Determine whether the true midHolocene response is non-additive

Determine whether the CESM2 ocean response to midHolocene climate changes is non-additive when multiple boundary forcings act together, by conducting single-forcing numerical experiments that perturb zonal surface stress, meridional surface stress, or heat flux independently.

Background

The paper demonstrates that the emulator's response can be reconstructed by linearly superimposing its responses to individual forcing components. However, this result establishes additivity only for the emulator, not for the underlying CESM2 ocean model.

The authors identify high-latitude sea-ice feedbacks and other nonlinear processes as possible sources of non-additivity, but the required single-forcing CESM2 experiments are unavailable. Such experiments would establish whether the true climate response contains interactions among the forcing components that the emulator fails to represent.

References

Although it contributes weakly to the equilibrium response, the orbital forcing is necessary for capturing the seasonal cycle changes in Figure \ref{fig:seasonal_cycle} with the correlation in Panel C dropping from 0.87 to 0.52 for $\mathcal{F}_{\mathrm{pi}$ (Figure S3). That the linearly superimposed responses capture not just the structure but the amplitude of the overall emulator response suggests that, for perturbations of this magnitude, the emulator combines forcing responses approximately linearly, consistent with the forcing pathways interacting only weakly through its internal state.

— Paleoclimate Boundary Conditions as an Out-of-Sample Test for the Forced Response of Ocean Climate Emulators  (2608.13494 - Subel et al., 13 Aug 2026) in Section 4, Contributions from Forcing Components; Section 5, Conclusion