"There's talk of a tipping point, where we thin the ice down sufficiently so that at some point large parts of it can't survive the summer melt season anymore, so we see this very rapid decline in ice cover," he said.
"It's quite conceivable that that tipping point we talk about has already been reached."
Particularly warm and sunny weather in the Arctic this summer has helped speed up the pace of the melt, Serreze said. But the sea ice decline is part of a decades' long trend.
In the dark days of the winter, some sea ice grows back. Overall, however, the ice pack has thinned.
"It's really a reflection of what's been happening over the past 30 years—this general pattern of warming, this general pattern of thinner and thinner ice, which makes it more vulnerable," he said.
The loss of sea ice is already having well documented impacts on the Arctic environment, such as shrinking polar bear habitat.
In addition, the melting sea ice will affect atmospheric circulation and precipitation patterns, Serreze said.
"Think of the Arctic as sort of the refrigerator of the Northern Hemisphere climate system. By losing that sea ice, we are greatly altering the efficiency of that refrigerator," he said.
Since different parts of the climate system are integrated, what happens in the Arctic will affect what happens elsewhere on the planet.
However, the climate models disagree on the nature of the potential impacts.
"That's the concern. It's the things that we don't know, it's the climate surprises in store," Serreze said.
If "we lose that sea ice, could we get a climate surprise because of that—a climate surprise that is difficult to deal with, like shifts in precipitation?"
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