News•November 3, 2016

Here’s How Much CO2 Will Make the Arctic Ice-Free

For every round-trip transatlantic flight or just two months of a home’s electricity use, 30 square feet of Arctic sea ice is lost, according to a new study that lays out in the simplest possible terms the relationship between heat-trapping greenhouse gas emissions and the precipitous decline of sea ice.

The further accumulation of 1,000 gigatons of carbon dioxide — about the limit that would keep global temperature rise under 2°C — would leave the Arctic Ocean effectively ice-free in the summer, the study, detailed in the journal Science, found. But keeping emissions in line with just 1.5°C of warming would allow ice to hang on in at least some parts of the Arctic.

A comparison of the extension of older sea ice in the Arctic in September 1984 and September 2016.
Credit: NASA Earth Observatory

“Our study now allows people for the first time to grasp intuitively how each of us affects the global climate system,” study co-author Dirk Notz, of the Max-Planck-Institute for Meteorology in Germany, said in an email. He and other sea ice researchers hope this clearer connection will help inform policy decisions to limit warming.

Summer Sea Ice Decline

The area of sea ice covering the frigid recesses of the Arctic naturally waxes and wanes with the seasons, reaching a peak at the end of winter and a low at the end of the summer melt season. But because of the excess heat trapped in the atmosphere by ever-rising levels of carbon dioxide and other greenhouse gases, that area has been steadily shrinking.

The largest losses have come at the summer minimum, which has declined by 13.7 percent per decade since satellite records began in 1979.

The summer minimum today is now about half of what it was just 30 years ago. This year saw the second lowest sea ice minimum on record and the lowest winter maximum.

“Things have already changed dramatically in the Arctic,” Walt Meier, a NASA sea ice researcher who wasn’t involved with the study, said.

This stark decline is of major concern because the loss of reflective sea ice causes more heat to be absorbed by dark ocean water, leading to even further warming at high latitudes. Sea ice also serves as a critical part of the habitats of Arctic species and protects coastal communities from storm-whipped waves.

After the shocking record low reached at the end of summer in 2007 (a record later bested in 2012), researchers were concerned about sea ice reaching a tipping point beyond which it would decline exponentially and irreversibly.

But the rebounding of ice between these record-setting years suggested this wouldn’t be the case, and the new study is the latest to suggest that over the long-term, sea ice decline is more directly related to carbon dioxide emissions.

Using both observations and computer models, Notz and colleague Julienne Stroeve, of the U.S. National Snow & Ice Data Center and University College, London, found that when looking at averages over 30 years, every metric ton of carbon dioxide emitted results in the loss of 30 square feet of sea ice. That amount of CO2 is what is emitted per person on a round-trip flight from New York to London, or by a car driving 2,500 miles.

“It is a useful, if simplified way of conceptualizing” sea ice loss, Meier said.

Notz and Stroeve’s explanation for the linear relationship between emissions and sea ice decline is that the ice is adjusting to a heat imbalance: As more heat is trapped by greenhouse gases, the ice retreats further north, where there is less sunlight. This year, open water reached up to 85 degrees north latitude, the furthest north it’s ever been noted.

Arctic sea ice extent hit a record low for October.
Credit: Zach Labe

Both Meier and Bruno Tremblay, of McGill University in Montreal, thought that explanation was likely too simplistic and that other factors, like ocean heat, played a key role. But, in general, the relationship the study lays out “brings [sea ice decline] into more concrete terms,” Tremblay said.

Ice-Free Arctic

The straightforward link between CO2 emissions and sea ice loss makes it easy to calculate just how much more CO2 can be emitted before the Arctic becomes ice-free, which researchers have defined as there being less than 1 million square kilometers (or about 390,000 square miles) of ice.

By their reckoning, emitting another 1,000 gigatons of carbon dioxide would cause the ice to reach that threshold. At the current rate of annual emissions — about 35 gigatons per year — that point would be hit sometime mid-century. A spate of exceptionally warm summers could mean it is hit earlier, while a stretch of cooler ones could delay it.

That amount of carbon dioxide would push global temperature to about 2°C above pre-industrial levels, “so reaching the 2°C global warming target means that sea ice is gone in summer,” Notz said.

But if emissions are significantly reduced, to only an additional 400 gigatons — or roughly in line with only 1.5°C of warming — ice could hang on in several spots.

Notz and Stroeve said they hope their findings will help inform the decisions being made at the national and international levels, including at the upcoming international climate negotiations in Morocco.

“We for the first time provide very concrete guidance on how policy decisions regarding total future emissions affect Arctic summer sea ice,” Notz said. “This has not been possible before.”