2013 in review; 2014 melt begins

The Greenland Ice Sheet had a far more typical melt extent and intensity in 2013 than in 2012, when summer surface melting set a record, compared to satellite observations since 1978. After the normal winter hiatus, the 2014 melting season has now begun again along the southern Greenland coastal areas. We will review the early progress of the 2014 season in our next post, in mid-June.

Overview of conditions

Greenland melt anomaly images

Figure 1. These maps show melt anomalies, or how the number of melt days in each year compared to the average number of melt days as recorded by satellite observations from 1981 to 2010. While 2012 set records for melt extent (center), and 2011 showed strong melt anomalies along the coasts (left), 2013 melt days were within a more typical range, on average (right). Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Figure 1 shows the cumulative number of days that the Greenland Ice Sheet experienced surface melting during 2013 (right image), along with comparison images for 2011 and for 2012, the record year for melt days.

Overall, 2013 melt intensity, expressed as the number of melt days relative to the 1981 to 2010 average, was slightly to moderately higher than average in the southern and western Greenland Ice Sheet but unusually low along the northern and northeastern coastal areas.

In particular, surface melt did not extend to the higher-elevation interior regions in the north as much as has been typical for the 1981 to 2010 period. A narrow band along the eastern coastline showed significantly greater than average melting, but here as well the surface melt conditions did not extend inland and uphill as they have in recent years.

Conditions in context

Figure 2. The graph above shows the daily extent of melt during 2012 on the Greenland Ice Sheet surface as a percentage, compared to the average from 1981 to 2010. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data||Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia|High-resolution image

Figure 2. The graph above shows the daily extent of melt during 2013 on the Greenland Ice Sheet surface as a percentage. The 1981 to 2010 average is shown by a blue dashed line. The gray area around this average line shows the two standard deviation range of the data. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

In contrast to 2012, the extent of melt was also closer to average during 2013, compared to the period 1981 to 2010.

Extensive melting began slightly later than usual, toward the end of May, but increased rapidly to above-average levels by mid June. A brief reduction in late June and early July was followed by a late-season re-advance in melt area in late July.

 

2013 compared to previous years

Figure 3. Melt extent departure from the average for 1978 to 2013. The area represented by the bars is the sum of the daily melt extent for June, July, and August of each year, with the average subtracted. This highlights the trend in melt, and the scale of past anomalous years. ||Credit: National Snow and Ice Data Center/T. Mote, University of Georgia|  High-resolution image

Figure 3. Melt extent departure from the average for 1978 to 2013. The area represented by the bars is the sum of the daily melt extent for June, July, and August of each year, with the average subtracted. This highlights the trend in melt, and the scale of past anomalous years.

Credit: National Snow and Ice Data Center/T. Mote, University of Georgia
High-resolution image

The 2013 summer in Greenland also saw a reversal of the recent trend in summertime loss of surface snow and ice mass by run-off, as would be expected given the reduced melting. Figure 3 illustrates the relative melt area departure from the average (sum of the daily melt areas over the ice sheet for June, July, and August in each year, with the average area for 1978 to 2013 subtracted). The very large increase in 2012 is clearly shown, as is the return during 2013 to conditions typical of the late 1990s.

Climate conditions during 2013

Graph of NAO

Figure 4. This graph shows the North Atlantic Oscillation Index (NAO) for June through August, for the period 1950 to 2013 (blue dashed line) and running 5-year average (red line). 2013 saw a marked difference from recent years, with average conditions similar to the 1990s and earlier.

Credit: Xavier Fettweis, University of Liège
High-resolution image

Figure 5.

Figure 5. This plot shows air pressure anomalies (left) and air temperature anomalies (right) at the 700 mb level for June to August 2013.

Credit: Courtesy NOAA/ESRL Physical Sciences Division
High-resolution image

Figure 7.

Figure 6. This graph shows anomalies for June through August for total meltwater runoff (blue), snowfall accumulation (red), and net surface mass balance (SMB, green) over the Greenland Ice Sheet from 1960 to 2013, compared to the period 1980 to 1999. The data are from the Modèle Atmosphérique Régional (MAR model), v3.3, and are in gigatons per year. MAR was run at a resolution of 15 kilometers and forced at its lateral boundaries by the ERA-40 reanalysis over 1958-1978 and ERA-Interim afterward.

