They examine past values and create climate models to predict future trends in temperature change. In reality, the temperature change has not been consistent across the planet. In the last 50 years, temperatures in the western United States have warmed. 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 .