{"id":17585,"date":"2020-04-15T05:01:00","date_gmt":"2020-04-15T05:01:00","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=17585"},"modified":"2020-06-09T12:09:05","modified_gmt":"2020-06-09T12:09:05","slug":"unusually-clear-skies-drove-record-loss-of-greenland-ice-in-2019","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/unusually-clear-skies-drove-record-loss-of-greenland-ice-in-2019\/","title":{"rendered":"Unusually Clear Skies Drove Record Loss of Greenland Ice in 2019"},"content":{"rendered":"\n
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Summer 2019 anomalies in number of melting days (a), snowfall (b), albedo (c), cloudiness (d), and temperature two meters above the ice (e). Image: Tedesco and Fettweis, 2019 <\/figcaption><\/figure>\n\n\n\n

Study identifies unprecedented atmospheric conditions behind devastating summer; suggests climate models may greatly underestimate future melting.<\/em><\/p>\n\n\n\n

By Sarah Fecht<\/strong><\/p>\n\n\n\n

Last year was one of the worst years on record for the\u00a0Greenland ice sheet, which shrunk by hundreds of billions of tons. According to a study published today in\u00a0The Cryosphere<\/em>,\u00a0that mind-boggling ice loss wasn\u2019t caused by warm temperatures alone; the new study identifies exceptional atmospheric circulation patterns that contributed in a major way to the ice sheet\u2019s rapid loss of mass.<\/p>\n\n\n\n

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\u00a0Marco Tedesco<\/a>\u00a0from Columbia University\u2019s Lamont-Doherty Earth Observatory.\u00a0<\/p>\n\n\n\n

The study used satellite data, ground measurements, and climate models to analyze changes in the ice sheet during the summer of 2019.<\/p>\n\n\n\n

The researchers found that while 2019 saw the second-highest amount of runoff from melting ice (2012 was worse), it brought the biggest drops in surface mass balance since record-keeping began in 1948. Surface mass balance takes into account gains in the ice sheet\u2019s mass \u2014 such as through snowfall \u2014 as well as losses from surface meltwater runoff.\u00a0<\/p>\n\n\n\n

\u201cYou can see the mass balance in Greenland as your bank account,\u201d said Tedesco. \u201cIn 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.\u201d<\/p>\n\n\n\n

Specifically, in 2019, the ice sheet\u2019s surface mass balance dropped by about 320 billion tons below the average for 1981-2010 \u2014 the biggest drop since record-keeping began in 1948. Between 1981 and 2010, the surface mass \u201cbank account\u201d gained about 375 billion tons of ice per year, on average. In 2019, that number was closer to 50 billion tons. And while a gain of 50 billion tons may still sound like good news for an ice sheet, Fettweis explained that it is not, because of another factor: the ice sheet is also shedding hundreds of billions of tons as icebergs break off into the ocean. Under stable conditions, the gains in surface mass balance would be high enough to compensate for the ice that\u2019s lost when icebergs calve off. Under the current conditions, the calving far outweighs the surface mass balance gains. Overall, the ice sheet lost an estimated 600 billion tons in 2019, representing a sea level rise of about 1.5 millimeters.<\/p>\n\n\n\n

Before now, 2012 was Greenland\u2019s worst year for surface mass balance, with a loss of 310 billion tons compared to the 1981-2010 baseline. Yet summer temperatures in Greenland were actually higher in 2012 than in 2019 \u2014 so why did the surface lose so much mass last year?<\/p>\n\n\n\n

Tedesco and co-author Xavier Fettweis, from the University of Li\u00e8ge, found that the record-setting ice loss was linked to high-pressure conditions (called anticyclonic conditions) that prevailed over Greenland for unusually long periods of time in 2019.<\/p>\n\n\n\n

The high pressure conditions inhibited the formation of clouds in the southern portion of Greenland. The resulting clear skies let in more sunlight to melt the surface of the ice sheet. And with fewer clouds, there was about 50 billion fewer tons of snowfall than usual to add to the mass of the ice sheet. The lack of snowfall also left dark, bare ice exposed in some places, and because ice doesn\u2019t reflect as much sunlight as fresh snow, it absorbed more heat and exacerbated melting and runoff.<\/p>\n\n\n\n

Conditions were different, but no better, in the northern and western parts of Greenland, because as the high pressure system spun clockwise, it pulled up warm, moist air from the lower latitudes and channeled it into Greenland.\u00a0<\/p>\n\n\n\n

\u201cImagine this vortex rotating in the southern part of Greenland,\u201d Tedesco explained, \u201cand 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 \u2014 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.\u201d<\/p>\n\n\n\n

But instead of bringing snowfall, these warm and moist clouds trapped the heat that would normally radiate off of the ice, creating a small-scale greenhouse effect. These clouds also emitted their own heat, exacerbating melting.<\/p>\n\n\n\n

Through these combined effects, the atmospheric conditions of the summer of 2019 led to the highest annual mass loss from Greenland\u2019s surface since record-keeping began.<\/p>\n\n\n\n

With the help of an artificial neural network, Tedesco and Fettweis found that 2019\u2019s large number of days with these high-pressure atmospheric conditions was unprecedented. The summer of 2012, one of Greenland\u2019s worst years, also saw anticyclonic conditions.<\/p>\n\n\n\n

\u201cThese atmospheric conditions are becoming more and more frequent over the past few decades,\u201d said Tedesco. \u201cIt 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.\u201d In other words, climate change may make the destructive high-pressure atmospheric conditions more common over Greenland.<\/p>\n\n\n\n

Current global climate models are not able to capture these effects of a wavier jet stream. As a result, \u201csimulations of future impacts are very likely underestimating the mass loss due to climate change,\u201d said Tedesco. \u201cIt\u2019s almost like missing half of the melting.\u201d<\/p>\n\n\n\n

The Greenland ice sheet contains enough frozen water to raise sea levels by as much as\u00a023 feet<\/a>.\u00a0Understanding the impacts of atmospheric circulation changes will be crucial for improving\u00a0\u00a0projections for how much of that water will flood the oceans in the future, said Tedesco.<\/p>\n
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