Mass balance of continental ice sheets
In recent decades, the melting of ice sheets, ice caps and glaciers has increased worldwide due to the warming of the ocean and atmosphere. Ice streams that end in the ocean are increasingly subject to submarine melting, which reduces the supporting effect on upstream glaciers and accelerates ice flow. In addition, warming increases meltwater runoff in summer, particularly in the Arctic. The GRACE/GRACE-FO data show that these ice mass losses account for more than half of global sea level rise and provide new insights into the underlying processes and drivers. This knowledge is crucial for developing and improving climate projections of sea level rise. Prior to the GRACE/GRACE-FO missions, satellite observations of the ice sheets were limited to measuring changes in the ice surface using radar and laser remote sensing. However, these methods only provide indirect measurements of mass change, as they are influenced by physical changes close to the surface, such as melt retention and the compaction of snow and firn. The GRACE/GRACE-FO missions have enabled enormous progress, as the change in ice mass can now be measured directly and with unprecedented temporal resolution. The GRACE/GRACE-FO mission duration increasing beyond 20 years means that the long-term mass trends and possible accelerations can be determined with increasing precision. At the same time, the monthly resolution of the GRACE/GRACE-FO data allow to assess the impact of individual melting or snowfall events on the mass balance. However, challenges remain, particularly for the Antarctic, as the GRACE/GRACE-FO data must be corrected for mass redistributions in the Earth's interior caused by the Earth adjustment to the retreat of past ice sheets. [1]
GRACE/GRACE-FO has shown that ice loss from the Greenland ice sheet has caused around a fifth of the global mean sea level rise of around 3.5 mm per year since 2003. During this period, there have also been individual years with very snowy winters, such as 2017 and 2018, but strong melting in summer has dominated throughout. As a result, GRACE/GRACE-FO have so far not recorded a year in which the Greenland ice sheet did not lose mass. This condition is a direct consequence of the increased warming in the Arctic compared to the global average. [1]
GRACE-FO measures record ice loss in Greenland
Grafik: GFZ
GRACE-FO data showed a record ice loss of 532 ±58 billion tons in Greenland in 2019. Compared to the relative to the long-term average (2003–2018), the ice sheet’s contribution to sea level doubled in that year. This record value was due to a stable high-pressure weather situation in summer, which led to clear skies over Greenland and thus favoured the absorption of solar radiation. At the same time, circulation patterns promoted the inflow of warm air from lower latitudes, which led to increased temperatures along the west coast of Greenland. Together with less snow accumulation in winter, this resulted in an overall mass balance in 2019 that was a further 68 billion tons more negative than in the previous record year of 2012. [2]
Large loss of ice mass in West Antarctica
Eva Boergens, GFZ
For the Antarctic ice sheet, the average mass balance determined by GRACE is -137 ± 41 billion tons per year (2003-2017), whereby uncertainties of the correction for mass changes in the Earth's interior are already taken into account here. However, there is also considerable annual variability of ±208 billion tons per year for the Antarctic. The greatest ice loss occurs in the Amundsen Sea area in the Pacific sector of West Antarctica, where glaciers and ice streams flow into the ocean at an increasing rate. In this area, GRACE has measured mass changes of -120 ± 14 billion tons per year and an annual acceleration which adds annual mass loss of -7 ± 2 billion tons from year to year. [1]
The GRACE/GRACE-FO satellites enable the detection of annual variations in the mass of the Antarctic ice sheet on a regional scale. These variations are caused by fluctuations in snowfall, which in turn is related to the transport of moisture and is modulated by global patterns of recurring climate variability such as El Niño/La Niña. In the years 2009 to 2011, pressure patterns in the atmosphere led to a southward transport of moisture that caused heavy snowfall of 300 billion tons in the Atlantic sector of the Antarctic ice sheet. The GRACE/GRACE-FO measurements thus provide quantitative data on snowfall in the Antarctic, which is essential for validating the mass balance in climate models and improving projections of sea level rise. [3]
ESA, CC BY-SA 3.0
Diagramm: Ingo Sasgen, AWI (s. Fußnote 3)
Robert Ricker, AWI, CC-BY 4.0
Diagramm: Ingo Sasgen, AWI (CC BY 4.0)
Observing mountain glaciers
Apart from the ice sheets, the GRACE/GRACE-FO missions have been extremely valuable for measuring mass trends in glacier regions. Although glaciers are very small-scale features, their regionally summed-up mass balances are measurable signals in the gravity field. The higher spatial resolution of GRACE/GRACE-FO at high latitudes, which is made possible by denser ground track coverage, is an advantage for recording these small-scale sources of mass change. GRACE/GRACE-FO makes it possible to estimate the contribution of glaciers to sea level rise at around 0.7 mm per year (2003–2017). However, in many areas the GRACE/GRACE-FO data must be corrected for the influence of mass signals that are not part of the glacier mass balance, e.g. hydrological variations, seasonal variations in snow storage in tundra areas or land uplift due to ice loss. The greatest mass losses are therefore recorded in Alaska, followed by northern Canada and the islands of Svalbard. [4]
Text: Dr. Ingo Sasgen, Alfred-Wegener-Institut
Further Information
- Sea level rise: Greenland's ice masses on the scale. Reklim Magazine
- Retreat of the ice sheets. Alfred-Wegener-Institut
- Polar Portal: Data and articles on ice mass loss in Greenland and the Arctic Ocean
Further Reading
[1] Tapley, B.D., Watkins, M.M., Flechtner, F., Reigber, C., Bettadpur, S., Rodell, M., Sasgen, I., et al. Contributions of GRACE to understanding climate change. Nature ClimateIn contrast to weather, which refers to daily or very short-term events, climate refers to an average condition in the atmosphere over a longer period of 30 to 40 years. All processes such as average temperature, precipitation, wind direction, wind s... Change 9, 358–369 (2019).
DOI: 10.1038/s41558-019-0456-2.
[2] Sasgen, I., Wouters, B., Gardner, A.S. et al. Return to rapid ice loss in Greenland and record loss in 2019 detected by the GRACE-FO satellites. Communications Earth & Environment 1, 8 (2020).
DOI: 10.1038/s43247-020-0010-1.
[3] Diener, T., Sasgen, I., Agosta, C., Furst, J. J., Braun, M. H., Konrad, H., Fettweis, X. (2021), Acceleration of dynamic ice loss in Antarctica from satellite gravimetry, Frontiers in Earth Science 9, 1266.
DOI: 10.3389/feart.2021.741789.
[4] Wouters, Bert, Alex S. Gardner, and Geir Moholdt. "Global glacier mass loss during the GRACE satellite mission (2002-2016). Frontiers in earth science 7 (2019): 96.