Greenland ice sheet reflectivity at record low, particularly at high elevations
by Jason Box, meltfactor.org, June 25, 2012
An updated compilation of NASA MODIS observations of Greenland ice surface reflectivity through 22 June, 2012 indicates that now, well into into the 2012 melt season, the ice sheet remains in a darkened state (see Greenland Ice Sheet Getting Darker).
The cause of the low reflectivity involves a combination of multiple factors:
- Abnormally intense melt at low elevations erases bright white snow, exposing a darker impurity rich bare ice surface. When the melt back of winter snow happens earlier, the anomaly grows.
- In areas where snow remains, temperature-driven snow metamorphism reduces reflectivity by rounding the sharp ice crystal edges that scatter visible light (Wiscombe & Warren, 1980; Warren, 1982; Dozier et al., 2009). This NOAA climate watch article includes a very useful photo. Fresh snow reflects ~84% of solar energy (Konzelmann & Ohmura, 1995). This fraction, called the albedo, decreases with increasing snow effective grain size.
- Increased snow liquid water content in areas of enhanced melting increases absorption of visible light; and
- Potentially less summer snowfall as in year 2011. Summertime snow events take the edge off the amplifying feedback by brightening the surface. With climate warming, the ratio of snowfall to rainfall decreases. It actually does rain on the lower elevations of the ice sheet. I measured 5 cm rainfall in a single 24-h period in June 1998 at Swiss Camp, located at 1,150 m elevation along the central western slope of the ice sheet.
- Atmospheric circulation that colleague Dr. Xavier Fetteweis at University of Liège, Belgium, has been examining for Greenland and who plans to post an analysis here.
- The possibility of increased snow impurities like carbonaceous soot from wildfires or diesel exhaust can lower ice sheet reflectivity.
Enhanced ice sheet melting is likely promoted by changes in the surrounding marine environment:
- Abnormally high sea surface temperatures (see the DMI’s nice web product and select anomaly from the drop down menu).
- A record setting low Arctic sea ice area (see NDISC’s operational sea ice extent visualization products).
- Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K. (2012). Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere Discuss., 6, 593-634, doi:10.5194/tcd-6-593-2012
- Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. Using satellite-derived surface albedo with calibrated regional climate modeled surface air temperature and surface downward solar irradiance, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback over twelve summer periods beginning in 2000. We find that while albedo feedback defined by the change in net solar shortwave flux and temperature over time is positive over 97% of the ice sheet, when defined using paired annual anomalies, a second order negative feedback is evident over 63% of the accumulation area. This negative feedback damps the accumulation area response to warming due to a positive correlation between snowfall and surface air temperature anomalies. Positive anomaly–gauged feedback concentrated in the ablation area accounts for more than half of the overall increase in melting when satellite derived melt duration is used to define the timing when net shortwave flux is sunk into melting. Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007 enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area approached positive values during this period. Thus, it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.