Pt2 – CO2, the Sun, and Warming in the Middle Ages – Viewer Questions on Climate Science

PAUL JAY: Welcome to The Real News Network. I'm Paul Jay in Baltimore. This is a continuation of our series of interviews with climate scientists answering your questions. As we said earlier in some of our climate interviews, that we would have a session for viewers to directly ask questions to climate scientists–and here we are. This part of our fundraising campaign, our spring/summer campaign. We have a $50,000 challenge grant. So if you click this "Donate" button–it's probably over here somewhere–and you donate, we will be able to keep doing programming like this. Now, without any further ado, let me introduce our climate scientists. Joining us from Paris, Valérie Masson-Delmotte. She's a French paleoclimatologist. She holds an engineering degree from l'École centrale Paris in physics and fluid transfer. Since 1997, she's been a senior scientist at the French nuclear energy commission, and she serves on the Intergovernmental Panel on Climate Change. And joining us from Colorado is Jeff Kiehl. Jeff is the senior scientist at the National Center for Atmospheric Research, where he heads the Climate Change Research section.

He's published over 100 articles on the effects of greenhouse gases on Earth's climate, the effects of stratospheric ozone depletion on climate, and the effects of aerosols on the climate system. He's also co-author of the book Frontiers of Climate Modeling. Thanks very much for joining us. JEFF KIEHL: Thank you. JAY: So we're going to go to our first question, and it's going to be for Jeff. And the first question is: One of the arguments given by skeptics is that they claim that warming is connected to activity of the sun, and one can see warming of some sort on Mars as well. Doesn't this disprove that CO2 is the issue?–I guess, is the argument. And this was sent by A. Barzawi, who put this up on our website. So, Jeff, what do you make of the argument this is really about activity of sunspots and such? KIEHL: Yeah, this has actually been a common argument used by a number of people who feel that the sun is the explanation for the observed warming on Earth. And most of this argument comes from the connection that people saw for a certain period of time: if you correlated or plotted the sun's output over the last 30 or 40 years, at the same time on the same graph you plotted the Earth's surface temperature, you saw that these–as the sun's activity increased, the planet was warming.

And so people immediately jumped to the gun and said, well, this shows that it's not increases in carbon dioxide that increase the temperature; it's the sun's output. Now, the problem with this argument is that they should have waited a few more–a couple more decades, because around 1980, yes, the solar–the temperature continued to increase, but the solar activity had peaked, because it was part of the 11-year solar cycle, and had started to go down. So this correlation or connection between solar activity and Earth's surface temperature has broken down. So there really isn't any strong evidence that the major cause of the warming that we've been seeing on earth is related to solar activity.

Solar activity does play a role, and it's been investigated how large of a role it plays, but it's actually a minor role compared to the increases in carbon dioxide. JAY: Valérie, do you want to add something to it? KIEHL: Excuse me? JAY: No, I'm asking Valérie if she wants to add something to the answer. VALÉRIE MASSON-DELMOTTE: I think, regarding total solar irradiance, it's clear there's no increasing trend over the last 40 years. But temperature has a clear increasing trend. So solar activity cannot be the driver of the warming over the last 40 years. It's likely that it's played a role at the beginning of the 20th century, but it's not the cause for the ongoing warming. JAY: Okay. So I'm–if you–. Sorry. Go ahead, Jeff. KIEHL: Could I say something about Mars? JAY: Yeah, please.

KIEHL: That is part of the question. And, I mean, the problem there is that the record, the temperature record, first of all, is not global. It's from one station, one site, observational site, on the surface of Mars. And it's a very, very short temperature record. So it would really be like us climate scientists going out and measuring the temperature on Earth at one place, Baltimore, and measuring it for a few years, and then going out and stating, oh, we know what the Earth's global climate is doing over a decades-long timescale. So this is a case of too little data, both in terms of locations and time, and making a grand inference about what's causing climate change on Mars–if it's even changing. JAY: Okay. We have–a couple of viewers have sent in questions that relate to other periods in Earth's history where temperatures have gone up and does not seem to be connected to CO2.

