At the same time, temperatures in the eastern United States have cooled. The cooling in the southeastern United States may be due to excessive sulfates in the air. Sulfates can scatter the sun's light before it reaches the earth's surface. Without heat from the sun, the earth's surface will cool. The map at the right displays temperature trends across the United States over the last 100 years.
Red circles represent temperature increases, while blue circles represent temperature decreases.
Large circles represent a 3 C change, medium circles represent a 2 C change, and small circles represent a 1 C change. Map courtesy of the United States Environmental Protection Agency . The increase in temperature has not been consistent through time, either. Some periods during the year have experienced greater and faster temperature increases than others. Average winter temperatures in areas between 50 and 70 North latitude have been increasing quickly, while summer temperatures have hardly changed.