Temperatures are increasing on a global scale, but at the regional level, the story gets complicated. In the central and eastern United States, for example, warming has not kept pace with other parts of the world over much of the last century.
As shown in the lower map, which is based on data from NASA’s Surface Temperature Analysis (GISTEMP), parts of the United States even cooled between 1930 and 1990. Areas of the greatest cooling are blue; those that warmed are red. Climate scientists have taken to calling the large area of cooling a “warming hole” because the areas surrounding it have warmed at a faster rate.
For more than a decade, researchers have puzzled over what’s causing the warming hole over the United States. Previous research has suggested natural variations in sea surface temperatures might be responsible, but a new study puts the focus on sulfates, a type of aerosol produced by coal power plants that’s known for causing acid rain. Sulfates are light-colored, and they cause cooling by scattering and reflecting sunlight. They also lower temperatures indirectly by making clouds more reflective and long-lasting.
The researchers, led by Eric Leibensperger when he was at Harvard University, used global climate models to estimate the cooling effect sulfates have had on the climate of the United States since 1950. As seen in the top map, they found that between 1970 and 1990—the period when sulfates were at their highest levels—average temperatures were nearly 1°Celsius (1.8°Fahrenheit) cooler in a core area centered on Arkansas and Missouri and about 0.7°Celsius cooler in a larger tear-drop region throughout the Midwest and Mid-Atlantic. The cooling effect extended into the North Atlantic Ocean as well; sulfate pollution lowered sea surface temperatures there by 0.3°Celsius.
Leibensperger’s research also shows that the cooling effect from sulfates is diminishing. The amount of the pollutant in the atmosphere has declined significantly in the last few decades due to the Clean Air Act. According to Environmental Protection Agency estimates, the amount of sulfur dioxide (a precursor to sulfates) released into the atmosphere fell by 58 percent between 1980 and 2010. Satellites have confirmed the decrease; the Ozone Monitoring Instrument (OMI) on the Aura satellite observed a sharp decline in sulfates over the eastern United States between 2005 and 2010.
As a response to the declining sulfate levels, Leibensperger’s modeling shows temperatures over the central and eastern United States have increased by 0.3°Celsius between 1980 and 2010. How much more warming can we expect as sulfate concentrations continue to decline? Not much, according to Leibensperger. Sulfate concentrations have declined so much already that the impact of future decreases won't be nearly as substantial.