The Committee on Climate Change says the business plan for Heathrow projects a 15% increase in aviation emissions by 2050.
If that increase is allowed, members say, ministers will have to squeeze even deeper emissions cuts from other sectors of the economy. The government said it was determined to keep to its climate change targets.
The Committee on Climate Change is a statutory body set up to advise the UK government on emissions targets.
It warns that creating the space for aviation emissions to grow will impose unbearable extra emissions reductions on sectors like steel-making, motoring and home heating.
The committee also says that in making the decision to allow a third runway at Heathrow, ministers appear to have jettisoned their policy that aviation emissions in 2050 would be frozen at 2005 levels.
Its chair, Lord Deben, wrote to the business and energy secretary Greg Clark, saying: ‘If emissions from aviation are now anticipated to be higher than 2005, then all other sectors would have to prepare for correspondingly higher emissions reductions.
‘Aviation emissions at 2005 levels already imply an 85% reduction in other sectors. My committee has limited confidence about the options (for achieving the compensatory cuts needed).’
Already since 1990, aviation emissions have doubled while economy-wide emissions have reduced by more than a third. Ministers see aviation as a special case because low-carbon technology for planes is not well advanced.
The committee says the Department for Transport appears to be planning to solve the aviation overshoot by buying permits to pollute from poor countries which have low levels of CO2 emissions.
This is permitted internationally under a new code recently agreed by the aviation industry.
But it is a departure from the government’s own existing policy – and rules stipulate that the change should have been checked with the committee before being agreed.
A committee spokesperson told BBC News: ‘The committee has consistently said the government should not plan to use credits to meet the 2050 target because these credits may not be available in the future and they may not be cheap.’
Doug Parr from Greenpeace said the affair showed climate change was still an afterthought from a government pursuing business as usual.
He said: ‘What ministers know full well but don’t want to admit is that a third runway means other sectors of the economy will have to bear the costs of further carbon cuts – whether it’s regional airports or the manufacturing and steel industries.
‘If that’s the plan, it’s time ministers came clean about it with those concerned and the British public.’
A spokesperson for the Department for Business, Energy and Industrial Strategy said: ‘The government agrees with the Airports Commission’s assessment that a new runway at Heathrow can be delivered within the UK’s carbon obligations.
‘We are considering how we will continue to reduce our emissions across the economy through the 2020s and will set this out in our emissions reduction plan, which will send an important signal to the markets, businesses and investors.
‘Our commitment to meeting our Climate Change Act target of an at least 80% emissions reduction below 1990 levels by 2050 is as strong as ever.’
But it’s not just on aviation that climate policies are struggling. The government’s long-awaited master plan for reducing long-term emissions has been delayed again – until early 2017.
The government did signal help for electric vehicles in the autumn statement, although critics say it has much more to do.
But the biggest challenge is the UK’s leaky housing stock: since the government scrapped its ill-fated Green Deal programme of home insulation it has had no nationwide plan to improve comfort and reduce emissions from existing homes.