text: Greenland is one of the brightest spots on planet Earth, but ominously enough, its gleaming surface darkens with each passing year, thanks to a strange series of physical processes, one of which cannot be seen with the naked eye. 
   
   
   
   According to a fascinating new study led by Columbia’s Earth Institute and published today the journal The Cryosphere, which finds that Greenland’s “albedo,” or reflectivity, has been decreasing since the mid 90s. By the end of the 21st century, Greenland could be ten percent darker than it is today. And that’s bad news for the ice sheet’s long term prospects, because,as anyone who has ever made the mistake of running in a black t-shirt on a hot summer day knows, darker surfaces absorb more heat. 
   
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   One would think that a massive chunk of land changing color would have attracted more attention. But the processes behind The Darkening are subtle, and in some cases, impossible for humans to see without the aid of technology. 
   
   The first culprit is tiny impurities—dust and soot grains—buried just beneath the surface of the ice sheet. As fresh snow melts in the summertime, deeper and dirtier layers of ice are exposed, rendering the surface a bit darker. As the researchers explain in their paper, especially warm summers can cause several years worth of impurities to become concentrated at the ice sheet’s surface. 
   
   The second reason has to do with a change in the reflective properties of the ice itself. As snowpack melts and refreezes, individual ice grains tend to get larger. Larger grains have less surface area and scatter less sunlight, therefore absorbing more solar energy. This effect, the authors say, is strongest in the infrared range, which is invisible to human eyes but can be seen with satellites. 
   
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   But if you thought humans were off the hook on this one, not so fast. The last few months marked one of the most freakishly hot Arctic winters in recent memory—and that’s a direct symptom of human-caused climate change. With more hot days in Greenland’s future, we can expect the ice sheet to darken more quickly. 
   
   “Rising temperatures are promoting more melting, and that melting is reducing albedo, which in turn is increasing melting,” lead study author Marco Tedesco said in a statement. “As warming continues, the feedback from declining albedo will add up.” 
   
   Contact the author at maddie.stone@gizmodo.com. 
   
   ||||| Research article 03 Mar 2016 
   
   Research article | 03 Mar 2016 
   
   Abstract. The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02decade−1 between 1996 and 2012. Over the same period, albedo modelled by the Modèle Atmosphérique Régionale (MAR) also shows a decrease, though at a lower rate ( ∼ −0.01decade−1) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo is confined to the regions of the GrIS that undergo melting in summer, with the dry-snow zone showing no trend. The period 1981–1996 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening. ||||| Greenland's Ice Is Getting Darker, Increasing Risk of Melting 
   
   Feedback loops from melting itself are driving changes in reflectivity 
   
   Greenland's snowy surface has been getting darker over the past two decades, absorbing more heat from the sun and increasing snow melt, a new study of satellite data shows. That trend is likely to continue, with the surface's reflectivity, or albedo, decreasing by as much as 10 percent by the end of the century, the study says. 
   
   While soot blowing in from wildfires contributes to the problem, it hasn't been driving the change, the study finds. The real culprits are two feedback loops created by the melting itself. One of those processes isn't visible to the human eye, but it is having a profound effect. 
   
   The results, published in the European Geosciences Union journal The Cryosphere, have global implications. Fresh meltwater pouring into the ocean from Greenland raises sea level and could affect ocean ecology and circulation. 
   
   "You don't necessarily have to have a 'dirtier' snowpack to make it dark," said lead author Marco Tedesco, a research professor at Columbia University's Lamont-Doherty Earth Observatory and adjunct scientist at NASA Goddard Institute of Space Studies. "A snowpack that might look 'clean' to our eyes can be more effective in absorbing solar radiation than a dirty one. Overall, what matters, it is the total amount of solar energy that the surface absorbs. This is the real driver of melting." 
   
   The feedback loops work like this: During a warm summer with clear skies and lots of solar radiation pouring in, the surface starts to melt. As the top layers of fresh snow disappear, old impurities, like dust from erosion or soot that blew in years before, begin to appear, darkening the surface. A warm summer can remove enough snow to allow several years of impurities to concentrate at the surface as surrounding snow layers disappear. At the same time, as the snow melts and refreezes, the grains of snow get larger. This is because the meltwater acts like glue, sticking grains together when the surface refreezes. The larger grains create a less reflective surface that allows more solar radiation to be absorbed. The impact of grain size on albedo – the ratio between reflected and incoming solar radiation – is strong in the infrared range, where humans can't see, but satellite instruments can detect the change. 
   