Credit: Xavier Fettweis, University of Liège
High-resolution image

Weather patterns were significantly different over Greenland during 2013 compared to 2012,  when high temperatures led to extensive melt. A dominant Arctic climate pattern, the North Atlantic Oscillation (Figure 4), was in its positive phase for the summer months (June through August) of 2013, sharply contrasting with a trend that had held for the previous six summers. As discussed in NSIDC Arctic Sea Ice News and Analysis on September 17, 2013, the positive phase of the NAO favors anticyclonic circulation over Greenland. The NAO generally produces warm and dry conditions over Europe and is associated with cooler and higher-precipitation conditions in Greenland and the central Arctic.

Consistent with this, lower-than-average air pressures (Figure 5, left) were observed over Greenland during the 2013 summer, as well as lower temperatures (Figure 5, right). Air temperatures over Greenland were 0.5 to 2 degrees Celsius (1 to 3.6 degrees Fahrenheit) below the 1981 to 2010 averages. Precipitation was also higher in summer 2013.

Figure 6 shows the pattern of total snowfall anomaly (the amount of snowfall relative to the average amount for 1980 to 1999); the meltwater runoff anomaly (the amount of mass lost to water runoff relative to the 1980 to 1999 average); and the net balance between snowfall and runoff (and evaporation of snow, a minor component of loss) called the surface mass balance (SMB), as calculated by the MAR regional climate model. Highlighted in this graphic are the major ice mass losses of the twelve years preceding 2013. Note that this accounting does not include ice that flows directly into the sea by glacier movement.

Pale by comparison

albedo image

Figure 7. This map shows average summer albedo anomaly for 2013 versus the 2000 to 2011 average, determined by satellite mapping. Data are from the NASA Terra satellite and the MODIS sensor (Moderate Resolution Imaging Spectroradiometer). The data set is the MOD10A1 collection, available from NSIDC. MODIS MOD10A1 data.

Credit: National Snow and Ice Data Center/Jason Box, Geological Survey of Denmark and Greenland (GEUS)
High-resolution image

During summer 2013, the albedo of the Greenland ice sheet surface was higher along the coastal part of the ice sheet than in recent years, indicating less wet snow and snow-free bare ice areas (ablation areas) along the ice sheet perimeter.

Reflectivity is highly variable on an ice sheet: dry snow is the brightest surface cover, above 85% reflectivity, (or, expressed on the 0 to 1 scale, albedo); wet snow has an albedo of about 0.6 to 0.8; coarse wet snow and slush is about 0.4 to 0.5; impurity rich bare ice with no snow cover has an albedo of 0.3; and clear deep water is near 0.10. As fresh snow ages, the grains become coarser and the albedo drops by as much as 10%. Melting in the snowpack has an even more dramatic effect, lowering reflectivity by up to 20%.

With less exposed bare ice near the coasts, the solar reflectance of these regions is higher than the recent average. In the interior, during 2013 summer snowfall events were widely spaced in time over the ice sheet interior, and so the snow surface tended to be older and therefore a bit darker than average.  

More to come

sample images

Figure 8. These images show the new derivative data sets for the Mote Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record data set. The lower left and diagonal panels show average annual and average monthly melt days extent maps, respectively, for the 1981 to 2010 climatology period melt season months April through September. The upper right image shows daily melt extent for 2012, including the two standard deviation range for the 1981 to 2010 period.

High-resolution image

Our next post will examine the early progress of the melt season. In the interim, new supporting data sets and analysis tools have been derived from the melt extent archives that will present a more complete picture of the melt season in 2014.

Several new data sets and graphic upgrades were generated since late September 2013. These changes are summarized in Figure 8 and include an analysis of a 30-year record of daily melt extent spanning the climatological reference period 1981 to 2010, as measured by the Mote melt algorithm (Mote, 2007). The 1981 to 2010 average is shown as a blue dashed line, and the gray area around this average line shows the two standard deviation range of the data. Annual and monthly average melt day maps were also generated, allowing an assessment of the impact of weather events and the trends of melt extent and intensity in various areas.

