NAIROBI, Kenya - If the sun warms the Earth too dangerously, the time may come to draw the shade. The "shade" would be a layer of pollution deliberately spewed into the atmosphere to help cool the planet. This over-the-top idea comes from prominent scientists, among them a Nobel laureate. The reaction here at the UN conference on climate change is a mix of caution, curiosity and some resignation to such "massive and drastic" operations, as the chief UN climatologist describes them.

Two young men exercise on a foggy morning in Calcutta, India, Wednesday, Nov. 15, 2006. [AP]

The Nobel Prize-winning scientist who first made the proposal is himself "not enthusiastic about it."

"It was meant to startle the policy makers," said Paul J. Crutzen, of Germany's Max Planck Institute for Chemistry. "If they don't take action much more strongly than they have in the past, then in the end we have to do experiments like this."

Serious people are taking Crutzen's idea seriously. This weekend, NASA's Ames Research Center in Moffett Field, Calif., hosts a closed-door, high-level workshop on the global haze proposal and other "geoengineering" ideas for fending off climate change.

In Nairobi, meanwhile, hundreds of delegates were wrapping up a two-week conference expected to only slowly advance efforts to rein in greenhouse gases blamed for much of the 1-degree rise in global temperatures in the past century.

The 1997 Kyoto Protocol requires modest emission cutbacks by industrial countries -- but not the United States, the biggest emitter of carbon dioxide and other heat-trapping gases, because it rejected the deal. Talks on what to do after Kyoto expires in 2012 are all but bogged down.

When he published his proposal in the journal Climatic Change in August, Crutzen cited a "grossly disappointing international political response" to warming.

The Dutch climatologist, awarded a 1995 Nobel in chemistry for his work uncovering the threat to Earth's atmospheric ozone layer, suggested that balloons bearing heavy guns be used to carry sulfates high aloft and fire them into the stratosphere.

While carbon dioxide keeps heat from escaping Earth, substances such as sulfur dioxide, a common air pollutant, reflect solar radiation, helping cool the planet.

Tom Wigley, a senior US government climatologist, followed Crutzen's article with a paper of his own on Oct. 20 in the leading US journal Science. Like Crutzen, Wigley cited the precedent of the huge volcanic eruption of Mount Pinatubo in the Philippines in 1991.

Pinatubo shot so much sulfurous debris into the stratosphere that it is believed it cooled the Earth by .9 degrees for about a year.

Wigley ran scenarios of stratospheric sulfate injection -- on the scale of Pinatubo's estimated 10 million tons of sulfur -- through supercomputer models of the climate, and reported that Crutzen's idea would, indeed, seem to work. Even half that amount per year would help, he wrote.

A massive dissemination of pollutants would be needed every year or two, as the sulfates precipitate from the atmosphere in acid rain.

Wigley said a temporary shield would give political leaders more time to reduce human dependence on fossil fuels -- the main source of greenhouse gases. He said experts must more closely study the feasibility of the idea and its possible effects on stratospheric chemistry.

Nairobi conference participants agreed.

"Yes, by all means, do all the research," Indian climatologist Rajendra K. Pachauri, chairman of the 2,000-scientist UN network on climate change, told The Associated Press.

But "if human beings take it upon themselves to carry out something as massive and drastic as this, we need to be absolutely sure there are no side effects," Pachauri said.

Philip Clapp, a veteran campaigner for emissions controls to curb warming, also sounded a nervous note, saying, "We are already engaged in an uncontrolled experiment by injecting greenhouse gases into the atmosphere."

But Clapp, president of the US group National Environmental Trust, said, "I certainly don't disagree with the urgency."

In past years scientists have scoffed at the idea of air pollution as a solution for global warming, saying that the kind of sulfate haze that would be needed is deadly to people. Last month, the World Heath Organization said air pollution kills about 2 million people worldwide each year and that reducing large soot-like particles from sulfates in cities could save 300,000 lives annually.

American geophysicist Jonathan Pershing, of Washington's World Resources Institute, is among those wary of unforeseen consequences, but said the idea might be worth considering "if down the road 25 years, it becomes more and more severe because we didn't deal with the problem."

By telephone from Germany, Crutzen said that's what he envisioned: global haze as a component for long-range planning. "The reception on the whole is more positive than I thought," he said.

Pershing added, however, that reaction may hinge on who pushes the idea. "If it's the US, it might be perceived as an effort to avoid the problem," he said.

NASA said this weekend's conference will examine "methods to ameliorate the likelihood of progressively rising temperatures over the next decades." Other such US government-sponsored events are scheduled to follow.