Standing shoulder to shoulder, Al Gore and British billionaire Richard Branson announced recently they would help save the Earth - by giving away US$25 million. And to take advantage of Branson's generosity, all you need is an invention that will fix global warming.

More specifically, the Virgin Earth Challenge prize, worth A$32 million, will be awarded to a person whose invention removes significant amounts of "anthropogenic, atmospheric greenhouse gases each year for at least ten years without countervailing harmful effects". Let's put the word 'significant' in perspective.

The Australian government thinks it can prevent up to four million tonnes per year of carbon dioxide emissions by requiring virtually every person in Australia to swap their wasteful incandescent light bulbs for more energy-efficient options. This is less than one tenth of one percent of current annual global emissions. For Branson and Gore, 'significant' amounts to something in the order of one billion tonnes per year - 250 times more than what Australia hopes to achieve. To make it happen every year - without side effects - is a truly monumental undertaking.

To the winner of the challenge goes the cold hard cash and to the rest of us go the spoils of a less-spoiled planet. Though the task may seem insurmountable, prizes have proved a potent incentive for innovation in the past. Branson himself is reaping the fruits of another large prize - licensing the technology that won the US$10 million Ansari Prize for the first private manned space vehicle for use by Virgin Galactic, his venture into space tourism.

While slowing global warming is arguably a bit more difficult than sending a few tourists into space, and though the prize won't be awarded until 2010 at the earliest, individuals and organisations are lining up to throw their hat in the ring. And since they're already queueing, we might as well start inspecting them to see if they've got what it takes to meet the challenge.

Here, fishy fishy
Some technologies aim to harness or improve on nature's own carbon removal and storage systems; its oceans, forests and soils. Others use a more artificial approach to carbon sequestration. An example of the former is Ocean Nourishment.

"Ocean Nourishment takes carbon dioxide from the atmosphere and converts it into fish," says Ian Jones of Sydney University's Ocean Technology Group. The idea is to dump nitrogen in the form of urea into the open ocean to stimulate the growth of plant-like microorganisms called phytoplankton. Like plants, phytoplankton perform photosynthesis - turning carbon into sugars that they use for food. Phytoplankton are near the bottom of the oceanic food chain, and more phytoplankton means more food for fish and other marine life. The theory goes: every animal is basically a mobile carbon storage device, so more animals in the oceans means better carbon storage.

The scheme was originally developed to restore the health of 'desertified' parts of the ocean with dwindling plankton populations, but snaffling excess atmospheric carbon dioxide is a welcome side effect. Early trials have been promising, but the biggest question is what will happen when it is scaled up.

Dawn Levy, a science writer at Stanford University in California, is wary of strategies like Ocean Nourishment and an analogous scheme that uses iron instead of nitrogen as fertiliser. "Monkeying with the food chain may have ripple effects - unknown consequences - throughout the system, and I doubt it would have a big enough effect to make a dent on the problem," she says.

There's carbon in them thar hills!
If the Earth Challenge were awarded today, many people would place their own money on geosequestration. Already well into the testing phase, variations on the geosequestration theme are probably the best known carbon-reducing strategies at the moment. Basically, geosequestration is the burial of carbon in rocks. Many possible locations have been mooted, including old oil fields, saline aquifers and unminable coal seams. Some schemes are looking to store carbon dioxide beneath the seabed.

For years mining companies have been pumping carbon dioxide and other gases into oil fields to pressurise oil pockets, helping push the oil to the surface. To many it makes sense to take advantage of this pre-existing infrastructure. The energy industry is well-placed to take the lead with geosequestration, as they can capture emissions at the source. In the North Sea, a Norwegian company has been separating carbon dioxide from natural gas and burying it for the last ten years, with over 10 million tonnes stored so far.

Critics of geosequestration say it amounts to sweeping carbon under the carpet, with a real chance that it could leak out at a later date. If this were to happen to carbon stored under the seabed, it could make the ocean more acidic – and ocean acidity is already a problem in some areas.

Kelpie Wilson, environmental editor at the online magazine truthout.org, believes everyone has overlooked something that applies to all forms of carbon sequestration. "When plants pull CO2 out of the air and use it to grow stems and roots, they recycle the oxygen back into the atmosphere. Are we in danger of burying a needful portion of our oxygen deep in the Earth?"

Fake plastic trees
Klaus Lackner of Columbia University in New York has several fingers in the Earth Challenge pie. One project of Lackner's that has attracted attention is using artificial trees to capture carbon dioxide. In fact, it was Lackner's daughter who performed the proof of concept experiment for the technology - for her high school science class. Blowing carbon dioxide through a solution containing sodium hydroxide (lye), she captured half of the CO2 as sodium carbonate, or soda ash.

Artificial 'trees' would be coated with a carbon-capturing chemical, their 'leaves' far more densely packed than a regular tree, as they need only be exposed to air rather than direct sunlight. It has been pointed out that the coating would need to be regularly recycled and refreshed – something trees do for no charge. The carbon captured using this method would then be piped away and stored using one of the geosequestration techniques.

As part of the industry group known as the Zero Emissions Coal Alliance (ZECA), Lackner is also behind proposals to minerally sequester carbon. In this process, carbon is transformed into a less reactive and – more to the point – less dangerous form than carbon dioxide.

One limitation of this method is the relative scarcity of raw materials for the reaction, which converts carbon dioxide into magnesium carbonate. Another extremely stable form of carbon is diamond. But if researchers could transform atmospheric carbon dioxide into diamonds they wouldn't need the prize money in the first place.

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It is genuinely exciting to think of the potential of these and other technologies that will emerge in the future. But how hopeful should we be?

Wilson has condemned the prize, which she calls "an engineer's wet dream", for encouraging inaction among the masses who blithely assume the inevitability of a techno-fix. This assumption is not shared by Jones and Lackner, who agree on the need to cut our fossil fuel use now. Others cry out for more support of renewable and non-polluting energies.

Levy, who thinks that the climate situation is more serious than the general public perceives, is more optimistic. "I think the prize will make a difference in both the short and long term. I am grateful people like Richard Branson and Al Gore are looking beyond profit and politics to address a problem that knows no borders."

A broader concern of Wilson's is one shared by many of us. When we set out to change the earth's climate on the grandest scale of all, how confident can we possibly be of precisely controlling its consequences? What if we overcorrect and remove too much carbon dioxide, killing all the trees and starving ourselves of oxygen in the process?

Last week the company Geopower Basel provided a timely warning to those who would save the Earth with technology. The geothermal energy project involved injecting pressurised water into 4.8 km deep bore holes near Basel, Switzerland, where the water is heated to 200 ° C. When the water returns to the surface as steam it drives a turbine to generate electricity. Unfortunately the work set off four minor earthquakes in the area.

Jones summed up the current pickle when he told the BBC, "Once you start managing nature you have to continue to manage nature, there is no use hoping that it will restore itself to a new equilibrium set up by humans."

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