Resources for analysis of trends and variations for the Greenland Today website will continue to expand as funding permits. We aim to build an interactive analysis tool similar to our Sea Ice Index web pages and to make daily data available as with the Sea Ice News and Analysis web pages. We are presently working on a table of results for the melt days and extents.

References

Fettweis, X., 2007. Reconstruction of the 1979-2006 Greenland ice sheet surface mass balance using the regional climate model MAR. The Cryosphere, 1, 21-40, doi:10.5194/tc-1-21-2007.  See also: https://www.aoncadis.org/dataset/CPL_MAR.html.

Hall, D. K., V. V. Salomonson, and G. A. Riggs. 2006. MODIS/Terra Snow Cover Daily L3 Global 500m Grid. Version 5. Boulder, Colorado USA: National Snow and Ice Data Center.

Mote, T. L., 2007. Greenland surface melt trends 1973–2007: Evidence of a large increase in 2007. Geophysical Research Letters 34, L22507, doi:10.1029/ 2007GL031976.

Acknowledgements

We would like to thank Xavier Fettweis of the University of Liege, Belgium, and Jason Box of the Geological Survey of Denmark and Greenland (GEUS) for their contributions to this post.

 

 

 

Late season warmth extends 2013 Greenland melt season…briefly

Greenland’s surface ice melt season reached a peak in late July, coinciding with a period of very warm weather. Greenland’s melt season this year will be closer to average than was 2012, with far less melting in the northern ice sheet and at high elevations. Nevertheless, an all-time record high temperature for Greenland may have been set in 2013.

Overview of conditions

Figure 1. Cumulative Greenland melt days image for 21 July – 19 August (30 days). This period spans the peak melt extents seen this year.

Figure 1. Cumulative surface melt days for July 21 to August 19, 2013 (30 days). This period spans the peak melt extents seen this year. Note that the color scale is 0 to 30 days, rather than 0 to 100 days for the daily figure. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record.
About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Surface melt on the Greenland ice sheet spread to the northern coastal regions and became especially frequent in the far northeastern corner of the island (Kronprins Christians Land). However, while some high-melt-extent years recently have seen elevations above 2,500 meters (8,200 feet) warm to the melting point, this rarely occurred in 2013, nor was there extensive melt in the northern interior portion of the ice sheet. A small region of the northwestern ice sheet, the drainage areas of Peterman and Humbolt glaciers, saw some inland melting. (However, some of these dark red pixels are mixed land areas, including ice plus rock and permanently frozen ground.) Melt lakes were prevalent along the central western coast in 2013 (as is typical of most seasons) but far less extensive in the northeastern and northwestern regions than in 2012. Melt lakes in Greenland may be seen in NASA Rapid Response-MODIS Arctic Subset images.

Conditions in context

plot

Figure 2a. The graph above shows the daily percent of the Greenland Ice Sheet surface that has shown melt, as of August 19, 2013 (red), along with the daily surface melt extent for 2012 (blue) and the average melt extent for 1981 to 2010 (dashed line). Two peak extent days are noted.

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Figure2a: Greenland melt extent data comparison for 2012 and 2013 to date. Two peak extent days are noted. Figure 2b:Melt extent map for the peak melt extent day to date this year, July 26.

Figure 2b. The map shows the melt extent for July 26, 2013, the peak melt extent date so far this year. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

The period between July 21 and August 19 (Figure 1) included the greatest percentage of surface melt extent days for Greenland during 2013, peaking at 44% on July 26 (Figure 2b). However, the overall melt extent for 2012 was far greater, exceeding 40% for several weeks (Figure 2a). Data posted by Xavier Fettweis at the University of Liège using a different melt algorithm (see their Figure 4) show that melt day frequency is slightly greater than average for northwestern Greenland near the coast (2 to 8 more melt days than average for the reference period, 1980-2011), and significantly greater than average for northeastern Greenland (8 to 16 days more). According to the MAR (Modèle Atmosphérique Régional) regional climate model, the main melt anomalies in 2013 have occurred along the northeast coast. These anomalies are also indicated in the satellite data, by red pixels in Figure 1. This has been due to the lower winter snowfall than average (resulting in a lower albedo) combined with warmer than average summer temperatures. However, the majority of the ice sheet showed mild to moderately lower-than-average melt duration for this season.

