The disappearing Arctic ice
On the heals of yesterday's post about the disappearing Greenland ice cap, there's this news article about the disappearing Arctic ice:
Arctic Ice Continues Record MeltingAn area of Arctic sea ice the size of Florida has melted away in just the last six days as melting at the top of the planet continues at a record rate.
2007 has already broken the record for the lowest amount of sea ice ever recorded, say scientists, smashing the old record set in 2005.
Currently, there are about 1.63 million square miles of Arctic ice, according to the National Snow and Ice Data Center in Boulder, Colo. That is well below the record of 2.05 million square miles set two summers ago and could drop lower before the final numbers are in.
In just the last six days, researchers say 69,000 square miles of Arctic ice has disappeared, roughly the size of the Sunshine State.
Scientists say the rate of melting in 2007 has been unprecedented, and veteran ice researchers worry the Arctic is on track to be completely ice-free much earlier than previous research and climate models have suggested.
"If you had asked me a few years ago about how fast the Arctic would be ice free in summer, I would have said somewhere between about 2070 and the turn of the century," said scientist Mark Serreze, polar ice expert at the NSIDC. "My view has changed. I think that an ice-free Arctic as early as 2030 is not unreasonable."
Unlike the melting of Greenlandic and Antarctic ice and alpine glaciers, the melting of Arctic ice does not result in sea level rise. Greenlandic, Antarctic and alpine ice is on land, and the melted water eventually runs into the oceans, raising sea levels, while Arctic ice is frozen ocean water to begin with-- that ice takes up about as much volume as the water it freezes.
(Some might argue that because ice is more voluminous than water, sea level might actually drop when Arctic ice melts, but that's faulty reasoning. It's true that ice takes up slightly more room than water at zero degrees C, but melted Arctic ice rises to a higher temperature, and thermal expansion cancels out whatever "gains" you might theoretically have seen.)
But while the disappearing Arctic ice news doesn't by itself raise the kind of immediate concerns as the disappearing Greenlandic ice, it's alarming all the same, because it is one of those non-linear global warming feedback loops we talk about.
See, sea ice is white, and so highly reflective-- much more so than is open sea water. This "albedo effect" is explained by the (U.S.) National Snow and Ice Data Center:
Albedo is a non-dimensional, unitless quantity that indicates how well a surface reflects solar energy. Albedo (α) varies between 0 and 1. Albedo commonly refers to the “whiteness” of a surface, with 0 meaning black and 1 meaning white. A value of 0 means the surface is a “perfect absorber” that absorbs all incoming energy. Absorbed solar energy can be used to heat the surface or, when sea ice is present, melt the surface. A value of 1 means the surface is a “perfect reflector” that reflects all incoming energy.Albedo generally applies to visible light, although it may involve some of the infrared region of the electromagnetic spectrum. You understand the concept of low albedo intuitively when you avoid walking barefoot on blacktop on a hot summer day. Blacktop has a much lower albedo than concrete because the black surface absorbs more energy and reflects very little energy.
Sea ice has a much higher albedo compared to other earth surfaces, such as the surrounding ocean. A typical ocean albedo is approximately 0.06, while bare sea ice varies from approximately 0.5 to 0.7. This means that the ocean reflects only 6 percent of the incoming solar radiation and absorbs the rest, while sea ice reflects 50 to 70 percent of the incoming energy. The sea ice absorbs less solar energy and keeps the surface cooler.
Snow has an even higher albedo than sea ice, and so thick sea ice covered with snow reflects as much as 90 percent of the incoming solar radiation. This serves to insulate the sea ice, maintaining cold temperatures and delaying ice melt in the summer. After the snow does begin to melt, and because shallow melt ponds have an albedo of approximately 0.2 to 0.4, the surface albedo drops to about 0.75. As melt ponds grow and deepen, the surface albedo can drop to 0.15. As a result, melt ponds are associated with higher energy absorption and a more rapid ice melt.
So, take away the sea ice and that energy that previously was radiated out to space is instead absorbed by ocean waters as heat. The process feeds on itself: the warmer waters melt more ice, and so the Arctic waters absorb still more energy, heat up, melts more ice and so forth. Like the disintegrating Greenland and Antarctic ice caps, the sped-up, self-reinforcing nature of the Arctic ice-water dynamic is not factored into the linear projections of the IPCC reports.
(The IPCC report on sea ice does, however, raise another issue-- the influx of fresh water into the North Atlantic. There are also still other potential issues to consider, such as changes in precipitation patterns in Canada, northern Europe and Russia.)
Scientists increasingly speak of an ice-free Arctic by 2030-- just 23 years from now-- an expectation that looks more on the mark every day.
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