Human activities produce large amounts of greenhouse gases (GHGs), primarily carbon dioxide (CO2), and thus contribute to global warming. The use of fossil fuels is the primary source of CO2 emissions, but the removal of trees from forested land has also contributed. Mature forests, having absorbed CO2 from the atmosphere while growing, store carbon in wood, leaves, and soil. That carbon is released when people clear forested land and destroy the wood. From 2000 to 2005, the loss of forests, primarily in tropical developing countries, accounted for approximately 12 percent of global GHG emissions. Slowing or halting deforestation in developing countries is a potentially low-cost way to help reduce global GHG emissions. For that potential to be realized, however, substantial challenges would need to be addressed-by providing technical and financial assistance to governments, by creating demand from private markets, or both.