More climate-heating carbon emissions, more people enduring noisy aircraft over their homes and, most likely, more air pollution – these would be the environmental impacts of building a third runway at Heathrow airport, a plan that has been backed by the chancellor, Rachel Reeves.

Despite many years of lobbying for a third runway, there is no current proposal that can be analysed for its environmental impacts. However, the last proposal was extensively assessed by the Airports Commission (AC) in 2017 and remains relevant.

Carbon emissions

The most profound impact would be the rise in CO2 emissions driving the climate crisis, which is already supercharging extreme weather disasters around the world. The AC estimated there would be an additional 4.4m tonnes of CO2 a year from a third runway.

The only significant development since then has been the nascent development of sustainable aviation fuels (SAF). However, the government is only planning for 10% of jet fuel to be sustainable by 2030, which its official advisers, the Climate Change Committee (CCC), says is already ambitious.

The CCC says overall UK emissions must fall by 63% by 2035, compared with 2019. But there is agreement among analysts that building an SAF supply chain to displace much of the existing demand for jet fuel, let alone that from a big expansion in flights, will be slow and difficult. SAF created by using renewable electricity is expensive and there is a limited supply of the waste fat that is used to make SAF. The government’s own forecast is that SAF will only cut emissions by 6.3m tonnes by 2040.

Small electric planes exist that can travel short distances but it will be many years, if ever, before large long-haul planes can be powered by batteries. Aircraft are becoming more fuel efficient but the rate of improvement is marginal compared to the increase in flights a third runway would bring.

Most importantly, the CCC has been clear on whether airport expansion is compatible with the UK’s legally binding targets: “no airport expansions should proceed” until a UK-wide management system is in place to ensure extra flights are balanced by cuts in aviation emissions overall. No such system exists.

A new study by the New Economics Foundation also suggests that all the reductions delivered by Labour’s plan to decarbonise the electricity grid by 2030 would be wiped out in five years if the third runway goes ahead, along with smaller expansions at Gatwick and Luton.

Noise pollution

Noise pollution is already a serious issue, with Heathrow being the UK’s busiest airport. The AC said a third runway would expose 12,000 to 28,000 more people to noise impacts exceeding 70 decibels. The Aviation Environment Federation (AEF) said a third runway would lead to 300,000 people experiencing being flown over by aircraft for the first time.

The number of flights is capped at 480,000 a year, but could go up to 720,000 with a third runway, according to Heathrow, an average of almost 2,000 flights a day. Newer aircraft are quieter, but the huge increase in flights and people affected would drown out this improvement.

Construction of a third runway would also have a substantial impact on the local environment, demolishing 800 homes, diverting five rivers and building a tunnel under the M25, according to the AEF.

Air pollution

Air pollution has long dogged Heathrow, partly from the aircraft and airport operations themselves, but also because it sits in a corner between the busy M25 and M4 motorways.

Heathrow has been in London’s ultra low emissions zone since August 2023. This means heavily polluting vehicles have to pay to enter the airport, which will be reducing air pollution. But expanding the number of travellers a year from 80 million to 140 million, as Heathrow has suggested with a third runway, is likely to increase air pollution. What happens will depend on how many passengers can be persuaded to take the train or tube to the airport.

However, the AC was less concerned about air pollution, placing “limited weight on suggestions that air quality represents a significant obstacle to the delivery of expansion at Heathrow”. It said measures such as providing electrical power to aircraft in stands could mitigate the problem. Nonetheless, a recent study found Heathrow was among the worst in Europe for exposing people to ultra-fine pollution particles.

Tim Johnson, the director of the AEF, said: “Heathrow’s third runway would be one of the most destructive infrastructure projects this country has ever considered. Heathrow is already the UK’s biggest carbon emitter so adding a third runway, and green lighting other airport projects, will leave our climate targets out of reach.

“The chancellor said things have changed since 2018, but the reality is that we have made little progress on reducing noise, while plans for cleaner tech and fuels are costly and not proven at scale.”

Shaun Spiers, the executive director at Green Alliance, said: “It’s crystal clear that pushing ahead with bigger airports and new roads will fly in the face of the UK’s climate targets.”