The idea of serious scientists and engineers gathering to discuss schemes for controlling the world's climate would a mere 10 years ago have seemed bizarre, or something from science fiction. But now, well into the 21st century, we are slowly and reluctantly starting to realise that global heating is real. We may have cool, wet summers in the UK, but we are fortunate compared with the Inuit, who see their habitat melting, and Australians and Africans who suffer intensifying heat and drought. We should not be surprised that public policy is edging ever nearer to geoengineering, the therapy our scientists are considering for a fevered planet.

Our senior scientific society, the Royal Society, met at the start of the month to launch the report "Geoengineering the Climate" and to hear from its representative scientists. The meeting was hosted by the president, Lord Rees, and the chairman was Professor John Shepherd, who chaired the study group. The goal, as Prof Shepherd explained in the Guardian in April, was to investigate theories of "intervening directly to engineer the climate system, so as to moderate the rise of temperature" and to "separate the real science from the science fiction".

Geoengineering is about deliberately changing the air, oceans or land surface of the world to offset global heating with the hope of restoring the cooler world we enjoyed in the last century. We are now fairly sure that the Earth has grown hotter by about one degree Celsius as a consequence of our own action in taking away as farmland the forests and other ecosystems that previously acted to keep the Earth cool. We also have increased by 6% the flow of CO 2 into the air by burning coal, oil and natural gas. If we started global heating, can we reverse it by engineering?

The first scientist to consider geoengineering seriously was the Russian geophysicist Mikhail Budyko. In the 1970s he proposed that we could offset global heating by spreading in the stratosphere a fine dispersion of particles that reflected sunlight back to space; he based the idea on the observation that volcanic eruptions that did this were followed by global-scale cooling. He suggested that we could mimic the effects of a volcanic eruption by putting an aerosol into the stratosphere. His idea was confirmed by the detailed observations and analysis of the effect of Mount Pinatubo's eruption in 1991. It injected 20m tonnes of sulphur dioxide into the stratosphere and this soon oxidised to form the white reflecting particles that offset global heating for three years. It is within our capacity to put this much sulphur dioxide into the stratosphere.

There are other ways of reflecting sunlight: large mirrors or diffusers of sunlight put in orbit around the sun. One of the more promising and controllable reflection methods was put forward by John Latham and Stephen Salter, who proposed spraying very fine droplets of sea water from the ocean surface to make the natural surface clouds, called marine stratus, whiter.

As well as cooling by reflecting sunlight away we could cool by removing the carbon dioxide or methane from the air. Klaus Lackner has proposed making artificial trees to do this; others, following the lead of Johannes Lehmann, would sooner see vegetation capture CO 2 and then, after harvest, turn the plant waste into charcoal and bury it.

Geoengineering implies that we have an ailing planet that needs a cure. But our ignorance of the Earth system is great; we know little more than an early 19th-century physician knew about the body. Geoengineering is like trying to cure pneumonia by immersing the patient in a bath of icy water; the fever would be cured but not the disease.

Many of us feel a sense of unease about using geoengineering to escape global heating. Most of the planetary therapies have side effects, potentially as severe as the disease itself. We know that the cooling by Pinatubo was accompanied by droughts; cooling alone does nothing to prevent the ocean growing ever more acid as the carbon dioxide dissolves in the water.

Before long, global heating could reach a level that makes geoengineering an enticing option. Indeed, cautiously applied it may help by buying us time either to adapt to climate change or to develop a practical scientific cure. We have, as yet, no comprehensive Earth system science; in such ignorance I cannot help feeling that attempts by us to regulate the Earth's climate and chemistry would condemn humanity to a Kafkaesque fate from which there may be no escape. Better, perhaps, to learn from the wiser physicians of the early 19th century; they knew no cure for common diseases but also knew that by letting nature take its course, the patient often recovered. Perhaps we, too, had better use our energies to adapt and leave recovery to Gaia; after all, she has survived more than three billion years and has kept life going all that time.