2011 Melting in Greenland
Year 2011 Greenland melting remains well above the (1979 – 2010) average;
close-to-record mass loss
M. Tedesco1, X. Fettweis2, T. Mote3 , N. Steiner1 and J. E. Box4
1) City College of New York – CUNY – NYC – USA
2) University of Liege, Liege, Belgium
3) University of Athens – Georgia – USA
4) Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, USA
Summary: Melting in Greenland in 2011 was still above the average (1979 – 2010 baseline period), exceptionally high over the west coast and reaching close-to-record simulated surface mass balance, bare ice exposure, albedo and runoff anomalies.
The melting index (e.g., the number of melting days times the area subject to melting) in 2011 estimated from spaceborne microwave observations using the approach in (Tedesco, 2007) did not break the previous record set in 2010 (e.g., Tedesco et al., 2011). However, 2011 is positioning itself 6th in terms of melting index, after 2010, 2007, 1998, 2002, 2005. An alternative approach using microwave data as well (Mote and Anderson, 2005) indicates that melt extent for the period June through August 2011 ranked third since 1979, following 2010 and 2007. Satellites data cannot produce estimates of runoff and liquid water content. However, these can be analyzed by means of models. The model used in this analysis (MAR, e.g., Tedesco et al., 2011) indicates that 2011 was comparable to the record season of 2010 with respect to runoff, surface mass balance, albedo and bare ice exposure. Strong negative surface mass balance anomalies occurred in 2011, according to MAR (e.g., the loss in 2011 and 2010 were much higher than the gained mass because of accumulation). Surface albedo simulated by MAR was consistently below or around 2 standard deviations below the mean for the period June – August (e.g., more solar radiation absorbed supporting more melting, see Figure 4 for a diagram). The bare ice area exposed during the summer of 2011 was also large with respect to the mean (close to up 3 standard deviations for the month of July), similarly to what happened in 2010.
We combined spaceborne observations and model outputs to provide an analysis of melting in 2011 over the Greenland ice sheet. The combination of the two approaches allows us to portray a more complete picture and to overcome limitations of the approaches taken separately.
close-to-record mass loss
M. Tedesco1, X. Fettweis2, T. Mote3 , N. Steiner1 and J. E. Box4
1) City College of New York – CUNY – NYC – USA
2) University of Liege, Liege, Belgium
3) University of Athens – Georgia – USA
4) Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, USA
Summary: Melting in Greenland in 2011 was still above the average (1979 – 2010 baseline period), exceptionally high over the west coast and reaching close-to-record simulated surface mass balance, bare ice exposure, albedo and runoff anomalies.
The melting index (e.g., the number of melting days times the area subject to melting) in 2011 estimated from spaceborne microwave observations using the approach in (Tedesco, 2007) did not break the previous record set in 2010 (e.g., Tedesco et al., 2011). However, 2011 is positioning itself 6th in terms of melting index, after 2010, 2007, 1998, 2002, 2005. An alternative approach using microwave data as well (Mote and Anderson, 2005) indicates that melt extent for the period June through August 2011 ranked third since 1979, following 2010 and 2007. Satellites data cannot produce estimates of runoff and liquid water content. However, these can be analyzed by means of models. The model used in this analysis (MAR, e.g., Tedesco et al., 2011) indicates that 2011 was comparable to the record season of 2010 with respect to runoff, surface mass balance, albedo and bare ice exposure. Strong negative surface mass balance anomalies occurred in 2011, according to MAR (e.g., the loss in 2011 and 2010 were much higher than the gained mass because of accumulation). Surface albedo simulated by MAR was consistently below or around 2 standard deviations below the mean for the period June – August (e.g., more solar radiation absorbed supporting more melting, see Figure 4 for a diagram). The bare ice area exposed during the summer of 2011 was also large with respect to the mean (close to up 3 standard deviations for the month of July), similarly to what happened in 2010.
We combined spaceborne observations and model outputs to provide an analysis of melting in 2011 over the Greenland ice sheet. The combination of the two approaches allows us to portray a more complete picture and to overcome limitations of the approaches taken separately.
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The melting index (e.g., the number of melting days times the area subject to melting) in 2011 estimated from spaceborne microwave observations using the approach in (Tedesco, 2007) did not break the record set in 2010 (e.g., Tedesco et al., 2011). 2011 is positioning itself 6th in terms of melting index, after 2010, 2007, 1998, 2002, 2005. The updated trend for the melt extent (e.g., the area of the ice sheet subject to melting) is 16,800 km2/year (close to the one estimated with 2010 of 17,202 km2/year). Figure 1 illustrates the 2011 anomaly for the number of melting days (e.g., the number of melting days in 2011 minus the average for the period 1979 – 2010) derived from spaceborne passive microwave observations using the approach mentioned in Tedesco, 2007. Red indicate areas where the number of melting days in 2011 was above average. Blue areas (mostly absent in the figure) indicate locations where this year’smelting was below the average. The ’hotter’ the color the longer the area was melting. In 2011, melting at high elevations (e.g. above 2500 m) was ~ 1 standard deviation above the 1979 – 2010 average (versus the ~ 2 standard deviations of 2010). Melting in 2011 was above the average over most of Greenland, with large positive anomalies (e.g., longer melting with respect to the average) occurring especially along the west and northwest, with melting lasting up to ~30 days longer than the average.
