Using three independent methods – isotope analysis of fossil trees and mosses, analysis of the distribution of lipids in fossil soil bacteria and an inventory of ancient plant distributions – the scientists determined that although the mean annual temperature on Ellesmere Island during the Pliocene epoch (2.6–5.3 million years ago) was about 19°C higher than today, carbon dioxide levels were only slightly higher than they are at present.
 
‘Our findings indicate that CO2 levels of approximately 400 parts per million (ppm) are sufficient to produce mean annual temperatures in the High Arctic of approximately 0°C,’ said Ashley Ballantyne of the University of Colorado at Boulder. ‘As temperatures approach 0°C, it becomes exceedingly difficult to maintain permanent sea and glacial ice in the Arctic. Thus, current levels of CO2 in the atmosphere of approximately 390 ppm may be approaching a tipping point for irreversible ice-free conditions in the Arctic.’

During the Pliocene, Ellesmere Island, which is in the High Arctic, hosted forests and now-extinct mammals including tiny deer, bears, three-toed horses, beavers and badgers. Today, it is one of the coldest and driest environments on Earth, where temperatures can drop to –38°C in winter. 

‘Our findings are somewhat disconcerting regarding the temperatures and greenhouse gas levels during the Pliocene,’ said team member Jaelyn Eberle of the University of Colorado Museum of Natural History. ‘I can’t help but wonder if the Arctic is headed towards conditions similar to those that existed during the Pliocene.’

September 2010