April 4, 2007 — Global warming may already be having an effect on the Arctic, which in 2005 only replaced a little of the thick sea ice it loses and usually replenishes annually, a NASA study said Tuesday.
Scientists from the U.S. space agency used satellite images to analyze six annual cycles of Arctic sea ice from 2000 to 2006.
Sea ice is essential to maintaining and stabilizing the Arctic's ice cover during its warmer summer months.
But "recent studies indicate Arctic perennial ice is declining seven to 10 percent each decade," said Ron Kwok from NASA's Jet Propulsion Laboratory in Pasadena, California.
"Our study gives the first reliable estimates of how perennial ice replenishment varies each year at the end of the summer.
"The amount of first-year ice that survives the summer directly influences how thick the ice cover will be at the start of the next melt season."
The team observed that only four percent, about 965,000 square miles, of thin ice survived the 2005 summer melt to replenish the perennial cover.
It was the weakest ice cover since 2000, and so there was 14 percent less permanent ice cover in January 2006 than in the corresponding period the year before.
"The winters and summers before fall 2005 were unusually warm," Kwok said. "The low replenishment seen in 2005 is potentially a cumulative effect of these trends.
"If the correlations between replenishment area and numbers of freezing and melting temperature days hold long-term, it is expected the perennial ice coverage will continue to decline."
Records dating back to 1958 have shown a gradual warming of Arctic temperatures which speeded up in the 1980s.
"Our study suggests that on average the area of seasonal ice that survives the summer may no longer be large enough to sustain a stable, perennial ice cover, especially in the face of accelerating climate warming and Arctic sea ice thinning," Kwok added.
