On several days last week, it was hotter in London than in Cairo. France has just declared a national emergency because of the heat. But the winemakers of Germany are ecstatic about this year’s vintage, and Asian manufacturers of air-conditioning systems are having a boom year for European sales.

That’s how most people see climate change: a gradual warming-up that will hurt some people and benefit others. The people of Tuvalu and others of the Netherlands will be inconvenienced as rising sea levels cover their homes, but they will either move or learn to breathe underwater. So no panic, and don’t take measures that might hurt economic growth.

But anyone who has been paying attention to the evidence coming out of the Greenland ice cores for the past 20 years should know that the real threat is not gradual warming. It is that the warming will trigger an abrupt and rapid cooling of the global climate, with catastrophic consequences for existing human populations. And the Europeans would be hit first and worst, for the mechanism that would cause this shift is the disappearance of the Gulf Stream.

Weather forecast

People talk about the “last ice age” as if it were over, but it’s not. The current cycle of global glaciation began around 3 million years ago, when the land that is now Panama rose above sea level, closing the old ocean channel between North and South America and forcing a major reorganization of ocean currents. Since then, ice sheets have covered around 30 per cent of the land surface of the planet most of the time, although this has been regularly interrupted by major melt-offs called “inter-glacials” when the ice coverage drops to about 10 per cent.

During the past million years these warm, wet episodes have come along approximately every 100,000 years. The present inter-glacial began about 15,000 years ago, and nobody knows for sure how long it will last. We do know, however, that the previous inter-glacial began about 130,000 BC, and lasted for 13,000 years — so we could already be in overtime on this one.

The good news is that we don’t automatically slide back into maximum glaciation. What happened in 113,000 BC was that the global climate flipped into a cool, dry, windy phase that was much less pleasant than our current balmy conditions: average temperature at least 5 degrees Centigrade lower than the present, and massive droughts all over the place. It took a further push — probably volcanic eruptions in Indonesia around 70,000 BC — to start the ice sheets growing again.

Bad to worse

The bad news is that even the “cool, dry, windy” phase would wreck human civilization. The whole enterprise of civilization has occurred within the warm, wet climatic bubble of the past 10,000 years. At least half the lands that now support agriculture would revert to tundra or semi-desert if we flipped back to the cool, dry and windy climate, and billions would die in the chaos of war and starvation.

When the flip happens, it isn’t gradual at all. The Greenland ice cores, a quarter-million-year record of annual snowfall that also tells us about average temperature, precipitation and even wind speed, contain an alarming message. When the climate mode shifts, global temperatures crash in 10 years or less — and stay down for centuries. The worst news is that the sudden flip into cool-and-dry is caused by gradual global warming.

The evidence in the Greenland ice cores is clear: the abrupt, high-speed flips in global climate known as Dansgaard-Oeschger events have happened many dozens of times. If we have a couple of more centuries of warm-and-wet conditions, we will have enough detail of global climate to postpone the next flip indefinitely. But if it goes over the edge now, it’s a calamity for everybody.

Europeans, whose agriculture could no longer feed even a tenth of their population, would be hit hardest of all. So why didn’t this prospect get more media attention during the recent unprecedented heat wave in Europe? Maybe all the science journalists were on vacation.