Greenhouse gas emissions since 1900 have probably overcome any cooling trends that took place up to 1900. 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 generally look dark from the sky, but agricultural lands, with their amber waves of grain, appear much lighter.
Dark colors absorb sunlight, and light colors reflect sunlight back out to space.
Changing from forests to crops therefore results in more sunlight being reflected back to space, which 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. This is an example of inadvertent geoengineering - we changed the reflectivity of the Earth and have probably caused a global cooling in the past. This is now probably being overwhelmed by our greenhouse gas emissions. So, when we talk about global warming, we can no longer take for granted that this global warming is starting from some natural climate state, undisturbed by human activities. Since about 1900, some cropland has been reverting back to forest, especially in the eastern United States.