AMOC Impacts on Surface Temperatures Across Changing CO₂ Concentrations

Abstract

As human emissions alter atmospheric CO₂ concentrations, surface temperatures and other major climate components are changing. In this thesis, I focus on one of these systems, the Atlantic Meridional Overturning Circulation (AMOC), to investigate the concurrent impacts this circulation may have on surface temperatures. Using an Earth system modelling approach, I jointly consider plausible CO₂ and AMOC changes (past, upcoming, and long‑term) and address diverse impacts on surface temperatures that have been overlooked thus far. In a first study, I investigate the past influence of the AMOC interannual variability on European winter cold extremes. I use a single‑model large ensemble approach to analyse the changes in such extremes and the physical mechanisms involved. I find that AMOC interannual variations can shift the distributions of cold extremes; however, this shift is much less evident in the recent period compared to a pre‑industrial climate. I explain the early shift and the contrast in the late period by changes in sea surface temperatures, sea ice, heat fluxes, and Euro‑Atlantic winter circulation regimes. Second, I focus on the AMOC carbon feedback, not only to quantify it but also to project its economic impact until the end of the century. The approach is to first design Earth system model simulations (which combine carbon‑cycle feedback and freshwater hosing experiments), and later incorporate the assessed feedback into an integrated assessment model. The results indicate that an AMOC weakening linearly leads to reduced ocean carbon uptake, and this reduction would give rise to economic damages, possibly turning AMOC weakening into a net cost to society. Third, I inspect the long‑term evolution of European heat extremes in the context of net‑zero emissions and a recovering and stabilising AMOC. This analysis is carried out in a set of net‑zero emissions simulations that extend for 1,000 years and allow for evaluating the effect of delays in emissions cessation. The findings suggest that the intensity and frequency of these extreme heat events might remain constant for many centuries, and that any delay in emissions cessation will bring significantly more intense extremes. Ultimately, I argue that combining such interdisciplinary approaches across timescales is essential for mitigating climate change and adapting to its uncertain impacts in the context of a changing AMOC.