By Andrew C. Revkin

The area of floating ice in the Arctic has shrunk more than in any summer since satellite tracking began in 1979, and it has reached that record point a month before the annual ice pullback typically peaks, experts said.

The cause is probably a mix of natural fluctuations, like unusually sunny conditions in June and July, and long-term warming from heat-trapping greenhouse gases and sooty particles accumulating in the air, according to several scientists.

William Chapman, who monitors the region at the University of Illinois Urbana-Champaign and posted a Web report on the ice retreat , said that only an abrupt change in conditions could prevent far more melting before the 24-hour sun of the boreal summer sets in September.

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