Future constraints and trends of the air-sea CO2 flux in the South-East Pacific region: a CMIP6 evaluation

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 (CO2) remains poorly constrained due to strong variability and spatial heterogeneity in the air-sea carbon flux (FCO2). 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 CO2 seasonality. In the northern SEP, intense coastal upwelling off Chile makes the region a major source of CO2, while the southern SEP, characterised by Patagonian fjords and islands, acts as one of the largest coastal CO2 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 FCO2 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 CO2 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 CO2 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 FCO2 changes faster than those in higher-latitude waters. Anthropogenic CO2 accumulation enhances background oceanic CO2 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 CO2 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.