Greenland is melting
Summer has arrived in the Arctic, and an early start to melting of the Greenland ice sheet is threatening the Arctic’s largest ice mass. In the past week, an area larger than the state of Texas was melting, covering more than 275,000 square miles. Normally, temperatures at Summit Station — so named because it lies atop the ice sheet at nearly 10,600 feet above sea level — have an average high temperature of only 12 degrees Fahrenheit in June. But on June 12, the temperature neared 32 degrees.
Two images from this event have captured the attention of international media. The first was a graph produced at the National Snow and Ice Data Center with a near vertical line showing the dramatic increase in the area melting.
Current weather conditions (-NAO pattern) have resulted in the first major Greenland Ice Sheet surface melt event of the summer
— Zack Labe (@ZLabe) June 13, 2019
The second was a photograph showing a dogsled team mushing through ankle deep meltwater on top of sea ice. The juxtaposition of the image and data provide a compelling message about the changing environment in and around the Greenland ice sheet.
— CNN (@CNN) June 18, 2019
The Greenland ice sheet is roughly the size of Alaska and contains enough ice to raise global sea level by more than 20 feet. Greenland gains ice each year through snow accumulation. It loses ice through icebergs breaking off the margin and meltwater that drains through the ice and into the ocean.
Each summer, a large portion of the surface of the ice sheet melts, with the most widespread melt typically occurring in mid-July. During the past two decades, the ice losses have been greater than the snowfall. A NASA satellite showed that the ice sheet shed an average of 280 gigatons (Gt, billions of tons) per year between 2002 and 2016. For reference, 1 Gt is roughly the amount of water needed to fill 400,000 Olympic size swimming pools.
The greatest loss of ice in the modern era occurred in 2012, when nearly the entire surface of the ice sheet experienced melt in mid-July that year, including Summit, which last had melt in the 1880s. The ice sheet lost a massive 627 Gt that summer, and floods were reported along rivers from raging meltwater.
The past three summers have seen more modest melt, but a warmer than normal spring and the extensive melt so far this year is likely to lead to massive losses this summer.
The satellite data are collected from a series of NOAA and Department of Defense satellites that record naturally emitted microwave radiation. Wet snow and ice appear bright in these images, compared to dark areas of dry snow.
Unprecedented melt area extent on the Greenland Ice Sheet. Technically the melt season (defined as 3 consecutive days of >1Gt loss) has not started yet! Complicated season with lots of surface melting, snowfall and refreezing. See @greenlandsmb for more. pic.twitter.com/QEusbTQ0S9
— Ice Alive (@icealivetweets) June 13, 2019
This contrast of dry and wet snow allows us to map the melt area each day during the past four decades. The near vertical line on June 12 was due to the rapid expansion of melt following the incursion of warm, humid air from the Atlantic Ocean over the ice sheet. The melt area increased to more than 275,000 square miles on June 12 and remained well above the long-term average for the remainder of the week, and the ice sheet lost more than 2Gt of ice per day based on model estimates by Danish Meteorological Institute (DMI).
Also that day, the dramatic photo of the dogsled was taken by Steffen Olsen of DMI in Inglefield Bredning, near the village of Qaanaaq. Four scientists and local hunters were trying to retrieve scientific instruments but were inhibited by the melting snow over the sea ice. DMI reported to The Guardian that they had retrieved the weather station monitoring equipment after the watery crossing. John Cappelen of DMI recorded a maximum temperature of 63 degrees Fahrenheit (17.3 degrees Celsius) at nearby Qaanaaq on June 12 and 59 degrees Fahrenheit (15 degrees Celsius) on June 13, which is exceptionally warm for 700 miles into the Arctic Circle.
The early onset to melt has important implications for the remainder of the summer. At the beginning of spring, fresh snow and compacted snow from previous years (called firn) is much colder than the freezing point. The snow and firn must be warmed to freezing before melt can start. Once melt is underway, the snow remains near the freezing point even after refreezing. Not all meltwater goes into the ocean; the firn has pores, like a sponge, that allows some of the meltwater to refreeze and acts as a buffer to sea level rise, but increasing surface melt is filling the pores, which will lead to more meltwater runoff into the ocean.
Meanwhile, snowmelt along the margin of the ice sheet also exposes bare glacial ice and allows surface meltwater ponds to fill. The refrozen snow and firn, bare ice, and meltwater ponds are much darker and absorb more sunlight than fresh snow, promoting even more melt.
For perspective, extensive melt in early June does not appear in the satellite record prior to 2000. The three years since 2000 that had extensive melt in early June ended with massive ice losses. Greenland melt is an increasingly significant contributor to global sea level rise, affects the ocean food web, and may alter ocean circulation and global weather patterns.
The early start to summer on the Greenland ice sheet is not occurring in isolation. Sea ice extent in the Arctic is near record low levels for June, numerous wildfires are being tracked in Siberia above the Arctic Circle, and new evidence suggests Arctic permafrost is melting faster than expected. Earth’s complexity and changes are scientifically interesting, but also have real implications for our Arctic policy, national security, and our coastal infrastructure.
Thomas Mote is a distinguished research professor of Geography and Atmospheric Science at the University of Georgia. He has published research for the past 25 years on Arctic climate change and the use of regional climate models and remote sensing to monitor the Greenland ice sheet. He provides the processed satellite data for the Greenland Today website at the National Snow and Ice Data Center.