We are – at the same time – thrillingly close and sickeningly far from solving our planetary fever. The world's leaders huddled in New York City yesterday to discuss man-made global warming, in a United Nations building that will soon be underwater if they fail. They all know what has to happen: their scientists have told them, plainly and urgently.

As man-made warming rises by up to 2.4C, all sorts of awful things happen – whole island-states in the South Pacific will drown, for example – but we can stop it. If we turn off the warming gases, the temperature will stabilise. But if we go beyond 2.4C, global warming will run away from us, and we will have lost the "Stop" button. The Amazon rainforest will dry out and burn down, releasing all the carbon stored in the trees; the vast amounts of warming gases stored in the Arctic will be belched into the atmosphere; and so 3C will turn ineluctably to 4C, which will turn to 5C, and the planet will rapidly become a place we do not recognise.

To stay the right side of this climatic Point of No Return, global emissions need to start falling by 2015 – just six years from now – and drop by 85 per cent by 2050. Our leaders need to agree this at the climate talks in Copenhagen in December. The scientific debate is over. The answer is in sight. Indeed, each one of the leaders could feel the solution on their skin and in their hair yesterday: it lies in the awesome power of the sun.

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Each day, the sun bombards our planet with 9,000 times more power than we need to run every car, warm every home, and power every electrical appliance on earth. If we can capture just a sliver of one per cent of it, we can kick fossil fuels into the melting dustbin of history. The technology exists. It is there, waiting for us. Professor Anthony Patt has shown that all the energy Europe needs could be provided by lining 0.3 per cent of the Sahara desert – an area the size of Belgium – with concentrating solar power technology. A consortium of Germany's leading corporations is raring to go. They just need the money. It costs a lot up front – $50bn – but this is nothing like as much we would spend chasing the last dribbles of oil into warzones, and defending ourselves as the planet goes into meltdown.

Every continent has the same option. The entire energy needs of the US could be met by covering 200 square kilometres of its empty deserts with solar plants: it would cost about 10 years' worth of oil purchases, with none of the wars, tyrannies, or blowback Islamism. China and India have similar options. It is achievable, with the kind of great effort we made to defeat the Nazis. We too could be a great generation – one that came close to the brink, but then came together in a great collective effort to change course. We would leave a lean, green civilisation that will run for millennia.

But instead, our leaders are fiddling with the old dirty technologies, too addicted and too addled to move us on and up. In Britain, we are actually turning back to coal, mining 15 per cent more this year than last. Professor Jim Hansen, the head of Nasa and the world's leading climatologist, calls coal power stations "death factories" that condemn millions to drown, or starve, or burn. Across Europe, solar power is being allowed to wither: Germany's biggest solar company, Q-Cells, has seen its stock fall from €100 to €10 in a year. The other market-leader, Spain, has seen a similarly disastrous fallback.

The World Bank, which receives £400m of your taxes every year, is promoting this soot-streaked vision across the planet. They have just spent $5bn helping poor countries to build power plants that will destroy them. Indeed, it just bankrolled the single biggest source of greenhouse gas emissions in earth – a coal plant in Gujarat, Western India.

How can this possibly be defended? US and European governments are engaged in the collective fantasy that coal can be rendered "clean" by "scrubbing" its carbon emissions from the chimney-stacks, and storing them somewhere forever. In the real world, one of the largest "clean coal" pilot plants in operation, Hazelwood power station in the Latrobe Valley, catches just 0.05 per cent of its carbon emissions. Professor Howard Herzog, the renowned expert on this technology, was recently asked what the chances of the technology achieving the cuts we need is. He replied: "Zero."

But a small number of people make a lot of money on coal and oil and gas. A shift to reaping power from the sun and the wind and the waves would render the rocks and barrels they have spent a fortune mining worthless – so they are prepared to pay politicians to keep the system working in their favour, and lavish billions on misinformation campaigns to keep us confused.

You can see this process working most clearly in the United States. Barack Obama is a highly intelligent man who has appointed some of the best scientists in the world to explain to him what needs to happen now. But he is trapped in a political system soaked in petrol. The lackey-filled House of Representatives has passed a woefully inadequate "Cap and Trade" bill, which – if it worked perfectly – would cut emissions by six per cent below 1990 levels. Even that won't happen: many of the permits oil companies are supposed to pay for will now be given away for nothing, producing no reductions at all. And even this feeble, sickly bill may not make it through Congress.

Meanwhile, China has hinted it would agree to more substantial restraint at Copenhagen if the rich world – responsible for 90 per cent of all the warming gases belched into the atmosphere so far – agrees to give one per cent of its GDP annually to poor countries to adjust to clean fuels. There's a lot to criticise the Chinese dictatorship for, but this isn't one of them. It's a reasonable request for simple justice. Poor countries have done very little to cause this crisis, but they will feel the worst, first. They deserve our reparations. Yet both the EU and US have damned this sane proposal as "totally unrealistic".

So are we, as a species, condemned to fall into the historical crack between a world powered by fossil fuels, and one powered by the sun? Will the fossil record discovered millions of years from now show we were just too irrational and too primitive to make that leap?

If we despair and wait glumly for the meltdown, we will make it so. Then we will have little choice but to try to survive as best we can in a radically altered landscape. But there is still a slim window in which sanity can prevail – and I believe, perhaps madly, that it can. It will require a global mass movement of extraordinary tenacity, pressuring governments everywhere, and overpowering the fossil fools. We can still change the tale of the 21st century from one of collapse to one of a species finding a way to live with its ecosystem, rather than against it.

It can be done. It must be done. Copenhagen is in three months. There, and in the years after when the deal must be implemented, we will learn something profound about ourselves. Are we a great generation – or the worst of all?

j.hari@independent.co.uk