Imagine if tomorrow the CIA and MI6 discover that Osama bin Laden has invented an incredible new weapon. This machine - stashed away in some dusty Afghan cave - doubles the intensity of hurricanes, causing them to drown a US city and kill nearly 2,000 people. It turns Spain and Australia dry in the worst droughts on record. It makes the oceans acidic, killing essential parts of the food chain. It is causes these acidic seas to rise and wash away whole nations like Bangladesh and Tuvalu. And if the machine is left switched on for too long, it will drown London and New York and Lagos and Kinshasa too.

This machine exists. It is called global warming - and we are our own Bin Laden. The world's scientists say our greenhouse gas emissions are causing this planetary cooking as surely as HIV causes Aids or smoking causes lung cancer.

If al-Qa'ida was unleashing this weather of mass destruction, we would do anything - anything - to stop them. But because the enemy is inside each one of us, we stagger on, building more airports and coal power stations and shrieking for cheaper oil. We are suffering from what psychologists call an "external context problem": this is so far outside anything we have experienced before, it instinctively seems it cannot possibly be true, no matter how much evidence washes at our feet.

This week, a small band of the sane is gathering to try to shake us awake. In the English countryside of Kent, thousands of ordinary people have set up camp to demand the British Government cancel its plans to build a new coal power station, with six others to follow. Coal is the worst warming-weapon, responsible for half of all the greenhouse gases humans have pumped into the atmosphere. It is twice as warming as the next worst fossil fuel - natural gas - and more than a hundred times worse than wind power. The Climate Camp protesters are refusing to be part of Generation Zzzzzzzz, drugged by celebrity and consumption. Armed only with the science, they are urging us to be rational, now, while we still can.

To grasp the urgency of the situation, let's look at one aspect of global warming that has been widely overlooked. As you lie on a beach this summer and stare at the ocean, you should be aware it is becoming rapidly more acidic - because of your emissions.

The oceans are the greatest carbon sink we have. They have inhaled a third of the carbon dioxide pumped by us into the atmosphere and buried it on the ocean floor. But there is a price. When CO2 combines with water, it creates a fizzy carbonic acid. You taste this acid on your tongue every day in your can of Coke. The more carbon the ocean soaks up, the more acid it produces. Since the start of the Industrial Revolution, the acidity of the seas has soared by 30 percent, and by the end of my life, it will have increased by 150 percent - unless we reverse course fast. "A change of that magnitude is more than we have seen in 20 million years," says Richard Feely of the National Oceanic and Atmospheric Administration in Seattle.

Turning the seas acidic sets off a series of disasters, only some of which can be predicted in advance. Disaster one: The collapse of the oceanic food chain. At the turn of the century, the US, Japanese and German governments were so impressed by the capacity of the oceans to mop up CO2 that they proposed compressing emissions from power plants and pumping the goo into the sea. So a series of tank-experiments were set up to see what would happen. Once the water became strongly acidic, the shells of dozens of sea creatures - from sea urchins to molluscs - simply dissolved, and they died. The food chain collapsed; almost everything else in the experiment died too.

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One of the creatures that is killed by acidity is the pteropod, a tiny little sea snail. That doesn't sound like a big deal - until you realise pteropods are the major food source for salmon, herring, cod and pollack. If they die, so does the staple food of hundreds of millions of humans. So long, and thanks for all the fish.

Disaster two: the death of coral. Acidic oceans dissolve coral like a fizzing paracetamol in a glass. So the coral reefs - the rainforests of the ocean, home to a quarter of all sea life - are dying at a rate that has staggered the scientists who study them. And the Reefer Madness gets worse: atolls like the Maldives and Tuvalu have foundations made of coral, so they will dissolve and collapse, if rising sea levels don't get them first.

Disaster three: the seas will lose their ability to soak up carbon dioxide. The creatures that currently "eat" carbon dioxide and sink to the bottom of the ocean - shelled plankton - are killed by acidity. The result? A sharp acceleration in global warming up here. There is even a fear the vast amounts of methane stored in the oceans will be destabilised and rise to the surface. The last time this happened, 55 million years ago, it caused warming so rapid most life on earth died. Think of it as the fart at the end of the world. That's why the biological oceanographer Professor David Hutchins says: "Frankly, ocean acidification is apocalyptic in its impact."

But remember: these are only some of the effects on the oceans - and the oceans are only one dimension of global warming. Suddenly the analogy with the al-Qa'ida psychosis doesn't seem so extreme. As the environmental writer Mark Lynas notes: "If we had wanted to destroy as much of life on earth as possible, there would have been no better way of doing it than to dig up and burn as much fossil hydrocarbon as we possible could."

We need a sea change before the seas change irreversibly. That's why I will be going to the Climate Camp. Where will you say you were when the carbon bomb was fired into the atmosphere?

--Johann Hari

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