Divergent Response of Ocean Regions to Temporarily Overshooting Paris Agreement Warming Levels

Under current mitigation implementations, it is of increasing likelihood that global warming will exceed the target set by the Paris Agreement (PA) of “well below 2°C”. Correcting for this overshoot through intense carbon dioxide removal could reverse global warming back to safe levels, but the biophysical impacts associated with pathways exposing the planet to dangerous warming levels is essentially unknown. This is particularly the case for the ocean ecosystem, where peak warming could lead to ecosystem threshold exceedance and non-reversible changes. Here, we investigate spatial asymmetries in the response of surface ocean ecosystem stressors to temporarily overshooting the PA target using a novel model framework.

To advance the knowledge on temporary overshoots, we utilized the Adaptive Emission Reduction Approach to design first-order emission pathways to reach given stabilization and overshoot peak temperature targets. With the help of this framework, we performed simulations with the Earth System Model GFDL-ESM2M that overshoot by 0.5°C and 1.5°C, and thereafter returns to the quasi-stabilized warming level of a simulation that respects the PA target.  

Our preliminary analysis shows important differences in regional ocean characteristics between simulations that overshoot the PA target and a simulation that stabilizes at the PA target, despite ultimately reaching the same global surface temperature. For instance, regional sea surface temperatures can differ by over 0.5°C in the extreme overshoot of 1.5°C in comparison to the PA stabilization simulation, even following the overshoot. This spatial heterogeneity is illustrated through the divergent oceanic response of the polar oceans; In northern latitudes, cooler temperatures are simulated through the expansion of the North Atlantic cold spot during the overshoot, which arises through decrease of heat transport of the Gulf Stream owing to the weakening of the Atlantic Meridional Overturning Circulation (AMOC) of around 6 Sv or 30 %. In turn, the Southern Ocean is substantially warmed regionally in the overshoot simulations versus the stabilization, likely originating from increasing cross-ocean transport of heat during the overshoots, an implication that is ongoing even after the temporary overshoots return to the PA warming level. Similar spatial heterogeneities are also found for other ecosystem stressors such as O₂ and pH, hinting at potential disruption of regional ecosystems. Our analysis indicates that increased assessment of regional ocean responses to temporary warming exceedance levels and their impacts for regional ecosystems are urgently needed in the scope of fully evaluating trade-offs associated with delaying climate action and overshooting the PA.