DATE: December 14, 2021
It’s almost a mantra in climate science: The Arctic is warming twice as fast as the rest of the world. But that figure, found in scientific studies, advocacy reports, the popular press, and even the 2021 U.N. climate assessment, is incorrect, obscuring the true toll of global warming on the north, a team of climate scientists reports this week. In fact, the researchers say, the Arctic is warming four times faster than the global average.
“Everybody knows [the Arctic] is a canary when it comes to climate change,” says Peter Jacobs, a climate scientist at NASA’s Goddard Space Flight Center, who presented the work on 13 December at a meeting of the American Geophysical Union. “Yet we’re misreporting it by a factor of two. Which is just bananas.”
Researchers have long known the world warms faster in the far north, because of a phenomenon known as Arctic amplification. The drivers of amplification include increased solar heating, as dark ocean water replaces reflective sea ice, along with occasional intrusions of tropical heat, carried to the Arctic by “atmospheric rivers,” narrow parades of dense clouds that drag water vapor northward.
Jacob’s co-authors include researchers who oversee several influential global temperature records, and they noted the faster Arctic warming as they prepared to release the global temperature average for 2020. NASA’s internal peer reviewer challenged the higher figure, suggesting the scientific literature didn’t support it. But the researchers have found the four times ratio holds in record sets from both NASA (3.9) and the United Kingdom’s Met Office (4.1), and they hope to soon include the Berkeley Earth record. (Their work also has company: In July, a team at the Finnish Meteorological Institute posted a preprint also arguing for the four times figure.)
The researchers found Arctic warming has been underestimated for a couple of reasons. One is climate scientists’ tendency to chop each hemisphere into thirds and label the area above 60°N as the “Arctic”—an area that would include, for example, most of Scandinavia. But the true definition of the Arctic is defined by Earth’s tilt. And, as has been known for centuries, the Arctic Circle is a line starting at 66.6°N. When researchers lump in the lower latitudes, “you’re diluting the amount of Arctic warming you’re getting,” Jacobs says. “That is not a trivial thing.”
The other difference is the choice of time periods over which the warming rate is calculated. Jacobs and his colleagues focused on the past 30 years, when a linear warming trend emerged for the Arctic. Analyses that look at longer term trends see less divergence between the Arctic and the world. That’s because before 1990, the Arctic’s temperatures fluctuated, and even cooled for decades because of air pollution, including light-blocking sulfate aerosols that swept in from the northern midlatitudes, says Mark England, a climate scientist at the University of California, Santa Cruz, who is unaffiliated with the new work. As the world moves off fossil fuels and curbs pollution, he says, “this scenario is not going to repeat itself again.”
Overall, the researchers make a valuable point, England says. “I’m one of the people guilty of using the 60° mark. I guess a large number of people are.” One open question, he adds, is how much of the fast Arctic warming comes from human-driven climate change versus natural variability. Some of the Arctic temperature rise could be due to multidecadal temperature swings in the Atlantic Ocean in the 20th century, which some scientists believe are driven by the ocean’s intrinsic variability. Even so, “introducing this rigor in terms of 66° is a welcome development and I’ll certainly be doing that going forward,” England says.
Mark Serreze, director of the U.S. National Snow and Ice Data Center, also welcomes the new analysis but points out that Arctic amplification is never a fixed ratio. As the researchers showed, the time span used to calculate the rate matters, as does the latitude and season—amplification is far larger in the winter. Serreze adds that Arctic warming has always been more uncertain than the rest of the world, because of the spottiness of the observational records. “As a result, I’m always in favor of looking at it as a range,” he says. “Two times to four times.”
Wherever the exact ratio of amplification sits, its influence is undeniable, researchers say. Thawing permafrost is undermining Indigenous villages, summer sea ice is vanishing, and water is sluicing off Greenland’s ice sheet in record amounts.
The team also sees the work as a cautionary tale, says Jacobs, who also works on communications for NASA. “When something is changing as quickly as the climate, numbers can get old and outdated quickly,” he says. “Before you realize it, you’re misinforming people by a factor of two.”