Farming new land can release lots of CO2 into the atmosphere
When a piece of land is cleared of perennial vegetation such as shrubs and trees, the dead plants are usually burned or left to decay. This releases carbon into the atmosphere, mostly as the greenhouse gas carbon dioxide, which contributes to global warming. The roots, where most plants store a significant fraction of their carbon, are generally left in place. They decay as well, but their carbon, stored underground, takes much longer to reach the atmosphere.
Tilling the soil for agriculture, however, releases much of this stored carbon relatively quickly. Tilling also stirs up humus — carbon-rich organic matter in the soil — in a process that leads to additional greenhouse gas emissions. Overall root and soil carbon loss depends on the characteristics of each site, but typically about 30% is released over a 20-year period of continued tilling and row-crop production.[[West, Tristram O., Gregg Marland, Anthony W. King, Wilfred M. Post, Atul K. Jain, and Kenneth Andrasko. “Carbon Management Response Curves: Estimates of Temporal Soil Carbon Dynamics.” (PDF) Environmental Management 33, no. 4 (2004): 507-518.]]
The release of carbon in the process of preparing land for biofuel crops, including corn for ethanol, is known as building “carbon debt” (see video). Based on energy accounting alone, corn ethanol appears only about 20% better than gasoline from the perspective of greenhouse gas emissions. When carbon debt is factored into the equation, any climate benefit at all is called into doubt. The question becomes how much land and what type of land is converted to farming — at home and even abroad — because of growing corn for ethanol (and instead of food).
For all of these reasons, scientists are now looking to cellulosic ethanol as a more climate-friendly alternative to corn ethanol.