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.

So what's going on with aviation in the UK?

The government is currently consulting on its aviation strategy with an initial "call for evidence," asking for views on forthcoming consultations on its six 'objectives'. This closes 11.45pm on Friday 13  October.

You can respond here or by emailing aviationstrategy@dft.gsi.gov.uk Update: we've had some glitches reported with the consultation form so you may do better typing responses to relevant questions into an email

Limiting carbon emissions is not one of the six key objectives, although number five is to "support growth while tackling environmental impacts". Whether an absolute limit to growth might be needed to avoid unacceptable climate or other environmental impacts doesn't seem to be up for discussion. (The other objectives are to 'help the aviation industry work for its customers'; to 'ensure a safe and secure way to travel'; to 'build a global and connected Britain'; to 'encourage competitive markets'; to 'develop innovation, technology and skills').

The consultation on environmental impacts will not even take place until the second half of 2018. Yet by then MPs will have already been asked to vote on building a new runway at Heathrow, in the absence of an overarching strategy or, apparently, any explanation on how aviation expansion can be compatible with cutting our emissions under the Climate Change Act or the Paris climate agreement.

How did we get here? In 2013, the Department for Transport was already predicting that aviation emissions would reach 47Mt by 2050, well over the intended limit of 37Mt. Despite having previously ruled out a new runway at Heathrow ('no ifs, no buts'), David Cameron set up the Airports Commission to look into the 'problem' of airport capacity in the South East, in other words, to decide between the competing claims of Heathrow and Gatwick.

The Airports Commission recommended Heathrow expansion. Their report included a rather hypothetical scenario in which the government is willing to expand Heathrow with one hand and introduce drastic measures to restrict flying on a national level with the other hand (a heavy carbon tax for example). This is both far-fetched and diametrically opposed to the government's approach as set out in the Aviation Strategy consultation. Although the government did not refer to this scenario in setting out their business case for Heathrow expansion, it enabled them to maintain that the Airports Commission had found it to be possible to expand Heathrow within existing carbon budgets.

The Committee on Climate Change wrote to the government in November 2016 asking for clarification: would the aviation emissions 'planning assumption' be kept to - in which case how would the business case for a new Heathrow runway be justified? Or if not, would they need to deepen the cuts demanded from other sectors in their forecast beyond the 85% cuts already required? Would this be possible?

The government have so far been remarkably reticent on providing official answers to these questions. But there is no indication that anything meaningful will be done to limit aviation emissions on a domestic level, with some 700 additional flights every day planned from a Heathrow third runway and alongside this, the Aviation Strategy consultation talking of growth at regional airports.

In this context, the statement in the Aviation Strategy consultation, "The UK’s carbon budgets have been set at a level that accounts for international aviation and shipping emissions, so that the UK is on a trajectory that could be consistent with a 2050 target that includes these emissions", is at the very least deeply misleading.

What's the problem with tackling aviation emissions 'on an international level'?

On the face of it, this would seem logical. The problem is that the international process is led by the aviation industry. Since their main priority is is continued expansion, the targets they set are inadequate, and the means proposed to achieve these are about as credible as click-bait diet tips.

The goal set under the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) is 'carbon neutral growth from 2020'. But to avoid catastrophic climate change, we need to cut emissions dramatically, not just level off.

CORSIA would rely heavily on offsetting (paying for projects to reduce emissions elsewhere), but this has been shown to be unreliable as a way of reducing global emissions. For example, a review found that 85% of the offset projects under the UN’s Clean Development Mechanism failed in the objective of reducing emissions. Projects may be double counted (included towards nations' emissions reductions targets) rather than additional. The UK Committee on Climate Change has said that the UK should cut its own emissions and not count offset credits towards its targets.

Importantly, aviation's climate change impact is around double what it appears from looking at CO2 alone. Factors such as nitrogen oxide (NOx), water vapour, contrails, and soot and sulphate aerosols vary, making exact calculations tricky. But it is clear that the CORSIA proposal which allows aviation to grow while offsetting only carbon dioxide emissions would leave perhaps half of its growing climate impact unaccounted for.

The CORSIA deal also predicts that a large part of new aviation emissions can be reduced by 'additional technologies and biofuels'. 'Additional technologies' are uncertain. Biofuels could be used to replace a small percentage of fuel, but are more expensive than petroleum based fuel. To cut costs and bridge the price gap, currently only one biofuel appears feasible for use at scale - palm oil (detailed analysis from Biofuelwatch). Palm oil cultivation has been associated with deforestation on a significant scale, most notoriously in Indonesia, where palm oil expansion has harmed wildlife such as orangutans, now critically endangered. Burning forest on Indonesia's peat soils to clear land for palm oil and other crops has triggered uncontrolled wildfires, emitting further vast quantities of carbon.  

You can read more in WWF's report explaining the problems with CORSIA.

The 'Clean Growth Strategy': an unreliable diet?

The long-awaited 'Clean Growth Strategy' is due out on Thursday. This is supposed to set out how greenhouse gas emissions will be reduced in line with the Paris, and the expectation is that while it will contain significant steps forward, aviation emissions will be ignored. This could be, then, the equivalent of having salad for lunch and dinner while stuffing your face with chocolate at midnight... but really, it is no joke. If the government wishes to discard climate change targets it should state so up front. Since it appears to be doing just that, a strong challenge in defence of the Climate Change Act is vital.