Unusually clear skies — not just rising temperatures — led to the largest loss of Greenland’s ice sheet in a single year.
(CN) — Greenland’s ice sheet shrunk by hundreds of billions of tons in 2020, the most in a single year in recorded history, according to a study released Wednesday — but it wasn’t caused solely by rising temperatures.
The study in the scientific journal The Cryosphere explains that this jaw-dropping amount of ice loss also stemmed from exceptional atmospheric circulation patterns that gave the world’s largest island unusually clear skies.
“Because climate models that project the future melting of the Greenland ice sheet do not currently account for these atmospheric patterns, they may be underestimating future melting by about half,” said lead author Marco Tedesco.
The scientist from Columbia University’s Lamont-Doherty Earth Observatory and his colleagues were able to analyze changes in Greenland’s ice sheet during the summer of 2019 by looking at satellite data, ground measurements, and available climate models. What they found was that although 2019 experienced the second-highest amount of runoff from melting ice — 2012 was the worst year for this phenomenon — it brought the biggest drops in surface mass balance since record-keeping began in 1948.
Measuring surface mass and its balance requires taking into account gains in the ice sheet’s mass, mostly through snowfall, as well as the losses brought on by surface meltwater runoff.
“You can see the mass balance in Greenland as your bank account,” Tedesco offered as an analogy. “In some periods you spend more, and in some periods you earn more. If you spend too much you go negative. This is what happened to Greenland recently.”
The ice sheet’s surface mass balance in 2019 experienced a drop of about 320 billion tons below the average for the years 1981 through 2010, the biggest drop since record-keeping began in 1948. Between 1981 and 2010, it was normal for the surface mass “bank account” to gain about 375 billion tons of ice per year on average. In 2019, that number dropped to a disheartening 50 billion tons.
While 50 billion tons may still sound like a lot, co-author Xavier Fettweis, from the University of Liège, explains that it is nowhere near being good news due to one other factor: icebergs.
The ice sheet continues to shed hundreds of billions of tons as icebergs naturally break off into the ocean. This is not an issue under normal conditions, as the gains in surface mass balance would be high enough to compensate for the ice that is lost from icebergs. But under current conditions, the calving far outweighs the surface mass balance gains, with ice sheet loss at an estimated 600 billion tons in 2019, representing a sea level rise of about .06 inches.
Before last year, 2012 was Greenland’s worst year for surface ice mass balance after suffering a loss of 310 billion tons compared to the normal baseline established from 1981–2010. Temperatures in Greenland were higher in 2012 than in 2019, however, begging the question: Why did the surface lose so much mass last year?
Tedesco and Fettweis discovered that the record-setting ice loss was linked to high-pressure conditions, known as anticyclonic conditions, that prevailed over Greenland for unusually long periods of time in 2019.
Due to the high pressure over Greenland, clouds could not form in the southern portion of Greenland, resulting in abnormally clear skies, letting in more sunlight to melt the surface of the ice sheets. Additionally, with fewer clouds there was about 50 billion fewer tons of snowfall than usual. This lack of snowfall left the dark, bare ice overly exposed in some places. Because ice does not reflect as much sunlight as fresh snow, it absorbed more heat — further contributing to the melting and runoff.
Conditions in the northern and western parts of Greenland were different, but no better off. As the high-pressure system spun clockwise, it pulled up warm, moist air from the lower latitudes and channeled it into Greenland. This also brought warm, moist clouds that, instead of promoting snowfall, trapped the heat that would normally radiate off the ice, creating a small-scale greenhouse effect.
“Imagine this vortex rotating in the southern part of Greenland,” Tedesco said. “And that is literally sucking in like a vacuum cleaner the moisture and heat of New York City, for example, and dumping it in the Arctic — in this case, along the west coast of Greenland. When that happened, because you have more moisture and more energy, it promoted the formation of clouds in the northern part.”
Tedesco and Fettweis used an artificial neural network to further analyze this phenomenon and found that 2019’s numerous days with these high-pressure atmospheric conditions was entirely unprecedented, though the summer of 2012 experienced similar anticyclonic conditions.
“These atmospheric conditions are becoming more and more frequent over the past few decades,” Tedesco said. “It is very likely that this is due to the waviness to the jet stream, which we think is related to, among other things, the disappearance of snow cover in Siberia, the disappearance of sea ice, and the difference in the rate at which temperature is increasing in the Arctic versus the mid-latitudes.”
In other words, climate change may make the destructive high-pressure atmospheric conditions more common over Greenland.
Current global climate models cannot capture the effects of a wavier jet stream. As a result, “simulations of future impacts are very likely underestimating the mass loss due to climate change,” said Tedesco. “It’s almost like missing half of the melting.”
The ice sheets of Greenland contain enough frozen water to raise worldwide sea levels by a staggering 23 feet, and Tedesco notes that understanding the impacts of atmospheric circulation changes will be crucial for improving projections for how much of that water will flood the oceans in the future.