The conclusion is based on three decades of data on the age and thickness of the ice that escapes east of Greenland from the Arctic each year. Scientists at the Norwegian Polar Institute found a marked difference in the ice level before and after it hit an all-time low in 2007.
In the years since, the data shows, the Arctic has entered what the researchers called a “new regime” — one that involves a trend toward an ice sheet much thinner and younger than before 2007, the researchers say. . They link the change to rising ocean temperatures in the rapidly warming Arctic, driven by human greenhouse gas emissions.
“Our analysis shows the long-term impact of climate change on Arctic sea ice,” they wrote in the journal Nature.
Walt Meier, a senior researcher at the National Snow and Ice Data Center at the University of Colorado Boulder, compared the 2007 low to a boxer getting a knockout punch. All the punches leading up to it weaken the fighter, but that biggest blow is too much for the boxer to overcome.
That’s not to say that the polar ice has completely turned off, but that it can’t recover quickly.
“You’re in a new situation, a new equilibrium, where you can’t easily get back to where you were,” said Meier, who was not involved in the new study.
The scientists’ analysis is based on data recorded in the Fram Strait, a passage between Greenland and the Norwegian archipelago known as Spitsbergen, through which sea ice regularly flows on its way to the North Atlantic. Underwater radar systems can detect the amount of ice flowing overhead, while satellites and buoys track the movement of ice and the time it spends in the Arctic.
They found that a dramatic change happened in 2007, when the Colorado Ice Research Center reported a record low sea ice cover that was 38 percent smaller than normal and 24 percent smaller than the previous record low, set in 2005.
Until 2007, they observed sea ice of different thicknesses and ages, often with bumps and ridges that come from older ice floes that have packed together. But in recent years, ice floes have become smoother and more uniform in thickness, an indication that they are younger and have a shorter lifespan. That’s a concern for several reasons: rising sea levels, loss of habitat for arctic creatures and a decrease in the albedo effect, in which ice reflects sunlight back into space. A less icy Arctic absorbs more solar heat.
Overall, ice floes spend 37 percent less time in the Arctic before escaping through the Strait of Fram to melt into the Atlantic Ocean, or about 2.7 years on average since 2007, the researchers found. The amount of ice thicker than 4 meters (about 13 feet) that passes through the strait fell more than 50 percent after the 2007 low.
The research confirms previous studies showing losses of nearly all of the oldest and thickest ice ever to cover the Arctic, and that ice floes are circulating faster around the Arctic and through the Fram Strait as the ice cover subsides.
The study voices the concerns scientists have harbored since the low point in 2007 (and since broken in 2012). Some at the time wondered if this was the start of an epic collapse. That didn’t happen, but there was no significant rebound either.
Researchers have been reluctant to be too clear about possible changes in the Arctic sea ice system as a whole because there is so much variation in ice cover from year to year, Meier said. The new study could change that, he said.
“They’re making a pretty good case and collecting a lot of data to say, yes, there’s a fundamental change and we’re in this new regime,” Meier said.
However, some disagree with any of the researchers’ conclusions.
“I’m not convinced it’s irreversible,” said Harry Stern, a mathematician and sea ice researcher at the University of Washington Applied Physics Laboratory. “If you reverse the conditions, you can reverse the changes in ice thickness.”
The study authors said this would take a long time even under the most optimistic global warming and emissions reduction scenarios. Even if carbon dioxide emissions fell to zero sometime in the next 50 years, it would take decades for the ocean to lose all the heat it has accumulated since humans began burning fossil fuels and emitting greenhouse gases.
“As the heat content of the ocean in the sea-icing regions … has increased,” the authors wrote in an email response to questions, “we suggest that the changes are irreversible, at least with the current climate.”