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The Finnish icebreaker MSV Nordica traverses the Northwest Passage through the Victoria Strait in the Canadian Arctic Archipelago on July 21, 2017.

David Goldman

When Canada and other countries signed on to the Paris climate agreement in 2015 they pledged to hold global warming to under 2 C by the end of this century and work to a more ambitious limit of 1.5 degrees.

Now, new research by Canadian and U.S. climate scientists reveals that between those two numbers lies a world of difference for the future of the Arctic and its peoples.

Two independent studies published Monday in the journal Nature Climate Change project that if average global temperatures can be stabilized at 1.5 degrees above preindustrial levels, the Arctic Ocean would retain its summer sea ice cover much of the time, going ice-free only about once every 40 years.

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While that would still produce some years of little to no ice in the Arctic – a situation that has never occurred in recorded history – those low ice years would be infrequent enough to potentially allow the Arctic ecosystem to retain many of its features and signature species.

In contrast, at 2 degrees, the change is pronounced, with the Arctic going ice free in the summer as often as once every five years.

“I think we were all surprised at how much difference half a degree of warming made,” said Alexandra Jahn, a climate scientist at the University of Colorado at Boulder and the author of one of the studies.

The results suggest that even a small shift toward the lower temperature target identified in the Paris agreement could translate into a critical difference for Canada’s northern frontier. They also reinforce the expectation that if emissions continue on their current course, which would warm the planet by 3 degrees over the coming century, the Arctic as we know it will become unrecognizable.

“At 3 degrees it’s virtually certain that the Arctic will be ice-free every summer,” said John Fyfe, a co-author of the second study, conducted at the Canadian Centre of Climate Modelling and Analysis, a federal research facility located at the University of Victoria.

Dr. Fyfe and his colleagues based their results on thousands of runs of a powerful computer simulation known as the Canadian Earth System Model 2. The effort was intended specifically to look at various outcomes of the Paris Agreement on the Arctic, something that had previously been inferred from existing studies that were not designed for that purpose.

In order to achieve a meaningful result, the team had to account for a bias in the model that predicts less ice than is typically seen based on real world observations. Their adjusted result ended up in close agreement with Dr. Jahn’s study, despite the fact that she was using a different computer model with the opposite bias.

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James Screen, a climate scientist at the University of Exeter in Britain, who was not involved in either study, said the converging results are an important incentive for policymakers to pursue a 1.5-degree limit on global warming, even though political and mathematical realities suggest that such a target will not be easy to achieve.

“There’s a lively debate going on about how feasible it is and how much more we could still emit and keep to 1.5 degrees,” Dr. Screen said. “The take-home seems to be that it’s either really hard or really, really hard.”

Nevertheless, he added, the new studies show that even incremental progress toward bringing down emissions below 2 degrees could have a meaningful outcome, particularly for northern environments and the people who depend on them.

In general, the Arctic is more sensitive than southern latitudes to climate change. For every one degree that the planet warms, the Arctic is expected to warm about two degrees. If the warming is sufficient to precipitate the disappearance of sea ice in the summer, it will disrupt an ecosystem that is based on a northern ocean largely hidden from the sun year-round and eliminate habitat for polar bears and other species.

Dr. Fyfe stressed that the results are based on long-term trends and do not speak to how fast climate change will progress in the Arctic from year to year.

But while climate scientists caution about misinterpreting short-term results, he said, it’s impossible not to be startled by ongoing signs of rapid change, including the recent news that this winter was the warmest on record for the Arctic, with temperatures surging above the freezing point at the North Pole in February.

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“Even for me … that’s a pretty dramatic result,” he said.

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