Effect of interannual climate variability on carbon storage in Amazonian ecosystems

The Amazon Basin contains almost one-half of the world's undisturbed tropic al evergreen forest as well as large areas of tropical savanna(1,2). The fo rests account for about 10 per cent of the world's terrestrial primary prod uctivity and for a similar fraction of the carbon stored in land ecosystems (2,3), and short-term held measurements' suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experien ced substantial interannual climate variability owing to frequent El Nino e pisodes in recent decades(5). Of particular importance to climate change po licy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage(6-8). Previous model a nalyses have demonstrated the importance of temperature in controlling carb on storage(9,10). Here we use a transient process-based biogeochemical mode l of terrestrial ecosystems(3,11) to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate v ariability and increasing atmospheric CO2 concentration during the period 1 980 to 1994. In El Nino years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere ( up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecos ystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere as sociated with deforestation in the Amazon Basin in the early 1990s(12). Soi l moisture, which is affected by both precipitation and temperature, and wh ich affects both plant and soil processes, appears to be an important centr al on carbon storage.