THEGREENGROK    Planetary Watch

Global Warming, Arctic Sea Ice, and Climate Feedbacks

by Bill Chameides | April 24th, 2012
posted by Erica Rowell (Editor)

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When it comes to Arctic sea ice, most of the attention has been on its extent — how much ocean does it cover. But a new study concludes that when it comes to climate, we should also be concerned with its age. (NASA)
When it comes to Arctic sea ice, most of the attention has been on its extent — how much ocean does it cover. But a new study concludes that when it comes to climate, we should also be concerned with its age. (NASA)

New study suggests that not all Arctic sea ice reflects the Sun’s energy the same.

The sea ice cover in the Arctic, as shown in the graph below, has been declining for decades.

There are lots of reasons to be concerned about this trend, not the least of which is the amplification of global warming through a positive feedback. (Since ice is a lot more reflective of sunlight than ocean water, melting sea ice allows more heat from the Sun to be absorbed instead of reflected, causing temperatures to rise more rapidly, thus melting more ice, and so forth, creating a positive feedback and lowering the Earth’s reflectivity, also known as its albedo.)

Mean sea ice anomalies, 1953-2011: Sea ice extent differences from monthly averages for the Northern Hemisphere. The more negative the number, the lower the extent of ice coverage. (Image courtesy of Walt Meier and Julienne Stroeve, National Snow and Ice Data Center, University of Colorado, Boulder)

But scientists have been discovering that disappearing sea ice isn’t the only thing going on in the Arctic. The whole character of the sea ice is changing. For one, it’s younger and thinner, and that means less reflective capacity.

Multiyear Ice vs. Seasonal Ice

Scientists put Arctic sea ice into two broad categories: seasonal and multiyear. While the age distinction is apparent in the terminology, there are other important differences.

Seasonal ice forms during the winter freeze and melts the following summer and reforms during the next winter. It tends to be flatter and more uniform than older ice.

This March, at the apparent maximum extent of the winter freeze, about 75 percent of the ice cover was new. How much of this will actually melt as seasonal ice? We won’t know until September.

Multiyear ice, which survives at least its first melt season (and is referred to more specifically as second year, third year, fourth year, or fifth+ year ice, depending on how long it sticks around), is thicker (has more volume) and tends to have more topography than seasonal ice.

In March, the oldest multiyear ice (fifth or more season) made up just two percent of the Arctic sea ice cover, dramatically down from about a quarter of the cover in the late 1980s.

Albedo Divergence

So in addition to the decline in total extent, we have also seen a shift in ice type — less multiyear ice and more seasonal ice. Should we care about there being more seasonal ice than multiyear ice? A new study published last week in the journal Geophysical Research Letters suggests that we should.

After taking measurements of the reflectivity (or albedo) of shorefast sea ice (i.e., ice that remains attached to the land) gathered over four years near Barrow, Alaska, authors Don Perovich, of Dartmouth College and the Army Corp’s Cold Regions Research and Engineering Laboratory (CRREL), and Christopher Polashenski of CRREL concluded that seasonal ice is significantly less reflective over the course of an annual cycle than multiyear ice.

Both multiyear and seasonal ice have albedos of about 0.85 during the winter. (Note: an albedo of 0.85 means that 85 percent of the Sun’s energy is reflected and 15 percent is absorbed. By comparison, open water has an albedo of about 0.07. More on albedo.)

But the albedos begin to diverge during the melt season, which for Barrow takes place between May and August. When melting begins, first the snow sitting on top of the sea ice melts, and then the ice itself (both seasonal and multiyear) begins melting, forming so-called “melt ponds” atop the ice.

Because seasonal ice is flatter, the melt ponds that form on it tend to cover more surface area than that of older sea ice whose greater topography limits the spread of the melt ponds. As a result, the albedo of seasonal ice falls faster and lower than multiyear ice. As the melt season continues, the ponds drain, increasing albedo for both types of sea ice, and then reform and evolve through the remainder of the season.

The low point for the albedo of seasonal ice, which by its definition continues to thin until it melts away completely to open water, is 0.07, the albedo of the open ocean. The albedo low for multiyear ice has been  estimated to be about 0.4.

When winter returns, new seasonal ice forms whose albedo will once again match that of the multiyear ice pack.

The net difference in albedo between the behavior of seasonal and multiyear ice is significant: Perovich and Polashenski estimate that over the course of the melt season, seasonal ice receives about 40 percent more heat from the sun than multiyear ice. That’s extra heat that not only melts the ice but more effectively warms the ocean, which further melts the ice….

Something to think about as we enter this year’s melt season.

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