Extraordinary melting in Greenland's ice sheet last summer was linked to warm air delivered by the wandering jet stream, a phenomenon that scientists have increasingly tied to global warming.
This interplay of climate phenomena, described in a new study in the journal Nature Communications, is more evidence of the complex ways in which the Arctic's climate is heading for "uncharted territory," said the study's lead author, Marco Tedesco.
The study adds to an emerging theory on the effects of the pronounced warming of the Arctic, where temperatures are rising faster than in more temperate zones, as models have long predicted. Known as "Arctic amplification," this moderates the normal temperature incline that drives the jet stream. If it makes the jet stream wobble, as some scientists suspect, it would suck warm air up into the Arctic—as was observed in Greenland last year.
The new study analyzes the severe shift in wind patterns last July that transported huge masses of warm, moist air from the Atlantic to the Arctic, dramatically melting the northern reaches of the ice sheet. Never before has the jet stream been seen to intrude so far into the Arctic during the summer, the scientists reported.
Accounting for these shifts is crucial to being able to model how much sea level will rise and how fast. Greenland's melting is one of the biggest contributors to rising seas, and if its ice were to disappear completely it could raise global sea level by as much as 20 feet.
"The models are not capturing these extreme events like the July melting," said Marco Tedesco, the study's lead author. "If we are changing the atmosphere in a way that has not been happening before, with greenhouse gases, then even if we have the tools to make projections based on observations, we won't know how to model for the future." Tedesco is a geophysicist with Columbia University's Lamont-Doherty Earth Observatory.
The unusual weather last July included a strong and persistent area of high pressure over Greenland. It led to new records in northwestern Greenland's surface temperature, melting and runoff, as well as record albedo loss—a darkening of the ice that increases heat absorption and accelerates melting.
It was also just one example of the profound influences flowing from changes in the jet stream, a river of air flowing west to east in a sinuous pattern around the northern hemisphere. In recent years, its wavy line has drifted farther north and south than usual.
Some researchers say these changes can explain intense warming in Alaska, the "polar vortex" weather that has frozen parts of the United States, as well as stronger storms in some regions and variations in tropical monsoons.
"Our results show the effects of a strongly warming Arctic and disturbed atmospheric jet stream on causing a record melt of the far northern reaches of the Greenland ice sheet last summer," said Edward Hanna, a climate change researcher at the University of Sheffield who was part of the research. In a previous study, Hanna also linked extreme Greenland ice sheet melting in 2012 to an unusual northward loop of the jet stream.
Overall in the summer of 2015, the melt season was about 30 to 40 days longer than average in the western, northwestern and northeastern regions of Greenland. For the first time since 2012, the melt area exceeded more than half the ice sheet.
Tedesco's study focused on the July 2015 heat wave in Greenland, but the jet stream continued to surprise researchers in the following months. Last winter, it again veered far northward, nearly brushing the North Pole, said Rutgers University climate researcher Jennifer Francis, a leading theorist of the far-reaching effects of Arctic amplification.
Francis said her research, along with other studies, has documented a big increase in the frequency of "high-amplitude" patterns, when the jet stream swings far north, along with an increase in the stationary highs that keep weather patterns blocked in for extended periods of time.
"The inference is, there is a clear mechanism here, and we should probably expect to see these big melting events on the surface of the Greenland Ice Sheet more frequently," she said.
That pattern was common last winter and led to record-high temperatures over large parts of the Arctic—including Greenland—that persisted for weeks and inhibited sea ice growth, according to the regular updates posted by the National Snow and Ice Data Center. Large areas of the Arctic reported the warmest conditions in 67 years of weather data, including the northern half of the Greenland ice sheet, U.S. ice researchers said on the Greenland Ice Sheet today website in April.
Winter sea ice topped out at a record low extent in March and has stayed at record low levels during the first few months of the melt season. Experts say it's possible that the sea ice will dwindle to another record low this summer.
Extreme twists in the jet stream recurred this spring, and according to the National Snow and Ice Data Center, the Greenland ice sheet melt season started early, in April, with melting across about 10 percent of the sheet. Such early melting has only been observed a few times since satellites started measuring it accurately in 1979.
A separate study, published in April in the International Journal of Climatology, found that stationary, or blocking, high pressure systems have become more frequent over Greenland since the 1980s. That research, by University of Sheffield scientists also linked the recurrence with flooding in Great Britain in 2007 and 2012.
Other researchers said the new study helps broaden the understanding of how atmospheric patterns will affect the Arctic in the long term.
Dirk Notz, director of the Arctic sea ice research unit at the Max-Planck Institute for Meteorology in Hamburg, Germany, said he's not convinced that the recent jet stream shifts are directly linked to global warming, but said the new study is part of an important effort to explain connections between changes in atmospheric patterns and extreme climate changes.
"We need to understand why things happen," Notz said. "I think there's a public perception that Arctic science focuses on monitoring only extreme events. That's a dangerous perception, because even if there's a record sea ice low this year, it will probably recover. From a science perspective, it's not the extreme events that are most interesting but the long-term trends."
Correction: A previous version of this story said the jet stream traveled east to west. It travels west to east.