They were everywhere in London on the weekend. The people in short sleeves or sandals. The ones with sunglasses ostentatiously hanging from the front of their shirts or balanced on top of their heads. The beer gardens and riverside pubs of the capital were heaving; corner shops ran out of ice-cream. Outside it was 17C (62F).

Monday was another warm day, without a cloud in the sky, and in the late afternoon the light took on a magical, honey-coloured hue. It brought to mind one of those summer evenings you remember from childhood, when you’d be in the park all day and your parents let you stay out until bedtime, and you felt like you were doing something deliciously naughty just by being there.

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Except it isn’t early summer: it’s February. And the entire developed world has not so much been doing something slightly naughty as systematically attacking the global ecosystem over a period of decades, and that’s how we go into this mess.

We should try to hold on to this fact as young, posh men the nation over develop a strange delusion that anyone would want to see their elbows; this is not supposed to be happening. Less than a month ago, there was video footage of extreme cold weather coming out of Chicago. Forks supported in midair by suddenly frozen noodles, water poured from kettles instantly freezing on its way to the ground: you know the sort of thing. OK, that was on the other side of the world, and was extreme and terrifying enough. But at least it was terrifying in the right direction.

On Monday, though, the temperature hit 20.3C in Ceredigion, west Wales: the highest February temperature ever recorded in Britain and the first time the thermometer had breached 20C in winter. The BBC weather account tweeted it out with a gif of the sunshine icon and the same excitable breathlessness with which Springwatch would announce it had found a new type of vole. My response contained a single word, repeated seven times. It began with F.

Because this isn’t good, is it? However enjoyable the unreasonable sunshine feels, whatever feeling of relief it instils in you after weeks of grey sky and Brexit, the idea of beach temperatures in February should be scaring the living shit out of you. It shouldn’t be possible to wander round London half-naked in February without bits of you falling off. There is a fairly direct inverse correlation between your ability to go out without a jacket at this latitude in winter and a polar bear’s likelihood of surviving the winter – yet the population of Britain wandered round with their shirts undone looking pleased with themselves. Nation of animal lovers my arse. Something has gone wrong.

John Lanchester’s latest novel, The Wall, portrays a dystopian, post-climate change Britain in which the entire country has been walled off to prevent “others”, from the ruined, flooded world beyond, getting in. The young resent the old because the change happened on their watch; the state kills with ease, and slavery has returned. But it is set in a place that is still recognisable as this country, in which people go to pubs, get pissed, commute on crowded railway lines and think the Lake District is a nice place to go. It’s a vision that’s haunting precisely because it is so banal. You can believe in this future. The world is ruined, and the British people have conspired as much as possible to ignore the fact.

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The country’s response to this unseasonably warm spell suggests Lanchester may be on to something. The 20 hottest years on record have all happened within the past 22 years; the five hottest were the last five. Yet the beaches and the beer gardens fill up, while the papers describe the weather as glorious and expend more words on the latest Westminster soap opera than on the looming climate crisis. The thing about an environmental apocalypse is that it doesn’t have a face.

There were those on Sunday who gave in to the heatwave a little too easily. They let themselves forget it was winter and found themselves, when the sun went down and the temperature dropped, suddenly shivering and unprepared for the cold. It feels uncomfortably like a metaphor.

• Jonn Elledge is assistant editor of New Statesman