But when discussing modifying Earth's climate, most scientists strongly emphasize two points: Such schemes should be attempted only after careful consideration and only as a last resort if disastrous climate changes begin to be felt. And they should never be a substitute for reducing emissions. At best, geoengineering represents a "thumb in the dike" – a temporary and partial solution. As Cascio puts it: "It's a stay of execution, not a pardon."
Even considering those reservations, some environmentalists remain far from persuaded. Geoengineering amounts to "an act of geopiracy," says the Swedish Society for Nature Conservation. "There is no reason for the governments or peoples of most of Africa, Asia, and Latin America to trust that the governments, industries, or scientists of the biggest carbon-emitting states will protect their interests."
Plans to modify the world's climate fall into two basic categories: One approach would reduce the amount of sunlight taken in by Earth and its atmosphere – sometimes called Solar Radiation Management (SRM). About 70 percent of sunlight is absorbed by Earth and its atmosphere, while the remaining 30 percent is reflected back into space. If just 1 percent more sunlight were reflected, Earth would cool by a measurable amount.
Other ideas concern removing carbon dioxide from the atmosphere, directly offsetting carbon dioxide emissions. (See sidebar at left.)
Perhaps the best-known sun-blocking concept involves lacing the upper atmosphere with particles of sulfur to make it more reflective. Real-world experience suggests that such a technique could work.