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Friday, June 1, 2012

Jason Box: Greenland Ice Sheet Getting Darker

Greenland Ice Sheet Getting Darker

The following provides detail to a story run by NOAA entitled Greenland Ice Sheet Getting Darker

by Jason E. Box, MeltFactor

Freshly fallen snow under clear skies reflects 84% (albedo= 0.84) of the sunlight falling on it (Konzelmann & Ohmura, 1995). This reflectivity 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 30% to 60% (Cuffey & 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 (20%) (Cuffey & Paterson). Thus, summer albedo variability exceeds 50% over parts of the ice sheet where a snow layer ablates by mid-summer, exposing an impurity-rich ice surface (Wientjes & 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 photo below shows how dark the ice sheet surface can become in the lowest ~1,000 m elevation in the “ablation area” after the winter snow melts away and leaves behind an impurity-rich surface. This dark area is where the albedo feedback with melting is strongest.

August 12, 2005, 8 p.m. local time, I took this photo from a helicopter flying over the ice sheet surface at ~1,500 feet altitude. This is how much darker the Greenland ablation area is than a fresh snow surface that blankets it in wintertime. Along much of the southwestern ice sheet at the lowest 1,000 m in elevation, impurities concentrate near the surface and produce this dark surface. Not all of the ice sheet is this dark, only the lower ~1/3 of the elevation profile of the ice sheet is. However, as melting increases on the ice sheet, so does the area exposed that is this dark.

Satellite observations from the NASA Moderate-Resolution Imaging Spectroradiometer (MODIS)  indicate a significant Greenland ice sheet albedo decline (-5.6±0.7%) in the June-August period over the 12 melt seasons spanning 2000-2011. According to linear regression, the ablation area albedo declined from 71.5% in 2000 to 63.2% in 2011 (time correlation = -0.805, 1-p=0.999). The change (-8.3%) is more than two times the absolute albedo RMS error (3.1%). Over the accumulation area, the highly linear (time correlation = -0.927, 1-p>0.999) decline from 81.7% to 76.6% over the same period also exceeds the absolute albedo RMS error.

Greenland ice sheet average reflectivity or albedo (multiply by 100 to get % units) for 12 summer (June-August) periods.

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. Like year 2010, 2011 albedos are more than 1 S.D. below the 2000-2011 mean.

Year 2011 albedo (multiply by 100 to get % units) over the Greenland ice sheet is the lowest observed in the 12 years since MODIS observations began day 65 year 2000. 11-day running median Greenland ice sheet albedo from Moderate Resolution Imaging Spectroradiometer (MODIS) MOD10A1 data. The dashed line represents the 2000-2011 daily average.

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).

This decline is not only over the lowest elevations, but occurs high on the ice sheet where melting is limited.

The greatest changes in reflectivity (or albedo, multiply by 100 to get % units) are found where a relatively dark surface of impurity rich "glacier ice" emerges once the snow cover melts. It's natural for snow cover to melt away at the lowest elevations of a glacier or ice sheet. However, the period of time over which the ice sheet surface is bare has increased significantly since year 2000 when these observations become available.

A significant albedo decline of 4.6±0.6% in the 2000-2011 period from a year 2000 value of 83.0% is observed for the accumulation area, where warming surface temperatures are enhancing snow grain metamorphosis.

References at link:


andrew said...

Now, that's scary.

rpauli said...

Tenney - this is a very telling and powerful post. Thanks