Changes in ocean heat content are a critical element of climate change, with the oceans containing about 90% of the excess heat stored in the climate system (60% in the upper 700 db). Estimates of these changes are sensitive to horizontal mapping of the sparse historical data and errors in eXpendable BathyThermograph data. Here we show that they are also sensitive to the vertical interpolation of sparsely sampled data through the water column. We estimate, using carefully constructed vertical interpolation methods with high-quality hydrographic (bottle and CTD) data, the observationally based upper ocean heat content increase (thermosteric sea level rise) from 1956 to 2020 is 285 Zeta Joules (0.55 mm yr-1), 14% (14%) larger than estimates relying on simple but biased linear interpolation schemes. The underestimates have a clear spatial pattern with their maximum near 15ºN and 12ºS, near the maxima in the curvature of the temperature depth profile.

There is large uncertainty in the future sea level change at regional scales under anthropogenic global warming. This study uses a novel design of ocean-only general circulation model (OGCM) experiments to investigate the ocean’s response to surface buoyancy and momentum flux perturbations, as part of the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP), and compares with results from coupled, atmosphere-ocean GCM (AOGCM) experiments. Much of the inter-model spread is driven by the response to surface heat flux perturbations. In a multi-model ensemble of OGCMs forced with identical surface heat flux perturbations, regional sea level and ocean heat content changes demonstrate considerable disagreement, especially in the North Atlantic. Spread in both residual mean advection and diapycnal diffusion changes contribute to much of the multi-model disagreement over regional heat content change. Residual mean advection changes are related to the large spread in simulated Atlantic meridional overturning circulation (AMOC) weakening (20-50%). We find approximately 10% more AMOC weakening in response to surface heat flux perturbations in AOGCMs relative to OGCMs with consistent ocean models. This enhanced AMOC weakening is driven by an atmosphere-ocean feedback which amplifies the surface heat flux perturbation. In the North Pacific, there is little agreement amongst the ensemble over which processes lead to ocean warming, with varying contributions from residual mean advection and diapycnal diffusion. For the Pacific basin, the atmosphere-ocean feedback reduces sea surface temperature (SST) warming by 0.5°C. In the Southern Ocean, the atmosphere-ocean feedback is not generally important for buoyancy and momentum flux perturbations.

The ocean has absorbed approximately 90% of the accumulated heat in the climate system since 1970. As global warming accelerates, understanding ocean heat content changes and tracing these to surface heat input is becoming increasingly important. We introduce a novel tracer-percentile framework in which we organise the ocean into temperature percentiles from warmest to coldest, allowing us to trace changes in ocean temperature to changes in air-sea heat fluxes and mixing. Applying this framework to observations and historical CMIP6 simulations, we find that 40% of heat uptake between 1970 and 2014 occurs in the warmest 10% ocean volume. However, the coolest 90% ocean volume outcrops over 23% of the ocean’s surface area, implying a disproportionately large heat input per unit area. Additionally, a cold bias in the CMIP6 models is traced to inaccurate sea surface temperatures and surface heat fluxes into the warmest 5 – 20% ocean volume.

Sea-level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientist and practitioners, builds on a framework of discussing physical evidence to quantify high-end global SLR for practice. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2 ˚C in 2100 (SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for +5 ˚C (SSP5-8.5) we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2 ˚C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while we emphasize the timing of ice-shelf collapse around Antarctica, which is highly uncertain due to low understanding of the driving processes.