Summary

Carbon is stored over thousands of years in many natural reservoirs in land and ocean ecosystems. This is vital for regulating global climate. These old carbon stores are vulnerable to climate change, which can cause this carbon to be released in the form of greenhouse gases such as methane. If these gases reach the atmosphere, they can drive further warming, which has implications for all life on Earth (see the figure) (1). But such positive feedback loops are a major uncertainty when predicting future climate change (2). On page 907 of this issue, Dyonisius et al. (3) describe their search for signals of old methane released to the atmosphere during the last deglaciation about 18,000 to 8000 years ago. This was when Earth last showed warming similar to what is predicted for our immediate future. They found that methane emissions from old carbon sources during this time were small, suggesting that substantial emissions of old methane may not be triggered in response to current and near-future climate change.