Among the most conspicuous evidence of climate change associated with global warming was the “alarming” rate at which Arctic ice had melted during the summer months, he said. The melting this year occurred at a much faster rate than in 2011 and outpaced the predictions of climate experts on the Intergovernmental Panel on Climate Change, he said.

By September, the level of Arctic ice was the lowest since satellite records began and had shrunk by nearly half — an area nearly the size of India — below the average minimum level in the 20 years before 2000, the organization reported.

The ice will reform in the winter but will be thinner than before and more vulnerable to further melting, Mr. Jarraud warned. “The trend is not only continuing but accelerating,” he said. “The more it melts, the faster it will melt.”

The ice melt will contribute to rising sea levels that are already 20 centimeters, or nearly 8 inches, higher than a century ago, Mr. Jarraud said, posing added risks in the event of extreme weather. Hurricane Sandy would have had less impact on New York if it had occurred 100 years ago when sea levels were lower, he said.

After a chilly start to 2012, average temperatures from January to October were 0.45 degrees Celsius, or 0.81 degrees Fahrenheit, above the average from 1961 to 1990, according to the World Meteorological Organization’s findings. A rise of only one degree Celsius was sufficient to increase the frequency of extreme weather events, Mr. Jarraud said.