An inquiry opened this week in Westminster that should be revolutionary. It is to decide, in a nutshell, whether the 50% of global carbon emissions embodied in the world’s built environment should be a factor in fighting the climate crisis. If we are all to account for the impact on global temperature rises of our eating, heating and travelling, why not our building? The inquiry is centred on a decision by M&S to demolish and rebuild its flagship art deco store in London’s Oxford Street, a structure that for some reason was overlooked for listing as historic. We are told that the “embodied” carbon that would be released by the redevelopment is 40,000 tonnes, reportedly the same as would be emitted by a petrol-driven car journeying from the Earth to the Sun. Goodness knows the distance for the forest of slabs and towers now rising out of control on London’s skyline. While this is not the first time embodied carbon has been a planning topic, it is certainly the most prominent. It has also moved to centre stage as M&S fancies itself as a net zero champion. It promises to include in its development a new store that will “revitalise” the street, adding that if refused permission, it will close, meaning that the building and we shall all be the poorer – classic developer’s blackmail. Such promises mean little, as London stores are opening and closing monthly. Walk round any British city and traffic restrictions, emission zones, cycle lanes and pollution warnings are on every side. Yet the many hoardings on these very same streets conceal a turmoil of demolishing, smashing, digging, concreting, towering and basementing, as if global heating had never been heard of. Westminster councillors wear hi-vis jackets and hard hats as party uniform, like Boris Johnson. Labour’s London mayor, Sadiq Khan, even approved the M&S project. It took a minister, Michael Gove, to call an inquiry into the redevelopment. How far the climate crisis is yet a planning issue is moot. M&S’s lawyers are protesting that no one told them about it beforehand. But the truth is that the retail worm has long been turning. Grand department stores are in terminal decline. Oxford Street has lost Debenhams, House of Fraser and Topshop. John Lewis is struggling, having closed eight shops already in Sheffield, York, Aberdeen and elsewhere. Even the sparkling new malls are in trouble. The biggest, Bluewater in Kent, has seen its book value plummet 70% in the last seven years. Lockdowns and online shopping have taken their toll. According to the property agency Lambert Smith Hampton, half of all Britain’s shopping centres are currently uneconomic, with a 20% vacancy rate. A 2021 Economist survey suggested a dramatic switch in offline shopping towards small, bespoke outlets that meet the public’s individual tastes, with more direct contact with staff. The concept of a traditional market is returning, grand as in Halifax and Derby or scruffy as in London’s Shoreditch. Farmer’s markets are booming. In America, even doctors are in on the act. Patriot Place outside Boston offers not just checkups and treatment for mild ailments, but scans for cancers and hip surgery. I can hear the NHS choking. Ask where younger Londoners most like to shop and the answer is somewhere intimate, old and attractive. Oxford Street’s crowds now burrow away into St Christopher’s Place, Fitzrovia and Soho. As for brash and impersonal Bluewaters and Westfields, they are for yesterday, as are the giant glass silos into which we used to lock office workers five days a week. In other words, combating the climate crisis can here move in lockstep with a humane urban environment. A conserved high street holds the future for offline shopping. It is for congregating and experiencing as well as spending. The evidence is in the busiest of London’s streets, historic Borough Market, Brick Lane, Portobello Road and Camden Lock. These are where people want to mingle, smell the city and spend money. Why do planners not understand this? That is why M&S should leave its handsome facade unsmashed and go elsewhere. Oxford Street’s future lies not in big stores, but in interiors, rear courtyards and back alleys. These should be converted for whatever purpose the market will bear: stalls, cafes, entertainers or just transient tenants. The secret, says the American urbanologist Edward Glaeser, is flexibility and informality. The city of the future is “young, gritty and fearless”, fostering “the human contact that enables our species to survive”. And it helps the planet into the bargain. Simon Jenkins is a Guardian columnist