THE planet’s climate was on the brink of entering a permanent ice age before humans intervened.
The ice caps at Earth’s 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 (Â鶹´«Ã½, 6 September, p 32).
According to a simple climate model developed by Thomas Crowley at the University of Edinburgh, UK, these oscillations…
