|December 17, 2015|
Greenland has shed more than 9 trillion tons of ice since 1900
|The Greenland ice sheet has shed an estimated 1 trillion tons of ice since 1900, with a doubling in the rate of ice loss from 2003 to 2010, according to a new study published in the journal Nature on Wednesday. |
The study, which was the work of 16 authors from five countries, used aerial imagery of Greenland from the 1980s to trace back the maximum extent of the Greenland Ice Sheet during a period of time when much of Greenland was colder than it is now, which was known as the Little Ice Age.
The Little Ice Age lasted from about 1450 to 1850, which was when the Greenland Ice Sheet was at its maximum extent during the past 1,000 years.
Researchers led by Kristian Kjeldsen from the Center for GeoGenetics at the Natural History Museum of Denmark sought to obtain direct measurements of Greenland's glaciers prior to 1992, rather than relying on computer modeling to arrive at estimates of historical ice sheet behavior.
"If we do not know the contribution from all the sources that have contributed towards global sea level rise, then it is difficult to predict future global sea levels," Kjeldsen said in a press release.
The aerial photographs allowed the researchers to see imprints from ice sheet movement on the surrounding landscape. For example, when ice begins to retreat, newly eroded parts of mountain slopes tend to appear in lighter colors than non-eroded areas, the press release said.
This boundary between lighter and darker areas of hillsides can point to the historical movement of glaciers, and is known as the trimline.
Other indicators of ice movement include sediments, large rocks pushed forward by ice that can then become covered by vegetation, and other features.
The researchers took advantage of thousands of aerial photos taken by the Greenland aerial photography survey between 1978 and 1987, and mapped the land features and glaciers in three dimensions. They then compared these maps to satellite data and other aerial photos to arrive at a more reliable estimate of what is known as the "mass balance" of the various glaciers that comprise the ice sheet.
The scientists looked especially closely at the period from 1900 to 1983, the two decades from 1983 to 2003, and the more recent period of 2003 to 2010.
Interestingly, they found that the same areas that are losing large amounts of ice today also lost considerable mass during the 20th century, including glaciers along the southeastern and northwestern coasts.
"One of the unique things about our results --- which distinguish them from earlier model studies - is that we not only estimate the total mass loss of the entire ice. But we can actually calculate changes all the way down to regional and local levels and say something about changes for individual outlet glaciers," Kjeldsen said in a statement.
The study indicates that the ice loss from Greenland amounts to about 10% to 18% of global sea level rise from the year 1900 to 2010. The total amount of sea level rise from the loss of the Greenland ice sheet, according to the study, is about 25 millimeters during that time period, or about 1 inch.
That may seem tiny, but the concern is that Greenland's melting has sped up dramatically in recent years, going from a mass loss of about 75.1 gigatons per year between 1900 and 1983, according to the study, to 186.4 gigatons per year between 2003-2010. (One gigaton is equal to about a billion metric tons.)
In addition, many climate scientists believe that the Greenland Ice Sheet, like the Antarctic Ice Sheet, has tipping points beyond which near complete melting of particular glaciers, if not the entire ice sheet itself, becomes inevitable over the course of several centuries.
Greenland is the world's largest island, extending more than 1,200 miles from its southern to northernmost points, and if all of its ice were to melt the oceans would rise by more than 20 feet. This would be disastrous for coastal megacities worldwide.
Current projections for sea level rise during the 21st century call for up to 3 feet of sea level rise by 2100, though some
Already, the melting of the Greenland ice sheet is one of the largest contributors to global sea level rise, accounting for about .02 inches of the .13 inches per year global average sea level rise (local rates of sea level rise vary significantly).
The speed-up in melting is due to increasing air and ocean temperatures, which are eating away at the ice sheet from above and below. This is especially the case at glaciers that terminate in the ocean via floating ice tongues known as ice shelves. As these are eroded from below by warming ocean waters, the glacial retreat speeds up considerably.
The total mass loss since 1900 is 9,013 gigatons, or about 9 trillion tons, with a margin of error of plus or minus 3.4 trillion tons.
The study came out on the same day that the National Oceanic and Atmospheric Administration (NOAA) released its annual physical of the Arctic region, known as the Arctic Report Card. NOAA found that more than half of the ice sheet was melting at one point in July of this year, which was the first time that had happened since an exceptional melt event in 2012.
Ice mass loss in 2015 totaled about 186 gigatons between April 2014 and April 2015, which was about 22% lower than the average mass loss for the 2002 to 2015 period, but 6.4 times higher than that during the preceding melt season.