Summary and Future Challenges

The observed increase in atmospheric CO2 results from direct anthropogenic carbon emissions and small imbalances in the natural carbon fluxes between the atmosphere, terrestrial biosphere, and oceans (Prentice et al. 2001). Globally, it appears that the terrestrial biosphere is acting as a net sink of carbon (Prentice et al. 2001). But, on a region-

by-region basis, terrestrial ecosystems may act as a net source or sink of atmospheric CO2, depending on the place in question (e.g., Dixon et al. 1994; McGuire et al. 2001; Prentice et al. 2001).

Terrestrial ecosystems are both a source and a sink of atmospheric carbon. On one hand, humans are releasing CO2 from the terrestrial biosphere through deforestation, cultivation, and other land use practices (Houghton 1999; McGuire et al. 2001). In addition, the effects of climate variability and climatic change on terrestrial ecosystems may be causing an additional source of carbon to the atmosphere (McGuire et al. 2001). On the other hand, a significant sink of atmospheric carbon in the terrestrial biosphere may result from several processes: CO2 fertilization, land use recovery, changing patterns of disturbance or forest harvest, or nitrogen fertilization.

At this stage, the exact magnitude of terrestrial carbon sources and carbon sinks is not clear: Estimates vary as much as by a factor of two. Furthermore, we do not yet have a completely satisfactory explanation for the mechanisms of the terrestrial carbon sink. Although hypotheses have been put forward, there is still no "smoking gun" that clearly identifies one (or a combination) of these mechanisms as the culprit. A combination of large-scale observations, process-based modeling, ecosystem experimentation, and laboratory investigations is still needed to explain the "missing" terrestrial carbon sink.

Only when we understand the size of the terrestrial carbon sources and sinks, and the processes that generate them, can we accurately begin to forecast the future evolution of atmospheric CO2 (Friedlingstein, Chapter 10, this volume). For example, different mechanisms for the terrestrial carbon sink will produce very different future behaviors. If CO2 fertilization is largely responsible for the terrestrial sink, then the sink might be expected to increase as CO2 levels continue to rise in the future. If the terrestrial sink is being primarily caused, however, by the recovery of past land use (e.g., ~50- to 100-year-old abandonment of agriculture in the temperate latitudes), then it is likely that the sink will eventually stop and disappear altogether in a few decades (Nabuurs, Chapter 16, this volume).

This is a fundamental question in understanding the future of the carbon cycle and the climate system: Will the terrestrial carbon sink continue to operate the same way in the future, as climatic changes become larger and larger? The bottom line is that the future evolution of atmospheric CO2 will be determined not only by human activity, but also by the terrestrial biosphere and ocean. To accurately forecast changes in future climate, we must understand not only the possible paths of human emissions, but also the dynamics of carbon sources and sinks within the terrestrial biosphere and oceans.

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