Winter ice cover may herald higher melt

Multiyear ice being flushed from the Arctic Ocean in the winter is not being replaced fast enough to prevent summer ice loss

Alan Bailey

Petroleum News

The opening of the Northwest Passage in the past couple of years has provided dramatic evidence of the reduction in Arctic sea ice coverage, with the sea ice minimum extent reaching a record low level in the summer of 2007, and almost dropping to the same level in the following summer.

And at the end of February, as the Arctic started to emerge from the perpetual dark of the polar winter, the sea ice in the region reached its maximum extent, before starting its annual retreat in the face of lengthening days and warming sunlight. As that annual ice ritual entered its retreating phase, data regarding the age and thickness of the ice suggested a continuing trend towards a shrinking northern icecap, according to scientists in the National Aeronautics and Space Administration and in the National Snow and Ice Data Center.

5.85 million square miles

But fortunately the melting and freezing of sea ice does not affect global sea levels because the ice floats in the ocean.

NASA is closely tracking the Arctic sea ice changes because ice forms the defining characteristic of the Arctic ecosystem and its specialized wildlife; Arctic ice constrains global warming by reflecting sunlight back into space; and the loss of sea ice is creating possibilities for new sea routes and new opportunities for natural resource exploration, said Tom Wagner, NASA cryosphere program manager.

But regular monitoring of the Arctic sea ice extent, an activity that became possible following the start of satellite observations in 1979, does not provide information about ice depths and volumes, critical parameters in the understanding of what is happening to the polar ice cap.

“The ice extent is only giving us a limited picture of the true health of the ice cover in the wintertime, and that’s because we’re only seeing the surface,” Meier said.

Multiyear ice

University of Colorado scientists have developed a technique for using the satellite data to infer the age of the ice, a technique that can distinguish multiyear ice from, say, ice less than one year old, or from ice one to two years old. Then, knowing that ice thickens with age, the age of the ice can be used as a proxy for ice thickness.

The results of this type of analysis have proved disturbing.

“What they have found is that the ice is getting younger,” Meier said. “The older ice is starting to disappear.”

In fact, the first-year ice that used to only cover quite a limited part of the central Arctic in the winter, and which mainly existed in coastal areas and areas outside the Arctic Ocean, has now encroached over huge areas that used to be occupied by multiyear ice.

10 percent older ice

At the end of February this year only about 10 percent of the Arctic sea ice was more than two years old, compared with an average of about 30 percent in the period between 1981 and 2000, Meier said.

And this year that dramatic loss of older and thicker ice will likely again lead to near record reductions in the minimum sea ice extent, as the relatively thin one-year ice succumbs to the summer melt.

“We’re seeing an ice cover that’s younger and thinner as we head into the summer, compared with previous years,” Meier said.

Driven mainly by winds, sea ice moves around continuously, Meier explained. And as part of those movements, some of the ice is flushed out of the polar region to melt at lower latitudes.

In particular, Arctic sea ice that includes substantial quantities of multiyear ice moves down the east side of Greenland into the North Atlantic during the winter. But, in the past, enough new winter ice has survived through the following summer to replenish the multiyear ice flushed southward out of the system. And this replenishment led to a relatively stable ice coverage extent from one year to the next.

But that did not happen when much of the young ice was lost during recent relatively warm summers, such as in 2005 and 2007, Meier said. Hence the massive loss of multiyear ice.

ICEsat satellite

“We haven’t been able to do this before,” said Ron Kwok, senior research scientist at NASA’s Jet Propulsion Laboratory. “… Eventually we’ll be able to figure out how much ice mass we’re losing every year, along with the decline in the ice area.”

NASA is still processing the data obtained since ICEsat’s launch, but results to date are already providing fascinating insights into ice volumes and characteristics. Currently, there are about 16,000 cubic kilometers of Arctic sea ice in the winter Kwok said.

“That’s enough water to fill Lakes Superior and Michigan,” he said.

ICEsat data indicate that sea ice in the open ocean typically grows to a thickness of about 6 feet in a year, Kwok said. Then, as the ice thickness increases, the rate of growth slows: Ice thickness typically reaches about 9 feet after two years, but it takes many years for ice to attain thicknesses of, say, 10 to 15 feet. On the other hand, where ice sheets pile up on top of each other, typically in situations where the wind thrusts the ice against a coastline, ice thicknesses can be as much as 30 feet.

“This is how really, really thick ice is formed,” Kwok said.

Long-term trends

For example, a relatively cool Arctic summer in 2008 resulted in the survival of quite a lot of first-year ice, thus giving rise to hopes of some stabilization in the ice cover. But the longer term trend shows that dramatic decline in the amount of thick multiyear ice.

And, while relatively cool weather in the Bering Sea this winter has resulted in an exceptionally large ice extent in the Bering, that ice is very thin — often less than three feet thick. In fact, the location of the southern ice edge in the Bering Sea tends to be particularly volatile over short timeframes; a cold northerly wind drives the edge south, while a warm southerly wind pushes the edge north, Meier said.

Arctic warming from aerosol particles

The particles, which can be emitted into the atmosphere as solids or liquids from a wide variety of sources including fires, volcanic eruptions and industrial activity, can affect the climate by either reflecting or absorbing the sun’s radiation. Reflection of the radiation causes cooling, while absorption of the radiation causes warming.

“There’s a tendency to think of aerosols as small players, but they’re not,” said Drew Shindell, a climate scientist in the NASA Goddard Institute for Space Studies and leader of a team investigating the regional climatic effects of aerosols, carbon dioxide and ozone. “Right now, in the mid-latitudes of the northern hemisphere and in the Arctic, the impact of the aerosols is just as strong as that of the greenhouse gases.”

Sulfates, black carbon

And, while U.S. and European emissions regulation has reduced sulfate emissions in recent years, black carbon emissions have risen steadily, especially from Asia. The net consequence has been an increase in the warming effect of the aerosols, the scientists concluded.

That warming effect has likely become especially prevalent in the Arctic, in part because of the proximity of the Arctic to major industrial sources of aerosol materials, and in part because precipitation that would otherwise flush particulate matter from the air is relatively sparse at high latitudes, Shindell said. In the Antarctic, where aerosols are less abundant, warming has been significantly lower than in the Arctic, he said.

“We will have very little leverage over climate in the next couple of decades if we’re just looking at carbon dioxide,” Shindell said. “If we want to try to stop the Arctic summer sea ice from melting completely over the next few decades, we’re much better off looking at aerosols and ozone.”

—Alan Bailey