The government has backed the construction of a third runway at London’s Heathrow Airport.

Chancellor Rachel Reeves announced the decision in a speech on measures aimed at improving economic growth, saying that the case for the expansion “is stronger than ever”.

The expansion of Heathrow has long been opposed by green groups and her announcement will be extremely controversial, not least because of its environmental impact.

Reeves claimed at the weekend that “a lot has changed in aviation” since plans for a new runway at Heathrow were first discussed decades ago.

She said “sustainable aviation fuel” would cut emissions, that there was huge investment going into electric planes, and that a third runway would mean fewer planes circling over London as pilots wait to land.

But the government’s independent adviser on cutting emissions, the Climate Change Committee (CCC), has repeatedly cautioned against airport expansion without a framework in place to manage overall national capacity.

Meanwhile, advocates of the expansion say it is vital for boosting growth – not least because demand is outstripping supply. Heathrow has no free landing slots available, meaning airlines have to buy slots from competitors if they want to expand their services.

BBC Verify has assessed Reeves’ claims about impact on the environment to see whether the government’s reasoning stacks up.

By burning traditional jet fuel – kerosene – aircraft release carbon dioxide, a planet-warming gas.

“Sustainable” fuels are alternatives to fossil fuels, made from renewable sources. They can come from agricultural waste and from used cooking oil.

The government argues they emit 70% less carbon emissions over the course of their lifetime. This is because the plants from which the fuels are often derived were absorbing carbon dioxide from the atmosphere when they were growing.

The previous government announced that by 2030, 10% of all jet fuel used in flights taking off from the UK must be sustainable as a way to reduce the impact of aviation on emissions. This target has been kept by Labour and it is increasing funding into sustainable fuels.

But there are a number of issues with using the rollout of these as a justification for airport expansion.

Sustainable fuels are currently used in a tiny fraction of jet fuel – the government target for 2025 is 2% – and scaling this up will be a major challenge. They are not completely carbon-neutral because of the energy used in producing, refining and transporting them and can vary widely between fuel types.

Many environmentalists argue that expanding UK airports is incompatible with the UK’s net zero targets because there is currently no viable widespread alternative to fossil-fuel based aviation fuel for powering planes.

Provisional official figures show that greenhouse gas emissions from UK international aviation in 2023 were 32.9 million tonnes of carbon dioxide equivalent. That was roughly 8% of the UK’s total greenhouse gas emissions (including international aviation) of 423.3 million tonnes in that year.

Aside from a major dip in the pandemic, there has been no substantial change in emissions from UK international air travel over the past 20 years.

There are already a number of small battery-powered electric planes and investment in the technology is increasing.

If planes are powered by electricity generated from renewables like wind and solar, flying could result in zero carbon emissions.

But the weight of batteries is currently regarded as a major obstacle to a large-scale expansion of electric-powered flights, particularly for long haul journeys.

Technological breakthroughs in battery weights are possible, but widespread electric air travel is not seen as a realistic prospect in the near future by most analysts.

Heathrow Airport has reported that in 2023 an average of 232 aircraft, more than a third of all arriving planes, were held in one of four “stacks” above London each day, where they circle at or above 7,000 feet until there is space to land at the airport. They spent an average of 6.85 minutes in a stack.

Circling at low altitude is less fuel efficient than cruising at high altitude, because of extra air resistance.

As a result, since 2014 Heathrow and air traffic control company Nats have been working with their counterparts across Europe to slow inbound flights down from as far as 350 miles away, when delays over London begin to build.

But it is difficult to estimate whether an extra Heathrow runway would reduce stacking, let alone reduce emissions.

The BBC has asked NATS for updated figures on how circling times over Heathrow have been changing and the impact on emissions, and we asked the Treasury for further details about the chancellor’s claim about circling flights.

Additional reporting by Gerry Georgieva and Mark Poynting