Another method of assessing surface melt extent from passive microwave satellite data (Mote and Anderson, 1995) indicated melt extent for the period June through August 2011 ranked third since 1979, following 2010 and 2007. The years were ranked based on the seasonal melt departure (SMD), the sum of the daily melt extent anomalies over each summer (Mote 2007). Year 2011 SMD was driven by more extensive than average June melting as well as intense melting in late July and late August. These annual ranking are sensitive to the length of the season selected. Expanding the season from 15 May to 15 September drops 2011 to the 6th most extensive melt year, following 2010, 2007, 2002, 1998, and 2005, in that order. [See the MEaSUREs site; http://climate.rutgers.edu/measures/snowice/ for details on this approach and the resulting climate data record.]
Spaceborne observations tell us only part of the story.
Because of the electromagnetic properties of liquid water in the microwave region, it is not possible to estimate snow or ice liquid water content values. Consequently, spaceborne microwave data can tell us only where and for how long melting occurs, not how ‘wet’ the snow/ice is. Besides, once the scene observed from space is wet, it is difficult to understand whether we are observing snow or ice actively melting. We lack measurement of melt intensity. Gauging melt intensity is crucial for mass balance studies that seek to answer: how much mass ice land ice is losing versus how much it is gaining?Land ice mass loss currently presents the single largest global sea level contribution. This is where models can help, as they can simulate these quantities.
Figure 2 illustrates the snowfall, runoff and surface mass balance for the period 1958 – 2011 obtained from the MAR model. MAR indicates that 2011 runoff and surface mass balance was comparable to the record setting year 2010, with strong negative surface mass balance anomalies. The loss in 2011 and 2010 was much higher than the mass gain from snow accumulation.
Another method of assessing surface melt extent from passive microwave satellite data (Mote and Anderson, 1995) indicated melt extent for the period June through August 2011 ranked third since 1979, following 2010 and 2007. The years were ranked based on the seasonal melt departure (SMD), the sum of the daily melt extent anomalies over each summer (Mote 2007). Year 2011 SMD was driven by more extensive than average June melting as well as intense melting in late July and late August. These annual ranking are sensitive to the length of the season selected. Expanding the season from 15 May to 15 September drops 2011 to the 6th most extensive melt year, following 2010, 2007, 2002, 1998, and 2005, in that order. [See the MEaSUREs site; http://climate.rutgers.edu/measures/snowice/ for details on this approach and the resulting climate data record.]
Spaceborne observations tell us only part of the story.
Because of the electromagnetic properties of liquid water in the microwave region, it is not possible to estimate snow or ice liquid water content values. Consequently, spaceborne microwave data can tell us only where and for how long melting occurs, not how ‘wet’ the snow/ice is. Besides, once the scene observed from space is wet, it is difficult to understand whether we are observing snow or ice actively melting. We lack measurement of melt intensity. Gauging melt intensity is crucial for mass balance studies that seek to answer: how much mass ice land ice is losing versus how much it is gaining?Land ice mass loss currently presents the single largest global sea level contribution. This is where models can help, as they can simulate these quantities.
Figure 2 illustrates the snowfall, runoff and surface mass balance for the period 1958 – 2011 obtained from the MAR model. MAR indicates that 2011 runoff and surface mass balance was comparable to the record setting year 2010, with strong negative surface mass balance anomalies. The loss in 2011 and 2010 was much higher than the mass gain from snow accumulation.
Figure 3 illustrates the 2011 standardized anomalies for near surface air temperature, surface albedo, snowfall, melt water production, bare ice area, and melt area all obtained from MAR for the months of April through August. Anomalies for the North Atlantic Oscillation (NAO) index are also reported. The temperatures in 2011 were higher than normal from mid June to mid August, according to a constant negative NAO index during these months. The constant negative NAO index induced anticyclonic and then dry conditions over the ice sheet, allowing to maintain a large bare ice extent through the whole melt season compared to 2010, when two snowfall events reduced temporarily the bare ice extent.
According to MAR, 2011 was characterized by an anomalously cold spring. Year 2011 melt onset was relatively late, beginning in June. The melt area for 2011 simulated by MAR is above 1 and 2 standard deviations in June and August, respectively, but within the mean during July. Nevertheless, surface albedo was consistently below or around 2 standard deviations below the mean for the period June – August (e.g., more solar radiation absorbed enhancing melting). The bare ice area exposed during the summer of 2011 was also anomalous with respect to the mean (close to up 3 standard deviations for the month of July), similarly to 2010. Figure 4 illustrates the simulated melt water, runoff, melt extent for the period May through August, and the bare ice extent for the same period simulated by MAR for the years 1958 through 2011. Both runoff and bare ice extent in 2011 are close to the 2010 record.
