Abstract/Summary

Global mean sea surface temperature (GMSST) is a fundamental diagnostic of ongoing climate change, yet there is incomplete understanding of multi-decadal changes in warming rate and year-to-year variability. Exploiting satellite observations since 1985 and a statistical model incorporating drivers of variability and change, we identify an increasing rate of rise in GMSST. This accelerating ocean surface warming is physically linked to an upward trend in Earth’s energy imbalance (EEI). We quantify that GMSST has increased by 0.54±0.07 K for each GJ m–2 of accumulated energy, equivalent to 0.17+/-0.02 K decade–1 (W m–2)–1. Using the statistical model to isolate the trend from interannual variability, the underlying rate of change of GMSST rises in proportion with Earth’s energy accumulation from 0.06 K decade–1 during 1985-89 to 0.27 K decade–1 for 2019-23. While variability associated with the El Niño Southern Oscillation (ENSO) triggered the exceptionally high GMSSTs of 2023 and early 2024, 44% (90% confidence interval: 35% to 52%) of the +0.22 K difference in GMSST between the peak of the 2023/24 event and that of the 2015/16 event is unexplained unless the acceleration of the GMSST trend is accounted for. Applying indicative future scenarios of EEI based on recent trends, GMSST increases are likely to be faster than would be expected from linear extrapolation of the past four decades. Our results provide observational evidence that the GMSST increase inferred over the past 40 years will likely be exceeded within the next 20 years. Policy makers and wider society should be aware that the rate of global warming over recent decades is a poor guide to the faster change that is likely over the decades to come, underscoring the urgency of deep reductions in fossil- fuel burning.