344 CARBON CYCLE measurements of trapped air demonstrate that PCO2 for the 18 000 years before 1800 was much lower than it is today, and consistently averaged 280 ppm There is a widespread agreement among scientists that the almost 100-ppm rise since the beginning of industrialization is unprecedented, being more than the increase during the last interglacial warming, which took place over thousands of years rather than over a mere 200 years Isotopic analyses of atmospheric CO2 confirm beyond doubt that this extra CO2 derives in large part from the incineration of fossil fuels such as coal, oil, and gas The major components of the anthropogenic perturbation to the atmospheric carbon budget are anthropogenic emissions, ocean and terrestrial exchanges, and their effects on the atmospheric CO2 increase (Figure 11) Models suggest that each year more than half of all the anthropogenic emissions from fossil fuel combustion, cement production, and tropical land clearing are offset by uptake in the oceans, by forest regrowth largely in the northern hemisphere, and by enhanced growth caused by increased levels of nutrients, mostly CO2 and nitrate from fertilizers Although several potential feedbacks exist to further counteract CO2 increase through the effects of climate on ocean and terrestrial biosphere uptake of CO2, it is not thought that these are significant in the short term However, the possible effects of climate change on ocean circulation could be very significant indeed Much current research is devoted to predicting how CO2 levels are likely to rise in the future and what impact this will have on global climate, sealevel, and biodiversity The greenhouse effect of CO2 is roughly logarithmic, which means that each factorof-two increase in CO2 produces roughly the same amount of warming Global circulation models predict a doubling of CO2 levels by the end of the twenty-first century (Figure 12) associated with global warming of between 1.5 and 4.5 C Although the effects of global warming will be felt worldwide, changes in temperature will be distributed unevenly, with by far the greatest impact felt in the high latitudes of the northern hemisphere Because the cycling of carbon in the terrestrial and ocean biospheres occurs slowly, on time-scales of decades to millennia, the effect of additional CO2 through industrialization inevitably represents a long-lasting disturbance to the carbon cycle Model predictions of future atmospheric CO2 levels indicate that they will continue to Figure 11 Major carbon reservoirs and fluxes (in gigatons and gigatons year 1, respectively) in the global anthropogenically influenced carbon cycle Reprinted from Kump LR, Kasting JF, and Crane RG (1999) The Earth System Upper Saddle River, NJ: Prentice Hall