It is a technology which may just save the planet: a machine capable of scrubbing carbon dioxide from the air, potentially halting global warming and returning the atmosphere to its former glory.
The prize for creating such a machine is not only worldwide fame, but also vast wealth - thanks in part to a $25m bounty put up by Sir Richard Branson. But with engineers around the world scrambling to build a planetary-sized air purifier, several researchers think they may already have the answer.
David Keith, an environmental scientist at the University of Calgary, has built a prototype device which does precisely this; sucking air into one end of a five-metre high vertical tower and pumping it out at the other end with 30% less CO2 in it.
Inside the tower the air is sprayed with droplets of a sodium hydroxide solution, which absorbs CO2 gas. This produces a solution of sodium bicarbonate, or baking soda. By adding calcium oxide - also known as lime - the sodium hydroxide can be recovered for reuse. This lime is then also recovered, again for reuse, by heating the resulting calcium carbonate - which finally leaves just the CO2. There is nothing magic about it, says Dr Keith; a similar reaction has been used by the paper industry for many years to chemically pulp wood.
One of the advantages of air capture is that you don't have to capture the carbon near its source, says Dr Keith. So the CO2 emissions being produced by traffic in Beijing can be captured from the atmosphere by machines in Bogotá. From a geopolitical perspective this is a huge bonus because it allows some nations to take action even if others are dragging their heels.
And unlike the carbon capture and storage processes used at power plants - where CO2 is captured from smoke stacks - air capture units can be built over the oil wells or deep aquifers where the CO2 would eventually be stored, so there is no need to produce more emissions transporting it. Alternatively it could also be used to make carbon neutral biofuels. Either way, it finally provides a means of reducing the amount of CO2 in our atmosphere, which is currently about 30% higher than pre-industrial levels and increasing by 25bn tonnes each year.
Earlier this year, Sir Richard Branson launched the Virgin Earth Challenge, a hefty reward for anyone that could develop a technology to reverse global warming. To collect this cash, however, contenders have to convince a panel of judges - including environmental heavyweights such as Al Gore and James Lovelock - that their technology is commercially viable.
This is where things get tricky. For Dr Keith's system to have any chance of making this happen at least tens of thousands of these structures would be needed, each hundreds of metres in diameter and more than a hundred metres high. And here's the catch. Building on this scale has a serious impact on how much it will cost, which is crucial if air capture is to take off. Taking all costs into consideration, he says that his process would operate for less than $100 per ton of CO2 captured, considerably more than $60 per ton it currently costs using smoke stack capture.
But one company believes it can do better. Global Research Technologies (GRT) in Tucson, Arizona, recently demonstrated a prototype based on the work of Klaus Lackner at Columbia University, in New York. This uses a proprietary material to absorb the CO2, which is then removed by washing it with a solution of sodium carbonate. "Chemically and energetically it's much easier than using sodium hydroxide," says Dr Lackner.
According to Allen Wright, president of GRT, a single device measuring 10 metres square could capture 1,000 tons of CO2 each year. On this scale, 1m devices would be needed to extract a billion tons of carbon from the atmosphere in a single year. Given current trends, this would be enough to start significantly reducing the CO2 in the atmosphere, he says. What's more, GRT believes that when they scale up their prototype it will be able to this at a competitive price.
But the race is far from won. "It is one thing to be doing research on a topic, but entirely another to have something that works at a practical cost," says James Hansen, one of the six Virgin Earth judges. Indeed Dr Keith describes Dr Lackner's solution as "ingenious", but questions whether the figures really add up.
Both Dr Keith and the GRT group accept that with the vast coal reserves this planet still has to offer, there is a potential for this technology to backfire by encouraging an increased use of fossil fuels. But they argue that all solutions to global warming face this problem. "In the absence of this technology we are going to burn that coal anyhow," says Mr Wright.
According to Dr Hansen, who is also head of Nasa Goddard Institute for Space Studies and an environmental scientist at Columbia University, in New York, the way to avert this is to set a tax on coal use that is equal to the cost of getting the CO2 out of the air.
Whether this works remains to be seen. In the meantime GRT is now planning on submitting to the Virgin Earth Challenge, but Dr Keith says he has no interest in following suit. "It's not very much money," he says. The costs of just demonstrating that the technology is cost-effective would dwarf the prize money, he says.