It is all about time for Extinction Rebellion, time that is running out.

The green group is not the first to try to save the planet. But there can have been very few protest movements who felt themselves on such a tight deadline.

That is because Extinction Rebellion reckons that governments have until just 2025 to impose a zero carbon economy. Or The Earth will be irreparably damaged.

Yesterday Amy Milner, 21, clutching a green flag with Extinction Rebellion’s trademark symbol of an egg timer in a circle, joined a couple of hundred other protestors to block Edinburgh’s North Bridge, part of a global wave of non-violent direct actions.

“We are here to let people know that time is running out to act on climate change before irreversible damage is done,” Ms Milner said. Her friend Martha, also 21, agreed. The protest, she said, caused inconvenience. “But it also raises awareness.”

Passers-by were mostly curious rather than angry. One man shouted abuse at the crowd, which numbered in the low hundreds. “Some of us are trying to get home,” he yelled.”And the climate has been warming for centuries.

The science might be clear about climate change and what needs to be done. But the politics is not. And that is at the core of a new kind of green protest, whether by school children inspired by Sweden’s Greta Thunberg or by Extinction Rebellion. Its logic? Politicians often accept the arguments for action. But they do not want to make the difficult decisions needed. So they need a spur. Cue the official line of Extinction Rebellion: that the UK and Scottish governments, engrossed in Brexit and other issues, are “complicit” on climate change.

Ms Milner and her friends understand there are other issues aside from the climate. But for them global warning is the story of their lives. Or their lives and deaths. She said: “None of the other things people worry about will matter if we don’t have a planet.”

Why direct action? Anna Fisk, 35, university lecturer, explained: “I’ve been going on demos since I was a teenager. It hasn’t worked. I’ve got involved in civilly-disobedient direct action as a last resort, because the ecological situation is desperate and we need drastic action.”

The climate crisis sparking yesterday’sTuesday’s protests might have been grim. But campaigners were determined to be in good spirits. They occupied North Bridge at around three in the afternoon and held on right through the evening. Scores of watching police were - for most of the time - content for the protest to go ahead, provided it did not spread.

And so organisers dished out Penguin bars and led the crowd in singing. One refrain went “Keep the coal in the hole. Keep the oil in the soil”. Another, to the to the tune of “Ye canny shove yer granny aff a bus”: “Oh, You canny make a profit when yer deid” and “If you all ye care about is money, yer a dummy”.

One protester, James, 30, carried a giant papier mache and bamboo skeleton called Marbhan on his back. Marbhan, he explained, is Gaelic for “corpse”. Two friends each used sticks to make the bone figure do a dance that was more jolly than macabre.

“He represents all the all the loss of life, the loss of species, which climate change will bring,” James said. “We need to move much faster to a zero carbon economy.”

There were arrests, but no aggression. By tea-time protestors, one in a wheelchair, had tried and failed to block nearby Princes Street. “Keep it proportionate,” shouted a police officer. It was not clear whether he was talking to his own or to campaigners. The wheelchair user was released back in to the main protest, only to be arrested later. Was her action proportionate to the climate crisis? She thought so.