News•April 12, 2013

Study: Arctic Summers Warmest in 600 Years

The Arctic has seen warmer summers over the past two decades than at any time in the past 600 years, according to a study published this week in the journal Nature. The study uses a sophisticated statistical approach, known as Bayesian modeling, to show that the extremely warm summers in high northern latitudes are evidence of an overall warming trend, rather than just a temporary fluctuation in an otherwise unchanging climate.

Surface temperatures in the Arctic were much warmer than average for the first decade of the 21st century, as shown in this image.
Click on the image to enlarge. Credit: NOAA.

That’s a crucial distinction: a natural fluctuation might reverse before the full effects of warm temperatures could set in, including the melting of Greenland’s ice cap, which could lead to a significant rise in sea level, and ice-free summers in the Arctic Ocean, which could trigger problematic weather anomalies in other parts of the globe.

Scientists have already shown that a warming climate will automatically generate more high-temperature records than a stable one. That’s because individual temperature measurements in a given location form a bell curve, with the greatest number of readings falling into the “normal” range for that location. A small number of readings will fall well above normal, however, and an equally small number will fall below.

If the normal range shifts, however, because the climate is warming overall, high temperatures that once came along very rarely will happen more often, while extreme cold temperatures will become even rarer. A 2012 paper by James Hansen, who recently retired from his post at NASA’s Goddard Institute for Space Studies, showed how this must be true for daily temperatures, but the same principle applies to seasonal averages.

The problem here is that people haven’t been taking comprehensive readings of temperatures in the high Arctic for very long, so they rely on proxies such as tree-ring thickness to stand in for temperature. And those proxies, said lead author Martin Tingley, an expert in climate statistics at Harvard, “carry significant uncertainties.”

Tingley likes to use an analogy: “I’m six-feet-four, which is significantly above average. So even if I’ve never met you, I can confidently guess that I’m probably taller than you are.” But if Tingley were about to enter a room with a thousand men already inside, he couldn’t be equally confident that he’d be the tallest.

It’s the same with summer average temperatures in the Arctic. “From a statistics perspective,” he said, “it’s very different to ask ‘is this summer warmer than the summer of 1473’ and ‘is this summer warmer than all summers in the past 600 years?’”

With precise records, you could just look it up, but the uncertainties in climate proxies make it much more difficult — and without going into the virtues and technical details of different statistical techniques, it turns out that Bayesian analysis is ideally suited to answer that broader question.

The answer, write Tingley and his co-author Peter Huybers, also of Harvard: “…we show that the magnitude and frequency of recent warm temperature extremes at high northern latitudes are unprecedented in the past 600 years.”

Tingley and Huybers’ analysis shows, moreover, that these recent extremes are best explained by an overall rise in average temperatures. That is right in line with an overwhelming body of evidence showing that man-made greenhouse gas emissions have already driven global temperatures higher, and will continue to do so as the century progresses.

Rapid Arctic climate change has resulted in a stunning decline in Arctic sea ice cover, with 2012 setting the record for the lowest ice extent since satellite observations began in 1979. The plunge in sea ice is helping to boost temperatures by exposing greater areas of dark, open ocean, which absorbs more incoming solar radiation that the brightly colored ice does.