If the world really does face a climate emergency, what ought you, personally, be doing about it? Should you, as increasing numbers of young people are doing, roam the streets at night letting down the tyres of SUVs?

The fast-growing movement that calls itself the ‘Tyre Extinguishers’ thinks this is an effective approach, and has targeted thousands of SUVs in cities around the world. My home town of Bristol – always quick to espouse a green cause – has seen at least 200 SUVs ‘extinguished’ in recent weeks.

Though they claim to be leaderless, the Extinguishers have a Twitter account where you can keep up-to-date with their latest ‘hits’, and a website that generously invites everyone to get involved. It even offers advice on deflating tyres, suggesting that a small bean – ‘we like green lentils, but you can experiment with couscous’ – be inserted into the valve before replacing the dust cap. SUV owners then wake to find flat tyres, and a windscreen leaflet (downloaded from the Extinguishers’ site) which explains: ‘You’ll be angry, but don’t take it personally. Your gas-guzzler kills… millions are already dying from climate change-related causes.’

It would be comical if the Extinguishers were not so frighteningly sure of their moral superiority. ‘Are you not worried about interfering in life-or-death situations?’ asked Channel 4 News of some masked and hooded men who had just deflated the tyres of a doctor’s car at 3 a.m. in Bristol’s Clifton Village. ‘No, no,’ they replied. ‘Because we are helping to save the lives of billions worldwide.’

Do they have a point? After all, many SUVs are unnecessarily big, require needlessly large amounts of material to make, take up a lot of road and, crucially, consume more fuel than smaller cars. The Extinguishers have been inspired partly by an alarming article published last December by the International Energy Agency (IEA), which said: ‘If SUVs were an individual country, they would rank sixth in the world for absolute emissions in 2021.’ Which sounds bad – until you delve into exactly what that means.

All the cars in the world account for just 8 per cent of manmade carbon emissions. The IEA estimates that 45 per cent of new cars are SUVs, and that the existing fleet produces about 2.5 per cent of global emissions. Not a huge amount, but similar to the national emissions of Indonesia, arguably sixth in the league of global emitters.

But what effect would swapping all these SUVs for something smaller really have on our atmosphere and climate? Suppose every SUV owner on the planet took the Extinguishers’ message to heart, and immediately ditched their SUV for a Ford Fiesta. The IEA reckons smaller cars consume around 20 per cent less energy than SUVs – so it is a reasonable guess that replacing every SUV on the planet with a hatchback would decrease global carbon emissions by roughly 20 per cent of that 2.5 per cent – or 0.5 per cent.

An unlikely maximum of half a per cent is hardly world-changing. The Extinguishers who get to bed late thinking they have done their bit to save the world should know just how little difference they are making. And it is not as if their actions don’t have negative consequences. Tyres can be damaged by the weight of the car when deflated, and driving on a flat tyre is dangerous. Accidents will happen and deaths are possible.

At the very least, people are inconvenienced, angered and turned against the Extinguishers’ cause. Their leaflet explains – ignorantly – that SUV owners will have no trouble relying instead on public transport. Really? How about the Bristol trauma surgeon whose tyres were ‘extinguished’ earlier this year? Or the Bristol MS sufferer for whom ‘bad days’ preclude walking to a bus stop, and whose car had its tyres flattened last month, despite displaying a disability badge?

Cars, we seem to forget, are incredibly useful. They permit drivers to go almost anywhere they like, whenever they want – a rather brilliant, highly democratic freedom that we have somehow come to view as a terrible thing. Cars are also increasingly clean and efficient. Their bad press started in the days when there were no catalytic converters, 20mpg engines and leaded petrol. Today’s machines, increasingly battery-powered, are very different.

In any case, the improbable scenario in which all the cars in the world were magicked away, and all the massive and diverse functionality they now provide was replaced with nothing, would, in saving that 8 per cent of all carbon emissions, only take us back to around the situation in 2014. Because, despite our personal actions – like cycling to work, or buying a Tesla – global emissions continue to rise.

It is not that these things don’t help at all – they just do not come near making a significant dent in the problem. A glance at the IEA graphs of who, where and what is mainly responsible will quickly show you where our focus should be. Since 2008, UK carbon emissions are down by 40 per cent; the US’s by 22 per cent and Europe’s by 17 per cent – rather giving the lie to the Extinguishers’ claim that they have to act because our governments refuse to. Chinese emissions, however, are through the roof – up 48 per cent, mainly the result of burning coal to generate power. Between 2008 and 2019 global carbon emissions, from all sources, rose by 15 per cent. Chinese coal burning was responsible for almost half this rise. All the SUVs in Britain emit the same amount of CO² as about three Chinese coal-fired power stations. There are now more than 1,100 of these in China, with dozens more under construction. Is deflating tyres in Clifton really going to save the planet?

The Extinguishers prefer not to see this. I suspect that they would rather enjoy a misplaced sense of agency gained from the thrill of attacking cars in the dark. They cling to that popular green philosophy:‘If we all do a little, we can together do so much.’ It is a heartwarming motto, but one that can justify almost any action, no matter how ineffectual or misguided. As the late polymath and environmental writer, Sir David MacKay, put it: ‘Maybe if we all do a little, we will achieve only a little.’