Warm temperatures for Greenland: Maniitsoq, July 30

Figure 3: Weather pattern map for Greenland and northeastern Canada, July 30, 2013 (top) and melt area map for July 31 (below left) and August 1 (below right). Strong southeasterly winds across the western coast brought very warm conditions to that region. The weather graphic was produced by Gorm Larsen for the Danish Meteorological Institute. See: http://www.dmi.dk/nyheder/arkiv/nyheder-2013/8/groenland-saetter-temperaturrekord/. Note that the question of a record warmest temperature for Greenland is still being resolved.

Figure 3: Weather pattern map for Greenland and northeastern Canada, July 30, 2013 (top) and melt area map for July 31 (below left) and August 1 (below right). Strong southeasterly winds across the western coast brought very warm conditions to that region. Note that the question of a record warmest temperature for Greenland is still being resolved. The weather map was produced by Gorm Larsen for the Danish Meteorological Institute.

Credit: National Snow and Ice Data Center/Danish Meteorological Institute
High-resolution image

A pattern of strong southerly winds in late July brought very warm temperatures to southwestern Greenland, possibly the warmest air temperature ever recorded on Greenland since 1958 (when extensive accurate records began). Temperatures at the small hamlet of Maniitsoq were measured at 25.9 degrees Celsius (78.6 degrees Fahrenheit) on July 30. These conditions were in part due to the downhill flow of the air off the western side of the Greenland Ice Sheet. Downhill air flow warms the air as a result of its compression as it goes to lower altitude (foehn or chinook effect). This weather event also led to a large region of melt along the western part of the ice shelf on July 31 and August 1, and the second-highest area of melt this year.

Three summers at Summit

Figure 4. The graph shows Summit, Greenland temperatures for 2011 (purple), 2012 (red), and 2013 to date (green). ||Credit: National Snow and Ice Data Center/Christopher Shuman and Michael Schnaubelt, UMBC JCET and Thomas Mefford, CIRES/NOAA |High-resolution image

Figure 4. The graph shows Summit, Greenland temperatures for 2011 (purple), 2012 (red), and 2013 to date (green).

Credit: National Snow and Ice Data Center/Christopher Shuman and Michael Schnaubelt, UMBC JCET and Thomas Mefford, CIRES/NOAA
High-resolution image

Summit is a research station located at the highest point on the Greenland Ice Sheet, at 3,216 meters (10,551 feet) above sea level. Above-freezing air temperatures were observed there by NOAA sensors for the first time in 2012, although ice cores indicate rare melting events in past centuries. The years preceding and following 2012 show more typical temperature conditions. Figure 4 shows a plot of temperatures recorded at the NOAA observatory at Summit, Greenland. The daily oscillation is a result of a day-night cycle. Although the sun remains above the horizon for much of the summer at Summit, it drops very low in the sky during evening hours and temperatures drop. Research shows that surface melting detectable by remote sensing (see Figures 1 and 2) can begin a degree or two Celsius (2 to 4 degrees Fahrenheit) below freezing due to solar heat absorption by the snow. (Thanks to Chris Shuman, University of Maryland Baltimore County with the support of the NASA Cryospheric Sciences Program, for this part of the discussion.)

Springtime melt in Greenland: Late start, rapid spread

Surface melting of the snow and ice of the Greenland Ice Sheet had a slightly late start, but quickly spread over a significant area, extending over more than 20% of the ice sheet in early June and reaching above 2,000 meters (6,500 feet) elevation in some areas. Small melt lakes have begun to form on the ice sheet, as seen by the new USGS/NASA Landsat-8 satellite.

Overview of conditions

Cumulative melt days for mid-May to mid-June in Greenland using the Mote method of determination of melt from satellite microwave data. Note that the color scale is 0 to 30 days, rather than 0 to 120 days for the daily figure. Lower figure shows the percent of the Greenland ice sheet that had some surface melting for each day of 2013 up to June 19th.

