The Ocean – sentinel for planetary warming

Almost the entire amount of planetary heating from climate change is buffered by the ocean. Today, the Earth is out of energy balance from human-induced emissions and consequently, heat is accumulating in Earth’s climate system from which about 90% is stored in the global ocean. Hence, measuring this accumulated heat induced from anthropogenic activities through an estimate of ocean heat content while integrating ocean temperature from the surface down to large depth allows us to track the state of planetary warming. The fraction of this surplus heat of anthropogenic origin that is reaching the deep ocean – i.e., which is not in active communication with the atmosphere – is stored and trapped for hundreds to thousands of years. Hence, the ocean does not only guide us on the current amount of planetary warming, but also on the committed – and even irreversible changes to come. The recent assessment of the Intergovernmental Panel for Climate Change (IPCC) has stated that global heating of the Earth system is unequivocal, and both satellite and in situ observations have shown that heat accumulation and hence the Earth energy imbalance has doubled over the past 2 decades. Detecting an acceleration of Earth heating has remained elusive to date, despite suggestive evidence of a potential increase in heating rates. Studying ocean warming from independent studies also tells us that since 1960, the warming of the world ocean has accelerated at a relatively consistent pace of 0.15 ± 0.05 (W/m²)/decade, while the land, cryosphere, and atmosphere have exhibited an accelerated pace of 0.013 ± 0.003 (W/m²)/decade. This has led to a substantial increase in ocean warming, with a magnitude of 0.91 ± 0.80 W/m² between the decades 1960–1970 and 2010–2020, which overlies substantial decadal-scale variability in ocean warming of up to 0.6 W/m². These findings withstand a wide range of sensitivity analyses and are consistent across different observation-based datasets. The long-term acceleration of Earth warming aligns qualitatively with the rise in CO₂ concentrations and the decline in aerosol concentration during the same period, but further investigations are necessary to properly attribute these changes, and to analyze regional implications and their link to marine extremes which are ongoing under the European project ObsSea4Clim. This global indicator is hence most fundamental for the use of the scientific community and the public to measure of how well the world is doing in the task of bringing anthropogenic climate change under control, which are the committed changes for hundreds to thousands of years, and to track increase and acceleration of planetary warming.