Three Decades of Greenland Ice Sheet Change
Posted by ESA Greenland Ice Sheet Climate Change Initiative.
This week there has been significant melting over a large area of the Greenland ice sheet. Temperatures even reached over 0°C for the second time this year at the very top of the ice sheet 3285m above sea level at Summit station where DMI operates a weather station.
The high melt rates have been due to warmer air moving over the ice sheet, which in combination with low snowfall over most of the ice sheet during the winter period means quite large amounts of melt might be expected this year. A large melt event this early is unusual but it’s not unprecedented a similar event happened in 2012 for example.
Specific melt events of this kind are controlled by local weather conditions in the North Atlantic but when these events are averaged over many years, you get the local background climate. In a paper published by scientists in a large European collaboration led by ESA and including DMI, DTU and GEUS, observations from satellites stretching back to the 1990s, including many shown here on the polar portal, have been used to give a well-rounded picture of how climate changes in Greenland have been affecting the ice sheet.
We show for example, that since the early 2000s the ice sheet has become thinner almost everywhere. For an ice sheet in balance with the local climate we expect to see a small increase in surface height year on year in the centre and a decrease around the edges as more snow falls than melts at higher elevations and the reverse happens lower down. However, scientists show that the ice sheet is now getting thinner almost everywhere (blue areas in the top row). “It’s quite striking that we see such big changes when we compare the early 1990s to the last few years” said scientist Sebastian Simonsen from DTU.
Comparison with climate model data that is used to calculate the surface mass budget of the ice sheet on the polar portal shows that almost all of the difference over most the ice sheet is due to changes in the surface mass budget, that is more melt and changes in snowfall rates.
However, in some areas the ice sheet flow is more important, such as the dark blue areas around Jakobshavn Isbrae in western Greenland and Helheim glacier in south east Greenland. In these places, an increase in the speed of ice flow associated with increases in the rate of iceberg calving and retreat of the glaciers is pulling more ice out of the interior and leading to stretching and thinning that the satellites can measure. “This is a process that is particularly hard for even the best of the current generation of ice sheet models to capture and shows where we need to keep working for improvements” continued Dr. Simonsen.
In the paper we also track 28 glaciers that calve icebergs all around the coast of Greenland. This is only a small but representative sample of the Greenland calving glaciers, including the 20 we also track on the polarportal. Many of them have been only poorly studied previously, but since the 1990s all except one of them has shown substantial retreat suggesting that ice loss from calving is a significant process and part of the consistent picture of an ice sheet in retreat.
Over the course of the study many different satellite sensors have been used. “It’s actually incredible how the technology has advanced over the last 30 years.” said lead author of the study Ruth Mottram, a climate scientist at DMI. “We have gone from just having short snapshots of, for example, ice sheet flow speeds or calving front changes to getting basically weekly repeats of much of this data. The ESA sentinel satellites have been absolute game-changers. They allow us to monitor the Greenland ice sheet remotely but in almost real time.”
Scientists from ENVEO, a remote sensing consultancy based in Innsbruck, Austria, have also been part of the project from the start and have produced a world-class ice velocity data set from Greenland based on Copernicus Sentinel-1 data. “For the first time we have a consistent and complete coverage of the ice sheet velocity repeated year after year”, said Jan Wuite of ENVEO who created this visualization showing how the ice flows from the interior of the ice sheet to the margins along relatively narrow ice streams. “While the ice velocity is relatively slow and constant in the interior, it increases drastically along the outlet glaciers showing pronounced seasonal changes and longer term trends” said Jan. “These data allow us to see if these changes might be related to meltwater or changes at the ice-ocean interface.” Thomas Nagler, CEO of ENVEO, added: “This highly successful collaboration between European scientists during the last six years has enabled us to develop a pre-operational monitoring system for Greenland using various remote sensing data, in combination with in-situ observations and modelling. This work is needed to provide up-to-date information on key ice sheet parameters in times of record climate change in these remote polar areas”.
The whole study ties together the many different process using GRACE data from a pair of satellites that by measuring changes in Earth’s gravitational field can measure changes in the total mass of the ice sheet and also displayed on the polar portal. Although GRACE ended in 2016, the team is looking forward to extending the record with the new GRACE-Follow On (GRACE-FO) mission when it becomes available. “We have calculated via two independent techniques that the Greenland ice sheet lost on average 255 ± 15 km3 of ice every year between 2003 and 2016” said DTU scientist Valentina Barletta. “That’s just a bit less than 1mm of sea level per year over the period where we have observations but we also see quite a lot of variability from one year to the next which we really need to understand, so it’s important to keep an eye on this”.
As well as straightforward monitoring, the ESA CCI team have used the observations to better understand the changes they have observed. “By combining GRACE data with climate model output we can assess how much of the ice is being lost by surface melt and how much by ocean processes like icebergs and ocean melt” said Ruth Mottram. “What we have also done in this latest paper is to break this down into regions on the ice sheet. There are some areas where the ocean interface is much more important and other areas where getting the surface processes right is more important.” She continues. “There are also some areas where we see models need to be improved.”
The satellite data in this latest study is used to test how well climate models work in Greenland. One of the final outcomes of the paper shows that although huge strides in understanding the ice sheet and the climate have been made over the last 30 years, ground observations are still vital. “One of the areas we discuss is in southern Greenland where the models and satellite observations don’t seem to agree very well” said Ruth Mottram “fortunately our colleagues at GEUS who were also part of this study have very useful data from their promice weather stations that we could use to identify and correct the problems with the model output. We also identified areas in this study where we need a lot more direct observations from both satellite and ground observations to help better understand the processes we want to model with both climate and ice sheet models. The really important point though is we have a long-term and consistent set of observations. This means we can now quickly identify anomalies and assess how the ice sheet is changing in really fine detail”.
More details are available in the open access scientific article:
Mottram, R.; B. Simonsen, S.; Høyer Svendsen, S.; Barletta, V.R.; Sandberg Sørensen, L.; Nagler, T.; Wuite, J.; Groh, A.; Horwath, M.; Rosier, J.; Solgaard, A.; Hvidberg, C.S.; Forsberg, R. An Integrated View of Greenland Ice Sheet Mass Changes Based on Models and Satellite Observations. Remote Sens. 2019, 11, 1407.
This work was carried out by the ESA Climate change initiative for the Greenland ice sheet funded via ESA-ESRIN contract number 4000104815/11/I-NB and in the framework of the Sea Level Budget Closure CCI Project funded via ESA-ESRIN contract number 4000119910/17/I-NB. HIRHAM5 regional climate model simulations were carried out by R.M. as part of the ice2ice project, a European Research Council project under the European Community’s Seventh Framework Programme (FP7/ 2007-2013)/ ERC grant agreement 610055.
Polar Portal is a collaboration between DMI, GEUS and DTU with funding from Dancea (Danish Cooperation for Environment in the Arctic) under the Danish Ministry for Energy, Utilities and Climate.
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