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Clouds Helped Enhance Greenland’s Record Melting

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When scientists saw melting across a whopping 97 percent of Greenland’s icy surface last summer, they were quick to note that such an event is rare, but not unprecedented. The last time it happened was in 1889, so while manmade global warming is clearly involved it isn’t necessarily the entire story.

A new new report in Nature on Wednesday has now helped flesh out the explanation: data from Summit Station, at the frozen island’s highest point, 10,551 feet above sea level, show that unusually warm temperatures in the region were enhanced by a blanket of low-level clouds that trapped extra heat from the Sun.

Extent of surface melt over Greenland's ice sheet July 8, 2012 (left) and July 12, 2012 (right), melting shown in pink. 
Click image to enlarge. Credit: NASA.

But while the events that led to the melt may have been unusual, said lead author Ralf Bennartz, an atmospheric scientist at the University of Wisconsin, in an interview, “I tend to believe we will see more of them toward the end of century.”

If so, the consequences could be dire: combined with faster-flowing glaciers dumping more ice into the sea, episodes of surface melting could accelerate the disintegration of Greenland’s 680,000 cubic miles of ice. If all of that ice entered the ocean, it would raise sea level by some 20 feet, inundating the world’s coastal regions, displacing hundreds of millions of people and destroying trillions of dollars worth of property.

Even with enhanced melting, that’s unlikely to happen for several hundred years — but climate scientists already expect ice and meltwater to drive sea level up by some 3 feet by 2100, enough to do enormous damage.

Bennartz and his colleagues can’t say at this point whether last summer’s cloudiness portends a trend. “The Arctic tends to be a cloudy place,” said Julienne Strove, of the National Center for Atmospheric Research, an expert on Arctic ice, in an interview, “and last summer was cloudier in general as you had several storms enter the Arctic.” In theory, clouds could be on the increase, as warmer temperatures force more water to evaporate from the oceans.

How Do We Know: Greenland's Melting Ice Sheet

Whether that adds to global warming, however, depends on what sorts of clouds they are: high, wispy cirrus clouds, made of ice crystals, would enhance the greenhouse effect by trapping heat from the Sun. Thick cumulus clouds, made of water droplets, would shield the planet from some of the Sun’s heat, keeping temperatures in check. The clouds that covered Greenland last July were intermediate: they were made of water droplets, but were thin enough to let some sunlight through.

“If those clouds had not been around there would not have been a melt event,” Bennartz said. “If they had been thicker, there also would not have been a melt event like the one we saw. What we do not know is what role clouds are going to play in the future.”

The instruments he and his colleagues used to study clouds have only been in place for three years, which is too short a time to detect any sort of trend. Warming temperatures alone, however, would likely increase melting over the rest of the century even without significant increases in cloud cover.

The new study raises the question of whether clouds played a role, not only in the melting of Greenland, but also in last summer’s record melting of sea ice in the Arctic Ocean. The latter event didn’t contribute to rising seas, because the ice that melted was already floating in the ocean. But it did expose a huge expanse of water to the Sun’s heat, which in turn warms the surrounding air — and ultimately the entire planet — in a self-reinforcing cycle known as “Arctic Amplification.”

Indeed, sea ice has been on a downward spiral ever since satellites first started to make observations in the 1970s, to the point where commercial shipping across the Arctic Ocean could soon be a real possibility.

Since nobody made a careful study of clouds over the sea ice, it would be hard to say for sure that they made the melting worse. In general, however, “last summer was cloudier than usual. So that certainly could have played a role,” Stroeve said.

Related Content
The Story Behind Record Ice Loss in Greenland
Greenland Melt Sets Record Weeks Before Summer Ends
Greenland Ice Melt Reaches Unprecedented Level
It's Official: Arctic Sea Ice Shatters Record Low
Greenland Ice Sheet Melt Nearing Critical 'Tipping Point'
Clouds & Climate: A Key Mystery for Researchers
Global Warming Has Pushed Arctic into 'New Normal'
Study Shows A Future In Trans-Arctic Shipping

Comments

By Dave (Basking Ridge, NJ 07920)
on April 3rd, 2013

There is always natural variability as well as GW at work. I think that always has to be kept in mind when attempting to attribute causes to isolated specific events versus trends. Certainly the well established strong and rapid warming trend in the Arctic is widely considered with high confidence to be entirely due to AGW. Against that overarching widely held conclusion, the underlying inference of this particular report is one of heavily implied attribution in so far as AGW “may” have acted in concert with a thermal push due to an unusual preponderance of warming types of clouds to cause the surface melt. Indeed it “may” have happened just like that. But at the moment, it is my understanding that that is not entirely obvious.

See “Variations in melt-layer frequency in the GISP2 ice core: Implications for Holocene summer temperatures in central Greenland”, Alley, R.B. and S. Anandakrishnan, Annals of Glaciology, Vol. 21, p. 64-70, 1995.

According to this, ice core data indicate that past surface melts (Greenland Ice Sheet), apparently similar to this recent event, have happened with a frequency of once every 80 to 250 years going back not just to 1889 but for at least 7000 years - and obviously therefore also before AGW.  In that context one could reasonably argue that this recent melt looks fairly normal. In other words, in view of that record, this recent melt could just as easily be part of that very long term melt pattern, suggesting the clear possibility of causes rooted in an unidentified quasi periodic natural event. Clearly AGW lowers the threshold for ice to melt. But in view of this it may not have been the main trigger or root cause for this particular event.

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By Patrick AUCOIN (MISSION BC Canada)
on April 4th, 2013

There has been a recent outcry in light of data released recently citing no actual Global temperature rise for the last 15 years. There is a great deal of conflicting information and it is fueling the assumption that CO2 emission reductions are a waste of time and is actually damaging to humanity! This is being propagated in many articles which question the CO2 connection to Global Warming. Every wrinkle is treated as a blunder for the science, who can argue that changing our lifestyles is painless and we should do it on questionable science. I appreciate all the great work being done and hope we can somehow turn this around so that we can all roll up our sleeves and work together to accomplish what needs to be done to ensure a safe future for our children.

I have been following this issue for a long time but am in no way near qualified to speak authoritively on this but I have a question I have not seen addressed. Would not the latent heat absorption of the massive ice melt we have been experiencing have a cooling effect on temperature rise?

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By Eric Peterson (Front Royal, VA 22630)
on April 4th, 2013

Patrick, here’s a website with that calculation http://psc.apl.washington.edu/wordpress/research/projects/arctic-sea-ice-volume-anomaly/  (scroll to the bottom).  Basically 5 x 10^21 Joules.  They compare that to US energy use which is apples and oranges.  For a more apropos comparison see http://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)  Basically the extra heat required to melt the ice each season is 1/3 of the daily solar input, so not much energy on a global scale and does not impact global average temperature.

But it is a lot of energy for the Arctic region.  Consequently the Arctic runs a bit more “normal” (closer to the old long term average) in the spring and runs a lot warmer in the fall as the ice refreezes and the refreezing releases heat.

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