Greenland contains the largest continuous mass of ice in the northern hemisphere; an area over 2 million km2. The frequency of Greenland surface melting has increased, likely as a result of human-induced climate warming, with the melt-area covering almost the entire ice sheet surface in 2012 (Ngheim et al. 2012; Box et al, 2013; Tedesco et al. 2013).
Although its fair to say that higher temperatures mean more melt, the response of earth’s glaciers and ice sheets to climate warming is complex, also depending upon a range of feedbacks (e.g. Box et al, 2012). For example, when ice melts, liquid water runs over its surface, sometimes collecting in pools and lakes. Liquid water is a more effective absorber of sunlight than snow or ice, so the overall reflectivity (also called albedo, Greek for ‘whiteness’) of the ice decreases. The result is faster ice melt. Melting promotes more melting.
Not only melt water that reduces Greenland ice sheet albedo. A variety of aerosol ‘impurities’ further reduce surface albedo. These include black carbon (BC) derived from incomplete combustion of fossil fuels, other industrial activity, biomass burning, and wildfire. Black carbon can be transported across the hemisphere through the atmosphere and deposited on ice, and currently the impacts remain uncertain (Hodson, 2014). Dark Snow field science is examining this question in detail based on 2013 field measurements and planned measurements for June-August, 2014.
The presence of microbes on the ice surface also alter albedo and can therefore influence melt rates (Yallop et al, 2012). They do so by growing and adding dark biomass to the ice, causing mineral fragments to aggregate and resist removal (flushing) by melt water, and by producing dark humic substances and pigments. There may even be a relationship between microbial activity and BC. Yet, it is not yet known whether microbes metabolise BC and reduce its impact, or cause it to “stick” to the ice surface and prevent its removal by flushing (Hodson, 2014).
Many microbes on the Greenland ice sheet inhabit ‘cryoconite’ holes; cylindrical tubes ‘drilled’ into the bright ice surface ice by dark cryoconite debris. The debris is a loose bonding of minerals encased in microbial biomass (e.g. Gribbon, 1979; Cook et al, 2010). Cryoconite holes on the Greenland ice sheet likely provide favourable conditions for photosynthesis by: 1.) maintaining light intensities that are high but not harmful because ultraviolet radiation is not transmitted by water; 2.) providing nutrients in melt water flowing in through the hole walls; and 3.) providing relatively long term (years) storage of microbes. This also promotes proliferation of bacteria and “grazers” that feed upon other microbes. These factors make cryoconite holes active and biodiverse ice sheet habitats (Hodson et al, 2008).
Despite being the most studied biological entity on the surface of the Greenland ice sheet, cryoconite holes remain poorly understood in terms of their biological community dynamics, thermodynamics, evolution and impact on albedo. Further, the characteristics of the holes themselves, and the microbes inhabiting them, have been shown to vary depending upon location on the ice sheet (Stibal et al, 2012), and these spatial patterns probably also evolve over time. Edwards et al (2014) recently found microbial communities to be extremely dynamic in response to environmental change, while Irvine-Fynn and Edwards (2014) showed that hydrological and glaciological processes might also influence microbial activity. Cryoconite holes will provide research foci for some members of the field team in summer 2014. For further information on cryoconite, see: here; here; and here.
by Joseph Cook and Jason Box
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