Expanding Heathrow would result in increased carbon dioxide emissions and could put the UK further off track on its climate goals, but the government is claiming it can offset that by investing in research on new low-carbon fuels for aircraft and on electric planes.

However, any such technology is still decades away, if it ever reaches commercial scale, making it certain that any new runway in the near future would be used by the same kerosene-fuelled, high-carbon aircraft that we have today.

The Climate Change Committee, the government’s statutory adviser on net zero, has warned repeatedly that airport expansion would breach the UK’s carbon budgets, which are set years in advance on a pathway to meet the goal of net zero emissions by 2050.

While the CCC cannot prescribe government policy, especially in the case of specific decisions, it has noted that any expansion of airports would be possible within the UK’s carbon budgets only if far steeper cuts in greenhouse gas emissions are made elsewhere within the economy. In its most recent report to parliament, the committee said: “Any plan to increase airport capacity needs to be based on realistic projections of future demand. These demand projections need to be consistent with climate change targets and take account of the costs to the sector of getting aviation to net zero emissions.”

Campaigners do not believe it is possible for an expanded Heathrow to operate within the UK’s carbon budgets. Alethea Warrington, the head of aviation at the climate charity Possible, said: “It is unlikely that this expansion can be done within the current carbon budget as a new runway would bring additional flights, with a huge chunk of additional emissions, with no way of removing them.”

Doug Parr, the policy director of Greenpeace UK, said: “Anyone who is serious about tackling climate change would be very cautious about making that problem even bigger without a coherent plan for dealing with it. Wishful thinking about future technological advances and cost reductions are not a coherent plan, and certainly not a plausible basis to allow the huge increase in emissions from a third runway at Heathrow. This policy amounts to announcing infrastructure now and kicking the carbon cost down the road for a future government to deal with.”

Yet there could be alternatives if the government is brave enough to grasp them. The CCC has advised that the aviation sector should be forced to “pay for permanent engineered removals to balance out all remaining emissions”. That would be expensive – ways of permanently removing carbon dioxide from the atmosphere, such as capturing the gas and storing it, are still under development.

Simpler than that, why not substitute train journeys for flying? In 2021, there were more than 3m flights within the UK between July to September alone and there are 15 flights a day to Brussels, which is just over two hours away by train from London.

But flights are often cheaper than trains, even over short distances – flying is in effect subsidised, through the tax system. To encourage people on to trains instead of domestic flights, the government would need to make rail journeys cheaper or flights more expensive. The former would require investment, and the latter would annoy some middle-class voters. Therefore neither seems likely.