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On the surface, it looks like any other weather-beaten offshore rig, with its towers of scaffolding, heavy-duty cranes and helicopter landing pad. Located in the North Sea's Sleipner West field, some 230 km off the Norwegian coast, the facility has pumped about 55 billion standard cubic meters of natural gas for Statoil, Norway's state oil company, over the past eight years. But beneath this particular rig lies what could turn out to be a cost-effective technique for fighting global warming.

Traditional drilling for fossil fuels like natural gas and oil releases millions of tons of carbon dioxide (CO2) into the atmosphere. CO2 is a greenhouse gas that is both naturally present in oil and gas fields, and is injected into the ground to boost the extraction process. Along with emissions from cars, fossil-fuel power stations and industry, oil and gas drilling contributes to the earth's rising temperatures. Beginning in 1996, Statoil has deployed a new method called carbon sequestration to stop the CO2 escaping: Statoil engineers remove the CO2 from the rising column of natural gas and send the greenhouse gas back into the ground, all in one continual process. So far the firm has stashed some 7.5 million tons of CO2 in a kind of emissions tomb known as a saline aquifer 1,000 m beneath the ocean floor. Statoil estimates there's room for 592 billion tons more, the equivalent of the CO2 emissions from all the power stations in Europe for the next 600 years. Canada's EnCana is also putting CO2 back into the ground, and BP and Gaz de France will be trying the technique soon. "Carbon storage is suddenly catching on," says Tore Torp, head of Statoil's CO2 research program. "Sleipner will not be a lone lighthouse for much longer."

Carbon storage and capture is not what environmentalists would call a green technology; its raison d'etre is to sustain and even increase the use of fossil fuels like oil, gas and coal (this time Next report also explores new developments in wind, solar and hydroelectric energy). But sustainable energy solutions — even imperfect ones — are needed in a world addicted to fossil fuels, and carbon sequestration could help the transition to clean, renewable fuels over the next 30 years. One reason for carbon sequestration's newfound popularity in Europe is that, starting in 2005, the E.U. will cap carbon emissions as part of its commitment to the 1997 Kyoto agreement on global warming. Installations will be assigned a carbon emission limit; if an installation exceeds its allowance, it will either have to pay a fine or purchase surplus allowances from other installations through a carbon emission trading system .

But even in areas where such schemes don't exist, the imperative for storing carbon is clear. The threat to the world's climate posed by rising levels of greenhouse gases is now accepted by the overwhelming majority of the world's scientists — even some Pentagon analysts have warned that global warming could pose a greater global security threat than nuclear war or terrorism. At the same time, energy demand keeps rising, and fossil fuels will continue to play the leading role in meeting world energy needs for decades to come. "We have a global energy system that is 85% dependent on fossil fuels, and it will take decades to change that," says Howard Herzog, who leads carbon sequestration research at the Massachusetts Institute of Technology. "Carbon sequestration is the only technology that can wean us off fossil fuels without too much of a shock to the system."

Statoil didn't develop the carbon sequestration technique to soothe its environmental conscience; it did so because reducing the amount of CO2 in the atmosphere makes good economic sense. In 1991, the Norwegian government introduced the world's first CO2 emissions tax, which today stands at j40 per ton of CO2 released. Under that scheme, Statoil would have had a CO2 tax bill for Sleipner West of j40 million a year. Instead, at a cost of about j70 million in equipment, they created the carbon sequestration system. "This approach will be one of the key solutions if emission reductions are really going to bite," says Torp.

Oil companies have taken the lead in carbon sequestration because they already possess the necessary technical expertise; for years they have transferred CO2 around as a way to extract more fuel. EnCana's Weyburn operation in Saskatchewan, Canada, for example, needs to pump extra CO2 into the ground to get more oil out of an old field. It imports 5,000 tons of CO2 a day from a U.S. coal gasification plant in North Dakota. Normally, oil companies either re-use any excess or release it into the atmosphere. In Weyburn's case, excess CO2 that would have been released into the atmosphere from the North Dakota plant is now used to pump more oil, and is then buried deep underground. EnCana expects the project to extend the life of the Weyburn field by more than 25 years.

