One of the problems scientists often have is teasing out meaning from noisy and variable real-world data. Sometimes patterns need statistical tests and techniques to coax out any patterns of behavior. But that's not a problem apparent in the latest research to reconstruct the levels of Arctic sea-ice for the last millennium and a half. In a paper published in Nature last week, Arctic scientists have attempted to map out the rise and fall of the North's ice-cap, using more than 1400 years worth of data. And the lead picture from their paper is striking. It shows sea-ice falling off of an unprecedented cliff in the last few decades:
Sea-ice levels have been seen to fall dramatically for much of the last 30 years. But we have only been accurately observing the ice-caps for the last 4 decades of the satellite era. To paint the line of wiggling Arctic sea-ice extent back across the centuries, the team turned to 'proxies' in the region around the Arctic. These are the various parts of the natural world which both roughly track local temperature, and can be accurately dated. All told the team looked at 69 proxies for historical Arctic conditions - from tree rings to ice cores to lake muds.
These proxies were melded to form an indicator of sea-ice level. This was shown to tie-in tightly to sea-ice area seen in the recent satellite data - as well as to the 200 years of direct observations of sea-ice that we do have. The broad narrative of the historical sweep of sea-ice, in the far north, could then be painted out with some confidence. And what stood out from the data was the unprecedented size and speed of the most recent ice-loss.
While global warming often reveals itself in other graphs as a sharp hockey-stick, here we appear to have something more dramatic - a sharply down-pointed scythe. For the authors, that implies something well out of the scope of natural climate cycles; man-made climate change is the only plausible culprit. The story doesn't end there, though. There are margins of error in working out this historical tale by proxy, as is shown by the pink areas in the graph above. Before 600 AD years the level of uncertainty from their proxies meant that the team couldn't draw the sea-ice line back with any confidence.
Other papers, however, using different proxies, have been more confident. It is believed that the Arctic ice-cap was last this small some 6-8,000 years ago. Then the tilt of the Earth's orbit meant the northern half of the planet received more energy from the sun that it does now. But as that tilt changed, the ice-cap regained its size, over the following millennia. Such slow-moving planetary tilts are unlikely to save us today. With our own climate experiment proceeding apace, and likely to melt the last shard of Arctic sea-ice within decades, it seems that we're the only ones who can blunt this particular scythe.