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.
In the U.S. central and eastern regions, a giant “warming hole” has created cooler areas that haven’t kept pace with the warming of other parts of the world during much of the last century.
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.
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