If a third runway is built at Heathrow, what will be the impact on the wealth of the nation? It will cost us somewhere between £23.4bn and £62.5bn. These are not my figures; they weren’t calculated by environmental groups, but by the Department for Transport.

For decades, government ministers have told us that airport expansion is essential for “growth”. As chancellor, Rachel Reeves claimed that a third runway at Heathrow would add 0.43% to GDP, figures we later discovered came from a report commissioned by the airport. The transport department’s own analysis concluded that the real addition to GDP would be between 0.03% and 0.05%: eight to 14 times less. Against this, it weighed the social and environmental costs of the project, to reveal a massive net loss. It found that a third runway would cause major harm to the health and wellbeing of up to 3 million local people.

But even these figures don’t capture it. The climate breakdown caused by the runway affects the entire planet, yet the costs to other countries aren’t counted. So again we must ask, why does the government support it?

Or does it? Though Heathrow expansion remains official policy, when asked at the weekend, Andy Burnham said the runway was “contested” and “principally it’s a matter for London and London MPs”. That sounds hopeful.

We also learned that the project has been delayed yet again. Though Heathrow announced last year that its plans were “shovel-ready”, it now admits the runway would take four years longer to build than it claimed: the estimated completion date is now 2039.

By then, or so we must hope, a government might have decided to take the climate impact seriously. If so, the new runway would need to be shuttered the moment it opened. Otherwise, according to the government’s Climate Change Committee, “almost all CO₂ emissions in 2050 will be from aviation”. Heathrow’s expansion alone, the committee says, would cause more carbon emissions by 2054 than “any other sector of the economy”.

And even this is not the end of it. The committee considers only the carbon dioxide planes produce, but the other impacts are even greater. A paper in the journal Atmospheric Environment suggests that, over 100 years, the nitrogen oxides, carbon monoxide, soot, volatile organic products, water vapour and other compounds released or formed in the upper troposphere and lower stratosphere by planes raise the heating caused by flying by 115%. Over 20 years, they raise it by an astonishing 517%.

Given that crucial Earth systems may be approaching their tipping points, and heating today triggers further greenhouse gas releases from forests, wetlands and permafrost, the 20-year horizon is crucial. But these other compounds are considered in neither international treaties nor the UK’s Climate Change Act. The impact of flying is officially and massively underestimated.

While the heating effects of almost all other sectors can be greatly reduced by technology, there are simply no good solutions for aviation. Successive governments have justified airport expansion plans with the promise of “sustainable aviation fuel”. Mostly they mean turning biomass into kerosene, which is even worse than using fossil fuels. If this substitute is to reach any kind of scale, it will either swallow up existing cropland, driving starvation, or it will cause the expansion of cropland into forests, wetlands and other habitats, accelerating the environmental crisis. An analysis by the Royal Society found that meeting existing UK aviation demand (never mind future growth) with energy crops “would require around half of UK agricultural land”. That’s the opposite of sustainable.

“Sustainable aviation fuel” can also mean synthetic fuels made from captured CO₂ and green hydrogen. But the Royal Society found that for existing aviation, we’d need to use between five and eight times the UK’s entire renewable electricity capacity. And still, aeroplanes would release a lethal chemical cocktail – not just CO₂ but those other compounds – into the high atmosphere, so only a fraction of the problem would be solved. Worldwide, a paper in the journal One Earth found that even with a wildly unrealistic programme of technological change, in which fossil jet fuels were phased out completely by 2040, the global heating caused by aviation would more than double by 2070, thanks to rising demand.

As for hydrogen-, ammonia- or battery-powered commercial aircraft, they all fail on technical or safety grounds, even before we ask where the power comes from. The “prototypes” endlessly showcased by the airlines, which never get beyond the artist’s impression, are nothing but “perceptionware”, designed to solve political problems, not technical ones.

If Heathrow expansion goes ahead, passenger numbers in the UK are forecast to rise from 299 million in 2025 to 491 million in 2050. As other sectors decarbonise, the proportion of climate breakdown caused by flying climbs and climbs, until planes account for almost all of it.

The only effective strategy is to reduce demand. There’s a progressive means of doing so: the frequent flyer levy. Wealth and flights are strongly linked. In 2024, 48% of England’s population did not fly at all. But 5% of the population took five or more flights abroad, and these account for 33% of all journeys. Increasing demand has been almost entirely driven by frequent flyers, who are highly concentrated in the highest income quintile. The levy would not be charged on the first flight someone took in any year, but it would escalate with every additional flight.

This is just, proportionate and – unlike all the techno-bollocks – immediately dispatchable. Otherwise, we continue to subsidise a wealthy, selfish minority with our money, our health, our wellbeing and the degradation of the living planet. How is that fair?

What government support for airport expansion shows is that “growth” is simply an excuse for giving powerful lobby groups what they want. When asked whether growth was more important than net zero, Reeves replied: “Well, if it’s the No 1 mission, it’s obviously the most important thing.” That’s another way of saying money is more important than human life. But, as the figures show, it was never about growth at all. It was always about meeting the demands of the airport lobby and wealthy passengers.

Now we wait on Burnham. Will the flight of reason continue? Or will human and planetary welfare at last outweigh the lobbyists?

• George Monbiot is a Guardian columnist