When it comes to deciding what should be done, most people, including the majority of environmentalists, shrink from the discomfort that goes with realistic thinking. George W Bush seems to have been persuaded that climate science is not a left-wing conspiracy to destroy the American economy. Along with the rest of our political leaders, however, he continues to insist there are no limits to growth. As long as we adopt new technologies that are supposedly environment-friendly, such as biofuels, economic expansion can go on as before.

At the other end of the spectrum, greens put their faith in sustainable growth and renewable energy. The root of the environmental crisis, they say - and here they agree with Bush - is our addiction to fossil fuels. If only we switch to wind, wave and solar power, all will be well.

In political terms, Bush and the greens could not be further apart, but they are as one in resisting the most fundamental fact about the environmental crisis, which is that it cannot be resolved without a major reduction in our impact on the Earth. This means curbing the production of greenhouse gases, but here fashionable policies can be self-defeating. The shift to biofuels, led by Bush but which is also underway in other parts of the world involves further destruction of rainforest, a key natural regulator of the climate. Reducing emissions while destroying the planet's natural mechanisms for soaking them up is not a solution. It is a recipe for disaster.

Yet standard green prescriptions are not much better. Many renewables are not as efficient or as eco-friendly as they are made out to be. Unsightly and inefficient wind farms will not enable us to give up fossil fuels, while large-scale hydroelectric power has major environmental costs. Moving over to organic methods of food production can have significant benefits in terms of animal welfare and reducing fuel costs, but it does nothing to stop the devastation of wilderness that goes with expanding farming to feed a swelling human population.

So conventional green nostrums are not all that different from Bush's business-as-usual policies. In each case, the end-result can only be a planet gutted of biodiversity, with humanity exposed to an increasingly hostile environment. To some extent, technology may be able to replace the biosphere that has been destroyed, but, like an obese patient hooked up to an artificial life-support system, we will be living on borrowed time. One day, the machine will stop.

The uncomfortable fact, which is ignored or denied by both ends of the environmental debate, is that an energy-intensive lifestyle of the kind enjoyed in the rich parts of the world cannot be extended to a human population of nine or 10 billion, the level forecast in UN studies for the middle of this century. In terms of resources, human numbers are already unsustainable. Global warming is the flipside of worldwide industrialisation, a side-effect of the dash for growth, and the reserves of oil and natural gas on which industry depends are peaking at just the point when demand for them is rising fast.

Contrary to the greens, there is not the remotest prospect that the world will renounce the use of fossil fuels. Ask any competent energy economist and you will discover that no expansion of renewables can satisfy the demand for energy that is being generated in China and India. Anyway, does anyone really expect the countries getting rich from hydrocarbons - Russia, Iran, Venezuela and the Gulf States - to give them up? As long as there is enough demand, these countries will continue extracting fossil fuels.

The only way forward is to curb the need for fossil fuels, while at the same time, since there is no way of giving them up altogether, making them cleaner. This means making full use of technologies many environmentalists view with superstitious horror. Nuclear energy has well-known problems of security and waste disposal and it is nothing like a universal panacea. Even so, demonising it is conventional green thinking at its delusional worst. Though solar power has potential, no type of renewable energy can replace the dirty fuels of the industrial past.

If we reject the nuclear option, we will inevitably end up going back to coal. There are emerging technologies that can make coal cleaner. That is no reason for turning our back on nuclear, which is already virtually emission-free. A similar reasoning applies to GM crops. Genetic engineering involves a type of human intervention in natural processes whose risks are not yet fully known. But the practical alternative is to carry on with industrial-style agriculture, whose destructive impact is all too clear.

Any feasible remedy for the environmental crisis involves high-tech solutions. The aim should not be to master nature or turn it into a mere resource for humans to exploit, as Bush and the greens, in their different ways, end up doing. Given the legitimate aspirations of people in developing countries, only a high-tech strategy has any chance of reducing the human footprint. But it will also be necessary to breach what has become the ultimate taboo and face up to the reality of population pressure.

Green activists, free-market economists and religious fundamentalists may not seem to have much in common, but they are all agreed there can be no such thing as overpopulation, or at any rate, nothing that can't be solved by better distribution, faster growth or a change in human values.

Actually, the perennially unpopular Rev Thomas Malthus was closer to the truth when, at the end of the 18th century, he argued that population growth would finally overtake food production. Industrial farming was supposed to make famine impossible. But it turns out to have been heavily dependent on cheap oil, and with farmland being lost as a result of the switch to biofuels, limits on food production are re-emerging. Far more than fantastical schemes for renewable energy, we need to ensure that contraception and abortion are freely available everywhere. A world of fewer people would be far better placed to deal with climate change than the heavily overpopulated one we are heading for now.

Despite unstoppable global warming, a humanly liveable world is still worth striving for. But it requires a sustained capacity for realistic thinking, which is not the strong point of the environmental movement. Along with the political classes, greens are in denial. While there is no technical fix for the human condition, intelligent use of technology is indispensable in coping with environmental disruption that is now unavoidable. It would be ironic if, because of their irrational hostility to high-tech solutions, the greens were to end up as much a threat to the environment as George W Bush.

· John Gray, a political philosopher, is author of Black Mass: Apocalyptic Religion and the Death of Utopia (Allen Lane)