ABSTRACT
During the 1980s an average of 5.5±0.5Gt (giga-tonnes, or billions of tonnes) of carbon were released to the atmosphere every year as CO2, due to the burning of fossil fuels, with an additional 1.6±1.0 Gt C per year due to tropical deforestation and other land use changes (Schimel et al., 1996). Regrowth of forests on abandoned agricultural land in the NH mid-latitudes might have absorbed 0.5±0.5Gt C per year, to give a range in total net anthropogenic emissions of 4.6-8.6 Gt C per year. Atmospheric CO2 increased at the rate of 3.3±0.2 Gt C per year during this time, which implies that 1.1-5.5 Gt C per year, or about one-third to two-thirds of total anthropogenic emissions, are quickly removed from the atmosphere at present. This carbon is thought to be removed primarily through absorption by the oceans and by an increase in the rate of global photosynthesis due to the stimulatory effect of the 25% increase in atmospheric CO2 which has occurred since the Industrial Revolution. Recent analysis of oceanic and atmospheric data indicate that the likely rate of oceanic uptake is 2.0±1.0 Gt C per year (Chapter 8.9). Thus, a consideration of the overall CO2 budget implies that the terrestrial biosphere is absorbing an extra 0-4.5 Gt C per year.
