7.3.1.2 Perturbations of the Natural Carbon Cycle from Human Activities

The additional burden of CO₂ added to the atmosphere by human activities, often referred to as ‘anthropogenic CO₂’ leads to the current ‘perturbed’ global carbon cycle. Figure 7.3 shows that these ‘anthropogenic emissions’ consist of two fractions: (i) CO₂ from fossil fuel burning and cement production, newly released from hundreds of millions of years of geological storage (see Section 2.3) and (ii) CO₂ from deforestation and agricultural development, which has been stored for decades to centuries. Mass balance estimates and studies with other gases indicate that the net land-atmosphere and ocean-atmosphere fluxes have become significantly different from zero, as indicated by the red arrows in Figure 7.3 (see also Section 7.3.2). Although the anthropogenic fluxes of CO₂ between the atmosphere and both the land and ocean are just a few percent of the gross natural fluxes, they have resulted in measurable changes in the carbon content of the reservoirs since pre-industrial times as shown in red. These perturbations to the natural carbon cycle are the dominant driver of climate change because of their persistent effect on the atmosphere. Consistent with the response function to a CO₂ pulse from the Bern Carbon Cycle Model (see footnote (a) of Table 2.14), about 50% of an increase in atmospheric CO₂ will be removed within 30 years, a further 30% will be removed within a few centuries and the remaining 20% may remain in the atmosphere for many thousands of years (Prentice et al., 2001; Archer, 2005; see also Sections 7.3.4.2 and 10.4)

About 80% of anthropogenic CO₂ emissions during the 1990s resulted from fossil fuel burning, with about 20% from land use change (primarily deforestation) (Table 7.1). Almost 45% of combined anthropogenic CO₂ emissions (fossil fuel plus land use) have remained in the atmosphere. Oceans are estimated to have taken up approximately 30% (about 118 ± 19 GtC: Sabine et al., 2004a; Figure 7.3), an amount that can be accounted for by increased atmospheric concentration of CO₂ without any change in ocean circulation or biology. Terrestrial ecosystems have taken up the rest through growth of replacement vegetation on cleared land, land management practices and the fertilizing effects of elevated CO₂ and N deposition (see Section 7.3.3).

Because CO₂ does not limit photosynthesis significantly in the ocean, the biological pump does not take up and store anthropogenic carbon directly. Rather, marine biological cycling of carbon may undergo changes due to high CO₂ concentrations, via feedbacks in response to a changing climate. The speed with which anthropogenic CO₂ is taken up effectively by the ocean, however, depends on how quickly surface waters are transported and mixed into the intermediate and deep layers of the ocean. A considerable amount of anthropogenic CO₂ can be buffered or neutralized by dissolution of CaCO₃ from surface sediments in the deep sea, but this process requires many thousands of years.

The increase in the atmospheric CO₂ concentration relative to the emissions from fossil fuels and cement production only is defined here as the ‘airborne fraction’.^[2] Land emissions, although significant, are not included in this definition due to the difficulty of quantifying their contribution, and to the complication that much land emission from logging and clearing of forests may be compensated a few years later by uptake associated with regrowth. The ‘airborne fraction of total emissions’ is thus defined as the atmospheric CO₂ increase as a fraction of total anthropogenic CO₂ emissions, including the net land use fluxes. The airborne fraction varies from year to year mainly due to the effect of interannual variability in land uptake (see Section 7.3.2).