Mass of ice lost (or gained) from Greenland expressed as time series
Sub-challenge: The mass of ice lost from Greenland expressed as time series
The impacts of climate change are especially obvious at the ice sheet of Greenland. The ice sheet encompasses a relatively low latitude and has warm-in-summer regions which have warmed by 2°C since the early 1990’s (Hanna et al., 2008). These regions are predicted to further warm between 2 and 12°C during the present century (Gregory, Huybrechts, & Raper, 2004). The Greenland ice sheet has been identified as one of the most sensitive “tipping elements” of global climate change, meaning that once the ice has melted, the decrease in temperature for returning to its previous state would be greater than the increase which caused the melt (Lenton et al., 2008). Over the last 30–50 years the ice sheet has undergone significant increases in its surface melt area and (modelled) runoff, as well as enhanced mass turnover (Hanna et al., 2011).
The Greenland ice sheet holds a massive amount of water and would be responsible for the equivalent of 6–7 m of contemporary sea level rise if it were to melt completely (Cuffey & Marshall, 2000). The estimation of change in total mass balance of the Greenland ice sheet is intricately related to competing changes in mass outflux and influx: while melt and runoff have increased in the past several decades, snow accumulation has also increased (Hanna et al., 2008). Adding to the complexity are anomalous events, detected in long term datasets from satellites, in snow accumulation and snowmelt (Nghiem et al., 2012)
Finding data regarding the changes in mass of the Greenland ice sheet expressed as a time series could provide the necessary information for understanding these changes.
The Gravity Recovery And Climate Experiment (GRACE) is a joint NASA-DLR satellite mission. The GRACE twin satellites are orbiting Earth at approximately 500 km and were launched in March 2002. The two satellites are separated by approximately 200 km in space and the relative distance between the two is measured very accurately. This information is used to derive monthly, global models of the Earth’s gravitational field (Barletta, Sørensen, & Forsberg, 2013).
On the website www.polarportal.org the Danish Arctic Research Institution presents updated knowledge on the condition of two major components of the Arctic: The Greenland ice sheet and the sea ice. Among the information available is an interactive map with accompanied graph showing the change in ice mass relative to June 2006, Figure 1 and Figure 2. Source: http://polarportal.dk/en/groenlands-indlandsis/nbsp/total-masseaendring/.
The webpage illustrates how the ice sheet gains mass through snowfall accumulating on the surface and how it shrinks through melting from the surface and discharge of icebergs from glaciers that end up in the sea.
The map depicted in figure 1 and 2 illustrates the latest GRACE satellite-derived mass changes.
The depicted graphs in figure 1 and 2 show the change in the total mass balance month by month, measured in gigatonnes (1 Gt is 1 billion tonnes or 1 km3 of water). The left axis on the graph shows how this ice mass loss corresponds to sea level rise contribution. 100 Gt corresponds to 0.28 mm global sea level. All mass changes in the image are relative to June 2006. Since the measurements began in 2002, the smallest April-to-April mass loss was 29 Gt during 2013-2014 and the largest was 562 Gt during 2012-2013.
The figures 1 and 2 are based on the monthly measurements of changes in gravity. Scientists at DTU Space have contributed to developing the methods used to derive ice mass changes from the gravity changes. The raw GRACE satellite data is carefully processed and validated before it is released to the user, and the product presented here might be delayed by 2-3 months. See Barletta et al. 2013, which is downloadable free of charge.
Figure 1: Greenland ice sheet mass gain/loss on April 2002 relative to June 2006 (polarportal.dk)
Figure 2: Greenland ice sheet mass gain/loss on January 2016 relative to June 2006 (polarportal.dk)
The GRACE satellite data was also used to create a time series from April 2002 until April 2015, quantifying mass change relative to the first measurement in April 2002, Figure 3 (Tedesco et al., 2015).
An analysis of the mass change data was performed, and this led to several conclusions:
- The melt area in 2015 exceeded more than half of the ice sheet on July 4th for the first time since the exceptional melt events of July 2012, and was above the 1981-2010 average on 54.3% of days (50 of 92 days).
- The length of the melt season was as much as 30-40 days longer than average in the western, north-western and north-eastern regions, but close to and below average elsewhere on the ice sheet.
- Average summer albedo, which is a measure of reflectance, in 2015 was below the 2000-2009 average over the northwest, and above the average over the southwest portion of the Greenland ice sheet. In July, albedo averaged over the entire ice sheet was lower than in 2013 and 2014, but higher than the lowest value on record observed in 2012.