There's apparently an increase in temperature that can be seen during medieval times, and that's often cited as an example that there is natural ebbs and flows in temperature, and one couldn't say CO2 had anything to do with what happened in medieval times. And then another viewer writes, in another period, which he says–he says after World War II there was a huge surge in recorded CO2 emissions, but global temperatures fell for four decades after 1940. So, Valérie, do you want to deal with this issue of medieval times? Or is that more of Jeff's bailiwick? Which one of you wants to take this on? MASSON-DELMOTTE: No, the medieval time is really something that I would like to comment on. JAY: Yeah. Go ahead. MASSON-DELMOTTE: Okay. So one ongoing effort is to characterize the climate at the global scale during this time interval. And this requires a huge collection of data from all latitudes, continents, oceans (it's a real challenge), and then agglomerating the sources of data to have an estimate of large-scale temperature. So far, we have estimates of Northern Hemisphere summer temperature, mostly overland.

Several reconstructions are available. And based on these reconstructions and their uncertainty, the ongoing warming seems to be larger than the warm episode of the medieval period. More important is the fact that the spatial structure of the warming during the medieval period is clearly different from the spatial structure of the ongoing warming which is detected both in the tropics and the high latitudes. The situation was different during the medieval period. Some areas had larger warming around the North Atlantic than others. And so the current understanding is based on these data, the documentation of the natural forcings that act on climate–these are the solar activity and the volcanic activity. And the last point is a characterization of the large-scale modes of variability linked with ocean currents in the North Atlantic, ENSO (El Niño-Southern Oscillation), and also patterns of variability of atmospheric circulation, called Arctic oscillation. So far, the explanation for the warm anomaly during the medieval period is probably the coincidence of relatively large solar activity, larger than the centuries before and after, but not much larger than today.

The second point is a lack of volcanic forcing. So it's a long interval with very few volcanic eruptions, which is, in fact, unusual. And the last point is probably a coincidence of modes of variability both in the tropics and in the high latitudes. So this is the current understanding based on the available data, the known forcings, and the use of climate models to relate causes to consequences, that is, simulations of the last thousand years, first by natural forcings, and the comparison between these simulations and the data. JAY: Well, Jeff, I can hear skeptics saying, then, well, if the explanation in medieval times is solar activity and natural forces, then why isn't that the explanation now? KIEHL: Because the rate of change is–and the magnitude is much larger than in the past. And you can see this if you ever have have looked at these figures that show annual temperature through time back 1,000 years. And once you get into the latter part of the 20th century, the temperature increase is significantly different than what you saw in the past.

But I want to go back to the specific question you asked about, you know, after World War II, carbon emissions went up, but the temperature didn't track along with the carbon emissions. The explanation for this is–well, first of all let me say, the expectation among some people is that as you increase the amount of carbon dioxide, the temperature should increase in lockstep with that increase in carbon dioxide. Now, that would be true in a very, very simple climate system, one that doesn't have dynamical oceans that can take up heat on different timescales and bury that heat and then bring it back to the surface, a climate system that doesn't have changes in, you know, terrestrial sinks of carbon and ocean sinks. Unfortunately, we don't live on such a simple planet–or maybe fortunately we live on a very complex planet. And so one really actually doesn't expect a lockstep, year-to-year [incompr.] CO2 increases, the temperature is going to, you know, gradually increase with each gradual increase of carbon dioxide.

Because of things like the ocean and its ability to take up heat and store it and then bring it back to the surface, this introduces periods where the temperature doesn't increase year after year in lockstep with the increases in carbon dioxide. You have fluctuations, natural variability in the climate system that keeps the temperature not one-to-one correlated with the increases in carbon dioxide. You know, a criticism that's been, actually, launched against our understanding of the climate system recently has been, oh, the last ten years of globally averaged surface temperature haven't shown, you know, a lockstep increase in temperature with carbon dioxide, and that, you know, climate models don't show this feature, and therefore climate models are wrong. Well, actually, that's a very false accusation. There's absolutely no convincing evidence of this. In fact, people have done very detailed studies of both observations and climate models and shown that this sort of flattening out of the temperature record on a decadal (ten-year) timescale has periodically occurred through the 20th century, and if you look at climate models, they do the same thing.

