During the past week, temperatures at the highest reaches of the Greenland ice sheet rose above freezing, melting snow at the Summit Station (10,550 feet above sea level) for the first time since July 2012 and perhaps only the third time in the last seven centuries.
Across lower elevations around the margins of the ice sheet, bare glacial ice melted at an unprecedented rate, losing 12.5 billion tons of water on Thursday alone, with daily losses likely exceeding any point in at least the past 70 years.
The Greenland ice sheet covers an area the size of Alaska with enough ice to raise global sea level by more than 20 feet. Greenland gains ice each winter from compacting snow accumulation and loses ice from melt water and icebergs discharged to the ocean. A dry, warm spring this year left a thin snow cover over bare glacial ice. Spring warming followed by a large melt event in June led scientist Jason Box of the Geological Survey of Greenland to predict record ice losses this year. The peak of the melt season passed in mid-July with more typical summer conditions, but the past week again saw a large increase in the area and intensity of melt across the Northern Hemisphere's remaining ice sheet.
These rapid changes point to the necessity for action on climate change and for improved observing systems to monitor the ice sheet.This latest heat wave was particularly unusual as a dome of warm air arrived from the east, associated with the remnants of the record-setting European heat wave last month. Most large melt events on Greenland involve warm, humid air masses arriving from North America and the western Atlantic Ocean. This surge of warm air started in North Africa, traveled north across Europe and then westward across the North Atlantic Ocean toward and across Greenland. Air masses originating over North Africa are known to reach Greenland -- Saharan dust has been traced to the ice sheet -- but this recent air mass was exceptionally warm. By following the path of the heat wave, scientists predicted the extensive melt several days in advance. Air temperatures at Summit hovered near or above freezing for more than 11 hours on Wednesday, which is nearly twice as long as the last melt event there in 2012.
Satellite data that I process for the National Snow and Ice Data Center showed more than 60 percent of the surface area of the ice sheet melting last Wednesday. While this is a smaller melt area than in mid-July 2012, the melt extent was second only to 2012 for late July. More detailed satellite images show that snow has melted from a large area along the western edge of the ice sheet, leaving more exposed ice than in 2012 in several areas. Numerous meltwater rivers and lakes, and dark, dust-encrusted ice reached areas higher on the ice sheet. Images comparing summer 2018 and summer 2019 show a larger area of bare ice and meltwater lakes this summer. Several of the Danish Meteorological Institute weather stations on the ice sheet have recorded greater melt rates in 2019 than in 2012.
In July 2012, the public watched video of melt water runoff raging down the Watson River, near Greenland's main international airport at Kangerlussuaq. The flood waters destroyed the bridge's approaches and prevented travel across the river, which have since been rebuilt. This past week, journalist Laurie Garrett recorded similar images of Watson River flooding. Meanwhile, further north near the U.S. Thule Air Force Base, less than 1,000 miles from the North Pole, scientist Pete Akers at the Institute of Geosciences and Environment in Grenoble reported temperatures above 60 degrees Fahrenheit, ice margin collapse, meltwater pouring into new channels and into the ocean.
While the ice losses on any individual day make only modest contributions to global sea level, the increasing frequency of heat waves and large melt events across Greenland during the past two decades contribute significantly to sea level rise. When the entire surface of the ice sheet melted in July 2012, climatologists and glaciologists told the public that these melt events will become more common in the future. While some recent years, such as 2017, have seen more modest melt seasons, we again witnessed record-setting losses from the ice sheet this summer.
Mass losses from Greenland this past week were already approaching levels not expected until 2070 based on the best available models. It is still too early to tell if the ice losses for the summer will exceed the losses in 2012, but it is clear that the Greenland ice sheet is rapidly responding to climate change, even faster than many scientists expected. These rapid changes point to the necessity for action on climate change and for improved observing systems to monitor the ice sheet.
