Ancient climate switch could signify sharp increase in today’s global temperature

A paper published in today’s Nature suggests that global warming could rapidly accelerate due to a positive feedback mechanism caused by water vapour or rainfall. The paper, which examines a period of rapid climate change 55 million years ago, during the Paleocene and Eocene, offers a clear indication of how gradual global warming can rapidly speed up, causing catastrophic effects.

Until recently, the prevailing view was that the temperature rise during the Paleocene/Eocene, of between five and ten degrees Celsius, was caused by a massive release of methane from the sea floor. Methane is a greenhouse gas, capable of causing a rise in temperature. However, work by scientists from the Universities of Sheffield and California, began to cast doubt on this theory, and further investigation found a new reason for this rapid change in climate.

Professor David Beerling, from the University of Sheffield and co-author the study explains, “Methane is an easy gas to look for as it leaves a negative isotope trail behind, meaning that we can detect a methane release event in sediments from any given era.

“When we examined sediments from across the Paleocene-Eocene boundary we found that methane had left a far bigger signature behind in sediments on land than in the sea. This effect was replicated across several continents. Our challenge was to explain why this amplification on land had occurred.

“Methane’s geochemical signature would only be higher in terrestrial sediments if plants had had more water available to them, either from the air in water vapour, or due to increased rainfall. When this occurs plants are able to photosynthesise better, which changes the isotopes they deposit on the ground in the form of dead matter. This, in turn affects the magnitude of the methane signature then found in the fossilized sediments.

These results suggest that a gradual increase in temperature causes a switch to a more humid/wetter climate state, which in turn amplifies global warming, causing it to warm more quickly. This can lead to sudden and dramatic increases in global temperature.

“The relevance of this research for today is that we are currently going through a similar process of gradual global warming to that that occurred during the Paleocene/Eocene. It suggests that the planet could again experience a rapid rise in temperature, if water vapour increases and amplifies the effects of current global warming.

“Scientists predicting the future effects of climate change rely upon computer models, since we have never lived through this kind of change before. However, by looking back to what has happened in the past we can better predict and test our models of future climatic change.”