Greenland ice sheet during the twentieth century – a missing link in IPCC’s climate report
Press release issued: 16 December 2015
Direct observations of the reduction and melting of the Greenland ice sheet during the last 110 years, made by an international team of climate researchers led by the Centre for GeoGenetics at the Natural History Museum of Denmark and including Professor Jonathan Bamber of the University of Bristol, have been published today in Nature.
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. However, most importantly, the observation-based results close a gap in the Intergovernmental Panel on Climate Change (IPCC)’s estimate of 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 twentieth century is a highly debated matter. 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 IPCC’s latest report from 2013.
Lead author Dr Kristian K. Kjeldsen from the Centre for GeoGenetics at the Natural History Museum of Denmark at the University of Copenhagen said: “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
This is the first time that researchers have made 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 around 1200AD to the end of the nineteenth century, which marks when the ice sheet was at its largest during the past millennium.
Changes in 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 colour than the non-eroded part where plants were growing along it. 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
The direct observations, which have been analysed by the researchers, were derived from aerial photos. During the period 1978-87 thousands of photographs were taken in Greenland, clearly revealing both trimlines 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 extent of the ice. From 1983 onwards satellite data and aerial photos provided the basis for similar calculations. Merged together these two methods provided a new technique 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 said: “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 extend up to 100km 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 south-eastern and north-western coast of Greenland contributed between 53 and 83 per cent of the entire mass loss for the individual periods.
Dr Kristian K. Kjeldsen said: “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 contributed by between 10 and 18 per cent to global sea level rise. Changes in global sea levels during the twentieth century as well as the individual contributing sources have been analysed 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.
The new results published in this Nature paper 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 to future IPCC reports: “Our paper contributes an estimate 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.”
‘Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900’ by Kjeldsen et al in Nature.
1. Based on actual observations, this study shows how much ice has disappeared during the twentieth century from the Greenland ice sheet.
2. Unlike previous model studies that provide an integrated number for the whole ice sheet, these results show the location of lost ice from the Greenland ice sheet.
3. The study proves that ice loss from the Greenland ice sheet has contributed to a global mean sea level rise of 25mm during the period from year 1900–2010 thus providing an important number to the latest IPCC report and adding to the closure of the twentieth century global mean sea level budget. The total mass loss is 9,013 ± 3,378 gigatonnes.