Last week, Science published a new study by Oregon State University researcher Andreas Schmittner and colleagues who found that an increase of carbon dioxide in the atmosphere may result in less warming than predicted. The researchers declared that their results "imply less probability of extreme climatic change than previously thought." Has the global warming apocalypse been called off?
Researchers know from physics that added carbon dioxide tends to increase the temperature of the atmosphere because it blocks the escape of heat from Earth into space. The term climate sensitivity conventionally refers to how much warming can be expected from a doubling of carbon dioxide concentrations in the atmosphere. It is generally agreed that by itself, doubling the amount of carbon dioxide in the atmosphere above pre-industrial levels would boost the average global temperature by about 1.2° Celsius (2.2° Fahrenheit). However, many climate researchers believe that other feedbacks (water vapor, clouds, etc.) add to the effect of carbon dioxide (and other greenhouse gases).
As humanity has burned fossil fuels, carbon dioxide concentrations in the atmosphere have increased from 280 parts per million (ppm) at the beginning of the Industrial Revolution to 389 ppm today. Temperature records agree since the late 19th century that the average global temperature has increased by 0.8°C (1.4°F).
The question of climate sensitivity has long been a vexed scientific problem. With regard to climate sensitivity, in 2007 the Fourth Assessment Report of the United Nation's Intergovernmental Panel on Climate Change (IPCC) estimated that "climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values." In IPCC parlance, likely means that there is a 66 percent probability that climate sensitivity falls between 2 and 4.5°C (3.6 to 8.1°F), with 3°C (5.4°F) as the best estimate.
Even more problematic is that previous researchers have been unable to rule out that doubling atmospheric carbon dioxide will not produce even bigger increases, say, in excess of 6°C (13°F), in global average temperature. Researchers have tried to pin down climate sensitivity by analyzing recent observational climate data, by tuning climate computer models to see how accurately they reproduce past climate, and by comparing paleoclimate data to current conditions.
The Science study combines paleoclimate data from the Last Glacial Maximum 21,000 years ago with computer modeling to arrive at a lower climate sensitivity figure. They estimate that climate sensitivity likely (66 percent probability) ranges from 1.7 to 2.6°C (3 to 4.7°F) with a median estimate of 2.3°C (4.1°F). The even better news is that the study finds that climate sensitivities larger than 6°C are "implausible." If carbon dioxide levels double, the world will likely get warmer, but not catastrophically hot. In other words, the article suggests that the sky is falling less.
The new study achieves this result by doing several runs of a moderately complicated climate model plugging in various values for climate sensitivity. The researchers then checked to see if the model outputs matched their new set of paleoclimate temperature data involving 435 temperature proxies, 322 ocean proxies, and 113 land proxies. Proxies like the prevalence of certain kinds of plankton fossils in ocean sediments and pollen on land. Plugging in a 6°C climate sensitivity produced a snowball Earth entirely covered with ice which clearly didn't happen whereas inserting values below 1.4°C generated too warm a world.
Naturally, the new paper landed in the midst of the fraught debate over man-made global warming. The editors at Investors Business Daily assert that "it's a bombshell—another in a long line of revelations showing the scientific fraud at the heart of the anti-global [sic] warming movement." On the other hand, climate change alarmist Joe Romm at the Center for American Progress headlines, "Media Misleads On Flawed Climate Sensitivity Study."
Of course, every study can be criticized and this one is certainly no exception. One of the reasons that comparing the Last Glacial Maximum to today is scientifically intriguing is that atmospheric carbon dioxide levels 21,000 years ago were only 190 ppm. Schmittner and his colleagues attempt to work backward from Ice Age temperatures to estimate climate sensitivity.
One of the chief criticisms lodged against the new study is that it significantly underestimates how cold the Last Glacial Maximum was. According to the new study, during the Last Glacial Maximum, the world was on average about 3.3°C (6°F) colder than today. In contrast, the IPCC estimates that Ice Age temperatures were between 4 to 7°C (7.2 to 12.6°F) colder than today. A colder Last Glacial Maximum implies higher climate sensitivities due to lower levels of atmospheric carbon dioxide. So figuring out which temperature regime is the more accurate is critical to determining the validity of the new study's estimates.
Another possibility suggested by a Science Perspective by University of Edinburgh researchers Gabriele Hegerl and Tom Russon published simultaneously is that adding carbon dioxide to a colder Ice Age climate will affect different climate feedback mechanisms. If so, adding carbon dioxide to our warmer interglacial climate will yield a different (and perhaps higher) climate sensitivity.
Assuming, however, that the new study's results stand up to further scrutiny, what does this mean in terms of future climate change? At the United Nations' Cancun climate change meeting last year, climate negotiators set a goal of avoiding an increase of more than 2°C (3.6°F) above pre-industrial temperatures. The folks over at the RealClimate blog calculate that if the IPCC's climate sensitivity estimate is right, at current rates of carbon dioxide increase this goal will be exceeded in 24 years. Assuming the new estimate is right adds 11 years.
"It is rare that a single paper overturns decades of work," notes Nathan Urban from Princeton University in a New Scientist interview. Urban, a contributing author to the new study, modestly added, "Quite often, it turns out that it's the controversial paper that is wrong, rather than the research it hopes to overturn. Science is an iterative process. Others have to check our work. We have to continue checking our work, too."
If their work ultimately checks out that would mean that while the world will get warmer, it will likely not become catastrophically hot. In other words, the sky would be falling less.
Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is available from Prometheus Books.