Discouraging news from the distant past

Government scientists looking back to a time when Earth was as warm as it is expected to get by the end of this century have come away with a disquieting conclusion: In the long term, climate may be 30 to 50 percent more sensitive to atmospheric carbon dioxide than previously thought.

U.S. Geological Survey scientists reconstructed environmental conditions from an era known as the mid-Pliocene warm period some 3 million years ago as part of an investigation led by UK researchers from the University of Bristol.

Assuming CO2 levels reach expected levels by the end of the century, the image on the left shows the warming of global temperatures (in degrees Celsius) from the climate's relatively short-term processes such as sea ice, clouds, and water vapor.  The image on the right includes long-term processes such as vegetation and land-ice.

Reporting their findings in the journal Nature Geoscience, the team reported that, for the levels of atmospheric carbon dioxide prevalent at the time, global average temperatures were "significantly higher" than would be expected from current estimates of the climate's sensitivity the greenhouse gas.

Current estimates of the climate's sensitivity to CO2 fail to take into account the longer-term positive feedbacks of the system, the scientists said.

"Earth is a dynamic system and climate models need to incorporate its multiple feedbacks as well as changes in both fast and long timescales," said Dan Lunt, a U-Bristol researcher and lead author of the article.

USGS scientist Harry Dowsett said the mid-Pliocene built "the most comprehensive global reconstruction of any warm period, and scientists did so by examining fossils to determine sea surface and deepwater ocean temperatures, vegetation, sea ice extent, and other environmental characteristics during that timeframe."

The era 3 million years ago is considered the closest comparison to what conditions could be by the end of the 21 Century, according to authoritative estimates by the Intergovernmental Panel on Climate Change.  During the mid-Pliocene, global average temperatures were about 3 degrees Celsius -- 5.5 degrees Fahrenheit -- warmer than today.

IMAGE: University of Bristol