The clouds, which form at temperatures of minus 110 degrees Fahrenheit (minus 80 degrees Celsius), "covered a larger area and persisted for a longer period of time than for any other Arctic winter during the past 20 years," said Ross Salawitch, an ozone researcher at NASA's Jet Propulsion Laboratory in Pasadena, California.

"These conditions heighten our concern regarding possible couplings between climate change and stratospheric ozone depletion," he said.

Scientists believe the unusually cold temperatures in the stratosphere are related to balmy winters at ground level -- which North America experienced this past year.

The findings were presented at the spring meeting of the American Geophysical Union in Washington, D.C.

Scientists are concerned because the thinning areas of the ozone layer now spread into populated areas of Europe.

The danger to humans is increased exposure to the sun's damaging ultraviolet radiation, which can lead to cancer and premature aging of the skin.

During the peak time this spring, the depleted ozone layer covered an area larger than the continental U.S. -- a finding that surprised the scientists.

Ozone is created and destroyed through natural processes in the stratosphere, which is located about 10 to 30 miles (16 to 48 kilometers) above Earth.

However, the introduction of human-made chlorofluorocarbons -- known as CFCs -- and the gradual warming of Earth may speed up the destruction.

The 1990s were particularly hard on the Arctic ozone layer. Its ozone was at a record low in 1997 and research has found that ozone is declining even during seasons when it should be at its highest.

NASA's project to study ozone depletion, called SAGE 3 Ozone Loss and Validation Experiment or SOLVE, involves more than 200 scientists from the U.S., Canada, Europe, Russia and Japan.