A new paper in Nature Geoscience finds that “transient climate sensitivity,” the fast temperature rise that accompanies increases in atmospheric CO2, may be much higher than previously thought. The reason is widespread sulfate pollution from man-made and natural sources, which has tempered the warming that the world would otherwise be seeing at current CO2 levels. This conclusion is based on research funded by YCEI and led by Yale atmospheric scientist Trude Storelvmo. Prior studies that did not fully account for the cooling influence of airborne particles may have underestimated how sensitive Earth’s climate really is to greenhouse gases such as CO2. Thus, as governments around the world take steps to improve air quality for better health, for example, by transitioning heavy industries to lower-sulfur fuels, they may inadvertently counteract efforts to limit the effects of climate change.
The concept of climate sensitivity has been used by scientists for many years to describe how Earth’s climate will respond on average to increased greenhouse gases such as carbon dioxide in the atmosphere. At the start of the Industrial Revolution, the atmospheric concentration of CO2 was about 300 ppm; today it is above 400 ppm and rising by about 2 ppm per year, mainly because of the CO2 that is released into the atmosphere by the burning of fossil fuels in the world’s energy systems. In 1979, MIT scientist Jule Charney led a study for the National Research Council that made the first modern estimates, using global climate models, of what is called the “equilibrium climate sensitivity,” a quantity that represents the Earth’s average temperature response to atmospheric CO2 in the long run, after the climate system has stabilized (reached equilibrium) at the higher concentration of CO2. Since carbon dioxide remains in the atmosphere for thousands of years, the “long run” can involve extremely long time scales by human standards.
The Charney report found that a doubling of CO2 would result in an average global temperature rise between 1.5 and 4.5 degrees Centigrade. Thirty-seven years after the Charney report, the range of estimates for equilibrium climate sensitivity—as given in the latest (2013) assessment report from the Intergovernmental Panel on Climate Change (IPCC)—is about the same.
The work by Storelvmo and her co-authors is specific to transient climate sensitivity, a quantity that represents the rise in average global temperature that would (more or less) immediately accompany a doubling of CO2 levels. The 2013 IPCC assessment report puts the transient sensitivity in the range of 1 to 2.5 degrees Centigrade. This temperature range is lower than the range for the equilibrium sensitivity in the Earth’s current state, partly because the transient sensitivity does not allow time for long-term feedback effects, such as the melting of glaciers or polar ice, to reinforce the immediate temperature rise from increased greenhouse gases. In many ways, however, the transient sensitivity is the number that is most relevant to efforts to mitigate the looming, short-term economic and social effects of global warming.
The new paper in Nature Geoscience suggests that values at the lower end of the range given in the IPCC report are unlikely to be realized in Earth’s actual climate system, if sulfate and other aerosol pollution is cleaned up. Says Storelvmo, “Our results imply that achieving the UN’s goal of limiting warming to 1.5 degrees above preindustrial levels will require immediate action in the form of a rapid transition to global net zero carbon emission.” According to Michael Mann, Distinguished Professor of Atmospheric Science at Penn State University, the work “… provides sobering evidence that Earth’s climate sensitivity may lie in the upper end of the current uncertainty range.”