Temporal Evolution of Ocean Heat Capacity and Its Impact on Near-Term Climate Sensitivity

In addition to Equilibrium Climate Sensitivity (ECS), the ocean’s thermal inertia plays a primary role in determining the near-term global warming rates driven by greenhouse gas forcing. To quantitatively understand the physical processes contributing to the difference of warming rates among CMIP6 models under the 1% CO2 increase scenario (1pctCO2), we optimally fit a climate emulator to derive the effective ocean heat capacity (κ_E) for each of the 70+ models after accounting for ECS discrepancies and decadal variability. Various metrics that describe the temporal curvature of models’ warming trends throughout the 150-year simulation under the 1pctCO2 scenario, such as TCR1 (Transient Climate Response at the CO2 doubling), TCR2 (the additional warming between CO2 doubling and quadrupling), TCR1/ECS, TCR2/ECS, and TCR2/TCR1, can be explained well by the derived κ_E. The derived κ_E can also reasonably predict model behaviors under the abrupt CO2 forcing scenario (4xCO2).

κ_E optimizations with varying complexity (e.g., single-phase constant, two-phase constants, single-phase linear changes, two-phase nonlinear changes, etc.) were also performed to evaluate how the temporal evolution of κ_E may affect models’ warming rates. We find that more sensitive (“hotter”) models (i.e., higher ECS) under the 1pctCO2 scenario tend to be more responsive initially (i.e., having larger TCR1/ECS) due to κ_E differences; however, under 4xCO2 forcing, hotter models tend to be less responsive initially due to κ_E. This contrast remains consistent across emulations with varying complexities. The interpretation using κ_E is consistent with the simple observation that, under the 4xCO2 scenario hotter models warm less (relatively) in the first 10 years compared to cooler models, while preferring to warm up more (relatively) in the later years.

The physical processes underpinning κ_E diversity in models and κ_E changes in time are investigated with particular attention. It was found that κ_E increases with time/warming for all CMIP6 models under the 1pctCO2 scenario, and for most models under the 4xCO2 scenario, leading to an even greater delaying effect on the global warming onset. The complex interconnection between ocean heat uptake and cloud feedback over different ocean basins, such as the Southern Ocean, will also be investigated. Physical mechanisms are corroborated by analyzing other extended “long-run” experiments in which some of the global models nearly approach full equilibrium.