Wyrd - OE: that which has become; fate-shaping; destiny-unfolding

Wednesday, 30 November 2011

Sizing the beast – can climates past foretell climes to come?

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.

Photo credit: Anne-Lise Heinrichs
That metric is what climate scientists refer to as the '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.

Going back to the future

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).

By running various LGM climate models, with different climate sensitivities, they picked out the most likely values – those that matched the LGM temperatures they had already mapped from the paleoclimate data. The sensitivity numbers came back satisfyingly tight – ranging somewhere between 1.4ºC and 2.8ºC of warming for a doubling of CO2. The most likely value was 2.3ºC.

3ºC or 2.3ºC - it's still a dragon

Worryingly, some in the media have taken this lowered sensitivity as requiring a big downgrading of the climate threat. 'Global warming much less serious than thought' trumpeted many a news outlet. It is as if the climate dragon has been shrunk to a climate poodle. But a 2ºC climate sensitivity was enough to shift us from the deep freeze of the LGM to today's relatively balmy conditions. Another 2ºC would take us on a dangerous path, one that could lead to equally dramatic changes. Our current increasing emissions are pulling us firmly along just such a path.

Maybe – if this single paper is right, and there are many scientists who contest it – then the dragon's roar could be notched down a decibel or two. But a dragon it still remains.

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