Did you hear about the time 22 flights were cancelled at London's Heathrow Airport because there was a polar bear on the runway? No? Well he was arrested and now faces up to three months in jail.

But this wasn't some sort of zoo escape that went very weirdly wrong — it was a direct action protest last summer that will see 13 people sentenced by a London court on Wednesday. The group has been told to expect jail time, a severity of punishment that has shocked many in the legal profession.

The thirteen are all members of Plane Stupid, activists who opposes the expansion of the airport — already one of the world's busiest — due to concerns over climate change. After getting onto the runway by cutting through the airport's perimeter fence in the early hours of July 13, the group chained themselves together and sat in a circle around the man who had dressed up as the North Pole's most famous beast.

The symbolism of the costume is obvious, with the polar bear's Arctic habitat disappearing at a staggering rate as a result of climate change. According to the US National Snow and Ice Data Center, Arctic sea ice cover has declined by 30 percent over the past 30 years as a likely result of human activity.

And while there are many contributing factors to climate change, the United Nations' International Civil Aviation Organization acknowledges that planes are particular polluters because their emissions are made at high altitude. According to the Intergovernmental Panel on Climate Change, aviation is responsible for 3.5 percent of climate change — a figure likely to rise to 5 percent by 2050.

It's in this context that Plane Stupid has mounted its campaigns, staging demonstrations at a number of airports across the UK since forming in 2005, as well as picketing the headquarters of major airlines.

While the group remained dormant for a number of years, it reformed last summer after the UK government's Airports Commission recommended that a new runway be built at Heathrow Airport — bolstering an expansion campaign backed by business interests despite business-friendly Prime Minister David Cameron promising in the run-up to the 2010 General Election "no ifs, no buts, there will be no third runway."

After staging their protest at Heathrow — causing five hours of delays in the process — the 13 were arrested and subsequently charged and found guilty of aggravated trespass, following a trial at Willesden Magistrates' Court.

At the conclusion of their trial in late January, Judge Deborah Wright said that while she believed the 13 were "principled" and "passionate," she could not "think of a more serious case of aggravated trespass."

They have been told to expect custodial sentences — involving up to three months in prison — meaning it could be the first time in over 20 years that anyone has been imprisoned in the UK for staging an environmental protest.

Their potential imprisonment has been met with dismay by progressive politicians, commentators, and environmentalists, who have come out to support the group.

In an impassioned statement provided to Plane Stupid, the UK shadow chancellor John McDonnell, whose parliamentary constituency Hayes and Harlington includes Heathrow Airport, said the concerns of the group were shared by many of his constituents.

McDonnell defended the right to non-violent direct action in the face of a government that appears set to break concrete promises it made to the electorate.

"It's almost inevitable that activists will lose patience with a process that they no longer trust and do what they can to solve the problems themselves," he wrote.

According to Blinne Ní Ghrálaigh_,_ a barrister and criminal law specialist at London's Matrix Chambers, the fact there are activists facing jail who have never previously been convicted of any criminal offense is "extremely surprising."

"The typical sentence for first time offenders convicted of aggravated trespass in these types of cases is a discharge or at worst a fine," she told VICE News.

While some of the activists admit to being scared about going to prison, they insist they have no regrets.

"Yes I'm afraid," said 28-year-old Danni Paffard, who says the significance of aviation lies not only in the level of pollution it is responsible for, but the fact the industry remains so unaccountable for its polluting.

"It's the fastest growing source of emissions and a key issue right now because it isn't covered by any global climate legislation," she told VICE News.

For 68-year-old Rob Basto, another of the 13 and holder of a PhD in Astrophysics who only became an activist in recent years, the protest was justified by the pressing threat climate change represents.

When he was 65, he was arrested for the first time in his life for climbing Blackpool Tower as part of a one-man protest. "When I found out we were in imminent danger, it just seemed natural to do something about it," he told VICE News. "Five or ten years ago, you needed to understand the science. Now, all you have to do is watch the news to know how bad things are."

According to another of the group, Ella Gilbert, who has an MSc in climate change and is working towards a PhD, one of the obstacles faced by Plane Stupid and other activists is the fact that people have become so accustomed to air travel.

"It's a lifestyle choice", she told VICE News. "People feel as though their lifestyle is being scrutinized".

Yet despite the ongoing growth of air travel, flying is an activity dominated by a very small group of people. In the UK, 70 percent of flights are taken by 15 percent of the population, while the Worldwatch Institute says just 5 percent of the world's population has ever flown.

Ella Gilbert, 23, has an advanced degree in climate change and is working towards a PhD. Photo by Al Simmons.

The activists stress they do not want to judge — they just want awareness raised of the damaging impacts of air travel. "Aviation is absolutely critical for the climate debate," said Paffard. "It's a difficult thing to talk about, though, because we all like to fly."

None of the activists were expecting to go to prison, but most of them were prepared for it. Margot Leicester, the mother of Edward Thacker, one of the 13, told VICE News while she was worried, she was extremely proud. She wants the democratic right to protest to be upheld and feels a sense of generational guilt for "causing" climate change while not having to face its worst affects.

As for the UK's Conservative government, the Heathrow 13 say its position is inconsistent — a majority supports the new runway and yet it has stated a commitment to cut greenhouse gas emissions by 80 percent by 2050.

But while the Conservatives have postponed major debate on the issue — in an apparent attempt to avoid disunity on a key concern for the capital ahead of the London mayoral election in ten weeks time — the activists remain united in the belief that, come what may, they did the right thing.

"I'm terrified about going to prison, from a personal point of view," said Paffard. "But I totally stand by what I did."