However, the flying-volcano approach might disrupt rainfall patterns and endanger regional water supplies, said atmospheric scientist Mike MacCracken of the Climate Institute, a Washington, D.C.-based think tank.
The Royal Society report "focuses a lot on global temperature, but that might not be the right metric," said MacCracken, who was not involved in the study. "Ensuring sufficient water for everyone might be more important."
Computer-controlled ships could ply the remote seas, pumping out seawater mist, which would encourage low, thick clouds to form, researchers say. The clouds would reflect sunlight back into space.
It would cost more than a billion dollars to launch a fleet of a few hundred of these ships, the new study says—a relatively small sum, as geoengineering costs go.
But the cloud ships' ability to change local temperatures and weather could raise fears that countries will clash over control of the clouds.
"Militaries have been exploring weather modification for a long time," but weaponized weather is unlikely, said Jason Blackstock, a physicist and international relations expert at the International Institute for Applied Systems Analysis in Austria.
Conflicts over cloud ships, Blackstock said, would more likely be aroused over countries' attempts "to tune the climate in ways that suit their own national interests"—for example to provide precipitation for crops.
Instead of trying to block sunlight via Earth's atmosphere, another approach would be to take the fight to outer space.
Huge mirrors or thin, reflective disks could orbit alongside Earth and block solar rays, some scientists say. (See "Shading the Earth" in National Geographic magazine.)
The approaches would be safe, with little in the way of side effects, the Royal Society says.
But it could cost a few trillion dollars and take decades to design, build, and launch, requiring "a space program many times larger than anything yet attempted."
Trees: Real and Unreal
Trees pull huge amounts of carbon dioxide, or CO2—a major greenhouse gas—out of the air, so planting more forests would be one of the most cost-effective ways of getting the gas out of the air, the study says.
But there's only so much land available, so scientists are now working on artificial trees that use chemical reactions to capture CO2. One proposed "tree" looks something like American-football goalpost strung with Venetian blind-like CO2 filters.
The artificially captured CO2 would have to be concentrated and pumped underground into caverns or old oil reservoirs.(Related: " Scrub CO2 From the Air, Win $25 Million—But How?")
Since this deals with the root cause of global warming—greenhouse gases—it is one of the most powerful approaches, other than avoiding CO2 emissions in the first place, the new report argues.
But it would likely be one of the most expensive schemes, costing perhaps tens of trillions of U.S. dollars to counteract today's warming.
Digging for a Solution
Dissolving mountains of rock might sound like a mad scientist's dream. But it's one of the proposals for speeding up the natural process of rock weathering, as a way of absorbing CO2.
Normal rainfall is slightly acidic, and over hundreds of thousands of years, it dissolves away mountains and other rocks.
The process pulls CO2 out of the air, locking it away in the form of minerals such as limestone.
A big operation for artificial rock weathering would need big mines, and a lot of electricity to chemically split seawater to make an acid that would be sprayed over the rocks.
The approach is "basically feasible," said Ken Caldeira, a climate scientist at the Carnegie Institution for Science in California and a co-author of the Royal Society report.
However, rock weathering's required huge mining operations "would require a huge amount of energy, which would make it very expensive," Caldiera added.
"It's something we might consider as an endgame."
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