2001 Farms caused cooling over last millennium say scientists A cooling of up to 2-degree Fahrenheit over land between 1000 and 1900 A.D. was caused by farms replacing forests and other natural vegetation, say researchers in Lawrence Livermore National Laboratory's Atmospheric Science Division. Previous studies had attributed cooling to natural climate variations. The Livermore research, however, suggests that much of this cooling could have been the result of human activity. Forests tend to look dark from the sky, but agricultural lands, with their literally amber waves of grain, are much lighter.
Dark colors tend to absorb sunlight, and light colors tend to reflect sunlight back out to space.
Changing from forests to crops results in more sunlight reflected back to space. This reflection of solar energy to space tends to cool the Earth, especially in regions such as the eastern and Midwestern United States, where huge tracts of land have been converted to crops, the Livermore scientists say. Greenhouse gas emissions in the 20th century likely overcame any cooling trends that took place up to that time, they believe. Growing more trees has been suggested as a way to soak up carbon dioxide, a greenhouse gas, from the atmosphere. However, earlier studies demonstrate that growing dark forests could actually heat the earth's surface because dark colors tend to absorb more sunlight, despite the uptake of carbon dioxide.