show the 5–95% range for 19 simulations from five climate models using only the natural forcings due to solar activity and volcanoes. Red shaded bands show the 5–95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings.
DR. RICHARD ALLEY: This wonderful figure from the IPCC is looking at the fingerprint of climate change. All of the different plots go from just more recently than 1900 up to 2000. That was the time that they could do best for this.
And in each plot, the black line is the history of temperature. This is for the globe, this would the globe's land, the globe's ocean, and then continent by continent up here, like Asia and Europe, and so on. So in each case, the black is what happened.
The blue models have been taken, and they've been told what nature did. What the sun was doing, what the volcanoes were doing. And the models then said this is the climate change that nature has caused.
In the pink, in each case, the model has been told what nature did, and what humans did. And what you will see, if you start down here, for example, with the global land, is that the warming back here is possibly caused by nature. The sun got a little bit brighter, and coincidentally, the volcanoes quit blocking the sun quite as much as they had done earlier. But recently, the dimming of the sun and some big volcanoes have tried to cool it off. Yet, the temperature went up.
And so what you can see in every one of these panels is that you can explain the climate changes that were happening early in the 20th century by natural causes because the human causes were not terribly large. But by the time you get to the later 20th century, if anything, nature tried to cool it off a little bit, yet the temperature went up. And so what we see across the globe, from Australia to North America, is that the fingerprint of climate change is now that of humans, not that of nature. Other fingerprinting exercises give the same answer, which is that we have taken over from nature in controlling climate change.
Activate Your Learning
Note that there are other lines of evidence confirming the relative significance of human influence suggested in the figure above. Suppose for a moment that you decide the satellite data is wrong, and the sun is really getting brighter. (This is not a sensible thing to do, but just suppose…) If this were correct, we know that more energy from the sun will warm the air near the Earth’s surface, but also will warm the air high in the stratosphere. Rising CO2 also warms the air near the surface, but rising CO2 cools the upper stratosphere. (Ultraviolet radiation heats the ozone there, which transfers energy to CO2 in collisions, and the CO2 then radiates the energy to space, so in the presence of much ozone high in the atmosphere where infrared radiation to space is easy, extra CO2 acts as a radiator and causes cooling of the adjacent air.) The observed pattern of changes—warming near the surface but cooling in the upper stratosphere—has the fingerprints of CO2, not the sun or other possible causes of climate change. Other fingerprinting exercises reach the same conclusion.
Taking all of this together, we now have very high scientific confidence that we humans are changing the composition of the atmosphere, primarily through the burning of fossil fuels, and that the rising concentration of important gases is causing warming. Feedbacks in the Earth system modify the initial warming and are acting to amplify the direct effects of our CO2 and increase the warming. The Earth is warming, based on a great range of independent data sets. This warming is occurring despite natural and human-caused cooling influences, and this warming has the pattern in space and time expected from our greenhouse gases plus the other influences on climate. The close agreement between what is happening, and what we expect to happen from our understanding of the climate system, confirms the science. And, because we are fairly confident that much more fossil fuel remains to be burned than we have burned already, the well-confirmed scientific understanding says that coming climate changes will be much bigger than those we have caused so far if we continue on the path we are now following. What that means is coming in the next module.