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 U.N. conference on climate change is a mix of caution, curiosity and some resignation to such “massive and drastic” operations, as the chief U.N. climatologist describes them.

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 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.

Tom Wigley, a senior U.S. government climatologist, followed Crutzen’s article with a paper of his own on Oct. 20 in the leading U.S. 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 0.9 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.