Short 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.
History of the Most Important Greenhouse Gases (launch image in a new window) [1]
PRESENTER: 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.&
First, 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.)
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)
So, yes, humans are increasing the greenhouse effect, primarily by producing CO2 by burning fossil fuels, with very little uncertainty.
Natural and Anthropogenic Warming (launch image in a new window [2])
PRESENTER: 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.
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.
Temperatures, Sea Level and Snow Cover (launch image in a new window [3])
PRESENTER: 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.
The temperature is going up. The figure shows a few of the indicators, but many more are known. Consider the next figure, for example.
Decadal Land-Surface Average Temperature (launch image in a new window [4])
PRESENTER: 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.
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 Operator's Manual
For recent updates on temperature, see NASA’s Goddard Institute for Space Studies (GISTEMP). [8]
Nature 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.
Models Using Natural and Anthropogenic Forcings (launch image in a new window [9])
PRESENTER: 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.
According to the model data shown in the IPCC figure above (SPM-4), can recent warming trends be explained by natural variability in factors beyond our control, such as solar activity and volcanoes? Imagine you are talking to a friend or relative who is not familiar with these models or is unclear on how to interpret them. Try your best to explain what the models show about recent climate change in your own words.
Click for answer.
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 are 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.
Links
[1] https://www.e-education.psu.edu/earth104/sites/www.e-education.psu.edu.earth104/files/Unit1/Mod4/Earth104_energy-images-Lesson4-energy-images-Lesson4-L4I4spm1.jpg
[2] https://www.e-education.psu.edu/earth104/sites/www.e-education.psu.edu.earth104/files/Unit1/Mod4/Earth104_energy-images-Lesson4-L4P2G2.jpg
[3] https://www.e-education.psu.edu/earth104/sites/www.e-education.psu.edu.earth104/files/Unit1/Mod4/Earth104_energy-images-Lesson4-L4P2G3.jpg
[4] https://www.e-education.psu.edu/earth104/sites/www.e-education.psu.edu.earth104/files/Unit1/Mod4/Earth104_energy-images-Lesson4-L4P2G4.jpg
[5] http://berkeleyearth.org/
[6] https://www.youtube.com/@Etheoperatorsmanual
[7] https://www.youtube.com/watch?v=E9eGzPxA1Dg
[8] http://data.giss.nasa.gov/gistemp/graphs/
[9] https://www.e-education.psu.edu/earth104/sites/www.e-education.psu.edu.earth104/files/Unit1/Mod4/Earth104_energy-images-Lesson4-L4P2G5.jpg