Calif. Researchers in Lawrence Livermore National Laboratory s Atmospheric Science Division have demonstrated a cooling of up to 2-degree Fahrenheit over land between 1000 and 1900 AD as a result of changes from natural vegetation, such as forests, to agriculture. Through climate model simulations, the LLNL research team made up of Bala Govindasamy, Ken Caldeira and Philip Duffy, determined that a previously recognized cooling trend up to the last century could, in part, be attributed to the land-use change. 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 amber waves of grain, tend to look 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 mid-western United States, where huge tracts of land have been converted to crops. In the 20th century, some of this cropland has been reverting back to forest, especially in the eastern United States. Greenhouse gas emissions in the 20th century likely overcame any cooling trends that took place up to that time. Growing more trees has been suggested as a way to soak up carbon dioxide, a greenhouse gas, from the atmosphere.