- Ice mass loss of 186 Gt over the entire ice sheet between April 2014 and April 2015 was 22% below the average mass loss of 238 Gt for the 2002- 2015 period, but was 6.4 times higher than the 29 Gt loss of the preceding 2013-2014 season.
- The net area loss from marine-terminating glaciers during 2014-2015 was 16.5 km2. This was the lowest annual net area loss of the period of observations (1999-2015) and 7.7 times lower than the annual average area change trend of -127 km2.
Figure 3: Cumulative change in the total mass (in Gigatonnes, Gt) of the Greenland Ice Sheet between April 2002 and April 2015 estimated from GRACE measurements. Each symbol is an individual month and the orange asterisks denote April values for reference (Tedesco et al., 2015).
A study by Zwally et al., 2003, derives mass changes of the Greenland ice sheet for 2003–07 from ICES at laser altimetry and compares them with results for 1992–2002 from ERS radar and airborne laser altimetry. The data is a comparison between two averaged points instead of an entire time series, nevertheless it is very useful and informative.
The data in this paper shows that the Greenland ice sheet continued to grow inland and thin at the margins during 2003–07, but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s. “The net balance changed from a small loss of 7 ±3 Gt a–1 in the 1990s to 171 ±4 Gt a–1 for 2003–07, contributing 0.5 mm a–1 to recent global sea-level rise. The mass changes were derived into two components: (1) from changes in melting and ice dynamics and (2) from changes in precipitation and accumulation rate. Increased losses from melting and ice dynamics (17– 206 Gt a–1) are over seven times larger than increased gains from precipitation (10–35 Gt a–1). Above 2000 m elevation, the rate of gain decreased from 44 to 28 Gt a–1, while below 2000 m the rate of loss increased from 51 to 198 Gt a–1.” (Zwally et al., 2003), Figure 4.
Figure 4: dM/dt is total rate of mass change, dMa/dt is the component driven by temporal variations in snow accumulation, and dMbd/dt is the component driven by ablation and ice dynamics, all averaged by 500m elevation bands over the ice sheet for the 1992–2002 and 2003–07 periods. Circled symbols are totals for all elevations weighted by area. Zwally et al., 2003
The total rate of mass change (dM/dt) was also divided into eight drainage systems (DS): two DS in the north (DS1, DS2), two in the southeast (DS3, DS4), two in the southwest (DS5, DS6) and two in the west (DS7, DS8) of Greenland (Figure 5). This provides an insight into regional differences in ice mass change.
Dynamic-/ablation-driven thinning was proven to be largest in DS3, DS4 and DS8 and very small in DS1, DS2, DS5 and DS6. Accumulation-driven mass increases are largest in DS3, DS4, DS7 and DS8.
The data used in this study was generated with the use of satellites. The ICEsat data ranges from 2003 till 2007 because during this period the satellite has been operational. The data is downloadable free of charge from the website of the National Snow and Ice Center (www.NSIDC.org). The ERS data is available on the European Space Agency website (http://earth.esa.int) although creating a free account is necessary for retrieving the data.
Figure 5: Components of mass change by drainage system. dMa/dt, dMbd/dt, dM/dt (Gt a–1) averaged over 500m elevation bands for the eight drainage systems for 1992–2002 (black) and 2003–07 (red) with totals for 1992–2002 (black symbols) and 2003–07 (red symbols). Zwally et al., 2003
In a paper by Hanna et al., 2011, the mass balance of the Greenland ice sheet was constructed with the use of climate data. The reconstruction of the Greenland Ice Sheet surface mass balance (SMB) used data from 1870 to 2010 and was based on merged Twentieth Century Reanalysis (20CR) and European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological reanalyses. The data used by Hanna et al. (2011) can be downloaded free of charge on the website of the National Oceanic and Atmospheric Administration (www.esrl.noaa.gov) and the website of ECMWF (www.ecmwf.int) The new SMB series was validated with global and regional climate and atmospheric circulation indices during this period. Figure 6 shows two figures, both depicting the change in surface ice mass per year, one over 40 years and the other over 20 years.
