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Monday, June 25, 2012

Jason Box: Greenland ice sheet reflectivity at record low, particularly at high elevations

Greenland ice sheet reflectivity at record low, particularly at high elevations

NASA MODIS MOD10A1 data from 1-22 June 2012 versus the previous 12 June periods spanning 2000-2011.

by Jason Box,, 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).

Ice sheet reflectivity this year has been the lowest since accurate records began in March 2000.
In this condition, the ice sheet will continue to absorb more solar energy in a self-reinforcing feedback loop that amplifies the effect of warming. It’s not a runaway loop, just an amplifier. A record setting melt season is likely if this pattern keeps up this year.

Perhaps most remarkable about the 2012 pattern is how much darker the snow and ice is becoming, not only at the lowest elevations around the ice sheet periphery where melting is always most intense, but in the higher elevation net snow accumulation area. June monthly average reflectivity is below the 2000-2011 average across the southern-central area where surface elevations are above 2,000 m (6,561 feet). A purple area about 1/4 the distance north of the ice sheet's southern tip at an elevation of 2,400 m (7,874 ft.) has reflectivity  7% below the already declining 2000-2011 June (12 year) average.

Consistent with the low albedo anomaly at high elevations is the shift of the summer radiation balance from negative (cooling) to positive (heating) (Box et al., 2012). In the 12 years between 2000 and 2011, the high elevation ice sheet net radiation (sum of upward and downward solar and infrared radiation) approached positive values. What I expect we will see if these low albedo conditions persist is 100% surface melting over the ice sheet. This would be a first in observations. It may not happen this year, but the trajectory the ice sheet is on, along with amplified Arctic warming, will have the ice sheet responding by melting more and more.

An updated visualization of the ‘noodle plot’ above will be maintained here.

The cause of the low reflectivity involves a combination of multiple factors:
  1. 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.
  2. 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.
  3. Increased snow liquid water content in areas of enhanced melting increases absorption of visible light; and
  4. 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.
  5. 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.
  6. The possibility of increased snow impurities like carbonaceous soot from wildfires or diesel exhaust can lower ice sheet reflectivity.
I don’t know the relative contribution of impurities versus the reflectivity reduction resulting from the first three melt factors. Yet, the pattern of concentrated low reflectivity around the ice sheet periphery  indicates the earlier loss of winter snow in the ablation area of the ice sheet where bare ice is exposed each year sometime during the melt season. That exposure is just happening earlier in the year. The pattern over the far northwestern ice sheet, over the Humboldt glacier, is a strong suggestion of increased melting.

Enhanced ice sheet melting is likely promoted by changes in the surrounding marine environment:
  1. Abnormally high sea surface temperatures (see the DMI’s nice web product and select anomaly from the drop down menu).
  2. A record setting low Arctic sea ice area (see NDISC’s operational sea ice extent visualization products).
There is certainly more to the story, such as the role of atmospheric circulation in pumping warm air up from the south, as in the case of the former record setting year 2011 low albedo anomaly. That circulation anomaly is described in a paper I’m completing the rebuttal for. This is after an intensive external and anonymous review process for the paper:
  • 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
I paste below the abstract of that study that contains yet more relevant information:
  • 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.
According to a cross validation with independent GC-Net AWS data, degrading MODIS instrument sensitivity identified by Wang et al. (2012) is not here detected in the MOD10A1 product.

1 comment:

Anonymous said...

The next large transformation I'm expecting is that Hudson bay becomes partially ice free during winters and then in Southern Greenland the speed of melt increases further. At the same time, of course, Barents and Kara Seas continue their winter loss.

Nice to hear the cautious prediction of the ocean level rise from the German group. I checked the elevations near my home city (again), it seems like the city can manage, though the harbour should move to neighboring town. The really big troubles here start at 5-7,5+ASL (GIS/part of WAIS)as much of the fields in the nearby areas get under water. Also started doing a map of +25ASL (some records of the Miocene state this was the level) but that got too large and scary very soon so I dropped it. probably +15 would make an interesting LARP here, maybe.