From Byrd Polar Research Center - Research Wiki
The following is from a manuscript in preparation for publication and is linked to a story run by NOAA entitled Greenland Ice Sheet Getting Darker...
Satellite observations of Greenland ice sheet reflectivity, or "albedo" indicate a significant ice sheet albedo decline (-0.056±0.007) in the June-August period over the 12 melt seasons spanning 2000-2011. Albedo decline, or darkening of the ice sheet is significant, allowing the surface to heat more from sunlight. Darkening of the ice sheet in the 12 summers between 2000 and 2011 permitted the ice sheet to absorb an extra 172 quintillion joules of energy, nearly 2 times the annual energy consumption of the United States (about 94 quintillion joules in 2009).
Freshly fallen snow under clear skies reflects 84% (albedo= 0.84) of the sunlight falling on it (Konzelmann and Ohmura, 1995). This reflectiveness progressively reduces during the sunlit (warm) season as a consequence of ice grain growth, resulting in a self-amplifying albedo decrease, a positive feedback. Another amplifier; the complete melting of the winter snow accumulation on glaciers, sea ice, and the low elevations of ice sheets exposes darker underlying solid ice. The albedo of low-impurity snow-free glacier ice is in the range of 0.30 to 0.60 (Cuffey and Paterson, 2010). Where wind-blown-in and microbiological impurities accumulate near the glacier ice surface (Bøggild et al. 2010), the ice sheet albedo may be extremely low (0.20) (Cuffey and Paterson, 2010). Thus, summer albedo variability exceeds 0.50 over parts of the ice sheet where a snow layer ablates by mid-summer, exposing an impurity-rich ice surface (Wientjes and Oerlemans, 2010), resulting in absorbed sunlight being the largest source of energy for melting during summer and explaining most of the inter-annual variability in melt totals (van den Broeke et al. 2008, 2011).
The observed albedo decline since the period of high quality satellite observations beginning in 2000 is largest in magnitude over the ablation area (-0.091±0.021 on average where an increasing dark bare ice area is exposed after winter seasonal snow cover ablates reveals a darker glacier ice surface with abundant impurities. The ablation area is where all previous cold season snow accumulation melts (or ablates) by the end of the warm season. The accumulation area is where there is net snow accumulation by the end of each year.
A significant albedo decline of 0.046±0.006 in the 2000-2011 period from a year 2000 value of 0.830 is observed for the accumulation area, where warming surface temperatures is enhancing snow grain metamorphic rates.
The albedo declines exceed the absolute root mean squared error (RMSE) found to be 0.035 using AWS data on monthly time scale.
According to linear regression, the ablation area albedo declined from 0.715 in 2000 to 0.632 in 2011 (time correlation = -0.805, 1-p=0.999). The change (-0.083) is more than two times the absolute albedo RMSE (0.031). Over the accumulation area, the highly linear (time correlation = -0.927, 1-p>0.999) decline from 0.817 to 0.766 over the same period also exceeds the absolute albedo RMSE.
Because of extreme 2010 melt and little snow accumulation during the melt season (Tedesco at al., 2011) and afterward, the ice sheet albedo remained more than two standard deviations below the 2000-2011 average in October.
Year 2011 albedo over the Greenland ice sheet is the lowest observed in the 12 years since MODIS observations began day 65 year 2000.
Like year 2010, 2011 albedos are more than 1 standard deviation below the 2000-2011 average.
Minimum ice sheet albedo is reached on average during the 3rd week of July.
The accumulation area is also susceptible to albedo feedback from grain growth metamorphism occurring in sub-freezing conditions.
- Bøggild, C.E., Brandt, R.E., Brown, K.J., Warren, S.G. 2010: The ablation zone in northeast Greenland: ice types, albedos and impurities. Journal of Glaciology 56, 101-113.
- Cuffey, K. M., & Paterson, W. (2010). The physics of glaciers Elsevier, ed (Vol. 4, p. 693).
- Konzelmann, T., & Ohmura, A. (1995). Radiative Fluxes And Their Impact On The Energy-Balance Of The Greenland Ice-Sheet. Journal of Glaciology, 41(139), 490-502.
- 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, The role of albedo and accumulation in the 2010 melting record in Greenland, 2011: Environ. Res. Lett. 6 014005, doi: 10.1088/1748-9326/6/1/014005.
- van den Broeke, M. R., Smeets, C. J. P. P., & van de Wal, R. S. W. (2011). The seasonal cycle and interannual variability of surface energy balance and melt in the ablation area of the west Greenland ice sheet. Cryosphere, 5(2), 377-390. doi: 10.5194/tc-5-377-2011
- van den Broeke, M. R., Smeets, P., Ettema, J., van der Veen, C., van de Wal, R. and Oerlemans, J.: Partitioning of melt energy and meltwater fluxes in the ablation area of the west Greenland ice sheet. The Cryosphere, 2(2), 179-189, 2008.
- Wientjes, I. G. M., & Oerlemans, J. (2010). An explanation for the dark region in the western melt area of the Greenland ice sheet. Cryosphere, 4(3), 261-268. doi: 10.5194/tc-4-261-2010
This research was supported by The Ohio State University Climate Water and Carbon initiative. David Decker and Russell Benson gathered and helped grid the MOD10A1 data. Co-authors of the paper in progress include:
- Xavier Fettweis, Department of Geography, University of Liège, Belgium}
- Julienne C. Stroeve, National Snow and Ice Data Center (NSIDC), Boulder, CO, USA & Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA}
- Marco Tedesco, The City University of New York, New York, NY, USA}
- Dorothy K. Hall, NASA Goddard Space Flight Center, Greenbelt, MD, USA
Follow my climate-cryosphere tweets on Twitter