Experimental evidence that microbial activity lowers the albedo of glaciers

Abstract

doi: 10.7185/geochemlet.1611Darkening of glacier and ice sheet surfaces is an important positive feedback to increasing global temperatures. Deposition of impurities on glaciers is primarily believed to reduce surface albedo, resulting in greater melt and mass loss. However, no study has yet included the effects of biological activity in albedo reduction models. Here, we provide the first experimental evidence that microbial activity can significantly decrease glacier surface albedo. Indeed, the addition of nutrients at ice meltwater concentrations to microbe-impurity mixtures resulted in extensive microbial organic carbon fixation and accumulation in Greenland Ice Sheet surface debris. Accumulated organic carbon, over the period of a melt season, darkened the glacial debris in our experiments from 31.1 % to 15.6 % surface reflectivity (used as an analogue for albedo in our calculations), generating a strongly absorbing surface. Our experiments are the first to quantify the microbially-induced potential melt increase for the Greenland Ice Sheet (up to an average of 17.3 ± 2.5 Gt yr -1 at present and up to ~85 Gt yr -1 by 2100, based on our first order calculations). Mass loss from glaciers will conceivably intensify through enhanced microbial activity, resulting from longer melt seasons and fertilisation from anthropogenic sources.

The role of microbes in snowmelt and radiative forcing on an Alaskan icefield

“…The generation of meltwater through microbes' albedoreducing properties motivates an hypothesis of bio-geophysical feedback on glacial landscapes 13,16 , such as the Greenland ice sheet. This feedback hypothesis, whereby microbes increase because they produce needed meltwater, is an active research area [13][14][15][16][17][18][19] , yet field experiments testing its assumptions are absent.Glacier microbiomes are water-limited 21,22 , because ice is generally not metabolically available, and oligotrophic, because their nutrient content equals that of precipitation plus deposition by airborne dust, pollen, and so on, with only limited N-fixation by local cyanobacteria 21,22 . Moreover, rapidly percolating water through large-grained snow may exacerbate both water-and nutrient limitation for algae in supraglacial snow-covered habitat 7 .…”

“…Fresh snow reflects >90% of visible radiation, but during melt its grain size and water content increase, reducing albedo and further increasing snowmelt 1 . Impurities, including black carbon 3 , dust 4 , and resident microbes [7][8][9][10][11][12][13][14][15][16][17][18][19] , also lower albedo; however, microbes differ from non-living particulates in several critical ways. Perennial populations of photosynthetic microbes actively resurface following overwinter burial by snow 20 , and depend on liquid water and nutrients for survival and reproduction 13,14,[20][21][22] .…”

“…The generation of meltwater through microbes' albedoreducing properties motivates an hypothesis of bio-geophysical feedback on glacial landscapes 13,16 , such as the Greenland ice sheet. This feedback hypothesis, whereby microbes increase because they produce needed meltwater, is an active research area [13][14][15][16][17][18][19] , yet field experiments testing its assumptions are absent.…”

Inorganic carbon addition stimulates snow algae primary productivity

“…In Sierra Nevada snowfields, snow algae abundance was negatively correlated to surface albedo [15] and a recent study quantified the role of snow algae communities in snowmelt on an icefield in Alaska [16]. Similarly, in the Arctic, red algal blooms darken the snow/ice surface, lowering surface albedo (by as much as 13% over the melt season) [17] and increasing melt rates [18,19].…”

“…In Sierra Nevada snowfields, snow algae abundance was negatively correlated to surface albedo [15] and a recent study quantified the role of snow algae communities in snowmelt on an icefield in Alaska [16]. Similarly, in the Arctic, red algal blooms darken the snow/ice surface, lowering surface albedo (by as much as 13% over the melt season) [17] and increasing melt rates [18,19].Allochthonous material delivered to snow and ice surfaces such as forest fire-derived black carbon, Saharan or pro-glacial mineral dust, volcanic ash, and anthropogenic pollution causes increased absorption of solar radiation and locally accelerated melting. These effects can be far reaching-a darkening of the Greenland ice sheet has been * Trinity L. Hamilton…”