"Global warming" is a phrase guaranteed to generate as much heat as the sun itself. There are many scientists who say we're in serious trouble if we don't reduce the burning of coal and oil -- and others who say it's all exaggerated, perhaps even an elaborate hoax that could crush the economy.
But there's a third camp. Call them the geoengineers. Maybe we can protect ourselves, they say -- literally by using smoke and mirrors.
They talk of shielding the planet from the sun's heat by pumping soot into the upper atmosphere to act as a natural sunscreen. Some have talked of adding iron to the oceans, so that plankton will bloom and absorb carbon dioxide. Or of making clouds brighter, so they'll reflect sunlight back into space. There's even talk of launching giant reflectors into orbit someday to shade the planet.
It may sound pretty far out, but it's serious enough that the Intergovernmental Panel on Climate Change, the group put together by the United Nations to deal with warming, is holding a meeting on geoengineering this week in Lima, Peru.
"Prevailing uncertainty in the sensitivity of the climate system," said the IPCC, "suggest that research is needed into geoengineering options as a possible complement to climate change mitigation efforts." (The IPCC shared the 2007 Nobel Peace Prize with former Vice President Al Gore, but has many critics.)
Michael Oppenheimer of Princeton University, who has worked both as a scientist and an environmental advocate, said discussion is healthy, as long as people don't use it as a substitute for finding new ways to get the energy we need.
"The subject can't be avoided, so let's talk about it sensibly," he said.
There have already been experiments in how geoengineering schemes might affect the global climate -- some of them not arranged by human beings.
In 1991, Mount Pinatubo, a volcano in the Philippines, erupted violently. It sent some 17 million tons of sulfur dioxide into the stratosphere, where it spread around the planet. By some calculations, the total amount of sunlight reaching Earth's surface over the next year was reduced by about 10 percent, and global temperatures dropped by 0.7 degrees Fahrenheit -- a significant number.
Ken Caldeira of the Carnegie Institution of Washington has run computer simulations of what would happen if future engineers tried to mimic the volcano's effect, or tried other ways to filter the sun's heat.
"Every computer simulation we've done," he said, "we've found that the amount of climate change can be reduced for most people most of the time."
He's at this week's conference in Lima speaking in favor of further study, but warning, like Oppenheimer, against thinking there's some easy techno-fix for the environment.
"It's relevant information that people in governments ought to know about," he said.
Activists Worry About Unintended Consequences
There are others who say they're nervous that the U.N. would even consider messing with the atmosphere. What if future engineers tried adding so-called aerosol particles to the atmosphere, they say -- and overdid it? Or what if some grand experiment went awry in a way that nobody could have anticipated?
In advance of this week's meeting, 125 advocacy groups from 40 countries signed a letter to the IPCC, "demanding a clear statement of its commitment to precaution." They said they worried that engineers in one country could try something at the rest of the world's expense.
Climate Change: Protecting Earth With Smoke and Mirrors
"The climate is a complex system," said Silvia Ribeiro, an organizer of the petition, in an accompanying statement. "Manipulating climate in one place could have grave environmental, social and economic impacts on countries and peoples that had no say on the issue. Scientists estimate that blasting particles into the stratosphere could alter monsoon and wind patterns and put at risk the food and water sources for two billion people."
The petitioners cited the eruption of the Puyehue volcano in Chile this month. Like Pinatubo, it may have a cooling effect -- but they said it also disrupted air travel for thousands of miles around it.
"If we could experiment with the atmosphere and literally play God, it's very tempting to a scientist," said Richard Odingo, a Kenyan earth scientist who attended a meeting on geoengineering in April. "But I worry."
Nothing is going to happen soon; most scientists agree none of the ideas proposed so far are yet possible. But people don't even know very much about them, which is why Princeton's Oppenheimer says a conference on geoengineering isn't such a bad idea.
"I never want to see it used," he said, "but I think we should be talking about it."
The Associated Press contributed to this report.