PICTURE A FOREST OF ARTIFICIAL TREES that soak up carbon dioxide at a thousand times the rate of real plants. Imagine an orbiting shield spanning a million square miles, shading the earth from the sun, or a man-made volcano that spews great clouds of sulfur dioxide to deflect sunlight and cool the atmosphere.

These ideas are controversial, expensive, and for the most part unproven, but don't laugh them off—they're inevitably going to become part of the debate about our future. They're hot topics among researchers in the growing field of geoengineering, a term that refers to using grand-scale technologies to manipulate the earth's atmosphere in ways that could combat global warming. Plenty of environmentalists, fearing a range of ecological side effects, are waving red flags, and many atmospheric scientists still call climate-altering schemes unrealistic. But these ideas are quickly gaining support—not just from late-night radio hosts but from mainstream scientists concerned that greenhouse-gas reductions aren't happening fast enough and won't, by themselves, be enough to stabilize the climate.

James Hansen, director of NASA's Goddard Institute for Space Studies and the man who famously sounded the climate-change alarm in 1988, made the case for geoengineering last February at the National Press Club, saying it's "probably needed" as an insurance policy in case of irreversible global warming. Paul Crutzen, the Danish chemist who won a 1995 Nobel Prize for his work connecting pollution and damage to the ozone layer, argued in the August 2006 issue of the journal Climatic Change that a threat as imminent as global warming requires a viable "escape route."

"There's a growing sense of desperation among scientists," says Marty Hoffert, a New York University physics professor and geoengineering proponent. "It comes from the immense gap between what they think should be done to address climate change and what governments are actually doing."

The idea isn't to stop pushing for tough federal emissions standards or slow the search for cleaner, renewable sources of energy. It's to add geoengineering solutions to our mix of options.

That doesn't sound so crazy in light of recent reports from the Intergovernmental Panel on Climate Change, a UN-sponsored consortium of more than 2,500 international scientists. A February IPCC report stated that humans are largely to blame for the warming trends seen over the past 50 years. "The effects of climate change are happening faster than we thought even five years ago," says Kristie Ebi, a public health consultant and contributing author for the IPCC. In May, the panel concluded that the most catastrophic effects of climate change may be unavoidable if global emissions don't begin to decline within a decade—a sobering reality check, especially as China and India continue to industrialize.

"We're putting all our energy into Plan A—to reduce emissions enough to prevent heating—but the situation may be so dire that we need a Plan B," says David Doniger, policy director of the Natural Resources Defense Council's Climate Center.

Plan B might include outlandish-sounding ideas like the simulated volcano concept that Crutzen has proposed. In 1991, when Mount Pinatubo blew its top in the Philippines, 20 million tons of sulfur dioxide particles clouded the atmosphere, slightly reducing temperatures around the world. Crutzen suggests releasing clouds of sulfur dioxide from high-altitude balloons, or blasting them skyward in artillery shells. The particles would float in the stratosphere for roughly two years before dispersing, and could lower sunlight's intensity by 2 percent throughout the world.

There are potential downsides, of course. The atmospheric injection might make our days hazier. Sulfur dioxide could produce acid rain if large quantities were to drift into the lower atmosphere. The project is iffy at best and could cost up to $50 billion. Still, the National Center for Atmospheric Research is working with Crutzen to test this idea using computer simulations.