The enormous, unprecedented pain and turmoil caused by the climate crisis is often discussed alongside what can seem like surprisingly small temperature increases – 1.5C or 2C hotter than it was in the era just before the car replaced the horse and cart. These temperature thresholds will again be the focus of upcoming UN climate talks at the COP26 summit in Scotland as countries variously dawdle or scramble to avert climate catastrophe. But the single digit numbers obscure huge ramifications at stake. “We have built a civilization based on a world that doesn’t exist anymore,” as Katharine Hayhoe, a climate scientist at Texas Tech University and chief scientist at the Nature Conservancy, puts it. The world has already heated up by around 1.2C, on average, since the preindustrial era, pushing humanity beyond almost all historical boundaries. Cranking up the temperature of the entire globe this much within little more than a century is, in fact, extraordinary, with the oceans alone absorbing the heat equivalent of five Hiroshima atomic bombs dropping into the water every second. Until now, human civilization has operated within a narrow, stable band of temperature. Through the burning of fossil fuels, we have now unmoored ourselves from our past, as if we have transplanted ourselves onto another planet. The last time it was hotter than now was at least 125,000 years ago, while the atmosphere has more heat-trapping carbon dioxide in it than any time in the past two million years, perhaps more. Since 1970, the Earth’s temperature has raced upwards faster than in any comparable period. The oceans have heated up at a rate not seen in at least 11,000 years. “We are conducting an unprecedented experiment with our planet,” said Hayhoe. “The temperature has only moved a few tenths of a degree for us until now, just small wiggles in the road. But now we are hitting a curve we’ve never seen before.” No one is entirely sure how this horrifying experiment will end but humans like defined goals and so, in the 2015 Paris climate agreement, nearly 200 countries agreed to limit the global temperature rise to “well below” 2C, with an aspirational goal to keep it to 1.5C. The latter target was fought for by smaller, poorer nations, aware that an existential threat of unlivable heatwaves, floods and drought hinged upon this ostensibly small increment. “The difference between 1.5C and 2C is a death sentence for the Maldives,” said Ibrahim Mohamed Solih, president of the country, to world leaders at the United Nations in September. There is no huge chasm after a 1.49C rise, we are tumbling down a painful, worsening rocky slope rather than about to suddenly hit a sheer cliff edge – but by most standards the world’s governments are currently failing to avert a grim fate. “We are on a catastrophic path,” said António Guterres, secretary general of the UN. “We can either save our world or condemn humanity to a hellish future.”