The Sun's ultraviolet rays could cause skin cancer in humans and biological damage to other living things if Earth were not shielded by the ozone layer high in the atmosphere. But polar stratospheric clouds made of nitric acid and water deplete this protective layer.

Scientists have known about the clouds for years, but U.S. researchers have just discovered the bands of frigid air in the stratosphere that help to create them, according to an article in the current edition of the journal Science.

And as Earth's surface gets warmer, due to heat trapped by so-called greenhouse gases, the stratosphere gets colder, making it an even better place to create the ozone-depleting clouds, NASA researcher Azadeh Tabazadeh said.

The more these high polar clouds proliferate, the slower Earth's recovery from ozone depletion, Tabazadeh said in a telephone interview from the NASA's Ames Research Center in California.

The polar stratospheric clouds do their work by sucking nitrogen out of the cold air. Because they are made up of large particles, each the size of a bit of road dust, the clouds are heavy and pull out the nitrogen as they fall toward Earth, Tabazadeh said.

Nitrogen is important because it reacts with the chlorine in human-made chlorofluorocarbons (CFCs). Now banned under international agreements, CFCs have long been identified as a prime cause of ozone depletion.

The polar stratospheric clouds pack a double punch, Tabazadeh said: They take away nitrogen, which can mitigate the effects of ozone depletion, and they also activate chlorine, which spurs ozone depletion.

Still, if Earth's climate stayed constant, the ozone layer should start recovering because CFCs are being limited. But Earth's surface climate is warming, which means the stratosphere is cooling.

"The surface warming causes a cooling in the stratosphere and the cooling promotes more ozone depletion," Tabazadeh said. "Global warming is actually affecting the ozone depletion."

"I think the best thing to do is try to control the global warming issue," she said. "And that could be controlled by less emissions of greenhouse gases and also less emissions of soot. It's very hard to regulate."