As proficient as the oil and gas industry gets at burying CO2, it's ultimately responsible for just a fraction of the world's CO2 emissions. To really tackle the carbon problem, researchers have to constrain the transport sector, which produces about one-fifth of all global CO2 emissions, and public utilities, including fossil-fuel power plants, which produce a third of all emissions. There is no effective way of sequestering CO2 emissions from automobiles, short of equipping every vehicle with its own capture technology and a place to store CO2 until it can be safely buried.

Power plants, on the other hand, produce enormous quantities of CO2 that can be centrally sequestered, and the U.S. is leading the charge for the effective storage of these gases. In 2003, the Bush Administration helped establish the Carbon Sequestration Leadership Forum, a group of countries working to develop FutureGen, the world's first near zero-emissions fossil-fuel power plant, within a decade. The $1 billion project would use coal to produce energy, and the resulting CO2 would be buried in aquifers and unmineable coal seams or sold to oil and gas companies for use in the extraction process.

Not everyone believes that carbon sequestration is a pollution panacea, however. "We could support carbon sequestration if it were proven to take CO2 out of the atmosphere safely and securely," says Bryony Worthington, a climate campaigner at Friends of the Earth U.K. Just how long CO2 can be safely buried under the earth is unknown. Statoil has been doing it the longest, but its Sleipner West rig has been in action for less than a decade. Worthington and other environmental activists argue that the $1 billion dedicated to FutureGen could be better spent. "The huge subsidies that will be needed to make sequestration competitive would be better used to support renewable energy technologies," says Worthington.

Even advocates acknowledge the risks. Carbon dioxide, for example, is extremely toxic. Could burying CO2 create a gas timebomb? Not according to John Gale, project manager of the International Energy Agency's Greenhouse Gas Research and Development Programme, based in the U.K. "You can't be 100% certain," he says, "but by the careful selection of storage reservoirs and the proper risk assessments, you should be able to minimize the likelihood of a catastrophe occurring." Oil companies certainly try; they conduct lengthy geological pre-screenings to ensure that there are no faults or weaknesses in fields, as well extensive risk assessment studies and 4-D reservoir modeling to determine the long-term fate of the CO2 prior to injection. Storage facilities are also constantly monitored to check for over-pressurization.

Yet even if Gale's advice is followed, no one can rule out an accident or leak. That's why some scientists are calling for more radical solutions, like converting CO2 into rocks. In this technique, CO2 is mixed with serpentine, a type of mineral, and water to create safe and stable — if bulky — carbonate boulders.

Another potential technique is deep-sea storage — shipping excess CO2 out to the deep ocean and pumping it down to the very bottom of the sea. Its advocates, such as the British Department of Trade and Industry (DTI) and Ola M. Johannessen, director of the Nansen Environmental and Remote Sensing Center at the University of Bergen, argue that the deep ocean has the potential to store between 1,000 and 27,000 gigatons of CO2; more than we are ever likely to produce before fossil fuels run out. The low temperatures and high pressure of the deep ocean, they say, will convert the gas into a harmless liquid and trap it for centuries.

Critics say that not enough is known about how large amounts of CO2 might change the oceans' chemistry and affect the health of coral reefs and fish. There's another hurdle too, as DTI discovered when it suggested burying Britain's CO2 deep in the North Sea: the dumping of industrial waste at sea is barred by international treaties such as the 1972 London Convention and the 1996 protocol that replaced it. Further constraints on oil-field dumping were set by the 1992 Convention for the Protection of the Marine Environment of the North-East Atlantic. Any deep-sea carbon-sequestration initiative would have to overcome these regulations. In the end, the most successful form of carbon storage can only buy more time to develop truly clean and renewable forms of energy. But that prospect is decades away. In the meantime, carbon sequestration can help stop the rot.

• Matthew Yeomans