Figure 1. Cumulative surface melt days for mid-May to mid-June in Greenland. Note that the color scale is 0 to 30 days, rather than 0 to 100 days for the daily figure. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data


Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

After the annual re-calibration of the melt algorithm in mid March (see March 18 post), very little melt was detected until May. A few southern coastal areas began melting in mid-May, followed by inland higher-elevation ice and all remaining coastal areas about June 3, when warmer conditions arrived. Surface melting reached the “Saddle” region of the ice sheet (located where the pale bluish band extends from the east to the west coastal zones in Figure 1) on June 11 and 13. Only the central eastern coast remains relatively melt free.

Conditions in context

Figure 2. The graph above shows the daily percent of the Greenland Ice Sheet surface that has shown melt, as of June 18, 2013 (red), along with the average surface melt extent for 1981 to 2010 (blue). ||Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia |High-resolution image

Figure 2. The graph above shows the daily percent of the Greenland Ice Sheet surface that has shown melt, as of June 19, 2013 (red), along with the average surface melt extent for 1981 to 2010 (blue).

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

At this point, the pace of melt is well above average, but well behind the early, intense start seen in the record 2012 season (see February 5 post).

After a spike in melt area in early June, cooler conditions have brought the melt area near the average extent of ~20% of the ice sheet.

 Rising temperatures

Figure 3

Figure 3: These images show temperature anomaly over Greenland at the 850 millibar level (approximately 1,500 meters, or 5,000 feet above sea level)  for May 15 to May 25 (top) and for June 5th to June 15, 2013, compared to the averages from 1981 to 2010. Temperatures were lower than average during May, then shifted to higher than average along the coasts in June.

Credit: Credit: NSIDC courtesy NOAA/ESRL PSD
High-resolution image

Cool conditions in April and May shifted to warmer-than-average weather along both coasts in early June, which initiated more widespread melt on the ice sheet. This shift roughly coincided with a larger change in the Arctic Oscillation from near-neutral conditions to slightly positive, and a shift from generally easterly and northerly winds to southwesterlies. The sea ice on both sides of Greenland remained at near-normal extent through the period.

Landsat 8 Images of early melt lakes

Figure 4. True-color Landsat 8 image of west-central Greenland near 69.9deg N, 51.0 deg W acquired on June 12 (just north of Jakobshavn Isbrae, Landsat acquisition location Path 11, Row 11). The image shows glaciers, coastal bedrock and nunataks, and several small fjords. White areas on the right side of the scene are winter snow (still melting away); light blue and grey areas are exposed ice where the 2012-2013 winter snow has melted away; deeper blue pockets are small incipient melt lakes.

Figure 4. This image from Landsat 8 shows melt ponds forming on the Greenland Ice Sheet. White areas on the right side of the scene are winter snow (still melting away); light blue and grey areas are exposed ice where the 2012-2013 winter snow has melted away; deeper blue pockets are small incipient melt lakes. Glaciers, coastal bedrock, several small fjords, and rocky crags (called nunataks) can also be seen in this true-color Landsat 8 image of west-central Greenland near 69.9 deg N, 51.0 deg W, acquired on June 12, 2013 (just north of Jakobshavn Isbrae, Landsat acquisition location Path 11, Row 11).

Credit: National Snow and Ice Data Center, USGS/NASA Landsat 8
High-resolution image

Melt ponds are already forming in some lower areas of the ice sheet, as shown in Figure 4. Ponds of meltwater on the ice sheet can have a significant impact on ice flow. As the ponds grow and deepen, they can force open cracks in the underlying ice and drain to the base of the ice sheet. This can lubricate the base of the glacier, causing it to flow more rapidly for a brief period. The drained water flows out under the ice sheet, emerging as small streams from beneath the ice at the edge.

