At first glance, the Copenhagen climate summit seems like a Salvador Dali dreamscape. I just saw Archbishop Desmond Tutu being followed by a swarm of Japanese students who were dressed as aliens and carrying signs saying "Take Me To Your Leader" and "Is Your Species Crazy?". Before that, a group of angry black-clad teenage protesters who were carrying spray cans started quoting statistics to me about how much carbon dioxide the atmosphere can safely absorb. (It's 350 parts per million they pointed out, before sucking their teeth.) Before that, I saw a couple in a pantomime cow costume being attacked by the police, who accused them of throwing stones with their hooves.

But the surrealism runs deeper and darker than this. Inside the Bella Centre, the rich world's leaders are defiantly ignoring their scientists and refusing to sign a deal that will prevent our climate from being dramatically destabilised. The scientific consensus shows the rich world needs to cut 40 per cent of our emissions of warming gases from 1990 levels by 2020 if we're going to have even a 50-50 chance of staying this side of the Point of No Return, when the Earth's natural processes start to break down and warming becomes unstoppable. Yet the scientists at Climate Analytics calculate our governments are offering a dismal 8-12 per cent cut – and once you factor in all the loopholes and accounting tricks, it becomes a net increase of four per cent.

Privately, government negotiators admit there's no way the negotiations will end with the deal scientists say is necessary for our safety. Indeed, it looks possible that this conference won't deepen and broaden the Kyoto framework, but cripple it. Kyoto established a legally binding international framework to measure and reduce emissions. The cuts it required were too small, and the sanctions for breaking it were pitifully weak – but it was a start. Kyoto's current phase expires in 2012, but the treaty's authors believed its architecture would be retained and intensified after that. The developing countries assumed that's what they were here to do. But the US is proposing to simply ditch the Kyoto infrastructure – won over decades of long negotiations – and replace it with an even weaker voluntary deal. In their proposal, every country will announce cuts and stick to them out of the goodness of their hearts. No penalties, no enforcement.

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So at the centre of this summit is a proposition stranger than any number of arrested cows or Nasa-quoting hoodies: we're playing Russian roulette with the climate, and our most powerful governments are filling the barrels with extra bullets, one by one.

Yet this conflagration here in Copenhagen is heartbreaking and heartwarming all at once. Our governments are showing their moral bankruptcy – but a genuinely global democratic movement is swelling to make them change course. Mass democratic agitation is the only force that has ever made governments moral before; it will have to do it again.

An army of dedicated campaigners is gathering here, and they are prepared to take real risks to oppose this sham-deal. The protest march on Saturday here must have been the most genuinely global demonstration in history. Under banners saying "There Is No Planet B", "Nature Doesn't Do Bailouts" and "Change the Politics, Not the Climate", there seemed to be people from every nation on earth. Lawrence Muli from Kenya's youth delegation told me: "We are having the worst drought in memory in Kenya. The seasons have changed in ways we don't understand. My family can't grow crops any more, so they are going hungry. I am here to say we won't die quietly."

Next to him was Bhuwan Sambhu from Nepal, who has seen his glaciers retreat dramatically in his short lifetime. Just behind them was Manuel Wiechers from Mexico City, who said his hometown has been devastated by the worst rains on record. At his side was Utte Richter, a 76-year-old German woman who said: "It would be immoral to stay at home when these decisions were being made, with everything they mean for the world. This system is near the end of the road, and we must change to a new way."

The same arguments are heard in the corridors of the Bella Centre, where the representatives of the poor countries are refusing to sign up to a deal that will dry out or drown much of their land. The government of Tuvalu – the low-lying island that is already being drowned by rising seas – has calmly, with great dignity, interrupted meetings that presume we can carry on emitting carbon, pointing out this means "we will die". Lumumba Di-Aping, the chief negotiator for the G77 block of developing countries, wept as he explained: "The more you defer action, the more you condemn millions of people to immeasurable suffering." He said our governments are acting "like climate sceptics. If they really believed global warming was happening, how could they do this?"

Today, these two strands of protest – inside the conference, and outside – will combine. Some of the delegates are expected to walk out of the Bella Centre talks in disgust. At the same time, brave young protesters supporting their message will be trying to break in, to express their revulsion at the betrayal of us all going on there. Of course, the parts of the global media that serve the interests of the polluting rich will be keen to shift the story on to "vandals" and "violent protest". There may be a minuscule minority of protesters who behave unacceptably. But in reality, there are two forms of vandalism about to happen in this city. There is the cutting of a few fences as part of an act of mass civil disobedience. It is an attempt to symbolically resist the much bigger act of vandalism – the trashing of our own habitat, by leaders too short-sighted and too money-addled to listen to the science.

Isn't it violent to knowingly condemn whole countries to drown? Isn't it vandalism to knowingly let the world's most crucial farming land crust over, its most precious rivers run dry, and its hurricanes become super-charged? Isn't that immeasurably worse than breaking a fence and cutting a cordon? Couldn't resistance to this destruction-machine justify this tiny act of destruction? The young protesters who will do this have proved themselves, so far, the sanest force in town. They have ensured that the corporate lobbyists punching holes in the deal are followed and shamed wherever they meet. They chant: "It's not your business – it's our climate."

When I hear the activists, I remember something Farley Mowat, the Canadian conservationist, wrote in the 1990s: "The last three decades of this century have witnessed the ignition of the most significant internal conflict ever to engage the human species. It is not the struggle between capitalism and communism or between any other set of 'isms'. It is the conflict between those who possess the means and will to exploit the living world to destruction, and those who are banding together in a desperate and last-ditch attempt to prevent the New Juggernaut from trashing our small planet."

This week, the small band of the sane got a little bit bigger and a lot more global. For today, it is vastly outgunned by the forces of ecological destruction, and it will certainly not be able to ensure a sane deal in Copenhagen. But think of all the other movements that were small at first and held up impossible dreams. They called him "Martin Loser King"; they said civil rights would never come; now everyone says he was right and there's a black President (although alas not a green one).

As Archbishop Desmond Tutu pointed out here, they said the Berlin Wall would never fall, and they said apartheid would never die; now they say we cannot make the transition from an economy powered by coal and oil to one powered by the sun, the wind and the waves. But unlike previous protest movements, we can't wait for it to accumulate speed over generations. Each tonne of carbon brings us closer to climatic – and climactic – tipping points. This is a leap human beings must make in one generation.

We know it can be done. We have the knowledge and the science. If we refuse to do it – out of inertia and denial and so a few fossil fuel corporations can carrying on raking in profit and bribing our politicians – that will be this summit's most surreal scene of all.

j.hari@independent.co.uk