Runaway global warming driven by the release of methane from the Arctic seems less likely than some scientists have feared.

Methane and its components can be locked up for millennia in permafrost — a frozen mixture of soil and ice — and in deposits of crystal-like structures called methane hydrates. Methane released by modern organic materials contains a form of carbon that methane from ancient sources does not, allowing scientists to distinguish between the two types.

Michael Dyonisius at the University of Rochester in New York and his colleagues analysed Antarctic ice cores to determine the origins of methane released during a warming period that ended the last ice age. The warming raised global temperature by roughly 4ºC — slightly more than the rise projected to occur by 2100 in most scenarios of human-induced climate change.

The team’s results suggest that methane emissions during that big thaw were dominated by emissions from wetlands, not by the release of ancient methane from melting permafrost and methane hydrates. The authors conclude that modern climate change is unlikely to trigger a massive release of ancient methane.