Who owns sensible trousers that aren’t navy blue or black? In an effort to better distinguish between the guards and the guarded, the prison dress code has ruled out most of my wardrobe. I’m left staring at a sizeable collection of brightly coloured leggings and weighing them against advice from sites like “first time in prison dot com” to “keep your head down”.

After a short trial, the so-called Heathrow 13, of which I am one, were found guilty of aggravated trespass and being “unlawfully airside” at the airport for protesting against re-emerging plans to build a third runway at Heathrow. Our sentencing is on Wednesday, and we’ve been told to “expect custodials”.

What ​on earth​ do you put on your 'out of office' if you might be back that afternoon, or maybe April?

The process of preparing myself for maybe, probably, inevitably being sent to prison this Wednesday has been an interesting one. Answers to questions like “Can I bring shampoo?” and “Do visits happen twice a week, or twice a month?” are surprisingly elusive. I can’t tell whether – as a non-smoking vegetarian in a barter economy based on exchanging tobacco and tuna – my ability to stockpile wealth is going to leave me quids-in, or deeply unpopular. And what on earth do you put on your “out of office” if you might be back that afternoon, or maybe April? Personal CD players seem to be ok, we think, but who owns CDs any more?

If we get sent to prison, this will be the first time any climate protester has been sent to jail, and the first time anyone has been jailed for aggravated trespass since it was brought in as part of the 1989 Criminal Justice Act to stop this kind of behaviour.

While I’m terrified at the thought of prison, I stand by the actions that we took. A third runway at Heathrow would produce as much emissions as the whole of Kenya, make a mockery of any pledges made at the UN climate talks in Paris last December, and David Cameron’s own election promise of “no ifs, no buts … no third runway”.

While our actions in occupying a runway may seem radical, looking at the implications of a new runway at Heathrow, it’s clear who the radicals really are.

If the government commits to build a new runway, it knowingly signs up to breaking its own “legally binding” climate change commitments on emissions reductions and attempts to tackle the defining global threat facing humanity.

Heathrow already produces illegal levels of air pollution, a blight that kills 9,500 people every year in London. According to a 2012 study, an additional runway is expected to treble Heathrow’s air pollution. UK airports contribute to 110 early deaths each year, mostly due to lung cancer and cardiopulmonary complaints. Fifty of these can be attributed to Heathrow alone. If you don’t believe me, take a look at an air pollution map of Greater London. Can you see that big yellow blob west of the the city? That’s Heathrow.

Flying is a modern-day miracle, but it is a luxury that comes at a price and we need to be honest about that. In the UK, 70% of the flights are taken by just 15% of the population. We’re not saying that ordinary people can’t enjoy travel; we’re saying it’s more likely that the only effect a third runway will have on ordinary people is making the air they breathe dirtier. And it’s not just about air pollution either: increased CO 2 emissions will make extreme weather events more common, like the terrible floods that damaged so many people’s communities in the north of England over Christmas. The third runway is costly, unnecessary and harmful to all of us.

A short time behind bars is nothing compared to the life sentence of climate change for people around the world, and generations to come. So as I hunt for appropriate clothes and sufficient reading material, mentally prepare for life without the internet and figure out what to do with advice like “don’t be alarmed by screaming in the night”, I feel no regret for the action I took.

While we’re on the wrong side of the law on this one, I am sure that we’ll end up on the right side of history.