Dramatic reduction of the Greenland Ice Sheet in the 20th century

Thursday 17 Dec 15

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Shfaqat Abbas Khan
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DTU Space
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For the very first time, climate researchers from the Centre for GeoGenetics at the Natural History Museum of Denmark, together with, among others, DTU Space publish in the scientific journal Nature their direct observations of the reduction and melting of the Greenland Ice Sheet during the last 110 years.

All previous estimations have been based on computer models which, although valuable, do not provide the same level of insight as direct observations. In this paper, the researchers can pinpoint where the ice sheet is particularly sensitive and what controls the loss of glacier ice in Greenland. The press release from the National Museum of Denmark states that, most importantly, the observation-based results close a gap in the Intergovernmental Panel on Climate Change’s (IPCC) estimate of the global sea level budget and should be taken under strong consideration for the next IPCC convention.

The fluctuating temperatures and their effect on the Greenland Ice Sheet during the 20th Century is often a frequently debated issue. One reason for this has been the lack of direct observations of the ice sheet from all of Greenland before 1992, which has made it difficult to estimate changes in both space and time during the earlier part of the twentieth century.

As a direct consequence there is no contribution included from the Greenland Ice Sheet to the global sea level budget before 1990 in IPCC’s latest report from 2013. Concerning this lack of data, lead author of the paper in Nature, postdoc Kristian K. Kjeldsen from the Centre for GeoGenetics at the Natural History Museum of Denmark, the University of Copenhagen says:

“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. In our paper we have used direct observations to specify the mass loss from the Greenland Ice Sheet and thereby highlight its contribution to global sea level rise”.

The results show that some areas of the Greenland Ice Sheet have lost considerable amounts of ice during the twentieth century. The mass loss along the southeastern and northwestern coast contributed between 53 and 83 per cent of the entire mass loss for the individual periods.

"The average mass loss rate over the past decade is much larger than at any other time over the last 115 years."
Shfaqat Abbas Khan, Associate professor, DTU Space

Converted to amount of water, the mass loss from the Greenland Ice Sheet from year 1900 until 2010 contributes to the global sea level rise between 10 and 18 per cent. Changes in global sea levels during the twentieth century as well as the individual sources have been analyzed in the IPCC report from 2013. However, no contributions from the Greenland and Antarctica Ice Sheets have been included. Furthermore, an estimate of sea level rise due to thermal expansion of ocean water is also missing for a large part of the twentieth century (1901-1990). The reason for this is that previous estimates lacked direct observations.

"Our understanding of the behavior of the Greenland Ice Sheet over the last century is greatly increased, and we see that the average mass loss rate over the past decade is much larger than at any other time over the last 115 years", says co-author Associate Professor Shfaqat Abbas Khan at DTU Space, the National Space Institute at the Technical University of Denmark (DTU), who was responsible for developing the methods needed to assess the mass loss of the Greenland Ice Sheet.

"To improve the projection of future sea level rise, a long-term data record that reveals the mass balance beyond short-term episodic events is required. When we look over a longer period, we simply get a more accurate picture of the trend," says Shfaqat Abbas Khan.

It is the first time that researchers make use of direct observations when estimating the extent of the changes observed over a long period of time over the entire Greenland Ice Sheet. Direct observations were derived from aerial photos. During the period 1978-87 thousands of photographs were recorded in Greenlandclearly revealing both trim lines and landforms. Together with present-day glacier positions the researchers mapped these features in three-dimensions, in order to reconstruct the volume of the former ice extent.

From 1983 and onwards satellite data and aerial photos provided the basis for similar calculations. Merged together these two methods provided a new method to map the thinning and mass balance of the glaciers.

The new results published in Nature will significantly further our understanding towards global sea level rise during the twentieth century and thus highlight its importance for the next IPCC panel convention to draft a new climate report. Professor Kurt H. Kjær explains how the research group’s data will make a substantial contribution in future IPCC-reports:

“Our paper contributes with an estimated mass loss from Greenland for the first part of the twentieth century, which is exactly the period where there is no data in IPCC’s report. As a consequence of this we are one step further in mapping out the individual contributions to global sea level rise. In order to predict future sea level changes and have confidence in the projections, it is essential to understand what happened in the past ”.

Read the press release from the Natural History Museum of Denmark in its entirety.