The Charney Report vs IPCC6: What’s changed in climate science in the last 40 years?

Papers: Carbon Dioxide and Climate, a Scientific Assessment by Charney et. al (1979);
Climate Change 2021: The Physical Science Basis by the IPCC (2021)

Right now in Glasgow, Scotland, representatives of world governments and other parties are currently gathering yet again to negotiate political solutions to climate change at COP26. This is the 26th semi-annual Conference of the Parties on climate change, but the history of our understanding of the problem — and attempts to deal with it — goes back even further than that. Speaking strictly of the science of global warming and its effects, what do we know now that the participants of the first COP did not?

One of the first major comprehensive scientific reports on the problem of climate change that was presented to policymakers was known as the Charney Report after its lead author, presented to the US Congress in 1979. Likewise, just this year the IPCC’s 6th report came out. Both of these reports are available to the public, and examining the differences and similarities between the two gives a fairly good idea of the major changes in climate science over the last few decades as relates to policy.

The Charney Report’s conclusion even back then was unequivocal: the climate will change in response to the anthropogenic carbon dioxide that had already been emitted. (By 1979, that information was already old news to the scientific community, courtesy of researchers going as far back as Svante Arrhenius and Eunice Foote in the 19th century. However, in 1979 this information was only just coming to the attention of lawmakers as well.)

The Report also summarized 5 areas of research that related to the question of how, exactly, the world would warm in response to greenhouse gas emissions. Back then, research was only just beginning into answering the question of “equilibrium climate sensitivity” –  essentially, in response to a given greenhouse gas forcing, how much would the world warm? The same metric is used today, including in the IPCC reports, although there is now some debate as to whether it is the most relevant metric for policymakers. In 1979, the areas of active research into equilibrium climate sensitivity that Charney and co-authors summarized were:

  1. How will global warming change clouds and their radiative effects (“cloud forcings/feedbacks”)? Could it lead to more warming?
  2. Are small aerosol particles increasing or decreasing global warming (the “aerosol effect”)?
  3. How much CO2 and heat from the surface will the deep ocean take up?
  4. How will climate change affect different parts of the globe (“spatial variability”)?
  5. What is the future pathway of human greenhouse gas and aerosol emissions?

In the last 40 years, scientists have been hard at work narrowing down the uncertainty in these areas in order to understand exactly how much the climate might warm in response to a certain amount of CO2 emissions. This is important for policy negotiations because it determines how much CO2 can be emitted while avoiding catastrophic impacts — policymakers cannot directly legislate the Earth’s temperature, but they could legislate or otherwise control CO2 emissions. However, politicians do not care about CO2 in itself but rather its impacts (such as temperature), which is why it is important that scientists investigate the relationship between CO2 emissions and temperature.

So what have they discovered? First off, the overall estimate of equilibrium climate sensitivity, or the average expected global temperature change if the atmospheric CO2 concentration goes up to 560ppm, remains at about 3 degrees centigrade of warming. This value was first calculated by Arrhenius and has not changed much since in over 100 years of climate science (although the error bars around that figure are constantly changing with new research). However, our understanding of why that value is what it is has improved dramatically, and our understanding of the effects of that potential change on the planet has also changed, in some areas of research more than others.

#1 on the list above from the Charney Report was uncertainty surrounding cloud feedbacks, which we still do not understand very well. Looking at a modern diagram of climate feedback uncertainties, we still don’t know whether the changes to clouds resulting from global warming will ultimately serve to increase or decrease climate change. But we do understand the processes involved much better.

Aerosol particles from industrial activities — such as soot and sulfur dioxide — were on average cooling the planet and acting against global warming. We’ve cleaned up non greenhouse-gas forms of pollution a lot since 1979, and so the planet is heating up faster than it would if we still had serious air pollution like we did back then.

We are still fairly uncertain on long term ocean CO2 uptake, although we know better now how the ocean circulation works. This won’t really change the overall equilibrium climate sensitivity, it will only change how long it takes to reach that equilibrium. We now know that, due to the slow speed of the deep ocean circulation, this process will take hundreds to thousands of years.

Back in 1979, scientists knew that the poles would warm more than the global average, but not much more than that. Today we worry also about changes to monsoons in Central America and Southeast Asia, the Gulf Stream that keeps Europe warmer than North America at similar latitudes, and more. We also worry about the “pattern effect”, or positive or negative feedbacks due to where exactly on the globe warming occurs. (We haven’t yet identified which pattern of warming would have such effects, but modeling studies suggest that a climate feedback due to the pattern of warming is possible.)

We now know more about how human emissions have varied over time. We know that aerosol emissions decreased due to the Clean Air Act in the US and similar policies, and that carbon emissions have continued to increase despite past attempts to limit them. However, we do not know what the emissions pathway of the future will look like, only that the Paris Agreement of 2015 will not be enough to avert serious climate change of 2 or 3 degrees (or more). The global policymakers gathering in Glasgow right now for the COP may be able to come to an agreement that gives us more certainty.

We also now know much more about feedback effects not mentioned in the Charney report, including the fact that they are possible in the first place. Two major categories of these “non-Charney” feedbacks are carbon cycle feedbacks and albedo feedbacks. Carbon cycle feedbacks include, for example, methane release from melting permafrost or ocean circulation changes, or carbon dioxide release from wildfires. (Wildfires also release aerosols that may have a cooling effect on the atmosphere — as well as soot and ash that may darken the Earth’s surface and contribute to albedo feedback effects that warm the Earth.) Other albedo feedback effects include desertification, or melting ice caps: any response to global warming, ecological or physical, that changes the color of the surface of the planet and thereby changes how much radiation reflects back into space instead of being kept in our atmosphere. Right now, the carbon cycle and albedo feedback effects are some of the greatest unknowns in climate science research today.

Despite some lingering unknowns, we have the information needed to make a crucial decision: to cut greenhouse gas emissions, immediately. In fact, we’ve had enough of this information since 1979. The biggest uncertainty left is this: will those in positions of power (at COP26 and elsewhere) act on this science, and soon?

The Charney Report vs IPCC6: What’s changed in climate science in the last 40 years? by Lee Brent is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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