A great lake under the ocean is oneof many schemes devised by scientists to cut carbon dioxide in the atmosphere and hence reduce global warming. But will such technofixes create new dangers?.

Japanese scientists want to create a great lake under the ocean, made not of water and salt but of carbon dioxide, liquified by the pressure of thousands of metres of ocean above. The scientists want to gather this polluting byproduct of the world's burning of oil and gas and pipe it somewhere it will do us less harm.

The undersea lake is one of a number of potential solutions to the global warming crisis. While there is still perceived to be no reasonable alternative to the burning of fossil fuels, scientists are wondering whether the byproduct, carbon dioxide, could not be siphoned off and shoved into some corner of the globe. The world produces six gigatonnes (six thousand million tonnes) of carbon dioxide a year, one sixth coming from power stations. A great undersea lake could easily store such quantities.

The lake could remain in its watery grave for thousands of years. By then, back on land, the days of fossil-fuel burning would be over and the greenhouse effect would be less acute. The great lake could buy us time.

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But technofixes like these have few takers. Environmentalists fear that such interference is like trying to compress a balloon - a new problem just emerges somewhere else. Witness the results of a 1994 pilot experiment, which tried another way of sucking carbon dioxide into the sea. Plankton, which absorbs a lot of carbon dioxide, lives on the surface of the ocean. The factor which limits the amount it can absorb appears to be iron. So, thought scientists, if we sprinkle the ocean with iron the plankton will absorb more carbon dioxide. But although the iron did provoke more plankton activity, organisms a rung up the food chain also tucked in to this unexpected feast and their numbers rose. These organisms just recycled the carbon dioxide back to the atmosphere.

Other solutions include growing more forests to absorb extra carbon dioxide. With improved forest management we could perhaps sequester around one gigatonne of the offending gas. But of course, it depends on what the forest is used for; if it is cut down and burnt then the gas is released into the atmosphere again.

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Another alternative is to stash the gas in underground nooks and crannies. Sam Holloway, a geologist at the British Geological Survey, has helped prepare a European Commission report on the underground disposal of carbon dioxide. The idea is to separate out carbon dioxide from power station emissions, compress it and pipe it under the land or the sea.

The carbon dioxide might be pumped into shallow gaps in the rock, such as the caves left from the mining of salt, or caverns specially hewn as a new home. The trouble with staying shallow is that there is insufficient pressure to turn carbon dioxide from gas to liquid. Short of pumping it in and sealing it off under pressure, which could cause all sorts of strains leading to shattering of rock later on, there seems to be no solution.

The answer, then, is to go deeper, where pressure and temperature work on the gas to turn it into a supercritical fluid, a kind of very dense gas. At these depths there is a wealth of reservoirs, great caverns in the rock. The only use to which humans have put these reservoirs so far, says Holloway, is to remove hot water, or hydrocarbons such as oil.

But most people have ruled out storage under the land, in Europe at least. This is because of the potential risks. In his report for the Commission of the European Communities non-nuclear energy research programme Holloway describes some rather dramatic ways of dying from sudden leaks. There is suffocation, for example. "At surface conditions, carbon dioxide is heavier than air. It therefore tends to cling to the earth's surface, forming a blanket-like cloud I pools of carbon dioxide may develop in valleys from which all oxygen has been driven away. At first, the danger of these clouds will not be noticed, because carbon dioxide is a colourless and odourless gas." A natural version of such a disaster happened in Cameroon in 1986. Large amounts of volcanic carbon dioxide escaped from Lake Nyos, killing more than 1,700 people.

Carbon dioxide can also kill by freezing. When a gas is released from a compressed state it expands, but to do this it requires energy which it gains by sucking the heat out of its surroundings. The result would be instant freezing of all nearby animals. Less colourful would be poisoning through a slow leak, contaminating drinking water supplies.

So an offshore solution is preferable. In any case, most European storage capacity is offshore, and most of that is in the North Sea. It has been estimated that the capacity there is great enough to cope with 20 years' of emissions from power stations.

But how deep should the storage go? In his report, Holloway says: "In the North Sea there are some very large, thick aquifers which dip at such a shallow angle that they are essentially horizontal over large areas. These appear highly suitable." There is a catch, though. Less dense than sea water, if the gas seeped through holes to the sea floor, it could bubble up to the surface. It would be impossible to ensure that the aquifer was fully sealed off.

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Nevertheless, the Norwegian oil company Statoil is to dispose of one million tonnes of carbon dioxide each year into an aquifer above the Sleipner Vest gas field in the North Sea. The carbon dioxide emerges from the field with the methane that the firm extracts. Statoil has to remove it anyway for the gas to be saleable.

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The gas fields lie deeper than the aquifers. The idea is to reach them just after their exploitation has come to an end and fill them with carbon dioxide before water starts to leak in instead.

But some scientists are attracted by the symmetry of filling empty oil fields. Once oil companies have extracted all the oil they can by other means, they sometimes try to tease out more by injecting carbon dioxide into the reservoir. Holloway has shown that if they exploited this approach to the full, the profits from the extra oil would equal the cost of separating out carbon dioxide from power station emissions and injecting it into these reservoirs. The advantage is that the technology is already tested, at least on land. Holloway says in his report, "The injection of substantial quantities of carbon dioxide into the ground is an everyday occurrence in the oilfields of West Texas and elsewhere."

Paul Freund, project director of International Energy Agency greenhouse gas research and development programme prefers the oil field to the aquifer option. "It seems so obvious I am not sure why it is not being done," he says. But even the undersea options are not completely safe. There are only a few bacteria living in saline aquifers, says Holloway, so little damage could be done there. The Japanese lake, however, could spell death for a lot of creatures about which we still know little.

Underwater storage also poses a threat to humans. One explanation for the Bermuda Triangle, a region where ships seem to disappear unusually often, is that a sudden release of gas from the ocean floor alters the sea's density, causing ships to sink because their support vanishes. Carbon dioxide stored underwater could cause similar disasters.

As a solution to global warming, underground disposal is probably most fraught with political, rather than technical problems. The oceans are seen by many environmentalists to be the one bit of nature that humans have not violated. Witness the strength of the protests about the dumping of the Brent Spar oil rig. But Freund sees underground disposal as a temporary, though essential measure. We would shift carbon dioxide damage from the top of the ocean to the bottom, he says.

Holloway's main recommendation is for more research. Freund says that meetings are starting with Statoil: "We have reached an understanding that we will help them develop a plan for monitoring and research on the Sleipner aquifer project."

But there is a catch and it lies in the place one would least expect: separating the carbon dioxide from the rest of the emissions. A typical power station belches a flue gas which, before it has been treated, contains between 3 and 16 per cent carbon dioxide. The carbon dioxide must be isolated or the storage space needed would be too big and the energy required to compress all the gas would be too great.

But the separation technology is so costly that it could add 40 per cent to the price of generating electricity. Looked at another way, it would take so much extra energy to separate out carbon dioxide that it would create an extra half tonne of carbon dioxide for every one tonne separated.

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Holloway says: "We need to do something about these carbon dioxide levels in the atmosphere. I assume that we will burn our fossil fuels over the next 100 years. This is one way that we can deal with it without profound changes in our way of life."