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Labour has said that Heathrow expansion does not meet their 'four tests' - but will allow MPs a free vote. SNP MPs could also tip the vote. The Committee on Climate Change have written a letter warning the government that aviation emissions must not be allowed to grow above the limit that was planned for, and expressing surprise that the transport minister's statement about Heathrow did not even mention climate change.

Please email your MP today (whatever party they belong to) about the importance of this vote.

The Transport Select Committee recently released their report on the government's plans to build a third runway at Heathrow. If you read the report in full, it is clear why this plan is completely incompatible with our climate obligations. Yet carbon budgets are given the briefest of mentions in the summary, and crucial issues tucked away in the ninth and final annex to the report, entitled 'Carbon'. Unsurprisingly, media coverage focused on those issues which were given more prominence, including air pollution, noise and public transport limitations.

But the committee's evidence-gathering can shine a light on some of the more extraordinary 'assumptions' being used by the government in its calculations around climate impact. These 'assumptions' are being used to avoid discussion of the damaging impact a third Heathrow runway would have on our climate targets, keeping MPs in the dark in the run up to this summer's Parliamentary vote on the scheme.

When international aviation and shipping were left out of the 2008 UK Climate Change Act, this caused concern. Would cuts elsewhere be cancelled out by increasing emissions from these sectors - the equivalent of our carbon bloated economy going on a diet, but one in which all calories are counted (except those from chocolate and cake)?

The Committee on Climate Change, set up under the Act, had a solution. They set a 'planning assumption' - a limit they recommended the government keep aviation emissions below - of 37.5Mt carbon a year. The committee sets gradually shrinking carbon budgets for other sectors of the economy to allow for this level of emissions from aviation. So the overall aim of the Act - to reduce UK emissions by 80% by 2050 - can be achieved in a meaningful way.

It is difficult to reduce emissions from flying, since it's inherently energy intensive. Recognising this, aviation was therefore given a generous allowance, limiting emissions at 2005 levels, rather than the dramatic cuts expected from other sectors.

But from the government's support for a third runway at Heathrow airport, and their current consultation on aviation strategy which focuses on growth, it appears they have no intention of keeping to these limits. The idea is that increasing emissions from a growing aviation industry can be dealt with internationally, freeing us from any obligation to attempt to limit emissions here. Unfortunately these solutions have about as much credibility as the click-bait diet ideas that pop up on the internet: "This one weird tip" which produces amazing weight loss. They seem too good to be true - and they are.