Arctic Ice: Why it Matters
August 29, 2012
Over the past three decades, the summer Arctic sea ice extent has declined roughly 40 percent, and the ice has lost significant volume, according to data from the Polar Science Center. Mark Serreze, director of the National Snow and Ice Data Center, told the Guardianlast year: “The extent is going down, but it is also thinning… There will be ups and downs, but we are on track to see an ice-free summer by 2030.”
A new study in this month’s Environmental Research Letters concludes that between 70 and 95 percent of the Arctic melt since 1979 has been caused by human activity. Man-made global warming has rapidly heated up the Arctic — the region has been warming about twice as fast as the global average. (See here for a good explanation of why.) What’s more, soot and other pollutants from smokestacks in Europe and Asia have traveled up to the Arctic. When those dark particles settle onto the snow and ice, they absorb sunlight and start sizzling.
Natural variability still plays a small role, however. This year, a large storm in August may have helped break up the sea ice and caused it to melt even more quickly. But NSIDC scientists say the long-term warming trend was the main driver — the slushy ice has become even more vulnerable to weather outbursts.
In the past, scientists have underestimated the pace at which Arctic sea ice would disappear. In 2007, the Intergovernmental Panel on Climate Change (IPCC) figured we wouldn’t see ice-free summers in the Arctic until the end of the century or so. But later observations suggested that sea-ice extent is shrinking far more quickly than the IPCC had forecast.
It appears that earlier climate models underestimated certain “feedback” effects. As Arctic sea ice melts, more and more of the ocean is exposed to sunlight. Since the darker ocean surface absorbs more sunlight than the bright ice, this warms the region even further. What’s more, a recent study in the Journal of Geophysical Letters found that IPCC models had low-balled the rate at which melted ice drifts off, further accelerating the collapse. That explains why some scientists have revised their forecasts, saying ice-free summers could come 40 or 50 years ahead of schedule.
Melting Arctic sea ice won’t, by itself, raise global ocean levels. But a warmer Arctic will cause Greenland’s ice sheet to melt — and that matters. Ice that’s floating in the ocean can’t raise sea levels when it melts, because the ice was already displacing its own volume. But as the exposed ocean absorbs more sunlight, the region will keep heating up. And that’s important when it comes to the vast ice sheet covering Greenland.
Greenland’s ice sheet is 1.9 miles thick and contains enough ice to raise global sea levels by about 25 feet (7.5 meters) all told. Back in 2007, the IPCC consensus was that Greenland’s ice sheet would remain fairly stable this century and wouldn’t contribute much to sea level rise. But more recent evidence suggests that this prediction is out of date. The melting of Greenland’s ice sheet appears to be accelerating of late, losing about four times as much mass last year as it did a decade ago. That’s partly due to warmer air. And it’s partly driven by rising ocean temperatures, as warmer water chews away at the edges of the ice sheet.
As a result, a recent study by the U.S. Jet Propulsion Laboratory predicted that sea levels are on pace to rise at least a foot by 2050, and possibly three feet by century’s end. (Longer-term forecasts depend on how rapidly Antarctica’s own massive ice sheets deteriorate.)
The changing Arctic could lead to more extreme summers and winters in the United States and Europe. It’s no shock that global warming will make summers even hotter. But could it also make winters colder? Perhaps. For that, we can thank the Arctic. As Jennifer Francis, a climate scientist at Rutgers, has been exploring, the amplified warming in the Arctic might well be contributing to extreme weather.
Why is that? First, the west-to-east jet stream appears to be slowing down, which allows weather patterns to persist in certain areas for longer. This could help account for the onslaught of snowstorms in the United States and Europe in 2009 and 2010, as well as prolonged heat waves like the one that hit Moscow in 2010. Arctic amplification can also increase the “waviness” of the jet stream surrounding the polar region. That could allow more frequent blasts of cold Arctic air to escape down into North America or Europe, leading to frigid winters.
The Arctic hasn’t yet reached the “point of no return,” but the world would have to cut emissions very quickly to stabilize the sea ice. One 2010 study in Nature found that it was still possible to halt the shrinking of Arctic sea ice. A more recentstudy in Science, looking at 10,000 years of Arctic melt, also concluded that we’re not yet at a “tipping point,” where the collapse in sea ice becomes inevitable. “The good news,” said Sven Funder, a co-author of the Science study, “is that even with a reduction to less than 50% of the current amount of sea ice the ice will not reach a point of no return.”
And yet, as climate blogger Joe Romm explained after the Nature study came out, we may have reached a practical point of no return. The Nature authors estimated that the world would have to cut greenhouse-gas emissions by 60 percent rapidly and keep cutting thereafter to avert the inexorable collapse of summer sea-ice. That’s unlikely. A (shrinking) number of climate scientists still think the world can cut emissions significantly and avert a 2°C rise in temperature. Yet the prognosis for Arctic sea ice looks a bit grimmer.