Cecilia Bitz, a professor of atmospheric sciences at the University of Washington who was not connected to the study, described its findings as “pretty breathtaking.”
“I’ve stood on sea ice, and I feel like when I’m standing on it, I occupy about a square meter of sea ice. To imagine my personal use of fossil fuels is causing about 50 or so square meters to disappear each year… it was very profound,” she told me.
She could not think of another study or statistic that was able to phrase a global change—the diminution of Arctic summer sea ice—in such an intimate way.
How does this clear relationship between global carbon dioxide and Arctic sea ice occur? Researchers still don’t know.
The authors sketch a broad conceptual model in this paper. As rays from the sun and warming rays trapped in the atmosphere heat up the frozen Arctic Ocean, the ice must compensate for the increased warming. It must equalize the system. In other words, it melts, and the ice edge moves further north to where the sun’s rays are less strong.
“You have this increased heat at the ice edge, so the ice will move further north to regain equilibrium, so more CO₂ will push it further north,” says Julienne Stroeve, a professor at University College London and a senior scientist at the National Snow and Ice Data Center.
“Of course it’s established that the Arctic sea ice doesn’t know much about global mean temperature, in the same way that nobody really knows much about global mean temperature—which is what makes it a very hard number to use to communicate anything about global warming,” said Notz. “What the similarity [between the two] implies is that the same mechanisms that determine the global mean temperature—namely, the rise in CO₂ concentration in the atmosphere—are the same mechanisms that also affect Arctic sea ice.”
But other researchers suspect that heat trapped in the oceans might still play some role in sea-ice loss. There’s little evidence that the northern Atlantic or Pacific are warming along the same kind of linear trend as the planet, which lead Notz and Stroeve to reject it as a cause. But Bitz said that might just be a lack of data: Oceanic heat data at high latitudes is still sporadically observed and poorly understood.
Notz and Stroeve’s paper will have two implications for the field. First, it will allow climate modelers to tighten their work on the northern pole. “The observations are over-sensitive compared to models,” Bitz said. “That helps us know how to make models better & how to interpret them now.”
But, second, it also suggests further that if the concentration of atmospheric carbon ever fell, sea ice in the Arctic Ocean could post a good recovery. In another study a few years ago, Notz and Stroeve asked a range of climate models to estimate what would happen if summer sea ice dwindled to nothing. The sea ice still made healthy recoveries in the winter.
“It’s not that once we’ve lost all summer sea ice, the winter sea ice will go automatically,” said Notz. “Winter sea ice will likely stick with us through the end of the century.”
And if humanity manages to limit its emissions, summer sea ice might stick around that long too.