In the past few decades, a handful of scientists have come up with big, futuristic ways to fight global warming: Build sunshades in the orbit to cool the planet. Tinker with clouds to make them reflect more sunlight back into space.

Their proposals were relegated to the fringes of climate science. Few journals would publish them. Few government agencies would pay for feasibility studies. Environmentalists and mainstream scientists said the focus should be on reducing greenhouse gases and preventing global warming in the first place. But now, in a major reversal, some of the world’s most prominent scientists say the proposals deserve a serious look because of growing concerns about global warming.

Worried about a potential planetary crisis, these leaders are ca- lling on governments and scientific groups to study exotic ways to reduce global warming, seeing them as possible fallback positions if the planet eventually needs a dose of emergency cooling. “We should treat these ideas like any other research,” said Ralph J. Cicerone, president of the National Academy of Sciences in Washington.

The plans and proposed studies are part of a controversial field known as geoengineering, which means rearranging the earth’s environment on a large scale to suit human needs and promote habitability. Dr Cicerone, an atmospheric chemist, will detail his arguments in favour of geoengineering studies in the August issue of the journal, Climatic Change.

Practising what he preaches, Dr Cicerone is also encouraging leading scientists to join the geoengineering fray. In April, at his invitation, Roger P. Angel, a noted astronomer at the University of Arizona, spoke at the academy’s annual meeting. Dr Angel outlined a plan to put into orbit small lenses that would bend sunlight away from earth ? trillions of lenses, he now calculates, each about two feet wide, extraordinarily thin and weighing little more than a butterfly.

In addition, Dr Cicerone recently joined a bitter dispute over whether a Nobel laureate’s geoengineering ideas should be aired, and he helped get them accepted for publication. The laureate, Paul J. Crutzen of the Max Planck Institute for Chemistry in Germany, is a star of atmospheric science who won his Nobel in 1995 for showing how industrial gases damage the earth’s ozone shield. His paper newly examines the risks and benefits of trying to cool the planet by injecting sulphur into the stratosphere. The paper “should not be taken as a licence to go out and pollute”, Dr Cicerone said in an interview, emphasising that most scientists thought curbing greenhouse gases should be the top priority.

Geoengineering is no magic bullet, Dr Cicerone said. But done correctly, he added, it will act like an insurance policy if the world one day faces a crisis of overheating, with repercussions like melting ice caps, droughts, famines, rising sea levels and coastal flooding.

“People used to say, ‘Shut up, the world isn’t ready for this,’” said Wallace S. Broecker, a geoengineering pioneer at Columbia. “Maybe the world has changed.” Michael C. MacCracken, chief scientist of the Climate Institute, a private research group in Washington, said he was resigned to the need to take geoengineering seriously. “It’s really too bad,” Dr MacCracken said, “that the US and the world can’t do much more so that it’s not necessary to consider getting addicted to one of these approaches.”

Martin A. Apple, president of the Council of Scientific Society Presidents, said, “Let’s talk about research funding with enough zeroes on it so we can make a dent.”

The study of futuristic countermeasures began quietly in the 1960s, as scientists theorised that global warming caused by human-generated emissions might one day pose a serious threat. But little happened until the 1980s, when global temperatures started to rise.

Some scientists noted the earth reflected about 30 per cent of incoming sunlight back into space and absorbed the rest. Slight increases of reflectivity, they reasoned, could easily counteract heat-trapping gases, thereby cooling the planet.

Dr Broecker of Columbia proposed doing so by lacing the stratosphere with tonnes of sulphur dioxide, as erupting volcanoes occasionally do. The injections, he calculated in the 80s, would require a fleet of hundreds of jumbo jets and, as a by-product, would increase acid rain. By 1997, such futuristic visions found a prominent advocate in Edward Teller, a main inventor of the hydrogen bomb. “Injecting sunlight-scattering particles into the stratosphere appears to be a promising approach,” Dr Teller wrote in The Wall Street Journal. “Why not do that?” Other plans called for reflective films to be laid over deserts or white plastic islands to be floated on the world’s oceans, both as ways to reflect more sunlight into space.

But government agencies usually balked at paying researchers to study such far-out ideas.

Critics of geoengineering argued that it made more sense to avoid global warming than to gamble on risky fixes. They called for reducing energy use, developing alternative sources of power and curbing greenhouse gases.

But international efforts like the Kyoto Protocol have so far failed to diminish the threat. Scientists estimate that the earth’s surface temperature this century may rise as much as 10?F.

“Climatic engineering, such as presented here, is the only option available to rapidly reduce temperature rises” if international efforts fail to curb greenhouse gases, Dr Crutzen wrote in his paper. “So far,” he added, “there is little reason to be optimistic.”

NYTNS