With carbon dioxide levels close to our own, the Arctic of the Pliocene epoch may have warmed much more than previously thought – and the modern Arctic could go the same way.
Ashley Ballantyne at the University of Colorado, Boulder, and colleagues analysed 4-million-year-old Pliocene peat samples from Ellesmere Island in the Arctic archipelago to find out what the climate was like when the peat formed.
At that time, CO2 levels are thought to have been close to current levels – around 390 parts per million – but global temperatures were around 2 to 3 °C warmer than today. It was the last warm period before the onset of the Pleistocene glaciation, and is used by climate researchers as a model for our future climate.
Previous studies using computer models have suggested that the Pliocene Arctic was also warmer than it is today – up to 10 °C warmer. A little warming can trigger a lot more in the Arctic because the loss of light-reflecting sea ice and the spread of plants across the land increase the amount of solar energy that is absorbed.
Ballantyne’s team estimated the temperature of the period at which the peat formed by measuring three things that are affected by temperature: the concentration of various chemical compounds, levels of a certain isotope in tree rings and the amount and types of fossilised vegetation.
The group’s analysis suggests the samples formed when average local temperatures were about -0.5 °C. That is 19 °C warmer than temperatures today – more than the previous computer models had estimated.
“These results should be alarming,” says Ballantyne. Although it could take centuries for current global temperatures to respond to rising CO2 levels, we can expect the Arctic to warm much more than the rest of the planet, he says.
Marci Robinson of the US Geological Survey in Reston, Virginia, agrees that Arctic warming is of concern, but doesn’t think it will match that of the Pliocene.
In a model study now in review, Robinson mathematically removed a seabed ridge that now stretches from Greenland to Scotland because it was much lower in the Pliocene. Without the ridge obstruction, warm water would have been able to travel more freely from lower latitudes to Arctic regions, allowing more warming, says Robinson. This would suggest that modern Arctic warming might fall about 5 °C short of that of the Pliocene. Even so, Robinson’s estimate means Arctic temperatures would rise by 14 °C.