Artificial upwelling via Stommel pipes under present and future climate conditions: an Earth system model assessment

Artificial upwelling has been proposed as a marine Carbon Dioxide Removal (CDR) approach to enhance the ocean's carbon sink. However, artificial upwelling disrupts the ocean thermocline as a side-effect through the upwelling of cold water, if done with e.g., wave-driven pipes. Large scale ocean thermocline disruption would lead to severe consequences for Earth's energy balance and the global carbon cycle during pipe-deployment and after pipe-termination. A different approach to realize artificial upwelling are counterflow Stommel Pipes. These pipes establish under specific salinity and temperature conditions a self-sustaining upwelling and simultaneous downwelling flow through efficient heat exchange between both streams, which mitigates direct ocean thermocline disruption risks. In this study, we simulate a specific Stommel Pipe design with 303, 643 and 1,103 m pipe length, under the assumption of high heat exchange efficiency and via a maximum technical potential approach in an Earth system climate model between the years 2025–2100 under a low and high future CO 2 -emission pathway to assess present to future pipe performance and impact on the Earth system and compare these results with the expected impact of wave-driven pipes. We find sustained upwelling intensities of up to 0.45 m 3 /s/pipe at the center of the Atlantic and Pacific subtropical gyres and an overall upwelling dependent increase in ocean net primary production. Stommel Pipe upwelling did not directly disrupt the ocean thermocline under our high heat exchange efficiency assumption but instead led to a regional increase in ocean stratification primarily due to salinity redistribution. The simultaneous upwelling of interior ocean nutrients and DIC to the surface caused CO 2 -emission pathway and upwelling intensity dependent net CO 2 outgassing to the atmosphere primarily driven by the South Pacific region (−249.0 Tg C/yr 2030), while the North Atlantic responded in lower upwelling intensity scenarios with minor CO 2 uptake (+7.1 Tg C/yr 2030). Conclusively, Stommel Pipes can regionally generate considerable upwelling without causing direct thermocline disruption under a high heat exchange efficiency assumption but have a region specific low to negative marine CDR potential if simulated as a stand-alone approach.