This image was acquired by the recently launched USGS/NASA Landsat 8 satellite, using its multi-spectral camera, the Operational Land Imager (OLI). Landsat 8 offers great opportunities for mapping and measurement of the world’s ice and its ongoing changes.  One improvement in the new sensor is the addition of a new spectral band, in the far blue visible range. This band (Band 1) may be useful for mapping lake area and depth. Knowing the total lake volume and rate of lake volume change is an important parameter for characterizing the intensity of the melt season and its potential effect on the rate of ice flow and surface melting for the ice sheet.

A report from the field

Figure 5: This photograph of melt runoff was taken near Kangerlussuaq, Greenland the week of June 17.

Figure 5: This photograph of melt runoff was taken near Kangerlussuaq, Greenland the week of June 17.

Credit: NSIDC courtesy Asa Rennermalm, Rutgers University
High-resolution version

With summer beginning, many Greenland researchers are now in the field, and reporting back on observed surface melting conditions. Thomas Mote from University of Georgia, who is in the Kangerlussuaq area with Asa Rennermalm of Rutgers University, reports indications that there was a fairly warm late winter, a cool spring, and heavy snow in May. This area has experienced strong melting, but much of it is the melting of the late spring snowfall. There is word of a 1-kilometer (0.6 mile) long meltwater lake about 7 kilometers (4 miles) inland on the ice east of Kanger. They did observe some fairly large meltwater streams and moulins.

A darker shade of pale

Figure 6. These color maps show average summer (June, July, and August) albedo for Greenland in 2000 and 2012. Albedo dropped by as much as 0.3 in some areas. The color scale ranges from caramine red (albedo of 1.0) to purple (albedo of 0.0); in the graphics above, the approximate observed range is 0.85 to 0.05. Coastal area albedos are typically below 0.15 (e.g. the western coastal area), and fresh dry powder snow is >0.90 (southeastern ice sheet in the 2000 image). Data are derived from NASA's Terra and Aqua satellites using the Moderate Resolution Spectroradiometer (MODIS) sensors. ||Credit: National Snow and Ice Data Center|High-resolution image

Figure 6. These color maps show average summer (June, July, and August) albedo for Greenland in 2000 and 2012. Albedo dropped by as much as 0.3 in some areas. The color scale ranges from caramine red (albedo of 1.0) to purple (albedo of 0.0); in the graphics above, the approximate observed range is 0.85 to 0.05. Coastal area albedos are typically below 0.15 (e.g. the western coastal area), and fresh dry powder snow is >0.90 (southeastern ice sheet in the 2000 image). Data are derived from NASA’s Terra and Aqua satellites using the Moderate Resolution Spectroradiometer (MODIS) sensors.

Credit: National Snow and Ice Data Center
High-resolution image

Surface melting on the ice sheet reduces the reflectivity of the snow, and this change is measured by albedo, the ratio of the amount of light reflected from a surface to the amount of incoming light. Over the past twelve summer seasons, surface melting has increased in Greenland, culminating with last year’s very extensive and intense melt season. Satellite data have been used to track this change in a recently submitted paper by Stroeve et al. (2013, in revision), showing that albedo has dropped by as much as 0.3 (30% reflectivity) in some areas.

Dry cold snow is the most reflective naturally occurring land cover type on Earth, with albedos as high as 0.95, but as melting begins, this drops to 0.70, and continues to drop with increased melt. Very wet snow has an albedo of 0.6 to 0.5, and exposed ice can be as low as 0.3. This darkening of a snow surface with the onset and progression of melt leads to an important climate feedback. With warm conditions, melting and darker snow leads to more sunlight absorbed by the snow, leading to further melting and darkening.

A note on the data

We thank our readers for writing us with useful suggestions to include more historical data on surface melt, as well as other statistical information. We are working to add such information as time and funding permit.

Reference

Stroeve, J., J. Box, Z. Wang, A. Barrett, and C. Schaaf. 2013. Re-evaluation of MODIS Greenland albedo accuracy and trends. Remote sensing of the environment, in revision.

An intense Greenland melt season: 2012 in review

Greenland’s surface melting in 2012 was intense, far in excess of any earlier year in the satellite record since 1979. In July 2012, a very unusual weather event occurred. For a few days, 97% of the entire ice sheet indicated surface melting. This event prompted NSIDC to build this Web site, with the help of two prominent experts on Greenland surface melting (Dr. Thomas Mote of University of Georgia, and Dr. Marco Tedesco of CUNY).

