Soot and re-frozen ice makes snowpack less reflective, in feedback loop that could accelerate sea level rise
By Tim Radford
Greenland is getting darker. Climatology’s great white hope, the biggest block of ice in the northern hemisphere, is losing its reflectivity.
According to new research, the island’s dusty snows are absorbing ever more solar radiation, which is likely to accelerate the rate at which the icecap melts.
The Greenland icecap covers 1.7 million square kilometres and contains enough ice to raise sea levels by seven metres. Right now, the rate of melting is on the increase, and meltwater flowing off the icecap could be raising sea levels by 0.6mm a year.
A powerful contributing factor, scientists report in The Cryosphere journal, could be that the ice has darkened over the last two decades. By 2100, the albedo – the climatologists’ term for the reflectivity of rock, sand, water or ice – could have fallen by 10%.
Soot blown in from wildfires further south – already fingered as one of the suspects by previous studies – may be part of the problem, but the researchers have a more complex agency in mind: the feedback loop.
In the summer, the surface ice starts to melt. As the top layers trickle away, old impurities are exposed, darkening the surface and making it more sun-receptive. As the snow freezes again, the grains of snow get bigger – as water becomes ice, it makes a glue for the snow grains – and the bigger grains make a less reflective surface.
Greenland still looks icy and snowy, and enough optical light is reflected to make snow-blindness a danger. But in the infra-red region of the spectrum − where the global warming happens in the thickening soup of greenhouse gases − it’s a different story.
“You don’t necessarily have to have a ‘dirtier’ snowpack to make it dark,” says the study’s leader, Marco Tedesco, founder of the Cryospheric Processes Laboratory, which is now based at Columbia University’s Lamont-Doherty Earth Observatory.
“A snowpack that might look ‘clean’ to our eyes can be more effective in absorbing solar radiation than a dirty one. Overall, what matters is the total amount of solar energy that the surface absorbs. This is the real driver of melting.”
“As warming continues, the feedback from declining albedo will add up. It’s a train running downhill, and the hill is getting steeper”
The latest study is not likely to settle the question of the future of the Greenland ice cap, if only because repeated studies keep delivering different conclusions.
Overall, climate scientists are increasingly sure that global climate change, as a consequence of global warming driven by ever-higher carbon dioxide levels in the atmosphere – itself a consequence of ever greater combustion of fossil fuels – is to blame. But big questions remain.
How much of the surface meltwater makes it to the ocean? What is going on where the ice meets the bedrock? And why have the glaciers started to accelerate on their journey towards the sea? What is the role of the warming ocean? Do clouds play a part?
Nevertheless, the darkening of the snows remains a potentially powerful contributing factor.
The Cryosphere science team used satellite information to compare summertime changes in Greenland’s albedo from 1981 to 2012. The darkening started around 1996 and the ice began absorbing 2% more radiation per decade.
On the other hand, the Global Fire Emissions Database revealed no statistically significant increase in soot released by forest fires in the northern hemisphere that could account for such darkening during that period, so questions remain.
But, over the same period, summer near-surface temperatures in Greenland increased by 0.74°C per decade to help accelerate the feedback. Computer models were used to settle the questions of grain size and albedo. Over the entire ice sheet, average albedo will fall by 8% over the rest of this century, and by as much as 10% on the western edge.
Professor Tedesco thinks these are conservative estimates. Global warming will mean more precipitation. As well as the winter snow, there will be more summer rain, which would also speed up melting. As average global temperatures creep up, higher altitudes are more likely to warm and melt.
“As warming continues, the feedback from declining albedo will add up,” he says. “It’s a train running downhill, and the hill is getting steeper.”
This article was produced by Climate News Network