It may be hard to imagine the world getting so hot that scientists and engineers would design a fleet of 55,000 mirrors, each bigger than Manhattan, and send them into space to deflect sunlight away from Earth.
Or that they would mimic a major volcanic eruption in order to cool the melting Arctic, shooting dust and other particles into the upper atmosphere where they would scatter the sun's light away from Earth.
Using geoengineering, the large-scale manipulation of the environment, to combat global warming has been proposed by scientists like Lowell Wood at Lawrence Livermore National Laboratory in California.
He argues that simulating a volcanic winter -- the cooling that follows major volcanic eruptions like Mount Pinatubo in 1991 -- is the most practical approach to managing global warming.
"It appears, of all the things I have heard discussed, to be the most economical and readily implemented," Dr. Wood says.
The idea of interfering with nature in such an aggressive and intentional way is seen as irresponsible by many other scientists and environmentalists. They worry that focusing on high-tech fixes will distract politicians and ordinary citizens from the measures that could be taken today to reduce the world's dependence on fossil fuels, which produce carbon dioxide when they are burned. Carbon dioxide and other greenhouse gases act like a solar blanket, trapping heat in the atmosphere.
"If we really knew we could do this, there is no question it would lessen efforts to push politicians to reduce carbon dioxide levels," says David Keith, an expert on geoengineering who holds the Canada Research Chair in Energy and the Environment at the University of Calgary.
But what if global warming is more severe or happens more quickly than scientists predict? The worst-case scenarios are bad enough, and would see cities like Vancouver, New York and Shanghai swamped by rising sea levels caused by melting Greenland and Antarctic ice. Other parts of the world could be periodically devastated by more severe droughts, hurricanes and other weather.
"You would like to have a backup system to try. To have an alternative," Mike MacCracken, with the Climate Institute in Washington, said.
Research needs to done, both to determine if geoengineering schemes would work and what unintended consequences they might have, said Dr. Keith, who recently moved to the University of Calgary from the Carnegie Mellon University in Pittsburgh.
He is working on what may turn out to be an economical way to capture carbon dioxide from the air and store it underground. It sounds more like tinkering than large-scale manipulation, but Dr. Keith considers his work to be geoengineering.
But he worries that if he is successful, it will give people a false sense of security.
"You don't want people to jump to conclusions that everything can be solved," he said.
When the time comes to make a decision, in 30 or 50 years, Dr. Keith said, politicians will need to know which geoengineering proposals will work and which won't, how much they will cost and what the risks are.
"It is essentially a statement of fact, that whatever we do now, people in 2050 are going to think about this. I'm not saying they are going to do it, but they are going to think about this."
He has written a number of articles about the options.
Mimicking a volcano by shooting particles that scatter light into the upper atmosphere could damage the ozone layer.
Using mirrors in space would cause the sun's light to flicker.
"That would be distracting," Dr. MacCracken said. To avoid brief, repeated eclipses, one giant mirror would have to be built, but Dr. MacCracken said that would require a manufacturing base on the moon.
John Bennett of the Sierra Club of Canada wondered if it wouldn't be easier to reduce our use of fossil fuels.
