Continued Sea Ice Decline in 2005
Since 1978, satellites have made continuous observations of Arctic sea ice. In that time, sensors have found an overall decline in its extent. Beginning in 2002, this decline steepened, with early onset of springtime melt north of Siberia and Alaska. Beyond summertime melt, Arctic sea ice further surprised researchers in the winter of 2004-2005. “Even if sea ice retreated a lot one summer, it would make a comeback the following winter, when temperatures fall well below freezing,” explains Florence Fetterer of the National Snow and Ice Data Center (NSIDC). “But in the winter of 2004-2005, sea ice didn't approach the previous wintertime level.” With the exception of May 2005, every month between December 2004 and September 2005 saw the lowest monthly average since the satellite record began.
This graph shows the five-day mean sea ice extent for July through September for the years 2002 through 2005. All four years were below the average sea ice extent for 1979-2000 (gray line). In fact, recent sea ice extent falls below the 1979-2000 average by an area twice the size of Texas. On September 19, 2005 (the latest date shown on this graph), Arctic sea ice extent fell to 5.35 million square kilometers (2.06 million square miles). It continued to decline until September 21, 2005, when it dropped to 5.32 million square kilometers (2.05 million square miles). This new low was 670,000 square kilometers (approximately 258,000 square miles) below the previous record low in 2002.
From 1979 through 2001, the rate of September Arctic sea ice decline was just over 6.5 percent per decade. The September 2002 minimum increased this rate to 7.3 percent. Incorporating the sea ice extent projection for 2005 increased the rate to approximately 8 percent per decade.
Patterns of natural variability play a part in Arctic sea ice decline. The Arctic Oscillation is a major atmospheric circulation pattern that can take a positive or negative mode. In its positive mode, it sets up winds that tend to break up sea ice and flush it out of the Arctic, and the thin ice left behind is more likely to melt. In its strongly positive phase in the early to mid-1990s, the oscillation may have made sea ice more vulnerable to summertime melt. Since the late 1990s, however, the Arctic Oscillation has exhibited a more neutral mode, while sea ice has continued to decline. Sea ice decline has persisted through different patterns of precipitation, wind, and local temperature variation. Researchers have found marked declines in sea ice difficult to explain without considering overall Arctic warming.
Sea ice decline is likely to affect future temperatures in the region. Because it is white or light in color, sea ice reflects much of the Sun’s radiation back into space, whereas dark ocean water absorbs more of the Sun’s energy. As sea ice melts, more exposed ocean water changes the Earth’s albedo, or fraction of energy reflected away from the planet. The increased absorption of energy further warms the planet. “Feedbacks in the system are starting to take hold,” says NSIDC’s lead scientist Ted Scambos. “There doesn't appear to be a way to turn this around, or even slow it down,” in a warming climate. Claire Parkinson, senior scientist of NASA Goddard Space Flight Center points out a potential mitigating factor, noting that “the reduced sea ice coverage will lead to more wintertime heat loss from the ocean to the atmosphere, and perhaps, therefore, to colder water temperatures and further ice growth.”
Still, recent trends caused concern. Walt Meier of NSIDC remarks, “Having four years in a row with such low ice extents has never been seen before in the satellite record. It clearly indicates a downward trend, not just a short-term anomaly.”
June marks the beginning of the melt season for Arctic sea ice, which reaches its minimum extent at the end of the season in September. In the past few Septembers, Arctic sea ice concentration (the amount of ice in a given area) has been markedly reduced. September 2002 set a new record low at 15 percent below average. It was followed closely by September 2003 and September 2004. So far, 2005 is shaping up to be another record-low sea ice year in the Arctic.
This image shows places where Arctic sea ice was above (blue) or below (red) average in June 2005, the end of the first month of the melt season. The images are made from data from the satellite-based, Scanning Multi-channel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I). Ice-free areas appear in light gray, and landmasses appear in dark gray. The black line shows the median ice edge for 1979 through 2000. Except for a small area in the East Greenland Sea, Arctic sea ice has retreated almost everywhere in June 2005. The month set a new record low: 6 percent below the long-term mean for June sea ice extent. During the melt season in any year, some areas may experience positive anomalies—higher than average sea ice concentration—or negative anomalies—lower than average ice concentrations. Most anomalies occur along the margins of the ice cap, but they can also occur near the pole at the end of the melt season. Few if any anomalies occur near the pole in June.
Different explanations have been proposed for Arctic sea ice decline, including the strong positive mode of the Arctic Oscillation (AO). This oscillation is an alternating pattern of atmospheric pressure at polar latitudes and mid-latitudes. In the early 1990s, the AO was in positive mode. In that mode, the AO produces a strong polar vortex, and resulting winds tended to flush older, thicker ice out of the Arctic. Since the late 1990s, however, the AO has been much more neutral, yet Arctic sea ice decline continues. Another explanation for declining sea ice is climate change. Global temperatures have risen, and climate models generally agree that one of the strongest signals of greenhouse warming is a loss of Arctic sea ice. Changes in surface albedo provide another, related explanation. Just as light clothes reflect the Sun’s heat on a hot day, bright sea ice reflects much of the Sun’s energy back into space. As sea ice melts, less energy is reflected back into space, and more energy is absorbed by the darker ocean waters. This creates a “feedback loop” in which sea ice decline fosters further decline.
Even after warm summers, Arctic sea ice has typically recovered in wintertime, but this has changed in recent years. Besides showing dramatic retreat in the summer, Arctic sea ice has begun to decline in the wintertime as well. Some scientists have begun to wonder whether Arctic sea ice has crossed a critical threshold from which it can’t recover.
Image courtesy of Ken Knowles and Terry Haran, National Snow and Ice Data Center. Information provided by Julienne Stroeve, Walt Meier, Florence Fetterer, Ken Knowles, and Mark Serreze, NSIDC.
Funny stuff on page 1 of this thread. Worth a look.