After spending the last 40 years trying to convince the world that global warming is real, the last thing climate scientists want to do is convey the impression there's an easy fix.

Nevertheless, as politicians dither and fossil fuels continue to burn, researchers are increasingly looking seriously at the idea of engineering the planet - geoengineering - to stave off the more extreme effects. A radical, if costly, intervention may be the only way to avert a planet-wide catastrophe.

Most of the many schemes being studied aim either to reduce sunlight to cool the planet or to capture and store carbon dioxide to halt the impact of fossil-fuel use. All have risks and costs, and side effects cannot be predicted or managed with certainty.

Without a doubt, scientists say, it is far better to reduce carbon-dioxide emissions than have to deal with the consequences. In 2006, former World Bank chief economist Nicholas Stern rang up the cost of climate change to the world economy at trillions of dollars over the next decade.

But if there's no choice, even the costliest proposals may look reasonable as ways to buy crucial time. "The hope is that we could buy a few decades to develop new energy sources," says John Latham, of the University of Manchester.

CLOUD BRIGHTENING

What it is A fleet of autonomous, wind-powered vessels would move about the oceans and spray tiny droplets of sea water into the air to enhance the reflectivity of marine clouds.

Hoped-for effect Clouds do a good job of reflecting incoming sunlight back into space. The sprayed droplets would add to their size and density, producing clouds that would scatter more light so the Earth would absorb less solar energy. Proponents suggest a 10-per-cent brightening of the clouds could achieve enough of a cooling to counterbalance the effects of a doubling of atmospheric carbon dioxide, which is expected by 2060.

Cost Relatively low (in the billions of dollars).

Risk Clouds are a complicated component of the Earth's climate system and changing their characteristics could lead to unintended consequences, especially in global precipitation patterns. One projection suggests it could produce drought in parts of South America.

Realism rating Medium. The technology to create a fine enough mist is still being developed. And this plan does not address the issue of carbon in the atmosphere and related problems such as ocean acidification.

STRATOSPHERIC AEROSOLS

What it is Sulphur-based chemicals would be injected into the upper atmosphere by balloon, rocket or high-altitude jet at regular intervals. The chemicals would stimulate the production of fine particles known as aerosols.

Hoped-for effect Similar to the natural cooling that occurs when a large volcanic eruption blasts millions of tonnes of sulphur dioxide into the stratosphere, where it combines with water to form droplets of sulphuric acid that reflect incoming sunlight back into space. The eruption of Mount Pinatubo in the Philippines in 1991 caused the average global temperature to drop by nearly one degree (Celsius) for about two years. It's estimated one to two million tonnes of sulphur would need to be delivered into the stratosphere each year to sustain a long-term cooling effect.

Cost Low to medium, depending on how longit would be needed. Estimated at $25-billion to $50-billion per year.

Risk Because the aerosols will eventually drift back down to Earth, they will contribute to air pollution and acid rain, though not a lot compared with the amount of sulphur that industry and power plants are pumping into the air already. Another risk is the possible impact on the ozone layer. Nevertheless, Paul Crutzen, the Dutch atmospheric chemist who won a Nobel for his work on ozone depletion, says the idea should be taken seriously.

Realism rating High. Unlike many proposals for geoengineering, this one has a natural analog and relies mostly on existing technologies. It would also have the speediest effect. A more far-fetched, science-fiction version of the scenario would replace the sulphur with smart, self-levitating, reflecting particles that would last longer and avoid polluting the atmosphere with sulphur.

SUN SHADES

What it is A giant screen, or a constellation of smaller screens, deployed in space and located millions of kilometres from Earth to intercept incoming sunlight.