This study uses 300 years of ocean mixed-layer temperature records preserved in sclerosponge carbonate skeletons to 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 instrumental records, with the pre-industrial period defined by nearly constant temperatures from 1700 to the early 1860s. The results show that global warming was already 1.7 ± 0.1 °C above pre-industrial levels by 2020, 0.5 °C higher than IPCC estimates. The study also highlights the divergent rates of warming between land and ocean surfaces, with land temperatures increasing at nearly twice the rate of surface oceans since the late 19th century. These findings have significant implications for near-term projections of global warming, indicating that the opportunity to limit global warming to no more than 1.5 °C by emission reductions alone has passed, and that the 2 °C threshold will be reached by the late 2020s at current emission rates.This study uses 300 years of ocean mixed-layer temperature records preserved in sclerosponge carbonate skeletons to 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 instrumental records, with the pre-industrial period defined by nearly constant temperatures from 1700 to the early 1860s. The results show that global warming was already 1.7 ± 0.1 °C above pre-industrial levels by 2020, 0.5 °C higher than IPCC estimates. The study also highlights the divergent rates of warming between land and ocean surfaces, with land temperatures increasing at nearly twice the rate of surface oceans since the late 19th century. These findings have significant implications for near-term projections of global warming, indicating that the opportunity to limit global warming to no more than 1.5 °C by emission reductions alone has passed, and that the 2 °C threshold will be reached by the late 2020s at current emission rates.