For people who think global warming is a myth, then the reduction in the arctic cap will be surprising.

To normal people, it is not.

This summer was the fourth in a row in which ice cap areas were sharply below the long-term average, said Mark C. Serreze, a senior scientist at the snow and ice center and a professor at the University of Colorado, Boulder.

A natural cycle in the polar atmosphere called the Arctic oscillation, which contributed to the reduction in Arctic ice in the past, did not appear to be a factor in the last several years, he said.

He said the role of accumulating greenhouse gas emissions had become increasingly apparent with rising air and sea temperatures.

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