AT 2.33pm last Friday, something inconspicuous, sophisticated, naïve and yet potentially momentous happened on Edinburgh's Princes Street.

A blue banner was unfurled from the gothic parapets of the Sir Walter Scott Monument. It read, in bold white letters: The Climate Clock Is Ticking. We Need A Better Plan Now.

Minutes later, the activists involved were arrested and taken to St Leonard's police station. They were later released without charge.

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The event was inconspicuous in that the banner billowed for less than 10 seconds. Its mayfly existence briefly slowed the Christmas shopping traffic.

It was sophisticated in that the activists scrawled their lawyer's phone number on the backs of their arms in case of arrest. They plotted their demonstration using scale drawings of the monument.

It was naïve only in the belief that rope bought from Homebase could hold the 15 metre banner without snapping in a stiff December breeze.

And it was momentous because earlier in the week the activists were at Stansted, helping to bring the UK's third-busiest airport to a halt. They are part of Plane Stupid Scotland, and now they are about to start a series of protests against airport expansion in Scotland.

If Plane Stupid Scotland was not an avowedly anti-hierarchical organisation, Dan Glass would be considered its leader. The affable 25-year-old Londoner arrived in Scotland in 2006 to study the psychological effects of climate change at Strathclyde University. Instead of working on his dissertation, he spent this year doing stunts such as blockading a private airfield in Edinburgh and attempting to superglue himself to prime minister Gordon Brown during an awards ceremony in July.

In the parlance of Plane Stupid, this, along with the Scott Monument banner drop and the Stansted invasion, is an "action". Over a pot of cinnamon tea shared with other members of Plane Stupid Scotland, he vows that over the next 60 days there will be "an escalation" of such events.

Aviation accounts for around a sixth of the UK's carbon emissions. "We cannot lose ourselves in a sea of words any more," said Glass. "We have to take action. And if that means breaking the law, so be it. We would do whatever it takes to cut carbon emissions to the necessary levels. If the government could put two and two together and had some joined up thinking, we wouldn't need to do this."

The next 60 days are seen as crucial. That is the length of the public consultation on the Scottish government's plans to expand Edinburgh, Glasgow, Prestwick and Aberdeen airports. Plane Stupid plans to highlight what it calls "the government's absurd logic" through engaging directly with communities near the airports, and direct intervention.

With such action comes the inevitable threat of arrest. The Plane Stupid Scotland members rate each other on how "arrestable" they are. At Stansted the Scottish branch provided food, media and legal support, keeping out of the way of arrest for fear of slowing down their plans for Scotland. "We need to use our arrests up here," said Glass.

Tilly Gifford has already been arrested once this year during an action. In April, the 24-year-old dropped a banner from the roof of the Scottish parliament.

"Yes, it was slightly terrifying, but now I'm a lot more clear about how it works," she said.

Juliana Napier, 23, has been arrested twice. "I have the most supportive parents," she said. "I come from quite a radical family. We used to go to the Faslane demonstrations when I was in my early teens. My dad makes fun of me. He's like, I've got an asbo, what do you have?'"

Much has been made of the time-honoured tradition of young protesters hailing from the middle-classes. Press coverage last week fixated on the presence of the grandson of a peer and the grand-daughter of a baronet at Stansted. Mention of it makes Glass, Gifford, Napier et al bristle.

"The people involved are so diverse, they are coming from all corners," said Glass. "They can't be stereotyped, as it is an issue that effects everyone. And I hate it when we are called radical. I think what we are doing is quite reasonable."

Professor Gerard de Groot, from St Andrews, whose latest book The 60s Unplugged sets out to burst any rose-tinted bubble around events such as the Paris student rebellions in 1968, was until recently sceptical about contemporary youthful uprisings.

That changed when he read an account of the Stansted protests.

"One of the protesters said something that caught my attention," he said. "She said: We are here because our parents have let us down. They are forcing us to live in a world that they made, but which will become unliveable'. That's exactly what they were saying in the 1960s.

"I don't think this is simply the idea that every generation rebels against their parents. This is something more fundamental. In its own way my cynicism about the 1960s has been tempered in the last month.

"So much of what has occurred with regards to the recession we're in now seems to be giving validation to what protesters in the 1960s were protesting about - the capitalist dream can't go on forever and the shallowness of consumerism. What was wrong with the 1960s was maybe they were a little bit too early."