   Aerial images of Greenland show rivers of meltwater and areas of dark ice. Greenland's surface is absorbing more solar radiation as melting increases grain size and brings old impurities to the surface. (Marco Tedesco/Lamont-Doherty Earth Observatory) 
   
   "It's a complex system of interaction between the atmosphere and the ice sheet surface. Rising temperatures are promoting more melting, and that melting is reducing albedo, which in turn is increasing melting," Tedesco said. "How this accumulates over decades is going to be important, because it can accelerate the amount of water Greenland loses. Even if we don't have a lot of melting because of atmospheric conditions one year, the surface is more sensitive to any kind of input the sun can give it, because of the previous cycle." 
   
   The study used satellite data to compare summertime changes in Greenland's albedo from 1981 to 2012. The first decade showed little change, but starting around 1996, the data show that due to darkening, the ice began absorbing about 2 percent more solar radiation per decade. At the same time, summer near-surface temperatures in Greenland increased at a rate of about 0.74°C per decade, allowing more snow to melt and fuel the feedback loops. 
   
   A likely cause for the large shift around 1996 was a change in atmospheric circulation, Tedesco said. The North Atlantic Oscillation, a large-scale natural weather cycle, went into a phase in which summer atmospheric conditions favored more incoming solar radiation and warmer, moist air from the south. Later records show those conditions shifted in 2013-2014 to favor less melting, but the damage was already done – the ice sheet had become more sensitive. In 2015, melting spiked again to reach more than half of the Greenland ice sheet. 
   
   While new snowfall can make the ice sheet brighter again, the dark material built up during the melt years is waiting just below the surface, preconditioning the surface to future melting, Tedesco said. 
   
   The scientists also ran a computer model to simulate the future of Greenland's surface temperature, grain size, exposed ice area and albedo. Over the current century, the model projects that the average albedo for the entire ice sheet will fall by as much as 8 percent, and by as much 10 percent on the western edge, where the ice is darkest today. Those are conservative estimates – the change could be twice that, Tedesco said. 
   
   The scientists looked into the hypothesis that soot from forest fires in China, Siberia and North America could be driving the increased darkening of the ice sheet. Using the Global Fire Emissions Database, they analyzed trends in black-carbon emissions from wildfires in those regions and Europe. While the amount of black carbon released by fires varied year to year, the scientists found no statistically significant increase during 1997-2012 to match the darkening of Greenland. 
   
   The study also raises questions about whether Greenland's high plateau is darkening as previous reports have suggested. The scientists found no long-term trend of darkening at the top, and they suspect that the Terra MODIS satellite sensor that has detected darkening in the past may actually be degrading, as previous studies have suggested. At lower elevations, the signal is much stronger. 
   
   "It is a very good paper which provides valuable new insights about the physical processes controlling the change in reflectivity of the Greenland ice sheet and specifically its darkening over time," said Eric Rignot, a senior research scientist at NASA's Jet Propulsion Laboratory who studies ice sheets but was not involved with the new study. "I also find it particularly interesting that the darkening indicated earlier by satellite sensors is now confirmed to be less, which is good news for the ice sheet. Yet the darkening of Greenland around its periphery remains a source of concern because it contributes to making the ice sheet melt away faster." 
   
   The feedback loops could be stopped with lots of snowfall and less melting, but that doesn't seem likely given the increasing concentration of greenhouse gases in the atmosphere, Tedesco said. And while warming is expected to increase precipitation, that precipitation includes increasing rainfall, which speeds up melting. Melting is also moving to higher elevations as global temperatures warm. 
   
   "As warming continues, the feedback from declining albedo will add up," Tedesco said. "It's a train running downhill, and the hill is getting steeper." 
   
   Co-authors of the paper are Sarah Doherty of the University of Washington; Xavier Fettweis of University of Liege; Patrick Alexander of NASA Goddard Institute of Space Studies and City University of New York; Jeyavinoth Jeyaratnam of the City College of New York; and Julienne Stroeve of University of Boulder. ||||| In a study that used satellite images stretching back decades, scientists have delved into the processes causing 'darkening' of the Greenland ice sheet – its diminishing ability to reflect the sun's radiation. 
   
   An aerial image of Greenland shows rivers of meltwater and areas of dark ice. Greenland's surface is absorbing more solar radiation as melting increases grain size and brings old impurities to the surface. 
   
   Greenland’s ice sheet has been darkening for the past two decades, and new research delves into the processes behind this phenomenon. 
   
