One of the great unknowns of climate science is predicting the effect of "feedbacks". As carbon dioxide levels and temperatures increase, will other parts of the global climate begin to be perturbed in a way that affects the direction or speed of global warming?
For instance, scientists have speculated at length about the effect of disappearing Arctic sea ice. A dark, open ocean absorbs more of the sun's heat than the highly reflective surface of an ocean capped by floating sea ice, and so should in theory result in a positive feedback – more heat could mean more melting of the sea ice.
Now comes evidence of another positive feedback in the form of the organic plant matter that has been stored for thousands of years in the permanently frozen soil of the northern hemisphere. Scientists estimate that two thirds of this permafrost will have melted by the year 2200, but perhaps more worrying in the short-term is that they suggest a tipping point could be reached within the next 20 years when the Arctic becomes a net source of carbon dioxide and methane.
Gargantuan amounts of carbon are locked in the plant matter of the permafrost, which has remained frozen for tens of thousand of years. What is expected to be released if the Earth continues to warm up as predicted amounts to about half of all fossil fuel emissions over the past 200 years.
This is a truly sizeable positive feedback that could undermine current targets for reducing man-made emissions of greenhouse gases. The more we investigate the nature of the Earth's climate, the more evidence we find for positive feedbacks that could make global warming worse.