The impact of warmer temperatures on Greenland ice
The Arctic is warming more rapidly than other regions. For example, while Earth’s average temperature has increased by one degree Fahrenheit over the past century, air temperature in Greenland has increased by about seven degrees Fahrenheit since 1991. Because warming is more rapid in the Arctic, the impacts are much more pronounced.
Greenland is the world’s largest island. It sits between the Arctic and Atlantic Oceans and is almost entirely covered with ice. A massive ice sheet blankets about 80% of the land surface.
Earth’s warming climate means that overall Greenland loses more ice than it gains each year. The amount of ice melt during the summer of 2007 was the largest since scientists first started making satellite measurements of the ice in 1979. According to University of Colorado climate scientist Konrad Steffen, the amount of ice Greenland lost in 2007 was “the equivalent of two times all the ice in the Alps, or a layer of water more than one-half mile deep covering Washington, D.C." The rate of ice melt in the western part of Greenland has sped up by about 30% since 1979.
Greenland’s accelerating rate of ice melt is one of many major changes in the region. The amount of sea ice in the Arctic Ocean has been decreasing in recent decades, with an especially steep decline in 2007. Permafrost in Arctic tundra has been thawing rapidly too. And warming is causing changes to Arctic ecosystems.
Warmer temperatures mean more melt days.
During summer, temperatures are warm enough for ice on the surface of the Greenland ice sheet to melt in many places. Warmer temperatures have lead to more days of melting ice.
The image to the left show regions of the Greenland ice sheet where melt occurred for more than three days between May 1st and September 30th of 1979. The image on the right shows the same for 2007. Darker red colors indicate areas with the highest number of melt days. The darkest red corresponding to places where there was melting ice during more than 75 days. Areas that had three or less days of melting ice are not colored.
Notice that areas with the largest number of melt days are located in southern Greenland and along the margin of the ice sheet.
All that melt water has to go somewhere.
Researchers were amazed to discover that melt ponds can disappear entirely in an hour or two. The water drains from the pond to the bottom of the ice sheet via chutes through the ice called moulins. The lower photograph shows the opening of a moulin on the surface of the ice sheet. Glaciologists suspect that melt water that flows to the bottom of the ice sheet through the moulins reduces friction, allowing the ice to move more quickly over the rocky surface. Researchers have noticed that the number of moulins is increasing.
As the glacier accelerates, it pushes ice into the ocean. The melt water draining to the bottom of the ice through moulins causes the ice to slip more quickly towards the ocean - a bit like slipping on a banana peel. Outlet glaciers, the routes that ice takes towards the sea, are moving more quickly now than they were in the past. For example, a large outlet glacier on the west side of Greenland, called the Jacobshavn Glacier, has nearly doubled its speed in the last decade. The flow velocity of other glaciers has increased by up to 50% during the summer melt period.
So the melt water contributes to rising sea level in two ways: (1) water from the ice surface finds its way to the ocean, contributing directly to sea level rise, and (2) the water's ability to speed flowing glacial ice towards the ocean also contributes to sea level rise. If all the ice that is on Greenland were to melt or calve into the ocean, global sea level would rise 7.2 meters (21 feet.) Currently, melting Greenland ice increases global sea level by about 0.5 millimeters each year. If the ice melts faster, then sea level will rise faster. A better understanding of how Greenland’s ice is changing will help us understand more about how sea level will change in the future.