The planet has already passed 1.5 °C of warming, according to a new measuring technique that goes back further in time than current methods. At the 2015 Paris Climate Accords, nations agreed not to exceed 1.5 °C, a guardrail of climate change.

“We have an alternate record of global warming,” says coral-reef geochemist Malcolm McCulloch, at the University of West Australia Oceans Institute in Crawley, and lead author of the study. “It looks like temperatures were underestimated by about half a degree.”

The Intergovernmental Panel on Climate Change (IPCC) uses a baseline for pre-industrial global mean temperatures that reference the earliest global instrumental temperature records. This period is around 1850–1900, when the first ship-based records of sea-surface temperatures became available.