Credit: NASA

“The Arctic is screaming,” declared Mark Serreze, senior scientist at the U.S. National Snow and Ice Data Center (NSIDC), when the extent of Arctic sea ice suddenly shrank in 2007. In that same year, Jay Zwally, a NASA climate scientist, warned that “the Arctic Ocean could be nearly ice-free at the end of summer 2012, much faster than previous predictions.”

In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) said satellite data showed that since 1978, “annual average Arctic sea ice extent has shrunk by 2.7 [2.1 to 3.3] percent per decade, with larger decreases in summer of 7.4 [5.0 to 9.8] percent per decade…” The agency predicted that both Arctic and Antarctic ice would continue to shrink and said that “in some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century.”

That was five years ago. Not all observers are convinced that Arctic sea ice is in trouble.

The Heartland Institute, in its Climate Change Reconsidered: 2011 Interim Report, noted:

Though semi-permanent sea ice exists around the North Pole, fringing sea ice in both the Arctic and Antarctic is an annual, seasonal feature. Fringing sea ice is therefore particularly susceptible to fast advance or retreat depending upon local oceanographic and atmospheric changes. Even quite major sea-ice changes are not necessarily due to climatic change.

The organization cited 2010 research by scientists for the British Antarctic Survey, who concluded that “it remains to be seen whether these changes in atmospheric circulation [which appear to be the proximate cause of the significant step-change in the date of sea-ice breakup] might be ascribed to human actions or simply to natural climate variability.” Heartland Institute concluded, “Clearly, the science pertaining to this matter is not settled.”

In January 2011, climate-change critic Christopher Monckton alleged:

In fact, the global sea-ice record shows virtually no change throughout the past 30 years, because the quite rapid loss of Arctic sea ice since the satellites were watching has been matched by a near-equally rapid gain of Antarctic sea ice. Indeed, when the summer extent of Arctic sea ice reached its lowest point in the 30-year record in mid-September 2007, just three weeks later the Antarctic sea extent reached a 30-year record high. The record low was widely reported; the corresponding record high was almost entirely unreported.

Arctic sea ice. Credit: NASA

Why such contrasting opinions about the trend in sea ice? One point to note is that Monckton was talking about global sea ice, not just Arctic sea ice. Still, does he have a point? We’ll return to that question, but first, here is some background about Arctic sea ice.

Changes in Arctic sea ice are tracked according to the ice cover’s size in September, when it reaches its yearly minimum — in other words, the sea-ice cover that has survived the North American summer. This chart from the National Oceanic and Atmospheric Administration (NOAA) shows a definite downward trend since the 1950s and 1960s.

Credit: National Oceanic and Atmospheric Administration

Similarly, this chart from NSIDC shows the overall average monthly sea ice extent based on satellite data going back to 1979. As with all things climate-related, year-to-year data show lots of ups and downs, but the long-term trend is obvious.

Credit: U.S. National Snow and Ice Data Center

Here’s a video from NSIDC showing the decline in minimum sea ice extent from 1979 to 2009, and here’s a similar video from NASA for the period of 1979 to 2007. The IPCC’s 2007 report said that the sea ice cover declined by about 23,000 square miles from 1979 to 2005.

In 2007, arctic sea ice extent dropped suddenly to its lowest point since measurements began in 1979. Until that year, sea ice extent had been declining by about 10 percent per decade, according to NASA. But the 2007 minimum was far below the previous record set in 2005 and 38 percent lower than the historical average.

Sea ice extent measures the area of sea ice coverage. But even more important is total sea ice volume, which is also decreasing. The volume of sea ice builds up over multiple years, so the percentage of older multiyear ice is an important indicator of the health of Arctic sea ice cover. Here’s a link to an animation showing the reduction in older sea ice over the past two decades.

The following chart from the Polar Science Center (PSC) of the University of Washington shows the trend in sea ice volume.

Credit: University of Washington Polar Science Center

Sea ice volume varies significantly throughout the year and normally reaches its peak in March. The PSC said the maximum Arctic sea ice volume in March 2012, at 20,800 cubic kilometers (km³), was 35 percent below the maximum in 1979.

What about Christopher Monckton’s assertion that global sea ice has remained unchanged for 30 years?

As I pointed out, Monckton is talking about global sea ice, not Arctic sea ice. His claim is that, even though Arctic sea ice is declining, Antarctic sea ice is increasing and balancing things out. A skeptic might argue that even if that were the case, Arctic and Antarctic sea ice aren’t the same.

Even more important, though, Monckton is wrong when he says overall global sea ice is not declining, according to statistician Grant Foster. In January 2011, Foster analyzed data from NSIDC and wrote:

Since satellite observations began, the rate of Northern Hemisphere sea ice loss is 3.3 times as fast as the rate of Southern Hemisphere sea ice gain. And that difference is statistically significant — way significant — it’s not just an artifact of the noise, it’s for real. Northern sea ice loss is vastly greater than southern sea ice gain. Monckton’s claim that “the quite rapid loss of Arctic sea ice since the satellites were watching has been matched by a near-equally rapid gain of Antarctic sea ice” is completely, utterly, demonstrably false.

