On a beautiful sunny day at the end of February, my colleagues from Plane Stupid and I scaled the roof of the Houses of Parliament, from where we dropped banners and paper aeroplanes made from confidential Whitehall documents. Our protest was timed to coincide with the end of the 'consultation' period over the plans for a third runway at Heathrow Airport.

There are two reasons we chose to make our point in this way. The first is that Britain simply cannot afford the massive leap in greenhouse gas emissions that will arise from this planned increase in air traffic. It is not just common sense that tells us this, but very credible science that maps out the emissions trajectory that the UK will need to take in order to meet our commitments to prevent catastrophic climate change. The current plans to more than double the number of flights using Britain's airports will scupper any chance we have of meeting the steep emissions reductions about to become law in the Climate Bill.

And this is where the second reason for Plane Stupid's campaign begins to become clear; the extraordinary level of collusion between the present Labour government and the UK's aviation industry. Brown's gang are well aware of the evidence that shows their aviation and climate policies are mutually incompatible and cannot possibly both succeed. But their response, rather than to revisit the aviation policy, has been to exclude emissions produced by international air transport from the targets in the Climate Bill. This decision, as well as contradicting and fatally undermining the stated purpose of the Climate Bill, also reveals a great deal about the close relationship between the Brown administration and their aviator mates.

This relationship was thrown into even starker relief by the so-called 'public consultation' process over Heathrow's expansion. As well as being barely comprehensible to an ordinary person (The Plain English campaign called for it to be withdrawn and rewritten) and, bizarrely, not containing any questions at all about climate change, it was actually impossible to say 'no' to the expansion plans by answering the questions posed in the questionnaire. None of this was an accident because this 'consultation' was a collaboration between the DfT and Heathrow's owners, BAA – a product of a joint enterprise called Project Heathrow which was put together to push through the expansion plans no matter what. This body has worked tremendously hard to find ingenious ways to override public opinion, as well as doctoring key data on nitrous oxide levels in order to appear to comply with EU restrictions on local air pollution.

There are lots of good reasons not to build a third runway at Heathrow – the destruction of communities, the burgeoning blight of aircraft noise, the soaring and probably illegal levels of local air pollution, and the cost to the British taxpayer of subsidising yet more air travel – before we even consider the unacceptable climate change impact. But there is only one argument in favour of the plan – the economic case, and even this is looking increasingly shaky. The case rests on a set of figures drawn from a couple of economic studies that were commissioned by the DfT, but paid for by, guess who? BAA. Other economic studies examining the financial benefits of expansion at Heathrow have produced wildly different results, yet somehow this work has not been allowed to inform the decision making process.

Gordon Brown's hilarious comment in response to our protest – that decisions in this country should be made "in this House, and not on the roof of this House", could not have been more apt. The whole point of our action was to highlight the fact that important decisions of great significance to the public are not being made in the House of Commons at all, but in corporate boardrooms by the people who stand to gain the most from these decisions, and at the expense of the rest of us whose views seem not to matter one jot.

The wider lesson this business over Heathrow teaches us is that in order to actually win the fight against climate change, we are not only going to have to make big changes to the way we as individuals live our own lives, we are also going to have to actively confront powerful vested interests who have undue influence over government policy. The outcome of the Heathrow struggle will clearly signal whether or not this government is actually institutionally capable of doing what needs to be done to reduce emissions in line with the climate science.

If, as we at Plane Stupid strongly expect them to, Brown's government gives a green light to the third runway plans, it will be the beginning of a massive and unprecedented campaign of peaceful direct action and civil disobedience to prevent its construction. The recent opening of Heathrow's Terminal 5 saw a FlashMob descend on the arrivals lounge to draw a line in the sand over any further expansion plans. We're starting to get organised (the FlashMob was the only thing that happened on time at T5 opening!). If our elected representatives are unable to make decisions based on the best interests of the British people over the interests of their corporate friends, then we the people will have to protect our interests from them. Plane Stupid are by no means the only ones who will be there to protect the village of Sipson from the bailiffs and bulldozers if and when they turn up to begin work. You should be there too, if you want to be able to look your grandchildren in the eye.

If you want to do your bit in the fight against climate change, get involved by emailing info@planestupid.com or calling 07948 219 493. Prevention is always better than cure, and there's still time to get these crazy airport expansion plans scrapped. If we're going to win this, we need more heroic people prepared to put their necks on the line for the future of civilization and the Earth's biodiversity. Are you such a person? If you're not ready to chain yourself to something yet, you can also donate to our cash-strapped campaign anonymously through the PS website. Every penny counts - the enemy are packing millions into their PR, while we're just a handful of mouthy volunteers.