Geoengineered Ocean Vertical Water Exchange Can Accelerate Global Deoxygenation

Abstract

Ocean deoxygenation is a threat to marine ecosystems. We evaluated the potential of two ocean intervention technologies, that is, “artificial downwelling (AD)” and “artificial upwelling (AU),” for remedying the expansion of Oxygen Deficient Zones (ODZs). The model‐based assessment simulated AD and AU implementations for 80 years along the eastern Pacific ODZ. When AD was simulated by pumping surface seawater to the 178–457 m‐depth range of the ODZ, vertically integrated oxygen increased by up to 4.5% in the deployment region. Pumping water from 457 m depth to the surface (i.e., AU), where it can equilibrate with the atmosphere, increased the vertically integrated oxygen by 1.03%. However, both simulated AD and AU increased biological production via enhanced nutrient supply to the sea surface, resulting in enhanced export production and subsequent aerobic remineralization also outside of the actual implementation region, and an ultimate net decline of global oceanic oxygen.

Could Artificial Downwelling/Upwelling Mitigate Oceanic Deoxygenation in Western Subarctic North Pacific?

“…AD and AU are simulated in the core region of the WSNP (∼440,000 km 2 , the shadow area in Figure 1) for the period of 2020-2100. Since engineered pipes are small and difficult to be resolved in the current coarse-resolution model, they are parameterized as suggested by Feng et al (2020) and Oschlies et al (2010). In AD case, dissolved ocean tracers are adiabatically transferred from a model grid box containing the pipe's upper end to another model grid box containing the lower end of the pipe at a constant velocity w (< 0).…”

“…The released water drives the upward motion of deep water, reducing oxygen in the upper ocean. Near the surface, oxygen is almost the same as in the non-engineered case, as a result of a tradeoff between compensatory upwelling of deep lowoxygen and enhanced air-sea oxygen flux during AD (Feng et al, 2020). Turning to the AU case, water experiences a more evenly distributed oxygen improvement.…”

“…In their simulations, Feng et al (2020) found that when implementing AD and AU in the eastern tropical Pacific ocean, elevated deep oxygen coincides with severe oxygen decline outside of the experimental site, reducing the amount of global oceanic oxygen. This undesirable consequence results from upwelled macro-nutrients, directly or indirectly brought by the engineering but not available to local phytoplankton growth, stimulating primary production and aerobic remineralization outside of the deployment site (Keller et al, 2014;Feng et al, 2020). Although AD and AU might not be suitable for the tropical oceans, their potential to improve the oxygen in other low-oxygen regions has not yet been explored.…”

“…With climate warming and eutrophication, the global ocean is undergoing deoxygenation, resulting in loss of biomass and significant changes in biogeochemical cycles (Diaz and Rosenberg, 2008;Breitburg et al, 2018). Management plans focused on reducing nutrient loads have made significant progress in restoring oceanic oxygen, yet uncertainty about their immediate effectiveness leads to calls for short-term solutions (Stigebrandt and Gustafsson, 2007;Conley, 2012;Feng et al, 2020). The arguably most promising solution includes using pipes to transport surface oxygen-rich water into deep water.…”

“…Compared to the tropical oceans, the WSNP region may provide AD and AU with a higher chance of success. In the WSNP, a less divergent surface flow regime facilitates the accumulation of engineered oxygenation, and biogeochemical responses are likely to be confined in the gyre region so that unwanted side effects outside of the deployment sites may not be as strong as in the study region of Feng et al (2020). To test the above hypothesis, we simulate continuous AD and AU that transport water between the surface and 280-650 m range of the WSNP.…”