Tarns speed melting of ice cap
Young sea ice is more susceptible to melting from within. Photo courtesy Stefan Hendricks, Alfred Wegener Institute.
FRISCO — German scientists say they’ve discovered another positive global warming feedback which could cause Arctic sea ice to melt faster than anticipated. During recent research expeditions in the Arctic they’re observed a large number of melt ponds on the surface, covering about half of the one-year ice.
“The ice cover of the Arctic Ocean has been undergoing fundamental change for some years. Thick, multi-year ice is virtually nowhere to be found any more,” said Marcel Nicolaus, of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research.
“Instead, more than 50 percent of the ice cover now consists of thin one-year ice on which the melt water is particularly widespread. The decisive aspect here is the smoother surface of this young ice, permitting the melt water to spread over large areas and form a network of many individual melt ponds,” Nicolaus said.
By contrast, the older ice has a rougher surface which has been formed over the years by the constant motion of the floe and innumerable collisions. Far fewer and smaller ponds formed on this uneven surface which were, however, considerably deeper than the flat ponds on the younger ice.
As part of their research, sea ice physicists at the institute have now measured the light transmission through the Arctic sea ice for the first time on a large scale, enabling them to quantify consequences of this change. In areas where melt water collects on the ice, far more sunlight and therefore energy is able to penetrate the ice than is the case for white ice without ponds. That means the ice is absorbing more solar heat, is melting faster, and more light is available for the ecosystems in and below the ice. The findings have been published inGeophysical Research Letters.
“We knew that an ice floe with a thick and fresh layer of snow reflects between 85 and 90 per cent of sunlight and permits only little light through to the ocean. In contrast, we could assume that in summer, when the snow on the ice has melted and the sea ice is covered with melt ponds, considerably more light penetrates through the ice,” he said.
To find out the extent to which Arctic sea ice permits the penetration of the sun’s rays and how large the influence of the melt ponds is on this permeability, the researchers equipped a remotely operated underwater vehicle with radiation sensors and cameras. In the summer of 2011 during an Arctic expedition of the research ice breaker POLARSTERN, they sent this robot to several stations directly under the ice. During its underwater deployments, the device recorded how much solar energy penetrated the ice at a total of 6000 individual points all with different ice properties.
“The young thin ice with the many melt ponds does not just permit three times as much light to pass through than older ice. It also absorbs 50 per cent more solar radiation. This conversely means that this thin ice covered by melt ponds reflects considerably fewer sun rays than the thick ice. Its reflection rate is just 37 percent. The young ice also absorbs more solar energy, which causes more melt. The ice melts from inside out to a certain extent,” Nicolaus said.
“We assume that in future climate change will permit more sunlight to reach the Arctic Ocean … particularly that part of the ocean which is still covered by sea ice in summer … The greater the share of one-year ice in the sea ice cover, the more melt ponds will form and the larger they will be.
“This will also lead to a decreasing surface albedo and transmission into the ice and ocean will increase,” he said. “The sea ice will become more porous, more sunlight will penetrate the ice floes, and more heat will be absorbed by the ice. This is a development which will further accelerate the melting of the entire sea ice area.”
However, at the same time the organisms in and beneath the ice will have more light available to them in future. Whether and how they will cope with the new brightness is currently being investigated in cooperation with biologists.
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