Johann Hari: Why we should all start shouting about airport expansion

The press coverage has howled about "bomb plots" and "anarchists" and "eco-fundamentalists" - so as they watched the Heathrow protest against Weather of Mass Destruction erupt this weekend, many ordinary people will be asking: why?

Why are so many people so disturbed by the idea of a third runway at Heathrow that they are prepared to scale buildings and face down the police to protest against it? In the late 1960s, a pair of psychologists called John Darley and Bibb Latané conducted an experiment that helps to explain what the protesters are trying desperately to do. The subject of the experiment - let's call her Linda - was taken to the top of a tall building and put in a room with three other people. She was introduced to the others as more random people were plucked off the street, and they were all told to fill in a questionnaire before the test began. The room then began to fill with thick black smoke.

Linda didn't know the other three people were actors who had been told not to react to the smoke in any way. What Darley and Latané discovered about human nature in the experiment was extraordinary. Linda would look at the smoke and try to make anxious eye contact with the others - but when she saw they were carrying on as normal, so did she. No matter how many times they ran the experiment, only when Linda - or any of the dozens of other subjects - could barely breathe would she stand up, interrupt the others, and say: "There's a fire!"

We are, collectively, sitting in that smoke-filled room, carrying on as if nothing is wrong. The hottest years on record have all happened in the past 20 years. Hurricanes have doubled in intensity since the 1970s. Half of Bangladesh is under water now, today. The Arctic ice is disappearing even faster than climate scientists feared. Well, everyone else is carrying on as if it's normal. Keep your head down, keep filling in the questionnaire; it will all be OK.

Building a third runway at Heathrow is one extreme symptom of this carry-on-the-smoke's-not-there mindset. The science shows unequivocally that every airport is a minor act of ecocide, and every flight helps to send the planet's climate spiralling a little bit further into chaos.

If the protesters - and the millions who can grasp the science they cite - cannot stop the expansion of air travel now, we will guarantee that Britain is unable to meet even the most measly carbon emissions targets. We will then feel our planet transform beneath our feet into a boiling, belching rock we do not recognise. That is why the protesters are at Heathrow - and that is why they are right.