evidence: A. Anthropogenic CO2 comes from fossil fuel combustion, changes in land use (e.g., forest clearing), and cement manufacture. Houghton and Hackler have estimated land-use changes from 1850-2000, so it is convenient to use 1850 as our starting point for the following discussion. Atmospheric CO2 concentrations had not changed appreciably over the preceding 850 years (IPCC; The Scientific Basis) so it may be safely assumed that they would not have changed appreciably in the 150 years from 1850 to 2000 in the absence of human intervention. In the following calculations, we will express atmospheric concentrations of CO2 in units of parts per million by volume (ppmv). Each ppmv represents 2.13 X1015 grams, or 2.13 petagrams of carbon (PgC) in the atmosphere. According to Houghton and Hackler, land-use changes from 1850-2000 resulted in a net transfer of 154 PgC to the atmosphere. During that same period, 282 PgC were released by combustion of fossil fuels, and 5.5 additional PgC were released to the atmosphere from cement manufacture. This adds up to 154 + 282 + 5.5 = 441.5 PgC, of which 282/444.1 = 64% is due to fossil-fuel combustion. Atmospheric CO2 concentrations rose from 288 ppmv in 1850 to 369.5 ppmv in 2000, for an increase of 81.5 ppmv, or 174 PgC. In other words, about 40% (174/441.5) of the additional carbon has remained in the atmosphere, while the remaining 60% has been transferred to the oceans and terrestrial biosphere.