The Greenland Ice Sheet contains a massive amount of fresh water, which if added to the ocean could raise sea levels enough to flood many coastal areas where people live around the world. The ice sheet normally gains snow during winter and melts some during the summer, but in recent decades its mass has been dwindling. For more information about the significance of the Greenland Ice Sheet and its surface melt, see About the Greenland Ice Sheet.

Warm conditions in 2012 were caused by a persistent high pressure pattern that lasted much of the summer. Since September, temperatures have remained warmer than average, but dropped well below freezing as autumn and winter arrived. We review the year’s events, and introduce some general characteristics of the Greenland ice sheet.

Overview of conditions

Figure 1. The number of melt days in 2012 on the Greenland Ice Sheet exceeded 120 for low elevation areas along the southwestern coast, and values above 100 days were seen in the far north and southeastern coastal areas. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Figure 1 shows the cumulative number of days of melt occurrence for 2012. The number of melt days in 2012 on the Greenland Ice Sheet exceeded 120 for low elevation areas along the southwestern coast, and values above 100 days were seen in the far north and southeastern coastal areas as well.

Overall, melt extent was the largest in the satellite record since 1979, and melting lasted almost two months longer than average. This was the first year in the satellite record that the entire ice sheet experienced melt at some point in the season.

During a peak melt event in July, even the summit areas of the ice sheet, nearly two miles above sea level, saw snowmelt conditions. While this has been observed in ice cores a handful of times in the past 1,000 years, it had not previously occurred in this century.

Conditions in context

Figure 2. The graph above shows the daily extent of melt during 2012 on the Greenland Ice Sheet surface as a percentage, compared to the average from 1981 to 2010. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Greenland’s 2012 melt season started early, surpassing the 30-year average for melt-covered area in mid-May, and remaining far in excess of typical conditions for June, July, and through mid August. For the peak melt days in early July and again in early August, more than 70% of the surface of the ice sheet experienced some melt, and the peak melt event on July 10 to 11 occurred over 97% of the ice sheet.

The overall pattern of melt is closely related to elevation. At the edges of the ice sheet, near the coast, melt is far more frequent. With increasing elevation, up to 2,000 meters (6,300 feet) or so, the number of melt days decreases from tens of days to just a few days. Above 2,500 meters, melt is rare, and in most years a large region of central and northern Greenland (up to 3,100 meters above sea level, or approximately 10,000 feet) sees almost no surface melt.

During 2012, a significant increase in melting days was seen at higher elevations as well, causing surface melting to percolate into older snow below and refreeze. Runoff of water from the ice sheet was intense. In one case, bridges and other structures adjacent or crossing a usually small melt-fed river, the Watson River near Kangerlugssuaq, were destroyed in mid July.

2012 compared to previous years

Figure 3. The image above shows the average cumulative melt days for the Greenland Ice Sheet for the period 1979 to 2007. Oranges and reds indicate greater numbers of melt days, while blues and greens indicate no or low numbers of melt days. Note: This image uses a different ice sheet mask than Figure 1. As a result, some coastal areas showing melt in Figure 1, such as the northern coast, are masked out in Figure 3. Data are from Greenland Ice Sheet Melt Characteristics Derived from Passive Microwave Data, from the Scanning Multi-channel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) instruments.

Credit: National Snow and Ice Data Center
High-resolution image

Figure 3 is a plot of the average annual number of melt days, for the period 1979 to 2007. These data come from a 2008 study of Greenland melt by Abdalati et al. based on the Scanning Multi-channel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) instruments, as part of the Program for Arctic Regional Climate Assessment (PARCA). These data provide a snapshot of past conditions over a longer time period.

The plot shows a more typical pattern of melt occurrence and number of melt days. A comparison with Figure 1 illustrates the extreme nature of the 2012 melt. Melt in 2012 was far more frequent and extensive along the northern west coast, the far north, and the southeast and southwest coastal areas of Greenland, as well as the almost unprecedented areas at high elevation. However, intense melt years have been the rule since 2006, particularly in 2007 and 2010.

