– Chemical records from sea sponge skeletons suggest that we passed the critical threshold of 1.5 °C of warming as early as 2010.
Sea sponges are used as a natural thermometer to peer back through time to well before humanity's industrial era, at a deeper layer of the ocean than previous measurements.
(Sea sponge skeletons store different ratios of strontium compared to calcium depending on the temperature of the water they're growing in).
The sponge data matches more closely to CO2 records than the IPCC's average does and correlates to historic known geological events, such as past volcanic eruptions.
Previous models, including the IPCC, calibrated their data using measurements taken from ships since the 1920s,
which were not very systematic and often lacked data from the Southern Hemisphere.
Strange weather we've been experiencing, including massive heatwaves in Europe in regions that don't usually have them,
are what we'd expect at around 2°C of post-industrial warming.