Climate researchers have published direct observations of the reduction and melting of the Greenland Ice Sheet during the latest 110 years, a step up from flawed numerical estimates used for all previous claims. In a recent paper, the researchers claim they can pinpoint where the ice sheet is particularly sensitive and what controls the loss of glacier ice in Greenland - and they close a gap in IPCC's estimate of global sea level budget. 

The extent of fluctuating temperatures and their effect on the Greenland Ice Sheet during the 20th Century is hotly debated. 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 the United Nations climate panel's (IPCC) latest report from 2013. Concerning this lack of data, first author on the paper in Nature, postdoc Kristian K. Kjeldsen from the Centre for GeoGenetics at the Natural History Museum of Denmark at 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."

Glaciers imprint on the landscape

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. The scientists were particularly interested in the changes of the ice sheet after the Little Ice Age, a period from c. 1200 AD to the end of the nineteenth century, which marks when the ice sheet was at its largest during the past millennium.

Changes of the Greenland Ice Sheet are observed when the ice retreats and leaves an imprint on the landscape. The vegetation along mountain slopes was removed by the advancing ice and once the ice begins to retreat the freshly eroded part of the mountain slope is seen as a lighter color than the non-eroded part where plants were growing all along.

The boundary between the lighter and darker parts on the mountain slopes is called a "trimline" and marks the maximum extent of the ice. Other signals of ice-movement can be rocks and sediments, which have been pushed forward by the advancing ice into elongated hills, and like the trimline, these features mark the maximum extent of the ice during the Little Ice Age.

Old aerial photographs lend a helping hand

Direct observations were derived from aerial photos. During the period 1978-87 thousands of photographs were recorded in Greenland- clearly revealing both trimlines and landforms. Together with present-day glacier positions the researchers mapped these features in three-dimension, 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 scientists have taken a much closer look at the three periods 1900-1983, 1983-2003, and 2003-2010. Senior author on the paper Professor Kurt H. Kjær from the Centre for GeoGenetics says, "The foundation for our study is a unique set of aerial photographs recorded by the then Danish National Cadastre and Survey, which cover both the ice-free land and extends up to 100 km onto the Inland Ice itself. The digital reconstruction of the past and present elevation, which is based on the aerial photos, is a first of its kind and allows for the unique surveying of the entire ice sheet and the landscape in front of the ice."

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 % of the entire mass loss for the individual periods. Postdoc Kristian K. Kjeldsen comments on this, "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."

Mass loss from Greenland is missing

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 %. 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).

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 substaintial 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. "

Published in Nature