GENEVA — The depletion of the ozone layer shielding Earth from damaging ultraviolet rays has reached an unprecedented level over the Arctic this spring because of harmful chemicals and a cold winter, the U.N. weather agency said Tuesday.

The Earth’s fragile ozone layer in the Arctic region has suffered a loss of about 40 percent from the start of winter until late March, exceeding the previous seasonal loss of about 30 percent, the World Meteorological Organization said.

Additional Photos

An iceberg is seen off Ammassalik Island in Eastern Greenland. The Arctic is a thermostat against overheating and a barometer of change, but now its own protective ozone layer that keeps out damaging ultraviolet radiation has thinned to record levels, the U.N. says. 2007 Associated Press file

The Geneva-based agency blamed the loss on a buildup of ozone-eating chemicals once widely used as coolants and fire retardants in appliances, and on very cold temperatures in the stratosphere, the second major layer of the Earth’s atmosphere, just above the troposphere.

Arctic ozone conditions vary more than the seasonal ozone “hole” that forms high in the stratosphere near the South Pole each winter and spring, and the temperatures are always warmer than over Antarctica.

Because of changing weather and temperatures, some Arctic winters experience almost no ozone loss while others with exceptionally cold stratospheric conditions can occasionally lead to substantial ozone depletion, U.N. scientists say.

This year the Arctic winter was warmer than average at ground level, but colder in the stratosphere than normal Arctic winters. U.N. officials say the latest losses — unprecedented, but not entirely unexpected — were detected in observations from the ground and from balloons and satellites over the Arctic.

Scientists who are concerned about global warming focus on the Arctic because that is a region where the effects are expected to be felt first.

Scientists say that significant Arctic ozone depletion is possible in the case of a cold Arctic stratospheric winter. Ozone losses occur over the polar regions when temperatures drop below minus 108 Fahrenheit, when clouds form in the stratosphere.

Average temperatures in January range from about minus 40 to 32 F, while average temperatures in July range from about 14 to 50 F.

“The Arctic stratosphere continues to be vulnerable to ozone destruction caused by ozone-depleting substances linked to human activities,” said WMO secretary-general Michel Jarraud. “The degree of ozone loss experienced in any particular winter depends on the meteorological conditions.”

The loss comes despite the U.N. ozone treaty, known as the 1987 Montreal Protocol, which has resulted in cutbacks in ozone-damaging chemicals, such as chlorofluorocarbons and halons, which were used in the making of refrigerators, air conditioners and fire extinguishers.

The 196-nation ozone treaty encourages industries to use replacement chemicals less damaging to ozone, the atmospheric layer that helps protect against the sun’s most harmful rays.

But because these compounds have long atmospheric lifetimes, it takes decades for their concentrations to subside.

U.N. officials project the ozone layer outside the polar regions will recover to pre-1980 levels between 2030 and 2040.