Satellites have been collecting data on the Greenland ice sheet since 1981. But since the mid-'90s, scientists have noticed the sheet has become progressively darker.
The satellites have been measuring the “albedo” of the Greenland ice sheet, essentially how much sunlight is reflected back from the surface towards space. The process is not viewable with the human eye, but the level of albedo is detectable through satellite instruments. Researchers from Columbia’s Earth Institute have collected and analyzed the data in a study recently published in the journal The Cryosphere.
Their findings state that if the current trend of darkening continues, parts of the sheet will be 10 percent less reflective by the end of the 21st century than they are today.
So what exactly is going on?
The study said soot from wildfires in China, Siberia, and North America getting into the ice is no doubt contributing. However, using the Global Fire Emissions Database, they found there was no increase in the number of wildfires from the mid-nineties, suggesting another culprit was at play.
They found that the real driver for the change was rising temperatures. From 1996 onwards, the ice began absorbing about 2 percent more solar radiation per decade. This is in line with the findings that the near-surface temperatures in Greenland have been increasing by 0.74°C per decade.
The snowpack of the sheet undergoes a constant process of thawing and refreezing throughout the seasons. However, the warmer and brighter summers are producing a bigger thaw in the ice. With each of these refreezings, the grains of ice get larger and larger. The larger the grains get, the less reflective a surface they have, hence they become darker.
The real issue with this darkening is the vicious circle it creates. As you’ll know if you touch the bonnet of a black car on a hot day, darker colors are less reflective so absorb more light and, importantly, heat. The darker the ice gets, the more vulnerable it is to melting, so it gets darker, and the cycle continues.
"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," lead author Professor Marco Tedesco, from the Columbia University's Lamont-Doherty Earth Observatory and NASA, said in a statement.
"As warming continues, the feedback from declining albedo will add up. It's a train running downhill, and the hill is getting steeper," Tedesco added.