Artificial sunblock
They said trying to cool off the planet by creating a kind of artificial sun block would delay the recovery of the Antarctic ozone hole by 30 to 70 years and create a new loss of Earth's protective ozone layer over the Arctic.

Ozone depletion
"What our study shows is if you actually put a lot of sulfur into the atmosphere we get a larger ozone depletion than we had before," said Simone Tilmes of the National Centre for Atmospheric Research in Boulder, Colorado, whose research appears in the journal Science.

Potential solution?
The sulfur injection idea has been proposed by a number of climate scientists as a potential solution to global warming.

Tilmes said the idea was intended to mimic the effects of a major volcanic eruption. Such eruptions in the past sent plumes of sun-blocking sulfur into an upper layer of the atmosphere known as the stratosphere that cooled temperatures on Earth.

Ozone in the stratosphere provides a protective layer high above Earth's surface that guards against harmful solar radiation.

Antarctica's ozone layer has been steadily thinning, resulting in a seasonal "hole" above the South Pole.

"We know that particles would result in the cooling of the planet," Tilmes said in a telephone interview.

What about the side-effects?
But such cooling would come with unintended side effects. She said sulfate injections could react with chlorine gasses in cold polar regions, triggering a chemical reaction that would further deplete atmospheric ozone.

Tilmes and colleagues looked specifically at the impact of plans to repair holes in the ozone over the poles and concluded that regular injections of sulfates over the next few decades would destroy between one-fourth to three-fourths of the ozone layer above the Arctic.

That would affect a large part of the Northern Hemisphere because of atmospheric circulation patterns, they said. The impact would be less during the second half of the century because of international pacts to ban the production of ozone-depleting chemicals.

In the Antarctic, a sulfate-injection scheme would delay the recovery of the ozone hole by 30 to 70 years, or at least until the last decade of this century.

Tilmes and colleagues used different measurements and computer models to make their predictions.

Risky business
She said her findings did not close the door on the idea of artificially cooling the planet in that way but raised a flag of caution.

"We need people to have atmospheric models to understand the process in more detail," she said.