Abstract
The South-East Pacific (SEP) plays a key role in the global carbon cycle. However, despite its importance, its ability to absorb anthropogenic carbon dioxide (CO 2 ) remains poorly constrained due to strong variability and spatial heterogeneity in the air-sea carbon flux (FCO 2 ). Uncertainties persist regarding the contrasting responses of the northern upwelling system and of the southern fjord-dominated region, as well as the mechanisms driving changes in CO 2 seasonality. In the northern SEP, intense coastal upwelling off Chile makes the region a major source of CO 2 , while the southern SEP, characterised by Patagonian fjords and islands, acts as one of the largest coastal CO 2 sinks in the Southern Hemisphere. Here, we analyse six global coupled atmosphere-ocean-carbon cycle-ecosystem models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to assess future projections of FCO 2 under intermediate and high-emission scenarios (SSP2-4.5 and SSP5-8.5). We also apply a first-order Taylor decomposition of the surface ocean partial pressure of CO 2 to quantify the relative contributions of its main drivers, providing a process-based interpretation of projected changes. By 2100, the SEP is projected to undergo substantial changes in carbon dynamics: under the SSP5-8.5 scenario, the multi-model mean shows that the northern SEP shifts from a CO 2 source to a sink, while the southern region becomes an even stronger sink with significant increases in variability over nearly the entire region. Under SSP2-4.5, this shift in the north is partially reversed and increases in variability are milder or non-significant over a large portion of the SEP, suggesting that moderate climate mitigation could preserve the region’s carbon dynamics. Upwelling zones are projected to experience FCO 2 changes faster than those in higher-latitude waters. Anthropogenic CO 2 accumulation enhances background oceanic CO 2 levels and DIC seasonality, while reduced temperature seasonality counteracts this amplification. These compensating mechanisms explain the projected sign reversal in the northern SEP, where DIC-driven anomalies weaken the upwelling influence and modulate future air-sea CO 2 fluxes. Our results highlight the sensitivity of eastern boundary upwelling systems to climate change and suggest that moderate mitigation scenarios may partially preserve present-day carbon dynamics, with important implications for regional and global carbon budgets.