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As mentioned above, the exposure of bare ice plays a major role on the increase of runoff and mass loss, either because it promotes enhanced absorption of solar radiation, much more than snow and because ice can melt faster than snow (see Figure 4 for a diagram explaining some of the feedback mechanisms).
How can we explain the differences between the record simulated by MAR and the results from spaceborne microwave data ?
Strong positive melting anomalies occurred in 2010 mainly because melting started earlier and lasted longer than usual. This aspect was captured by spaceborne microwave data because melting anomalies were largely driven by the length of the melting season. Because of this, the surface mass balance and runoff simulated by MAR for 2010 were in agreement with the melting index record derived from spaceborne observations. In 2011, however, melting did not start until late in the season and it did not last as long as in 2010. The exposure of bare ice promoted strong melting with the 2011 season being characterized by relatively short but intense melting. This was not captured by spaceborne microwave data because of their limitation in estimating the amount of liquid water within the snowpack. In summary: the 2010 season was largely driven by a longer season and therefore captured by microwave data, where the 2011 season was characterized by a relatively short but intense melting season, with the albedo feedback mechanism playing a major role (as in 2010) and large bare ice areas subject to melting.
References (in alphabetical order):
Mote, T.L., and M.R. Anderson, 1995. Variations in Melt on the Greenland Ice Sheet Based on Passive Microwave Measurements, J. of Glaciology, 41, 51-60.
Mote, T. L., Greenland surface melt trends 1973–2007: Evidence of a large increase in 2007, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L22507, 5 PP., 2007 doi:10.1029/2007GL031976
Tedesco , M. , X. Fettweis, M. R. van den Broeke, R. S. W. van de Wal, C. J. P. P. Smeets, W. J. van de Berg, M. C. Serreze and J. E. Box. 2011. The role of albedo and accumulation in the 2010 melting record in Greenland, Environ. Res. Lett. 6 014005
Tedesco M., X. Fettweis, M. R. van den Broeke, R. S. W. van de Wal, C. J. P. P. Smeets, W. J. van de Berg, M. C. Serreze, and J. E. Box , Record summer melt in Greenland in 2010 , EOS AGU, Volume 92, Number 15, 12 April 2011
Tedesco, M., Snowmelt detection over the Greenland ice sheet from SSM/I brightness temperature daily variations, Geophys. Res. Lett., 34, L02504,doi:10.1029/2006GL028466, January 2007
How can we explain the differences between the record simulated by MAR and the results from spaceborne microwave data ?
Strong positive melting anomalies occurred in 2010 mainly because melting started earlier and lasted longer than usual. This aspect was captured by spaceborne microwave data because melting anomalies were largely driven by the length of the melting season. Because of this, the surface mass balance and runoff simulated by MAR for 2010 were in agreement with the melting index record derived from spaceborne observations. In 2011, however, melting did not start until late in the season and it did not last as long as in 2010. The exposure of bare ice promoted strong melting with the 2011 season being characterized by relatively short but intense melting. This was not captured by spaceborne microwave data because of their limitation in estimating the amount of liquid water within the snowpack. In summary: the 2010 season was largely driven by a longer season and therefore captured by microwave data, where the 2011 season was characterized by a relatively short but intense melting season, with the albedo feedback mechanism playing a major role (as in 2010) and large bare ice areas subject to melting.
References (in alphabetical order):
Mote, T.L., and M.R. Anderson, 1995. Variations in Melt on the Greenland Ice Sheet Based on Passive Microwave Measurements, J. of Glaciology, 41, 51-60.
Mote, T. L., Greenland surface melt trends 1973–2007: Evidence of a large increase in 2007, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L22507, 5 PP., 2007 doi:10.1029/2007GL031976
Tedesco , M. , X. Fettweis, M. R. van den Broeke, R. S. W. van de Wal, C. J. P. P. Smeets, W. J. van de Berg, M. C. Serreze and J. E. Box. 2011. The role of albedo and accumulation in the 2010 melting record in Greenland, Environ. Res. Lett. 6 014005
Tedesco M., X. Fettweis, M. R. van den Broeke, R. S. W. van de Wal, C. J. P. P. Smeets, W. J. van de Berg, M. C. Serreze, and J. E. Box , Record summer melt in Greenland in 2010 , EOS AGU, Volume 92, Number 15, 12 April 2011
Tedesco, M., Snowmelt detection over the Greenland ice sheet from SSM/I brightness temperature daily variations, Geophys. Res. Lett., 34, L02504,doi:10.1029/2006GL028466, January 2007
