The world may already have crossed a series of climate tipping points, according to a stark warning from scientists. This risk is “an existential threat to civilisation”, they say, meaning “we are in a state of planetary emergency”. Tipping points are reached when particular impacts of global heating become unstoppable, such as the runaway loss of ice sheets or forests. In the past, extreme heating of 5C was thought necessary to pass tipping points, but the latest evidence suggests this could happen between 1C and 2C. The planet has already heated by 1C and the temperature is certain to rise further, due to past emissions and because greenhouse gas levels are still rising. The scientists further warn that one tipping point, such as the release of methane from thawing permafrost, may fuel others, leading to a cascade.