In this chapter, we focus on the land sink of anthropogenic CO2, because humans have a history of using the terrestrial biosphere for our purpose and because efforts to control atmospheric CO2 levels involve deliberate manipulation of the biosphere. We present atmospheric evidence for the land sink and use information about its interannual variations to infer its stability.
The Mauna Loa CO2 record is a clear documentation of the increasing concentration of CO2 in the atmosphere as a result of anthropogenic activities. By 1999, the atmospheric CO2 abundance had increased by 25% since the beginning of the preindustrial era. The cumulative increase, together with the concomitant increase in CH4, N2O, CFCs, and other greenhouse gases, presents a total radiative forcing of ∼2–3 W/m2 to the climate system in the 1990s. This forcing is countered to some degree by the increase in sulphate and other aerosols in the atmosphere.
The decreasing 14C/12C ratio in tree rings (Suess, 1955) proves that the atmospheric CO2 increase is due to the addition of fossil (14C-free) carbon. However, the CO2 increase rate, as determined from the atmospheric record, is only 50%–60% that emitted by fossil fuel combustion (Figure 3.1). Thus, the land and oceans have absorbed the remainder of the fossil fuel CO2 as well as the CO2 released due to land use modification.