Johann Hari: How much proof do the global warming deniers need?

Thank God man-made global warming was proven to be a hoax. Just imagine what the world might have looked like now if those conspiring scientists had been telling the truth. No doubt Nasa would be telling us that this year is now the hottest since humans began keeping records. The weather satellites would show that even when heat from the sun significantly dipped earlier this year, the world still got hotter. Russia's vast forests would be burning to the ground in the fiercest drought they have ever seen, turning the air black in Moscow, killing 15,000 people, and forcing foreign embassies to evacuate. Because warm air holds more water vapour, the world's storms would be hugely increasing in intensity and violence – drowning one fifth of Pakistan, and causing giant mudslides in China.

The world's ice sheets would be sloughing off massive melting chunks four times the size of Manhattan. The cost of bread would be soaring across the world as heat shrivelled the wheat crops. The increased carbon dioxide in the atmosphere would be fizzing into the oceans, making them more acidic and so killing 40 per cent of the phytoplankton that make up the irreplaceable base of the oceanic food chain. The denialists would be conceding at last that everything the climate scientists said would happen – with their pesky graphs and studies and computers – came to pass.

This is all happening today, except for that final stubborn step. It's hard to pin any one event on man-made global warming: there were occasional freak weather events before we started altering the atmosphere, and on their own, any of these events could be just another example. But they are, cumulatively, part of a plain pattern where extreme weather is occurring "with greater frequency and in many cases with greater intensity" as the temperature soars, as the US National Climatic Data Centre puts it. This is exactly what climate scientists have been warning us man-made global warming will look like, to the letter. Ashen-faced, they add that all this is coming after less than one degree of global warming since the Industrial Revolution. We are revving up for as much as five degrees more this century.

Yet as the evidence of global warming becomes ever clearer, the momentum to stop it has died. The Copenhagen climate summit evaporated, Barack Obama has given up on passing any climate change legislation, Hu Jintao is heaving even more coal, David Cameron has shot his huskies, and even sweet liberal Canada now has a government determined to pioneer a fuel – tar sands – that causes three times more warming than oil. True, the victims are starting to see the connections. The Russian President, Dmitri Medvedev, had been opposed to meaningful action on global warming until he found the smoke-choked air in the Kremlin hard to breathe. But if we wait until every leader can taste the effects of warming in their mouths, the damage will be irreparable.

Given the stakes, the reasons why so many people still refuse to accept the evidence can seem oddly trivial. A common one is: "It snowed a lot in the US and Britain last year. Where was your warming then, eh?" But scientific theories are based on patterns, not individual events. You might know a 90-year-old woman who has smoked a pack of cigarettes every day of her life, and is totally healthy. (I do.) It doesn't disprove the theory that smoking causes lung cancer. In the same way, one heavy snowfall doesn't prove anything if it is part of a wider overall pattern of dramatic warming. And that snow probably was. While it snowed a lot in a few places, there were at the very same time harsher, more bitter droughts in many more places – making it globally the fifth hottest winter ever recorded, according to the US National Oceanic and Atmospheric Administration. (All the others were in the past decade). And that winter is your punchline proof that warming isn't happening?

But the broader public mood, smeared like sun-screen over us all, isn't active denial. No – it's the desire to endlessly postpone this issue for another day. In 1848, a 25-year-old man called Phineas Gage was working on constructing the American railroads. It was his job to lay explosives to clear rocks out of the way – but one day his explosive went off too soon, and a huge metal rod went through his skull and out the other side. Amazingly, he survived – but his personality changed. Suddenly, he was incapable of thinking about the future. The idea of restraining himself was impossible to grasp. If he had an urge, he would act on it at once. He could only ever live in an eternal present. As a civilisation, we are beginning to look like Phineas Gage on a planetary scale.

Yet scattered among us there is a fascinating group of people who are offering a path to safety. Every summer since 2006, ordinary British citizens have built impromptu camps next to some of the most environmentally destructive sites in Britain, and taken direct action to shut their pollution down. So far, it has worked: they played a crucial role in the cancellation of the third runway at Heathrow and a big new coal power station at Kingsnorth.

That's how earlier this week I found myself on a high wooden siege tower in a camp in the Scottish hills, staring down across a moat towards the glistening, empty offices of the Royal Bank of Scotland. You own this bank: 84 per cent of it belongs to the taxpayer after the bailouts. Yet it is using your money to endanger you, by financing the most environmentally destructive behaviour on earth, like burning the tar sands. The protesters chose to come here democratically – everything at the climate camps is done by discussion and consensus – because they have a better idea. Why not turn it into a Green Investment Bank, transforming Britain into a global hub for wind, solar and wave power? Why not go from promoting misery across the world to being a beacon of sanity?

So the protesters risked arrest in marching on RBS's offices because they know the stakes. As Professor Tim Flannery, one of the world's leading climate scientists, explains: "My great fear is that within the next few decades – it could be next year, or it could be in 50 years, we don't know exactly when – we will trap enough heat close to the surface to our planet to precipitate a collapse, or partial collapse, of a major ice shelf... I have friends who work on the West Antarctic Ice Sheet, and they say [when a collapse happens] you'll hear it in Sydney... Sea levels would rise pretty much instantaneously, certainly over a few months. We don't know how much it would rise. It could be 10 centimeters, or a metre. We will have begun a retreat from our coasts... Once you have started that process, we wouldn't know when the next part of the ice sheet would collapse, we don't know whether sea level will stabilise. There's no point of retreat where you can safely go back to... I doubt whether our global civilisation could survive such a blow, particularly the uncertainty it would bring."

Nature doesn't follow political fashion. Global warming may not be hot today, but the planet is – hotter than ever. When you stare out over the wave of Weather of Mass Destruction we are unleashing, who looks crazy – the protesters, or the people who have yet to join them?

You can follow Johann Hari on Twitter at twitter.com/johannhari101

j.hari@independent.co.uk