300 years of sclerosponge thermometry shows global warming has exceeded 1.5 °C

Anthropogenic emissions drive global-scale warming yet the temperature increase relative to pre-industrial levels is uncertain. Using 300 years of ocean mixed-layer temperature records preserved in sclerosponge carbonate skeletons, we demonstrate that industrial-era warming began in the mid-1860s, more than 80 years earlier than instrumental sea surface temperature records. The Sr/Ca palaeothermometer was calibrated against ‘modern’ (post-1963) highly correlated (R2 = 0.91) instrumental records of global sea surface temperatures, with the pre-industrial defined by nearly constant (<±0.1 °C) temperatures from 1700 to 1800 and then from 1840 to 1860. The intervening period is excluded due to volcanic cooling of ~0.2 °C mainly from the massive Tambora eruption in Indoneasia in 1815 which caused the year without a summer.  From the commencement of Industrial-era warming in early 1860s increasing ocean and land-air temperatures overlap until the late twentieth century, when the land began warming at nearly twice the rate of the surface oceans. Hotter land temperatures, together with the earlier onset of industrial-era warming, indicate that global warming was already 1.7 ± 0.1 °C above pre-industrial levels by 2020. Our result is 0.5 °C higher than IPCC estimates, with 2 °C global warming projected by the late 2020s, nearly two decades earlier than expected.