Dark Snow: From Arctic to Himalayas

July 9, 2014

darksnow490

Guardian:

When American geologist Ulyana Horodyskyj set up a mini weather station at 5,800m on Mount Himlung, on the Nepal-Tibet border, she looked east towards Everest and was shocked. The world’s highest glacier, Khumbu, was turning visibly darker as particles of fine dust, blown by fierce winds, settled on the bright, fresh snow. “One-week-old snow was turning black and brown before my eyes,” she said.

The problem was even worse on the nearby Ngozumpa glacier, which snakes down from Cho Oyu – the world’s sixth highest mountain. There, Horodyskyj found that so much dust had been blown on to the surface that the ability of the ice to reflect sunlight, a process known as albedo, dropped 20% in a single month. The dust that was darkening the brilliant whiteness of the snow was heating up in the strong sun and melting the snow and ice, she said.

The phenomenon of “dark snow” is being recorded from the Himalayas to the Arctic as increasing amounts of dust from bare soil, soot from fires and ultra-fine particles of “black carbon” from industry and diesel engines are being whipped up and deposited sometimes thousands of miles away. The result, say scientists, is a significant dimming of the brightness of the world’s snow and icefields, leading to a longer melt season, which in turn creates feedback where more solar heat is absorbed and the melting accelerates.

In a paper in the journal Nature Geoscience, a team of French government meteorologists has reported that the Arctic ice cap, which is thought to have lost an average of 12.9bn tonnes of ice a year between 1992 and 2010 due to general warming, may be losing an extra 27bn tonnes a year just because of dust, potentially adding several centimetres of sea-level rise by 2100. Satellite measurements, say the authors, show that in the last 10 years the surface of Greenland’s ice sheet has considerably darkened during the melt season, which in some areas is now between six and 11 days longer per decade than it was 40 years ago. As glaciers retreat and the snow cover disappears earlier in the year, so larger areas of bare soil are uncovered, which increases the dust erosion, scientists suggest.

Research indicates that the Arctic’s albedo may be declining much faster than was estimated only a few years ago. Earlier this year a paper in Proceedings of the National Academy of Sciences reported that declining Arctic albedo between 1979 and 2011 constituted 25% of the heating effect from carbon dioxide over the same time.

According to Danish glaciologist Jason Box, who heads the Dark Snow project to measure the effect of dust and other darkening agents on Greenland’s ice sheet, Arctic ice sheet reflectivity has been at a near record low for much of 2014. Even a minor decrease in the brightness of the ice sheet can double the average yearly rate of ice loss, seen from 1992 to 2010.

“Low reflectivity heats the snow more than normal. A dark snow cover will thus melt earlier and more intensely. A positive feedback exists for snow in which, once melting begins, the surface gets yet darker due to increased water content,” says Box on his blog. Both human-created and natural air pollutants are darkening the ice, say other scientists.

Nearly invisible particles of “black carbon” resulting from incomplete combustion of fossil fuels from diesel engines are being swept thousands of miles from industrial centres in the US, Europe and south-east Asia, as is dust from Africa and the Middle East, where dust storms are becoming bigger as the land dries out, with increasingly long and deep droughts. Earlier this year dust from the Sahara was swept north for several thousands miles, smothered Britain and reached Norway.

According to Kaitlin Keegan, a researcher at Dartmouth College in the US state of New Hampshire, the record melting in 2012 of Greenland’s northeastern ice-sheet was largely a result of forest fires in Siberia and the US.

Evolution of impurity index. Map of impurity index iimp anomaly averaged over the May–June period. The anomaly is calculated with respect to the 2003–2013 average. The grey areas correspond to pixels outside the ice sheet or with insufficient high-quality data. Only data from 16 May to 26 June were used, to eliminate artefacts due to low solar elevation

Evolution of impurity index. Map of impurity index iimp anomaly averaged over the May–June period. The anomaly is calculated with respect to the 2003–2013 average. The grey areas correspond to pixels outside the ice sheet or with insufficient high-quality data. Only data from 16 May to 26 June were used, to eliminate artefacts due to low solar elevation Source: Nature Geoscience Dumont et al 2014

Nature Geoscience:

The surface energy balance and mass balance of the Greenland ice sheet depends on the albedo of snow, which governs the amount of solar energy that is absorbed. The observed decline of Greenland’s albedo over the past decade123 has been attributed to an enhanced growth of snow grains as a result of atmospheric warming12. Satellite observations show that, since 2009, albedo values even in springtime at high elevations have been lower than the 2003–2008 average. Here we show, using a numerical snow model, that the decrease in albedo cannot be attributed solely to grain growth enhancement. Instead, our analysis of remote sensing data indicates that the springtime darkening since 2009 stems from a widespread increase in the amount of light-absorbing impurities in snow, as well as in the atmosphere. We suggest that the transport of dust from snow-free areas in the Arctic that are experiencing earlier melting of seasonal snow cover4 as the climate warms may be a contributing source of impurities. In our snow model simulations, a decrease in the albedo of fresh snow by 0.01 leads to a surface mass loss of 27 Gt yr−1, which could induce an acceleration of Greenland’s mass loss twice as large as over the past two decades5. Future trends in light-absorbing impurities should therefore be considered in projections of Greenland mass loss.

 

4 Responses to “Dark Snow: From Arctic to Himalayas”

  1. indy222 Says:

    Peter (or Jason),
    Were albedo effects from dust/ash at all incorporated into the AR4 or AR5 climate models, even as just an approximate guess? Or was it assumed zero albedo change as we evolve. We very much need to see the AR5 model predictions modified with these many feedbacks which were not considered, to get a more realistic idea of what we’re facing. Granted the conservative approach is frightening enough – it appears that glacial moulins, softening ice at the glacier base, the positive feedback of more bare snow leading to more dusty surface snow and melt, thawing permafrost, loss of low clouds (Sherwood et al last year)…. correct me if I’m wrong but I don’t believe any of these were considered in the AR5 models used in the published graphs and predictions which planners are using in figuring costs and mitigation ideas. Even a rough guess from observational data, as Rahmstorff etal did for the glacial speed rate is at least not biased LOW by assuming zero. One the one hand you have the Guy McPherson’s extrapolating feedbacks exponentially to human extinction by mid century, which seems absurd. But those sort of projections get some currency among some just because of the non-incorporation of known positive feedbacks. I teach climate change and would love some source that is more realistic (if uncertain) than the IPCC’s.

    • greenman3610 Says:

      will run that by Jason. am guessing not since the effects have not been well constrained as of yet, there
      have been several significant pubs since AR5

  2. indy222 Says:

    Well here’s more on the grim albedo effect as a positive feedback, from Flanner et al. (2014) http://www.nature.com/ngeo/journal/v4/n3/full/ngeo1062.html

    Note the final line in the abstract:

    “On the basis of these observations, we conclude that the albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m−2 K−1, substantially larger than comparable estimates obtained from 18 climate models.”


  3. […] Guardian: When American geologist Ulyana Horodyskyj set up a mini weather station at 5,800m on Mount Himlung, on the Nepal-Tibet border, she looked east towards Everest and was shocked. The world's…  […]


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