Sizing Up the Beast – Can Climates Past Foretell Climes to Come?
Anne-Lise Heinrichs
It's a tricky beast, this climate change dragon. We only know it is there thanks to the fitful light cast by the flickering torchlight of climate science. Sometimes its shadow looms large; sometimes its shadow shifts and shrinks. But if we're going to gauge the scale of the devastation it could wreak – and equip our dragon-slaying hero with the tools needed – we need to get a better measure of the beast.
That metric is what climate scientists refer to as 'climate sensitivity'. It is a measure of how much the globe's average temperature will rise, if the amount of CO2 in the air were to double. To gain a more accurate tail-to-snout reckoning, researchers often find themselves turning back to ancient lore – and poring over the records of climate change past. By looking at how temperature and CO2 have varied with the ebb and flow of our Ice Age's warm and cold periods, they can get a better idea of how sensitive the world is now – as we face our own catastrophic CO2 gamble.
Many papers, and much painstaking toiling over ancient climatic data, have cast a rough net around the climate's sensitivity. But it is a rather loose net – the IPCC, in 2007, said it could range anywhere from ′2°C to 4.5°C, with a most likely value of about 3°C'. Recent research has done little to tighten that band. Now a study released in Science has proclaimed a much tighter fit to the climate dragon – and controversially, it has even shrunk the beast a little.
The paper fitted together the climate data puzzle for the last ice age – 21,000 years ago – when ice sheets sat over much of Europe and America. CO2 levels back then were much lower than our fossil-fuel inflated levels – 180 ppm rather than today's 390 ppm. Naturally enough, temperatures then were much colder too. The paper's authors sought to pin down those temperatures by using all the paleoclimate evidence they could find from the Last Glacial Maximum (LGM).
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