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Author Topic: Cryoconites - the critical organic component of GIS decline  (Read 1854 times)

Lewis C

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The reason that a greater focus on cryoconites seems warranted is that the propensity for snow droughts under the rising occurrence of the Greenland high appears pretty much ideal for the acceleration of their expansion across the GIS.

If the paper by Wientjes et al (2011) is correct in implying that the ice-sheet's entrained and windborne dust impose no significant darkening effect, as the clean white ice under emptied melt-lakes attests, then it is the microbial component of cryoconites that are capable of imposing very significant albedo loss to coincide with the peak of the melt season, thereby amplifying meteorological melting influences.

See this from Nasa, taken on August 17 2010 by their EO-1 satellite of an area of SW Greenland centered on 68.91N x 48.54W, for an image of the contaminated surface ice in contrast to underlying ice exposed by former melt pools:
http://eoimages.gsfc.nasa.gov/images/imagerecords/80000/80677/greenlandponds_ali_2010229_lrg.jpg

Given that the melt area since '79 has on average extended right across the GIS as far north as 67 degrees (NSIDC), and that this releases water through the permeable surface to form a fairly impermeable ice layer at the level that it freezes, the cryoconites' need to be able to form mini-pools to flourish is increasingly being met across potentially huge areas of what has been the accumulation zone as the accumulation rate declines.

In effect the ablation zone is gaining elevation, which is critical to the rate at which the ice-sheet's internal decay advances. To my mind it is not simply the area of additional melt lakes forming over newly impermeable ice that is troubling, but their location. For example, along the west coast for each five metres of extra altitude melt-lakes form at, there is around 1,000 kms2 of extra area whose moulins deliver melt water within the encircling bedrock watershed to increase the internal voids and thus the volume of the retained water under the ice sheet.

We already have around 60,000 kms2 in the SW and 20,000 kms2 in the SE whose melt-lake moulins are within the watershed and are feeding that reservoir - as can be seen by overlaying the watershed and maximum melt-lake altitude lines on a topographical map. Moreover that water is not heading for Jacobshaven but for the lowest point it can find, specifically the 300km-wide depression at about 68 degrees North. Only when the water-table in the arterial melt-water system and in any caverns it carves out exceeds the level of the bedrock watershed inland of Jacobshaven does it start to overflow to the sea. And at the end of the melt season and the end of that outflow it then has all winter to transfer its remaining heat into the base of the ice sheet.

Given that the melt-water arteries must lead first to the depression and only then to the outlet, it follows that the entire contents of the system is replaced with freshly warmed water each summer, thus delivering an annual pulse of heat energy into the ice sheet's underside, which is already warmed by heat emissions from the underlying bedrock.

From this perspective the potential of snow droughts to assist the expansion of the cryoconites' microbial ecology not only along the coasts but particularly up and across higher elevations looks like a highly significant factor in the coming GIS melt rate, and in the advance of the date where the ice-sheet base is decayed to the point of starting to collapse into its underlying water reservoir. With an average of well over 2,000 tonnes of overlying ice per square metre of the base, the resulting changes may be somewhat abrupt.

Regards,

Lewis