National Geographic : 2004 Sep
irnn ice in the Arctic The annual breakup of sea ice (above and opposite) off the coast of Alaska happens weeks earlier than it once did. Since 1978 the area of perennial Arctic sea ice has decreased by 9 percent per decade, but the degree to which the ice has thinned has been harder to assess. Scientists using submarine sonar data documented a 40 percent thinning in the past 30 years. Others have put the estimate at about 15 percent. Some predict that the ice could be absent in summer by 2100.