   The study, published Thursday in The Cryosphere, a journal of the European Geosciences Union, considers the processes by which an ice sheet loses its ability to reflect sunlight – its albedo – terming this diminishing capability as “darkening.” 
   
   Researchers found positive feedback loops, whereby the more an ice sheet melts, the quicker that melting process becomes, which could have significant implications for projections of the ice sheet’s future contributions to sea level rise. 
   
   “We knew that these processes had been happening,” says lead author Marco Tedesco, a professor at Columbia University's Lamont-Doherty Earth Observatory and adjunct scientist at NASA Goddard Institute of Space Studies, in a telephone interview with The Christian Science Monitor. “What’s new is the acceleration of the darkening, which started in 1996.” 
   
   The research looked at two factors that affect the rate of melting: impurities in the ice, and the size of grains in the snowpack. 
   
   The first of these is often visible to the human eye, with higher concentrations of impurities literally darkening the appearance of the snow. 
   
   And, as darker substances tend to absorb more energy from the sun, more impurities will mean the ice melts more quickly. 
   
   “Then there are things that happen that are invisible to our eyes, which make the snow ‘darker’ – not dirtier, but more absorbent of energy from the sun,” Dr. Tedesco tells the Monitor. “The grains get bigger, for example, which decreases the albedo, but we don’t see this effect because it takes place on the infra-red.” 
   
   This second process, whereby the grains in the snowpack increase in size, happens as a consequence of melting and refreezing, the meltwater acting as a glue, binding grains together. 
   
   The more it happens, the bigger the grains become, and the less reflective is the surface of the ice sheet, which in turn leads to faster melting – one aspect of the positive feedback loop. 
   
   “In a snowpack with larger grains, it might look much cleaner to our eyes, but that clean snow pack is much 'darker' than a dirty snow pack,” Tedesco explains. 
   
   With regard to the impurities, more melting also results in faster melting: as the ice sheet melts, water runs off, but impurities will often stay. And, as the top layers melt, impurities lower down become exposed, gradually increasing their concentration at the surface. 
   
   “So, you have a combined effect of increased dirt due to melting and increased grain size, also due to melting,” says Tedesco. “It is a positive feedback process that amplifies the melting effect.” 
   
   And although the effects of increasing grain size are invisible to the human eye, the same is not true for satellite instruments. 
   
   So it was that Tedesco and his team were able to analyze satellite data stretching back decades, beginning in 1981, to draw conclusions about the changes in the albedo of the Greenland ice sheet. 
   
   They found little change in the first decade, but starting in 1996, the data indicated that, due to “darkening,” the ice sheet became about 2 percent more absorbent of solar radiation each decade. 
   
   Taking this fresh insight, the scientists ran some computer simulations, which produced a predicted drop in albedo of the Greenland ice sheet of up to eight percent over the course of the current century. 
   
   “It’s the first time we’ve really tried to explain the processes behind darkening, and it’s important because a darker ice sheet can melt in a much faster way,” says Tedesco. “In turn, this understanding is significant because if you miss it, it may have big implications on the accuracy of future projections.”
target: – The white, reflective surface of Greenland's snowpack is getting darker and less reflective, all thanks to what the Christian Science Monitor calls "positive feedback loops"—the idea that a little bit of melting leads to more and faster melting. "We knew that these processes had been happening," says Columbia professor Marco Tedesco, lead author of a new study in the journal Cryosphere. "What’s new is the acceleration of the darkening, which started in 1996." In fact, parts of Greenland may actually be 10% darker—technically it is losing its "albedo," or reflectivity—by the end of the century, researchers predict. "It's a train running downhill, and the hill is getting steeper," Tedesco says. So what happened in 1996? One likely culprit is the natural change in atmospheric circulation called the North Atlantic Oscillation, in which summer atmospheric conditions "favored more incoming solar radiation and warmer, moist air from the south," Tedesco writes in Columbia's Earth Institute. This melting brought impurities such as soot from previous melts to the surface, making it darker and, in turn, prone to melting even faster. And because individual ice grains tend to get larger when snow melts and refreezes, less light scatters across those larger surfaces and more is absorbed, reports Gizmodo. This effect appears strongest in the infrared range the human eye cannot see but was found in satellite images. When the atmospheric conditions naturally shifted back in 2013, the damage had already been done, with the ice sheet more vulnerable to melting. (Check out how old the soil is under some parts of Greenland's ice pack.)