THE planet鈥檚 climate was on the brink of entering a permanent ice age before humans intervened.
The ice caps at Earth鈥檚 poles formed only in the past 30 million years, as levels of carbon dioxide in the atmosphere fell. Then around 2.5 million years ago, as the overall cooling trend continued, something strange happened: the climate began see-sawing ever more wildly between conditions like today and ice ages every 20,000 to 50,000 years (New 杏吧原创, 6 September, p 32).
According to a simple climate model developed by Thomas Crowley at the University of Edinburgh, UK, these oscillations were a sign that the climate was set to flip to a new stable state: a permanent ice age lasting tens of millions of years or more. This flip could have occurred in about 100,000 years from now, or possibly earlier. 鈥淚t鈥檚 not proven, but it鈥檚 more than just an interesting idea,鈥� he says ().
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However, by pumping so much CO2 into the atmosphere we have delayed this transition indefinitely. 鈥淲e are probably very comfortably away from it happening now,鈥� says Crowley.