The rule of unintended consequences threatens to strike again.

Some researchers have suggested that injecting sulphur compounds into the atmosphere might help ease global warming by increasing clouds and haze that would reflect sunlight. After all, they reason, when volcanoes spew lots of sulphur, months or more of cooling often follows.

But a new study warns that injecting enough sulphates to reduce warming would wipe out the Arctic ozone layer and delay recovery of the Antarctic ozone hole by as much as 70 years.

The paper appears in the online edition of the journal Science.

Its lead author says research shows that "trying to artificially cool off the planet could have perilous side effects."

Simone Tilmes, of the National Center for Atmospheric Research in Boulder, Colo., also says that although "climate change is a major threat, more research is required before society attempts global geoengineering solutions."

One study worries that fixing climate will destroy ozone, while another raises the possibility that recovery of the ozone hole over Antarctica will worsen warming in that region.

A full recovery of the ozone hole could modify climate in the Southern Hemisphere and even amplify Antarctic warming, scientists from the University of Colorado at Boulder, the National Oceanic and Atmospheric Administration and NASA report in a paper scheduled for Geophysical Research Letters.

Although temperatures have been rising worldwide, there has been cooling in the interior of Antarctica in summer, which researchers attribute to the depletion of ozone overhead.

"If the successful control of ozone-depleting substances allows for a full recovery of the ozone hole over Antarctica, we may finally see the interior of Antarctica begin to warm with the rest of the world," said Judith Perlwitz of the Cooperative Institute for Research in Environmental Sciences, a joint institute of CU-Boulder and NOAA.

The authors used a NASA supercomputer to model interactions between the climate and stratospheric ozone chemistry. A return to pre-1969 ozone levels would mean atmospheric circulation patterns now shielding the Antarctic interior from warmer air to the north will begin to break down during the summer, they concluded.

Volcanic eruptions cooled climate

The idea of reversing global warming by injecting sulphates into the air was suggested by eruptions such as the 1991 blast by Mount Pinatubo in the Philippines, which produced a brief cooling.

The massive 1815 eruption of Tambora in what is now Indonesia produced such a strong cooling that 1816 became known as the "year without a summer" in New England, where snow fell in every month of the year.

But Tilmes said she knew that volcanic eruptions also temporarily thin the ozone layer, which protects people, plants and animals from the most dangerous ultraviolet rays from the sun.

So she and colleagues calculated the effect of suggested sulphate injections and concluded that the result, over the next few decades, would be to destroy between one-fourth to three-fourths of the ozone layer above the Arctic. This would affect a large part of the Northern Hemisphere because of atmospheric circulation patterns.

The sulphates would also delay the expected recovery of the ozone hole over the Antarctic by about 30 to 70 years, or until at least the last decade of this century, they said.