The Importance of Arctic Sea Ice

In a NASA video, Tom Wagner, the organization’s cryosphere program manager, said Arctic sea ice is important is because “it’s kind of one of the main controls on weather over the whole planet.” He explains:

The Arctic Ocean is kind of like a mirrored hat upon the surface of the Earth, and as that ice begins to decrease and melt away, the sunlight encounters dark ocean, where it gets absorbed and begins to heat the ocean up.

Arctic ice reflects sunlight, helping keep Earth's temperature lower. Credit: NASA

Arctic waters in the North Atlantic are crucial for worldwide ocean circulation. Under normal conditions, the extremely cold and dense Arctic waters sink into the deep ocean and create a worldwide “conveyer belt” that is the basic driver of ocean currents around the planet (the NASA video sums this up well — its less than four minutes and worth watching).

Wagner noted that ocean circulation sets the planet’s weather and climate, and as the Arctic Ocean undergoes transformation, things change:

What we’re really concerned about is that, over the last 30 years … [we've got] a lot more open water. What we’re trying to understand is how those changes are correlated with warming of the planet overall.

While the average global temperature is rising, the Earth’s poles are warming faster, meaning Arctic and Antarctic regions stand to be affected disproportionately. “It looks like the Arctic, just in the past decade, has warmed about 1 to 4 degrees,” said Wagner.

Are Polar Bears in Trouble?

The long-term decline in Arctic sea ice extent has affected the size and health of polar bear populations, as the iconic animals are dependent on sea ice for habitat.

A recent study by the United States Geological Survey (USGS) said:

In the Beaufort and Chukchi seas, the duration between melt and freeze onset has increased, and summertime sea ice extent has decreased. As summer ice habitats melt in this region, polar bears that do not follow the receding pack ice may be forced to swim long distances to areas of higher sea ice concentration or to land.

The researchers used satellites to track movements of polar bears and found that the decline in sea ice is forcing stranded animals to swim unusually long distances. They concluded:

Despite the ability of polar bears to swim long distances, this behavior places them at risk of drowning and imposes greater energy expenditure… Long-distance swimming is likely an additional indicator of the negative effects of sea ice loss on polar bears in the Beaufort and Chukchi seas.

(View a report on long-distance swimming by adult female polar bears in the Beaufort and Chukchi seas by Anthony M. Pagano and his colleagues, presented at the 20th International Conference on Bear Research and Management in July 2011.)

A polar bear breaks through thin Arctic Ocean ice. Credit: U.S. Geological Survey

Polar bears do their hunting and feeding on sea ice. As the ice cover melts earlier in the summer and freezes later in the fall, bears have less time to feed. This impacts the viability of populations, because successful reproduction depends on females developing weight and storing fat.

In a study on climate change’s impact on polar bear litter size, published in February 2011 in Nature Communications, Péter K. Molnár and his colleagues said that in the early 1990s, 28 percent of female polar bears failed to reproduce because of the lack of energy. The researchers predict that if spring sea ice breakup were to occur one month earlier than when it did in the 1990s, the failure rate could grow from 40 to 73 percent, and mean litter size would decrease by 22 to 67 percent. If sea ice breaks up two months earlier, it could result in 55 to 100 percent reproductive failure and a mean litter size decrease of 44 to 100 percent.

Did Arctic Sea Ice Recover in 2012?

This past April, average sea ice extent appeared to have grown greatly, according to a report from NSIDC. Sea ice extent was near the long-term April average and even reached a record high for the Bering Sea. Does that mean the decline in Arctic sea ice has stalled?

No, said Marika Holland, a sea ice expert at the National Center for Atmospheric Research (NCAR): “Sea ice exhibits large natural variability due to year-to-year variations in weather.”

By the end of winter, overall ice cover was extensive, but NSIDC’s report said that cover “is made up of dispersed floes spread thinly across the ocean surface.” NSIDC scientist Walt Meier said, “While extent briefly reached the average, the amount of thicker, older ice has remained far below normal, as has the total ice volume.” The ice cover is expected to melt off quickly and has already begun to do so in the Beaufort and Bering seas.

But Is 33 Years of Data Really Enough?

The satellite systems used to measure sea ice have only been in place since 1979. Could the apparent decline since then be just a temporary fluke?

Research on changes in Arctic sea ice published in Nature in late 2011, by Christophe Kinnard and colleagues, confirms:

Although extensive uncertainties remain, especially before the 16th century, both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years.

Explaining why they did their study, the researchers said that science had previously confirmed that “Arctic sea ice extent is now more than 2 million square kilometers less than it was in the late 20th century.” But because satellite data only goes back several decades, there was a need for a longer-term record “with which to assess natural sea ice variability.” To build that longer-term picture of sea ice extent, the researchers used proxy data — ice core records, tree-ring chronologies, lake sediments and historical documents — to reconstruct variability in sea ice cover going back to the year 561 CE.

The researchers wrote:

Until now, the question of whether or not current trends are potentially anomalous has therefore remained unanswerable… Enhanced advection of warm Atlantic water to the Arctic seems to be the main factor driving the decline of sea ice extent on multidecadal timescales and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming.

[Editor's Note: This article is the third in a series examining key arguments about climate change. The two previous articles examined whether global temperatures are really increasing and whether sea levels are really rising.]

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