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. The proposal comes from prominent scientists, among them a Nobel laureate. The reaction here at the annual 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.
The Nobel Prize-winning scientist who first made the proposal is himself "not enthusiastic about it."
"It was meant to startle the policymakers," 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 at Moffett Field, California, NASA's Ames Research Center hosts a closed-door, high-level workshop on the global haze proposal and other "geoengineering" ideas for fending off climate change.
When he published his proposal in the journal Climatic Change in August, Crutzen cited a "grossly disappointing international political response" to warming.
The Dutch scientist, awarded a 1995 Nobel Prize 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 poured so much sulfurous debris into the stratosphere that it is believed it cooled the Earth by 0.5oC for about a year.
Wigley ran scenarios of stratospheric sulfate injection 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 huge dissemination of pollutants would be needed every year or two, as the sulfates precipitate from the atmosphere in acid rain.
The US scientist 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.
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.
NASA said this weekend's California conference will examine "methods to ameliorate the likelihood of progressively rising temperatures over the next decades."