Interannual variability in the global carbon cycle

Abstract

The annual growth rate of atmospheric CO2 has varied between 1 and 5 Pg C yr−1 over the last decades. Most of this variation is associated with terrestrial and oceanic exchanges of carbon which seem to vary independently. Three processes contribute to the annual flux of carbon from terrestrial ecosystems: changes in land use, natural disturbances, and metabolic changes caused by variations in climate. Because rates of land‐use change are often not available on an annual basis, estimates of the flux of carbon attributable to land use change may underestimate year‐to‐year variability. Limited data reviewed here suggest that the interannual variability of this flux is generally small for two reasons. First, although rates of land use change may vary substantially from year to year at a local scale, variability is generally less at regional and global scales because high rates of deforestation in one area do not necessarily coincide with high rates in other areas. Second, less than 50% of the carbon lost to the atmosphere as a result of land use change is lost in the year of disturbance; the rest is released in subsequent years. The interannual variability of the flux of carbon from land use change is thus less variable than rates of land use change and probably accounts, globally, for less than 5–10% of the observed variation in the annual growth rate of CO2 in the atmosphere. Natural disturbances are estimated to account for a similar fraction of the variation. The most important contributor appears to be the effect of short‐term changes in climate (temperature and precipitation) on terrestrial metabolism. Over the period 1980–1995, year‐to‐year differences in the flux of carbon from terrestrial metabolism have almost been as large as variations in the growth rate of atmospheric CO2.