Abstract
Artificial upwelling (AU) is a novel geoengineering technology that brings seawater from the deep ocean to the surface. Within the context of global warming, AU techniques are proposed to reduce sea surface temperature at times of thermal stress around coral reefs. A computationally fast but coarse 3D Earth System model (3.6° longitude × 1.8° latitude) was used to investigate the environmental impacts of hypothetically implemented AU strategies in the Great Barrier Reef, South China Sea, and Hawaiian regions. While omitting the discussion on sub-grid hydrology, we simulated in our model a water translocation from either 130 or 550 m depth to sea surface at rates of 1 or 50 m3 s–1 as analogs to AU implementation. Under the Representative Concentration Pathway 8.5 emissions scenario from year 2020 on, the model predicted a prevention of coral bleaching until the year 2099 when AU was implemented, except under the least intense AU scenario (water from 130 m depth at 1 m3 s–1). Yet, intense AU implementation (water from 550 m depth at 50 m3 s–1) will likely have adverse effects on coral reefs by overcooling the surface water, altering salinity, decreasing calcium carbonate saturation, and considerably increasing nutrient levels. Our result suggests that if we utilize AU for mitigating coral bleaching during heat stress, AU implementation needs to be carefully designed with respect to AU’s location, depth, intensity and duration so that undesirable environmental effects are minimized. Following a proper installation and management procedure, however, AU has the potential to decelerate destructive bleaching events and buy corals more time to adjust to climate change.
Highlights
Shallow-water coral reefs sustain some of the most biodiverse ecosystems on the planet, providing numerous ecosystem services and values to both humans and the environment (Deloitte Access Economics, 2013)
Under control conditions, the model predicted an increase in the areaaveraged annual mean Sea surface temperature (SST) over the 80 years of 1.9–2.4◦C for the three test regions
“deep” scenarios completely eliminated bleaching threat level 1 (BL-LV1) condition over the modeled 80-year period, with the exception of “deep_low” at Great Barrier Reef (GBR) (Supplementary Figure S4). These results imply that increasing upwelling rate and depth increase the effectiveness of Artificial upwelling (AU) in mitigating coral bleaching caused by heat stress
Summary
Shallow-water coral reefs sustain some of the most biodiverse ecosystems on the planet, providing numerous ecosystem services and values to both humans and the environment (Deloitte Access Economics, 2013). Thriving in the euphotic zone at tropical latitudes, corals face increasing temperature-induced bleaching events (Aronson et al, 2002; Hughes et al, 2018). Through photosynthesis, most of the energy required by their hosts, the loss of zooxanthellae can lead to coral starvation and death, and to reef degradation. Sea surface temperature (SST) will rise in the decades and increase the probability of extreme marine heat events (Frölicher and Laufkötter, 2018) and the frequency and intensity of mass coral bleaching events contributing to reef degradation worldwide (Hughes et al, 2017). Climate change adaptation measures that could provide a longer time window for corals to cope with rising temperature, are needed along with the climate mitigation efforts
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