The potential response of historical terrestrial carbon storage to changes in land use, atmospheric co₂, and climate

Modeling and measurement studies indicate that ocean and land ecosystems are currently absorbing slightly more than 50% of the human CO2 fossil emissions (Prentice et al., 2001). However, a significant question remains regarding the sources and sinks of carbon over land governed by changes in land covers and physiological processes that determine the magnitude of the carbon exchanges between the atmosphere and terrestrial ecosystems. Most of these processes are sensitive to climate factors, in particular temperature and available soil water (Post et al., 1997). It is also likely that these processes are sensitive to changes in atmospheric CO2. Moreover, the climate variation is not uniformly distributed throughout the Earth’s surface or within ecosystem types. Therefore, simulations of terrestrial carbon storage must take into account the spatial variations in climate as well as non-climate factors that influence carbon storage, such as land-cover type and soil water holding capacity, that interact with climate. Estimates should also account for land-cover changes with time. Because the changes in land cover, mainly from forest to croplands or forest to pasturelands, shorten the turnover of carbon above and below ground, they act to reduce the sink capacity of the biosphere. Historical changes in biospheric carbon storage and exchange with the atmosphere are commonly simulated with globally aggregated biospheric models (Jain et al., 1996; Kheshgi and Jain, 2003), mostly in response to changes in atmospheric CO2 and climate, or changes in land-cover types (Houghton and Hackler, 2001; Houghton, 2003).