But ways to remove the gases that humans have already put into the atmosphere are still a long way off, experts say.
"There's nothing out there that's being commercially developed at the moment," Walker said.
"People are putting ideas out there that involve everything from enriching algal growth in the oceans by adding iron filings through to solar-powered scrubbers placed in the desert which would process CO2 out of the atmosphere," Walker said.
But, he added, "these ideas are either on paper or in labs on a small scale."
The Virgin Earth Challenge is a way of adding incentive for researchers to more rapidly bring carbon-scrubbing solutions into commercial use.
The idea follows the lead of challenges such as the Ansari X Prize, which awarded ten million U.S. dollars for the successful flight of the first privately built manned spacecraft.
So far, most of the technologies in the works that might apply for the Virgin prize focus on "air capture," whereby greenhouse gases are sucked directly from the atmosphere.
One proposal suggests "planting" artificial trees—structures that employ chemical-coated slats or "leaves" to absorb CO2 in the surrounding air (related photos: Earth's carbon cycle).
A similar idea involves huge CO2-sucking towers standing 390 feet (120 meters) high and about 330 feet (120 meters) in diameter. The towers would spray a sodium hydroxide solution into the air to trap carbon dioxide.
This project—involving eight research institutions in the United States, Canada, and Germany—is based at the Climate Decision Making Center at Carnegie Mellon University in Pittsburgh, Pennsylvania.
Team member Joshuah Stolaroff of Carnegie Mellon said that the air-capture towers could be located anywhere, because CO2 has nearly the same concentration around the globe.
He said, for instance, that all the towers could be placed in Antarctica "and capture the entire world's emissions."
Stolaroff sees a day when humans' greenhouse gas emissions cease completely. But even then, he believes, air-capture technologies such as his team's towers would be useful.
"In the future, when most carbon emissions have been eliminated, previously emitted carbon can be captured from the atmosphere to reduce concentrations back toward pre-industrial levels," Stolaroff said.
The researcher said that the system would be used in parallel with other carbon capture and storage technologies to reduce CO2 concentrations as quickly as possible.
Stolaroff admits that air capture is likely to be expensive.
Whether it becomes economically viable in the future "depends on how far the technology has progressed by that time and how much we value a low-carbon climate.
"Our research suggests that, given a reasonably funded research-and-development program, carbon capture from air could quite possibly be available at prices that make it worthwhile on a very large scale," he said.
But others aren't so confident.
Harry Audus is manager of the International Energy Agency's greenhouse gas research program based in Cheltenham, England.
The basic problem with air capture, he said, "is that it doesn't look very attractive from an engineering point of view. It's just the sheer scale of the quantity [of CO2] you're talking about."
It's much easier and cheaper to catch the gas at the source, especially from stationary sources such as power stations, Audus said.
Dealing with CO2 emissions from other sources, such as aircraft, requires new solutions.
Audus said that taking carbon dioxide from the atmosphere would allow Virgin boss Branson "to carry on flying his planes."
But air capture is "very expensive," he said. "You never know. But if you start looking at the basic science and the physics involved, it doesn't look a very likely option."
Nevertheless Stolaroff, of Carnegie Mellon, is hopeful that better air-capture technology can be developed in the future.
As for the 25-million-U.S.-dollar challenge?
"We are considering whether to enter," he said.
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