Figure 6: Linear least squares regression trends of Greenlands ice sheet surface mass balance for (a) 1870–2010 and (b) 1990–2010. Note the different scales and regional/temporal disparities of change. (Hanna et al., 2011)
Data use, availability and gaps
A time series with the changes in mass of the Greenland ice sheet is easily found. Multiple websites have interactive tools to show the Greenland ice mass over the course of time. The most prominent webpages are:
However, these tools are all based on the GRACE satellite data, which ranges from 2002 to present. Other data regarding ice mass can be found in scientific papers, but these require more effort to find, to read and to check for relevance. The data found in the scientific papers do not show actual timelines but show comparisons between averages or use cumulative depictions of ice mass change. This data does range further back in time; an average of 1992 until 2002 and 1870 (e.g. Hanna et al., 2011). The datasets used by the papers are all available free of charge, however this data is heavily processed for it to be used for expressing Greenland Ice Sheet mass balance. Therefore, specialist knowledge is required to expand the existing information regarding the ice sheet mass when using these datasets.
The websites where the datasets can be downloaded are:
- www.NSIDC.org (GRACE & ICEsat)
- https://earth.esa.int (ERS)
- www.esrl.noaa.gov (20CR)
- www.ecmwf.int (ECMWF)
Conclusion and lessons learned
This challenge was to present a time series showing the mass of ice lost from Greenland. This data is readily available with easy and free access. The data shows that the mass balance of the Greenland ice sheet is a complex system which has strong regional differences. Lessons learned are:
- The GRACE satellite dataset is the only dataset which is translated into a timeseries.
- When searching for Greenland ice mass change most information found is based on the GRACE satellite data.
- Because the only time series is derived from the GRACE dataset the time period only ranges back to 2002.
- Other depictions of ice mass variation are either comparative or cumulative.
When interested in the change in mass of the Greenland ice sheet not all data is equally accessible. This might result in a skewed image of the process. Creating a platform where all data is depicted might provide a more complete image. When possible, a model in which all current data and knowledge regarding the Greenland ice mass temporal change can be incorporated and be plotted over time would give the most complete image.
- Barletta, V. R., Sørensen, L. S., & Forsberg, R. (2013). Scatter of mass changes estimates at basin scale for Greenland and Antarctica. The Cryosphere, 7, 1411–1432. https://doi.org/10.5194/tc-7-1411-2013
- Cuffey, K. M., & Marshall, S. J. (2000). Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet. Nature, 404, 591. Retrieved from http://dx.doi.org/10.1038/35007053
- Gregory, J. M., Huybrechts, P., & Raper, S. C. B. (2004). Climatology: Threatened loss of the Greenland ice-sheet. Nature, 428(6983), 616.
- Hanna, E., Huybrechts, P., Cappelen, J., Steffen, K., Bales, R. C., Burgess, E., … Savas, D. (2011). Greenland Ice Sheet surface mass balance 1870 to 2010 based on Twentieth Century Reanalysis, and links with global climate forcing. JOURNAL OF GEOPHYSICAL RESEARCH, 116. https://doi.org/10.1029/2011JD016387
- Hanna, E., Huybrechts, P., Steffen, K., Cappelen, J., Huff, R., Shuman, C., … Griffiths, M. (2008). Increased Runoff from Melt from the Greenland Ice Sheet: A Response to Global Warming. Journal of Climate, 21(2), 331–341. https://doi.org/10.1175/2007JCLI1964.1
- Lenton, T. M., Held, H., Kriegler, E., Hall, J. W., Lucht, W., Rahmstorf, S., & Schellnhuber, H. J. (2008). Tipping elements in the Earth’s climate system. Proceedings of the National Academy of Sciences, 105(6), 1786–1793.
- Nghiem, S. V, Hall, D. K., Mote, T. L., Tedesco, M., Albert, M. R., Keegan, K., … Neumann, G. (2012). The extreme melt across the Greenland ice sheet in 2012. Geophysical Research Letters, 39(20), n/a--n/a. https://doi.org/10.1029/2012GL053611
- Tedesco, M., Box, J. E., Cappelen, J., Fausto, R. S., Fettweis, X., Hansen, K., … Wahr, J. (2015). Greenland Ice Sheet.
- Zwally, H. J., Li, J., Brenner, A. C., Beckley, M., Cornejo, H. G., Dimarzio, J., … Wang, W. (2003). Greenland ice sheet mass balance: distribution of increased mass loss with climate warming. Retrieved from https://icesat4.gsfc.nasa.gov/cryo_data/publications/ZwallyETALJGlac2011Jan.pdf