The upward march of melt

Figure 4. This illustration shows a generalized cross-section of glacier facies, zones with distinct physical characteristics resulting from the conditions that formed or changed the snow. The snow cover is completely stripped away in the ablation facies. The entire year’s accumulation is raised to the melting point and wetted in the soaked facies. In the percolation facies, the annual increment of new snow is not completely wetted nor raised to the melting point, and the amount of percolation decreases with altitude, becoming negligible at the dry snow line. Little or no melt occurs in the dry snow facies.

Credit: National Snow and Ice Data Center/C.S. Benson
High-resolution image

The perennial pattern of melt seen in Figure 3 has had a large impact on the character of the ice sheet. Figure 4, from Benson 1996, is a summary of what happens in the snow and ice during a summer melt season. The term facies refers to areas or zones of the ice with distinct characteristics that provide clues to the conditions that formed or changed them.

At low elevations, all the winter snow is melted away early each year, and bare ice is exposed and melted. This is the ablation zone, where old ice flowing out from the center of the sheet is melted as part of the annual melt season, and the water flows over the surface to the coast. Above this, there is a zone where annual melt may or may not remove all of the winter’s snow. If the melt completely saturates the snow with water, lakes and rivers are seen on the ice surface, forming the saturation zone or wet snow zone. Where the surface melt water drains into the snow but remains more or less in place and re-freezes as an ice layer, we get a percolation zone. Above this is the dry snow zone where melt rarely occurs.

A major signal of climate change for Greenland is the steady climb of these facies uphill as melt seasons and summer temperatures increase. The changes in facies also pre-condition the surface of the ice sheet for even more melting.

Not-so-arctic weather

Figure 5. This image of Greenland summer 2012 air temperature anomalies at the 925 hPa level (about 3,000 feet above the surface) shows that all of Greenland experienced warmer than average temperatures, with the strongest warming along the west coast. Temperatures are compared to the 1981 to 2010 average.

Credit: National Snow and Ice Data Center, from NOAA/ESRL Physical Sciences Division
High-resolution image

Summer warmth this year in Greenland was intense and widespread. In areas already prone to summertime melt, the period June through August of 2012 was more than 2 degrees Celsius (4 degrees Fahrenheit) warmer than the average for 1981 to 2010, and greater than 1.5 Celsius (3 degrees Fahrenheit) for nearly the entire ice sheet. This resulted from a very persistent high pressure ridge that dominated the weather, creating clear skies, light winds, and low precipitation.

Edward Hanna at the University of Sheffield and colleagues recently published a study of Greenland temperatures based on weather station data. They found warming to be much stronger on the west side of Greenland than on the east since 1991. The data from 1991 to 2012 show that some locations in western Greenland have warmed 2 to 4 degrees Celsius (4 to 7 degrees Fahrenheit) during summer, while some locations along the west and northwest coasts of Greenland warmed as much as 10 degrees Celsius (18 degrees Fahrenheit) during winter.

At present, the Greenland Ice Sheet is in its winter mode. As the daily images show, it is currently exhibiting little or no melt as would be expected for this time of year. NSIDC will continue to post daily image updates throughout the 2013 melt season, and provide periodic analysis as conditions warrant.

References

Benson, C. S. 1996. Stratigraphic studies in the snow and firn of the Greenland ice sheet. U.S. Army Corps of Engineers: Snow, Ice, and Permafrost Research Establishment Res. Rep. 70.

Hanna, E., S. H. Mernild, J. Cappelen, and K. Steffen. 2012. Recent warming in Greenland in a long-term instrumental (1881–2012) climatic context: I. Evaluation of surface air temperature records. Environmental Research Letters 7, no. 4, doi:10.1088/1748-9326/7/4/045404.

Tedesco, M.,  X. Fettweis, T. Mote, J. Wahr, P. Alexander, J. Box, and B. Wouters. 2012. Evidence and analysis of 2012 Greenland records from spaceborne observations, a regional climate model and reanalysis data. The Cryosphere Discuss., 6, 4939–4976, doi:10.5194/tcd-6-4939-2012.