Some scientists think that the world's halting attempts to reduce carbon emissions are bound to fail. So they have proposed various schemes for counteracting the global warming impact of fossil fuels. The Gaia scientist James Lovelock proposed an unusual and untested idea in a recent paper. He suggested that we install millions of pipes to bring nutrient-rich water to the surface to feed carbon sequestering organisms. Other scientists are working on schemes as diverse as mirrors that reflect part of the sun's energy, increased aerosol pollution to stop sunlight getting to the earth, and improving plankton growth by adding iron to the oceans. All these schemes are 'offsets'; they seek to counter-balance the impact of human activities with schemes to reduce CO2 elsewhere. The technology optimists believe that one or more of these techniques can completely counteract human effects. The cost often seems very reasonable – in the billions rather than the trillions – and the technological challenges seem not insuperable. The pessimists say these schemes will have huge unintended effects, possibly worse than climate change itself, and that toying with 'geo-engineering' projects, as they are called, simply delays the day that the world starts to realise it must cut fossil fuel use. Geo-engineering deals with the symptoms, not the causes, of global warming. And none of the proposed schemes deal with the adverse effects of higher CO2 concentrations, such as increased ocean acidity.

This article argues that all the major geo-engineering proposals have substantial pitfalls, but that it makes clear sense to increase the research funding into these schemes. The opponents and proponents of geo-engineering have got locked into an almost theological debate as to the ethics of climate modification but this argument should be secondary to the need to have well-defined back-up plans in the event of increasingly rapid deterioration of the global climate.

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Geo-engineering The idea of geo-engineering has a long history. John von Neumann thought that climate modification could be used to ensure drought in the Soviet Union during the Cold War. Von Neumann later foresaw 'forms of climatic warfare as yet unimagined'. More peaceful schemes for climate modification in the 1960s included spreading heat-absorbing soot across the Arctic ice in order to encourage melting and eventually increase temperatures across northern Canada and Siberia. In the 1970s the US military made a failed attempt to increase rainfall over the Ho Chi Minh trail to impede movement by North Vietnamese forces.

By the 1970s scientists were already beginning to propose schemes for future emergency remediation of the atmosphere to counter rapid increases in temperature or other effects of global warming. Contrary to many people's impressions, scientific papers on the dangers of fossil fuel burning began to be written as long ago as the late 1950s.

The main ideas for geo-engineering to counter global warming have evolved gradually, usually over a period of decades. They can be divided into two main categories: improving ocean take-up of carbon and reflecting more of the sun's energy.

Ocean seeding of iron to increase plankton growth and sequester carbon.
The Lovelock scheme is a variant of this.
Mirrors in the upper atmosphere to reduce the amount of the sun's energy reaching the earth.
Increasing the reflective aerosol content of the upper atmosphere, also to reflect more sunlight.

Seeding the oceans with iron In some parts of the world the growth of ocean plankton is impeded by a lack of iron in the water. The advocates of ocean seeding say that seeding the ocean with more iron will enhance the growth rate of micro-organisms. Many experiments have confirmed that this is the case. This growth absorbs carbon. Approximately half of all photosynthesis on the planet is carried out by plankton, so this form of carbon capture is potentially extremely useful. It is also true that in many parts of the world's ocean plankton volumes appear to have fallen substantially, partly as a result of the warming of the oceans. So improved iron availability in the water may restore some part of the missing plankton. Increased plankton availability may also improve the volume of fish and birds up the food chain.

This much is largely agreed; the arguments come over whether the plankton stores carbon for a significant period of time.

The proponents of ocean seeding of nutrients say that when the organisms die they will sink to the ocean bottom and the carbon in their cells will be stored for centuries. The sceptics respond by expressing doubts as to whether the plankton sink, or simply rot near the surface; or, if they do sink, whether the carbon will be stored on the sea floor. So far, the evidence supports the sceptics.

