Tropical soils contain one-third of the carbon stored in soils globally , so destabilization of soil organic matter caused by the warming predicted for tropical regions this century could accelerate climate change by releasing additional carbon dioxide (CO ) to the atmosphere . Theory predicts that warming should cause only modest carbon loss from tropical soils relative to those at higher latitudes , but there have been no warming experiments in tropical forests to test this . Here we show that in situ experimental warming of a lowland tropical forest soil on Barro Colorado Island, Panama, caused an unexpectedly large increase in soil CO emissions. Two years of warming of the whole soil profile by four degrees Celsius increased CO emissions by 55 per cent compared to soils at ambient temperature. The additional CO originated from heterotrophic rather than autotrophic sources, and equated to a loss of 8.2 ± 4.2 (one standard error) tonnes of carbon per hectare per year from the breakdown of soil organic matter. During this time, we detected no acclimation of respiration rates, no thermal compensation or change in the temperature sensitivity of enzyme activities, and no change in microbial carbon-use efficiency. These results demonstrate that soil carbon in tropical forests is highly sensitive to warming, creating a potentially substantial positive feedback to climate change.