Earth’s air temperature passed the agreed 1.5°C warming limit around 2010, according to measurements from the skeletons of sea sponges in the Caribbean, but some climate scientists aren't convinced.
The world is already 1.8°C warmer than it was in pre-industrial times, having passed the 1.5°C limit in 2010 or 2011, claim researchers who used sea sponges to work out how temperatures have changed in the Caribbean over the past 300 years.
“The increase in global mean surface temperature has been half a degree greater than the current accepted estimates,” says Malcolm McCulloch at the University of Western Australia. “What our work says is that we are a decade more, or further, advanced in the global warming scenario.”
However, other climate scientists say data from a single region isn’t a reliable way of working out past global temperatures.
The 2015 Paris Agreement called on countries to limit warming to 1.5°C above pre-industrial levels, but didn’t define exactly what this meant. Climate scientists putting together reports for the Intergovernmental Panel on Climate Change (IPCC) have therefore defined it as the average temperature between 1850 and 1900.
By this time, the planet had already begun warming as a result of emissions from the burning of fossil fuels. But because there were very few temperature measurements before 1850, there is huge uncertainty over how much fossil fuel-driven warming there was early on during the industrial age. So the choice of 1850 to 1900 as a baseline was a pragmatic one.