Climate change often evokes images of smokestacks, and for good reason: The single largest source of carbon emissions related to human activity is heat and power generation, which accounts for about one-quarter of the carbon we put into the atmosphere. Often overlooked, though, is how we use land, which contributes almost as much. The erosion and degradation of soil caused by plowing, intense grazing and clear-cutting has played a significant role in the atmospheric accumulation of heat-trapping gases. The process is an ancient one. Ice cores from Greenland, which contain air samples trapped thousands of years ago, reveal increases in greenhouse gases that correspond with the rise of farming in Mesopotamia.