Duo Chan, Geoffrey Gebbie, Peter Huybers

Accurate historical temperature estimates are crucial for understanding current warming levels and informing policy decisions. Trends in global land and ocean temperatures diverge, however, before 1945. Inter-calibration of coastal land-ocean temperatures indicates that this divergence arises from under-corrected biases in sea-surface temperature linked to late 19th century instrumentation changes. Harmonizing land and ocean temperatures yields a steadier warming since 1850, better aligning with reconstructions from proxies and results from models driven by external radiative forcing. Our estimates also suggest that the average temperature over 2019–2023 is 1.36C warmer than the 1850–1900 baseline, or 10% higher than most existing estimates. 20-year average global mean temperatures are likely to exceed 1.5C by 2028, regardless of emissions scenarios, though the likelihood of surpassing 2C remains below 50% for scenarios with substantial emission reductions.