by Johann Hari

"Geo-engineering" sounds like a bland and technical term but it is actually a Messianic movement to save the world from global warming, through dust and iron and thousands of tiny mirrors in space. It is also the last green taboo.

Environmentalists instinctively do not want to discuss it. The wider public instinctively thinks it is mad. But now, the taboo has been breached. James Lovelock, one of the founding fathers of modern environmentalism, proposed a way to slash global warming without cutting back on a single fossil fuel.

"Geo-engineers" believe man should consciously change the planet's environment, using technology, to counter the effects of global warming.

They are like a chef who realizes she has accidentally put in too much cayenne, so reaches for lashings of oregano to balance it out, only this time the recipe is the atmosphere of the planet Earth. Ken Caldeira, a geo-engineering expert at the Carnegie Institute, says: "In effect, we're already engineering the climate by emitting so many greenhouse gasses. We just don't want to admit it. You can argue that the only reason difference between what we're doing today and what geo-engineering advocates are proposing is a matter of intention. And frankly, the atmosphere doesn't care about what's going on in our heads."

Grand geo-engineering schemes come in two main flavors. The first tries to increase the oceans' capacity to absorb carbon from the atmosphere. At the moment, the oceans are, along with the rainforests, the most effective natural mechanism for taking carbon out of the atmosphere. So geo-engineers ask: Is there anything we can we do to supercharge them?

The simplest proposal is to sprinkle vast amounts of iron along the surface of the world's seas. This would create the ideal conditions for a surge in the quantity of plankton, the friendly microorganisms that "eat" carbon while they are alive. When they die, they sink to the bottom of the ocean -- taking the carbon with them, for centuries, to a watery grave. It has been tried in a number of small-scale experiments off the coast of the Galapagos Islands, and it did indeed cause dead seas to spring to life with carbon-sucking plankton.

Enter James Lovelock, with a similar proposal. He suggests another way to spur the oceans to sink massive amounts more of carbon dioxide. His plan is to build vast vertical pipes across the world's seas. They would pump water from the bottom of the oceans -- rich in nutrients, but mostly dead -- to the top. This rich water would be ideal for microorganisms such as salps to breed in. They too "eat" carbon n and then excrete it, where it sinks to the floor of the ocean.

The second school of geo-engineering projects tries to reflect much more of the sun's energy back into space, so it doesn't stay here and cook us. For example, we know that when volcanoes erupt, they release huge amounts of tiny sulphuric dust into the atmosphere that serve as a blanket and measurably cool the planet down. When Mount Tambora blew in 1815, for example, it was known as the "year without summer." So scientists such as the Nobel Prize-winner Paul Crutzen have suggested we may have to artificially simulate this effect, by spraying sulphur into the atmosphere: in effect, fighting pollution with pollution. The U.S. National Academy of Sciences has gone even further, suggesting that 55,000 small mirrors placed in the upper atmosphere would be enough to counter about half the impact of global warming.

So why have greens been reluctant to discuss these solutions? They have a very good reason. All the evidence suggests that, in reality, they cannot work -- but they sound just plausible enough to join denialism as another hallucinatory excuse to do nothing while the planet boils.

To understand why, you need to look to the conservative philosopher Edmund Burke. In the 18th century, Burke argued that the functioning of human societies was so complex it could not be fully understood by the rational mind. If you pulled out one thread for impeccably rational reasons -- by, say, abolishing the monarchy n you would find that dozens of other threads would come loose too, in ways you couldn't have predicted and would never have wanted. Burke was seriously wrong about human societies -- but, by a strange historical quirk, his approach applies quite well to understanding the ecosystem of the planet.

Look again at the geo-engineering schemes we're discussing and you'll see how. Plans to make the plankton and salps "eat" the carbon for us bump up against an unintended consequence. Too much organic matter sinking all at once triggers the release of methane n the most warming gas of all. What about pumping sulphur into the atmosphere? Ken Caldeira explains: "One of the problems...is that it would destroy the ozone layer, so you might solve the problem of global warming, but then we'd all die of that."

Nor do any of these schemes deal with the other great problem caused by our greenhouse gas emissions. They are making the oceans more acidic, killing off shell and coral formation at the bottom of the food chain. So even if we somehow blunted the global warming effect, the increased carbon in the atmosphere would still kill the oceans -- and ruin our sources of food.

Yet soon, the fossil fuel industry may start marketing geo-engineering as The Solution, an alternative to cutting back. The scientist Josh Tosteson puts the necessary response well when he asks: "Do we really have the capacity to understand complex systems at the level of the globe well enough to make conscious perturbations that result only in the consequences that we want, and nothing else?" (Burke couldn't have put it better.)

It is far smarter to try to stay close to the carefully balanced ecosystem that has evolved over millions of years than to cack-handedly engineer our own, with the extremely limited knowledge we have.

Looking to smoke and mirrors in space, or James Lovelock's beautiful pipe dream, to save us from having to cut back our carbon emissions is achingly tempting. I love the belching world we live in, and I wish it could be made to work. But carrying on pumping out greenhouse gases because of the possibility of geo-engineering is like telling an alcoholic that he doesn't need to quit drinking, because in a few years you'll give him a liver transplant -- with a few rusty old knives you found in your garage.

Yet if we don't slash emissions now, in just a few decades' time we will be inescapably smacking into these geo-engineering choices. Do you save us from runaway global warming for a while, at the cost of destroying the ozone layer forever? Do you cool the oceans while letting them become acidic and die? Do you want to have to make that call?

Johann Hari writes for The Independent in Britain; j.hari@independent.co.uk