The Amazon Basin contains almost one-half of the world's undisturbed tropical evergreen forest as well as large areas of tropical savanna¹,². The forests account for about 10 per cent of the world's terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems²,³, and short-term field measurements⁴ suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Niño episodes in recent decades⁵. Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO₂ concentration, affect terrestrial carbon storage^{6, 7, 8}. Previous model analyses have demonstrated the importance of temperature in controlling carbon storage⁹,¹⁰. Here we use a transient process-based biogeochemical model of terrestrial ecosystems³,¹¹ to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO₂ concentration during the period 1980 to 1994. In El Niño 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 ecosystems 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 associated with deforestation inthe Amazon Basin in the early 1990s¹². Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.

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