Overestimation? The projected global mean sea-level rise is about 25% lower at the end of this century in the eddying model

Ocean eddies strongly affect global mean sea-level projections. René M. van Westen and Henk A. Dijkstra. Science Advances  Apr 9 2021, Vol. 7, no. 15, eabf1674

DOI: 10.1126/sciadv.abf1674

Abstract: Current sea-level projections are based on climate models in which the effects of ocean eddies are parameterized. Here, we investigate the effect of ocean eddies on global mean sea-level rise (GMSLR) projections, using climate model simulations. Explicitly resolving ocean eddies leads to a more realistic Southern Ocean temperature distribution and volume transport. These quantities control the rate of basal melt, which eventually results in Antarctic mass loss. In a model with resolved ocean eddies, the Southern Ocean temperature changes lead to a smaller Antarctic GMSLR contribution compared to the same model in which eddies are parameterized. As a result, the projected GMSLR is about 25% lower at the end of this century in the eddying model. Relatively small-scale ocean eddies can hence have profound large-scale effects and consequently affect GMSLR projections.

DISCUSSION

For the two different versions of the CESM (HR-CESM and LR-CESM), the overall responses to the increase in CO2 [GMST, contributions by glaciers to the GMSLR (34), thermo(steric) effects (21), and surface mass balance changes of the GrIS (35)] are quite similar and compare also well to 31 CMIP6 models analyzed. The projected temperature change and snowfall anomaly over the AIS for the HR-CESM and LR-CESM are also similar to the ones reported in Gregory and Huybrechts (29).

However, the Antarctic basal melt (7) strongly deviates between the HR-CESM and LR-CESM. The HR-CESM and LR-CESM simulations provide GMSLR projections of 5.4 ± 0.3 cm (95% confidence level) and 15 ± 0.8 cm (95% confidence level) through basal melt in 2100, respectively, which gives a factor 2.8 difference. The LR-CESM GMSLR projection of basal melt is within CMIP6 projections, but the HR-CESM projects quite lower GMSLR values with respect to CMIP6 ones. These differences in basal melt are related to the different horizontal resolutions in the ocean component of the models.

The Southern Ocean is a rather complex region where the large-scale ocean circulation, mesoscale ocean eddies, sea-ice formation, and atmospheric processes all play an important role in the response under global warming. Mesoscale ocean eddies are highly relevant for the redistribution and transport of heat and salt (20, 22, 36, 37) and are critical for the correct momentum balance for the large-scale circulation. Explicitly resolving ocean eddies in the HR-CESM does not only lead to a better representation of the present-day subsurface temperature distribution surrounding Antarctica (compared to LR-CESM) but also to a different response under global warming. For the HR-CESM, we find changes on both the large scale (e.g., in the ACC, sea-ice fields) and the regional scale (Weddell and Ross gyres and the Antarctic Coastal Current), while in the LR-CESM (and CMIP6 models), these occur only on the large scale.

Because of the extreme computational costs, there is unfortunately only one high-resolution simulation available for the analysis done here (HR-CESM control and HR-CESM). More of those simulations are required to provide a broader range of GMSLR projections, also under different climate change scenarios. However, the results here already indicate that sea-level projections based on low-resolution climate models should be interpreted with great care, in particular, regarding estimates of the effects Antarctic basal melt.