What is the cause for that flattening out of, say, [incompr.] ten-year timescales? It's the storage of heat in the ocean, because that's sort of the long-timescale regulator of climate system. JAY: Okay. If you want–. MASSON-DELMOTTE: Yeah, I would like to add one point. JAY: Yeah, go ahead, Valérie. Yeah. MASSON-DELMOTTE: So there are two comments I would like to make. The first one is that the effect of CO2 on climate is not linearly proportional to the concentration of CO2. That's one thing. It's more logarithmically related to the concentration of CO2. So you cannot expect the climatic effect of CO2 to have the same shape as the increase in the atmosphere. That's one point. And the second point is that after World War II, where the use of fossil fuel increased dramatically, emissions of CO2 increased, but also the emission of convection aerosols. These are pollution particles, especially composed of sulfates, that have a climate effect.

So they are quickly washed out by rain, but when they are in the atmosphere, concentrated around the industrial areas, these particles reflect part of the incoming solar radiation. So at first order, they have a rough cooling effect, they have indirect effects on the formation of clouds. And it's probable that–it's likely that in the 1950s to 1980s, the emissions of pollution aerosols, especially in Europe, former Soviet Union, and the U.S. and Canada, had a significant effect on the Northern Hemisphere's temperatures. And so they acted against the effects of greenhouse gases. JAY: And since then, there's far less aerosols in the atmosphere is the point. KIEHL: Well, actually, the sources of the aerosols have been shifting. I mean, you know, up into the mid '70s or so, it was the West that was emitting lots of sulfur dioxide, which became particles in the atmosphere.

But, you know, we've placed technology scrubbers on smokestacks that have dramatically reduced the amount of emission of carbon dioxide. However, other countries that are now burning lots of fossil fuel, coal, are emitting lots of aerosols into the atmosphere–sulfur dioxide, which becomes aerosols. And this is an interesting issue. I mean, it's a sort of a dilemma that if you were to actually put clean air scrubbers on all the smokestacks around the world, you'd be taking out all of that aerosol production, and you'd actually see the global temperature increase. There–you know, people have done calculations to estimate how much the planet would warm or accelerate in its warming if you were actually to clean the air up from dirty [crosstalk] JAY: But in terms of the role of aerosols in the post-World War II period, do we think there was more aerosols then than there is now? KIEHL: No. No, because our industrial activity, the amount of energy, fossil fuel, that we were burning then was less than we are now or less than, say, the mid '60s, before we put scrubbers on smokestacks. MASSON-DELMOTTE: It was not located at the same places.

KIEHL: Right. Emissions locations have changed through time. That's what I was saying is that now the emissions are mainly coming from, you know, parts of the world that are still burning lots of coal and don't have cleaners, scrubbers on their smokestacks. JAY: Right. So the issue of temperature not going up lockstep with carbon emissions after World War II has more to do with the effect of oceans and the other issues you talked about. KIEHL: Well, aerosols and just the intrinsic nature, variability in the climate system, I think those are two processes that we know were and are still active in the Earth's atmosphere climate system that can explain these periods where even though carbon dioxide emissions are going up, the temperature isn't going up just in step with it. JAY: Right. Okay. So if you would like to ask more questions about this issue, you can either do it below here in the comment sections–it's probably the easiest way to do it, and because then we'll know which questions relate to which segment that we're doing of this series.

We will do another segment, several other segments. Please come back soon for those. And don't forget the "Donate" button. This is all part of our matching-grant spring/summer fundraising campaign. And if you want see us do more Real News, then we need you to click that. Thanks for joining us on The Real News Network..