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28-Year Record of Greenland Ice Sheet Health

Posted by William Colgan on January 14, 2021
Climate Change, New Research, Sea Level Rise / No Comments

We have a new open-access study about Greenland Ice Sheet mass balance – or health – in the current issue of Geophysical Research Letters. In this study, we present a new 28-year record of ice-sheet mass balance. This record is relatively unique for two reasons.

Firstly, because of its length. The most recent ice-sheet mass balance inter-comparison exercise (IMBIE2) clearly highlighted how the availability of ice-sheet mass balance estimates has changed through time. During the GRACE satellite gravimetry era (2003-2017), there are usually more than twenty independent estimates of annual Greenland Ice Sheet mass balance. Prior to 2003, however, there are just two independent estimates. Our new 1992-2020 mass balance record will therefore provide especially welcomed additional insight on ice-sheet mass balance during the 1990s.

Figure 1 – Greenland Ice Sheet mass balance estimated by IMBIE2 between 1992 and 2018. The number of independent estimates comprising each annual estimate is shown. Prior to 2003, there are only 1 or 2 independent estimates of ice-sheet health each year.

Secondly, because of its consistency. This new mass balance record has been constructed by merging radar altimetry measurements from four ESA satellites (ERS-1/2, ENVISAT, CryoSat-2 and Sentinel-3A/B) over nearly three decades into one consistent framework. While all four of these satellites use the same type of Ku-band radar altimeter, to date, their measurements have usually been analyzed independently of each other. This time, however, we use machine learning to merge the elevation changes measured by these similar-but-different satellites into a common mass balance signal through space and time. This makes our new record the only satellite altimetry record that spans the entire IMBIE period.

Figure 2 – Comparison of our new multi-satellite radar-altimetry derived record of ice-sheet health (“Radar-VMB”) with two records estimated by the input-output method (“Colgan-IOMB” and “Mouginot-IOMB”), as well as one record estimated by satellite gravimetry (“GRACE-GMB”).

When we compare our new radar altimetry record of mass balance to two existing input-output records of mass balance, we find good agreement in the capture of Greenland’s high and low mass balance years. These other two multi-decade records are derived from the input-output method, in which estimated iceberg calving into the oceans is differenced from estimated surface mass balance (or net snow accumulation) over the ice sheet. While the input-output method often has limited spatial (and temporal) resolution, our radar altimetry derived record can resolve spatial variability in mass balance across the ice sheet every month since 1992.

Figure 3 – Our multi-satellite radar-altimetry derived map of declining ice-sheet health over the (a) the 1992-1999, (b) the 2000-2009, and (c) the 2010-2020 periods.

While our new long-term record provides a new overview of the health of the Greenland ice sheet, it can also be helpful to understand the processes that influence ice-sheet health. For example, we see a sharp increase in mass balance between 2016 and 2017. When we look at this event in detail, we can attribute it to unusually high snowfall in fall 2016, especially in East Greenland, and unusually little surface melting in summer 2017, throughout the ice-sheet ablation area. We estimate that the 2017 hydrological year was likely the first year during the 21st Century during which the ice sheet was actually in a state of true “mass balance” – or equilibrium – as opposed to mass loss.

The development of this new dataset was primarily funded by the European Space Agency (ESA), with a little help from the Programme for Monitoring of the Greenland Ice Sheet (www.promice.dk). Our multi-satellite Ku-band altimetry mass balance record is now available as tabulated data – both for the ice sheet, as well as the eight major ice-sheet drainage sectors – at https://doi.org/10.11583/DTU.13353062. Within the next two years, the ongoing Sentinel-3A/B satellite missions are clearly poised to extend Greenland’s radar altimetry record to three decades. This will allow us to start assessing ice-sheet health using the statistics of a 30-year climatology record. This keeps us excited at the prospect of updating this record in the near future. Stay tuned!

Simonsen, S., V. Barletta, W. Colgan and L. Sørensen. 2021. Greenland Ice Sheet mass balance (1992-2020) from calibrated radar altimetry. Geophysical Research Letters. L61865. doi:10.1029/2020GL091216.

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Greenland data rescue: An appeal

Posted by William Colgan on November 24, 2014
Communicating Science, Glaciology History, New Research / No Comments

As described in this month’s newsletter No 7, the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) is nearing completion of its comprehensive database of surface mass budget observations from the Greenland ice sheet melt area and peripheral glaciers. We now have just over 2400 unique observations spanning from the 1938 Freja Glacier expedition to the present. Approximately half these observations have never been published. These historic measurements were fragmented across studies, most of which were pre-digital or unpublished, effectively making this highly valuable data inaccessible to the global research community. Despite our best efforts, however, we are still missing data from a handful of known expeditions. For example, does someone you know perhaps have a copy of Alfred Wegener’s 1930 Qaamarujuk Glacier observations? There is a chance we might even be unaware of some expeditions, especially recent private sector prospecting work. Please get in touch with Horst Machguth (homac@byg.dtu.dk) of the www.promice.dk team if you can help us out with this community data assimilation project!

Colgan, W., H. Machguth and A. Ahlstrom. 2014. Data Rescue: Greenland Surface Mass Budget Database. PROMICE newsletter No 7. Ed. S. Andersen and H. Pedersen.

database_map

Map of the location, with temporal description, of the Greenland ice sheet melt area and local glacier surface mass budget observations presently contained in the database. The grey sites are the missing data (from a manuscript in preparation).

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