A 300-year record of ocean mixed-layer temperature changes preserved in the calcium carbonate skeletons of long-lived sclerosponges shows that industrial-era warming began in the mid-1860s, more than 80 years before instrumental sea surface temperature records. Using a Sr/Ca palaeothermometer calibrated against modern instrumental records, the study demonstrates that global warming has exceeded 1.5°C above pre-industrial levels by 2020, with a temperature increase of 1.7±0.1°C. This 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. The study highlights the importance of long-term temperature records in understanding the pace and extent of global warming, and the need for accurate pre-industrial baselines to assess current warming trends. The findings suggest that land temperatures are warming at nearly twice the rate of surface ocean temperatures, and that the current rate of warming will likely lead to a global surface temperature increase of 2.5°C by 2035 and 2°C by 2040. The study also emphasizes the urgency of reducing emissions to meet the Paris Agreement goals of limiting global warming to no more than 1.5°C above pre-industrial levels.A 300-year record of ocean mixed-layer temperature changes preserved in the calcium carbonate skeletons of long-lived sclerosponges shows that industrial-era warming began in the mid-1860s, more than 80 years before instrumental sea surface temperature records. Using a Sr/Ca palaeothermometer calibrated against modern instrumental records, the study demonstrates that global warming has exceeded 1.5°C above pre-industrial levels by 2020, with a temperature increase of 1.7±0.1°C. This 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. The study highlights the importance of long-term temperature records in understanding the pace and extent of global warming, and the need for accurate pre-industrial baselines to assess current warming trends. The findings suggest that land temperatures are warming at nearly twice the rate of surface ocean temperatures, and that the current rate of warming will likely lead to a global surface temperature increase of 2.5°C by 2035 and 2°C by 2040. The study also emphasizes the urgency of reducing emissions to meet the Paris Agreement goals of limiting global warming to no more than 1.5°C above pre-industrial levels.