Sea ice in the Arctic has without doubt dwindled during the past 20 years, but the cause is not entirely clear. Although some scientists have pointed the finger squarely at global warming, a new study in the current issue of Polar Geography suggests that a low-pressure weather system near Iceland too deserves some blame. This lowwhich together with the Bermuda-Azores High drives the so-called North Atlantic Oscillation (NAO)pulls warm air into the Arctic to the east of Iceland and flushes out cold air to the west. "When the Icelandic Low is weak, it will still bring warm air northward," says study author Claire Parkinson of NASA's Goddard Space Flight Center, "but not as much as when the Icelandic Low is strong."

Parkinson analyzed two decades worth of satellite data to track variations in Arctic sea ice. Whereas the overall amount decreased between 1979 and 1999, regional differences indicated that local factors other than climate warming were involved. In particular, she found that as the Icelandic Low grew stronger between 1979 and 1990, it helped warm the Arctic and ice levels dropped off (top image). During the following decade, however, the situation reversed (bottom image). "This regional pattern of reversals in the ice extent trends is highly suggestive of an Icelandic Low impact, or more broadly, of an impact from the NAO," Parkinson says. "Whether the ice cover as a whole will continue to exhibit the decreases that it experienced over the 1979 to 1999 period might depend on the strength and phase of the NAO, as well as on long-term trends in the climate system."