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Climate scientists are most concerned about positive feedback systems. For example, a rise in polar temperatures melts the icecaps, which changes a "white body" that reflects heat into a "black body" that absorbs it, which, in turn, further increases the polar temperature. Another example: sea water holds more dissolved CO2 when it is cold; as it warms, it releases CO2 into the atmosphere, which increases the greenhouse effect.

Today another piece of positive feedback bad news has hit the headlines:

Western Siberia is heating up faster than anywhere else in the world, having experienced a rise of some 3C in the past 40 years. Scientists are particularly concerned about the permafrost, because as it thaws, it reveals bare ground which warms up more quickly than ice and snow, and so accelerates the rate at which the permafrost thaws.

Siberia's peat bogs have been producing methane since they formed at the end of the last ice age, but most of the gas had been trapped in the permafrost. According to Larry Smith, a hydrologist at the University of California, Los Angeles, the west Siberian peat bog could hold some 70bn tonnes of methane, a quarter of all of the methane stored in the ground around the world.

The permafrost is likely to take many decades at least to thaw, so the methane locked within it will not be released into the atmosphere in one burst, said Stephen Sitch, a climate scientist at the Met Office's Hadley Centre in Exeter.

But calculations by Dr Sitch and his colleagues show that even if methane seeped from the permafrost over the next 100 years, it would add around 700m tonnes of carbon into the atmosphere each year, roughly the same amount that is released annually from the world's wetlands and agriculture.

It would effectively double atmospheric levels of the gas, leading to a 10% to 25% increase in global warming, he said.

Thanks to Earl Mardle for picking this up!

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