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Ice Sheet Assessments

2017-2018 Ice Sheet Assessment

Aaron Letterly
31 October 2018

Greenland Surface Melt in 2017 and 2018

The lowest cumulative surface melting of the Greenland ice sheet since 1996 occurred in 2017 (NSIDC). The large mass of ice that covers over 80% of Greenland’s surface experienced lower-than-average surface air temperatures from June through August (Figure 1), but higher-than-average temperatures returned with a vengeance during late summer and early fall (Figure 1). The unseasonably warm September and October were responsible for the latest melting event during the satellite record, which occurred between October 29-31, 2017. Even though below-average temperatures over Greenland’s interior and southeast kept melting from accelerating during the summer, the melting events at the end of the season still resulted in an annual loss of ice mass that was greater than any year between 1979 and 1994.

Figure 1: June-August 2017 temperature anomaly at 2 meters (°C) based on ERA-Interim (left) and September 2017 temperature anomaly at 2 meters (right). Both anomalies are based on the 1982-2016 mean. Data are from ClimateReanalyzer.Org.

In 2018, heavy snowfall in the winter and into the spring covered much of the ice sheet in snow. In terms of the surface energy budget, significant melt events during the summer and slightly warmer-than-average conditions did not outweigh the large amount of highly-reflective snow deposited on the eastern Greenland coast. This may have led to a net increase in mass of the Greenland ice sheet, or at the very least a smaller loss than normal.

2017 and 2018 in a Broader Context

While the last 15 years have seen a Greenland ice sheet that is predominately decreasing in mass, 2017 and 2018 were rare in that during consecutive melting seasons, the overall mass of the ice sheet did not significantly decrease from the preceding winter, and perhaps even increased. However, this relatively small “growth” of the ice sheet does little to buck the trend of a shrinking ice mass.

Figure 2 shows the average Greenland albedo from June-September for two different points in the satellite record, 1988 and 2017-2018. It is clear that while the interior of Greenland is still completely snow-covered, widespread decreases in the albedo of coastal regions are the result of the ice sheet receding, exposing darker bare rock and soil. These decreases in albedo are further highlighted in the image showing albedo differences between 1988 and 2018. Very few, if any, areas of Greenland have experienced an increase in albedo over the last 30 years.

Figure 2: Greenland albedo from June-September for 1988 (left), the mean of June-September albedo for 2017-2018 (center), and the differences in albedo from 1988-2018 (right).

Ice Mass Variation Since 2002

NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites have provided over 15 years of data, creating a short but important climate record of the changes to ice sheets covering both Greenland and Antarctica. Both ice sheets have been decreasing in mass over the data record, with the decrease accelerating from 2009 onward. Figure 3 shows the time series of ice mass anomalies compared to the 2002-2016 average. Data is currently available through the middle of 2017, so the changes in Greenland’s ice sheet described above are not yet fully recognizable in the plot. The Antarctic ice sheet covers a much larger area than the Greenland ice sheet, with variations in the sign and magnitude of the mass change over the continent. For example, ice mass in Eastern Antarctica has increased over the last 15 years, while ice mass in the Antarctic Peninsula and Western Antarctica has decreased. Nonetheless, the average annual decrease in ice mass over the Antarctic ice sheet is 127 (±39) gigatons per year. The Greenland ice sheet has been losing 286 (±21) gigatons per year, but this yearly rate does not yet take into account the full effect of ice accumulation in 2017 and 2018.

Figure 3: Ice mass anomaly (in gigatons) relative to the 2002-2016 mean for the Greenland ice sheet (magenta) and Antarctic ice sheet (cyan). Data are from GRACE.

References

NSIDC, 2018: 2017 review and 2018 season kick-off, National Snow and Ice Data Center, http://nsidc.org/greenland-today/2018/04/2017-review-and-2018-season-kick-off/, April 9, 2018.

NSIDC, 2018: Greenland melt in July and August: Three spikes and out, National Snow and Ice Data Center, http://nsidc.org/greenland-today/2018/10/greenland-melt-in-july-and-august-three-spikes-and-out/, October 10, 2018.

Wiese, D. N., D.-N. Yuan, C. Boening, F. W. Landerer, and M. M. Watkins (2017) Antarctica Mass Variability Time Series Version 1 from JPL GRACE Mascon CRI Filtered