Emulating Thermosteric Sea-Level Rise Using a Three-Layer Energy Balance Model

Although the mass loss of land ice is projected to be the largest contribution to sea-level rise in the coming centuries, thermal expansion will also be an important contributor and its accurate projection is primordial to understanding sea-level change over (multi-)centennial timescales. Earth System Models (ESMs) are the main tools for projecting thermosteric sea-level rise. ESMs, however, are computationally demanding and therefore long, multi-centennial simulations are challenging. In this study, we use a physical-based emulator that simplifies the climate system by using three vertically stacked layers, allowing us to mimic the energy balance response of ESMs to reproduce their thermal expansion simulations. We use this emulator to extrapolate simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) from 2100 to 2300 and validate our method with CMIP6 runs that are available over that time scale. Overall, the three-layer emulator outperforms its two-layer predecessor in simulating thermal expansion up to 2300, providing a reduction of up to 78% in cumulative error for the projection period covering 2100 to 2300. Finally, we demonstrate how using temperature output from the three-layer model can help us capture non-linearities in dynamic sea-level change better than its two-layer counterpart. The latter is a first step towards building more reliable emulation approaches for oceanic processes affecting regional sea-level change.