Deforestation accounts for about 20% of CO2 emissions globally
Most of the ongoing increase in carbon dioxide emissions is caused by the burning of fossil fuels. Not all, though: the Intergovernmental Panel on Climate Change (IPCC), the world’s leading authority on the subject, estimated in a 2007 report that land use change, dominated by deforestation, caused about 1.6 billion tons of carbon emissions per year in the 1990s, versus about 6.4 billion tons annually from burning fossil fuels and making cement.[[Denman, K.L., G. Brasseur, A. Chidthaisong, P. Ciais, P.M. Cox, R.E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S Ramachandran, P.L. da Silva Dias, S.C. Wofsy and X. Zhang, 2007: Couplings Between Changes in the Climate System and Biogeochemistry. (PDF) In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. pp 516-18.]] The report gave a range of uncertainty of plus-or-minus about 65% for the emissions from land use change, because these emissions are difficult to track, and plus-or-minus about 6 percent for the fossil fuels and cement. These ranges translate to land use change accounting for between 8 and 31 percent of total annual carbon dioxide emissions, with 20 percent as a reasonable estimate.
If, as some argue, the expansion of corn production to produce ethanol ultimately results in increased deforestation, the notion of corn ethanol as a climate-friendly alternative to gasoline becomes dubious.