This hasn't stopped attempts to use carbon sequestration by plankton as a commercial opportunity. Planktos, a US company, is just about to start an experiment by tipping one hundred tonnes of powdered iron into the Pacific, not far from the Galapagos Islands. The company sees the project as an attempt to understand whether seeding the ocean with iron can sequester carbon for long periods. Success would enable the company to start selling commercial offsets. Its critics say the experiments are just another form of pollution.

The dangers from the project are probably quite limited. The increase in oceanic iron will be measured in parts per trillion or less than the impact of Chinese sandstorms on the iron content in the western parts of the Pacific. This year's experiments by Planktos will probably show significant plankton growth, but are unlikely even to begin to convince the sceptics that the carbon taken up will be stored productively. But if it were to work, it would surely be quite inexpensive per tonne of carbon sequestered.

Plankton blooms off the coast

The Lovelock scheme James Lovelock – along with Chris Rapley of the Science Musuem – proposes that the world considers placing millions of vertical pipes several hundred metres long in the world's seas and oceans. Cold water would come to the surface as a result of a one-way valve and the upward and downward moving of the pipe. Cold water from the deep contains more nutrients and the scheme is intended to encourage the growth of tiny sea creatures such as salps. These organisms excrete carbon-rich waste, which then falls to the ocean floor. As with the iron dust idea, the increased growth of sea-level organisms helps to capture carbon. Once again, the key question is how long the carbon is held for before returning to the atmosphere.

The scientists behind the proposal think that the pipes may also have the effect of increasing algae growth. This will add to the ocean's output of dimethyl sulphide, a chemical known to stimulate cloud formation, possibly helping to block sunlight from reaching the earth.

Lovelock and Rapley don't propose this scheme because it is the best way of dealing with climate change. They seem to be advancing the idea in despair at the slow pace of political endeavours to check emissions growth. By coincidence, their scheme is in the early stages of commercial development by Atmocean, a US company based in New Mexico. As with Planktos, the immediate commercial applications of this technology seem non-existent. Their entrepreneurial drive must be based on a view that the carbon market will eventually reward businesses like these two for sequestering CO2. It has to be said that this is a brave gamble.

Mirrors in the atmosphere The global warming impact of increased atmospheric CO2 can be counterbalanced by decreasing the amount of solar energy reaching the earth's surface. Instead the light energy could be reflected. The percentage of the sun's energy that needs to be reflected is quite small; perhaps one or two per cent.

This could be done by placing billions of small mirrors in the high portions of the atmosphere reducing the amount of light energy getting to the earth's surface. Despite this idea resurfacing regularly at scientific conferences, there appears to be no large-scale research trial planned.

Sulphate or other aerosols in the high atmosphere Fine dusts thrown into the atmosphere reduce the strength of the sun's rays reaching the earth. We know this works because of the measurable impact of large volcanic eruptions that have spewed small particles into the air. The explosion of Mount Pinatubo in 1991 chilled the world by about 0.5 degrees the following year. Sulphates can be used as a countervailing pollutant to CO2.

Edward Teller, the main proponent of the US 'Star Wars' missile defence programme, wrote an article in 1997 suggesting that blasting enough reflective dust to chill the atmosphere would only cost a billion dollars a year.

This idea received powerful support over the last year from Nobel scientist Paul Crutzen who advocated further research into sending light-reflecting particles such as sulphates to the edges of the atmosphere. He and others have stressed that they view such schemes as last resorts but that the world needs to research these ideas further because of the possibility of extremely rapid warming at some stage in the future.

It does seem clear that we need to research this idea, however unattractive it seems. Blocking a portion of the sun's rays does not reduce the CO2 in the atmosphere so problems such the increasing acidification of the oceans will continue. The eruption of Mount Pinatubo did cool the world, but it also significantly changed rainfall patterns, causing drought in some areas.

In 1971 British meteorologist Hubert Lamb said that before we engaged in geo-engineering it would be 'an essential precaution to wait until a scientific system for forecasting the behavior of the natural climate...has been devised and operated successfully for, perhaps, a hundred years'. He was right, but we probably don't have the luxury of waiting.