Positive Effects of Geoengineering on Ocean Acidification and Aragonite Saturation Levels

In the past geoengineering has been considered as a promising strategy for global cooling, although it has had some drawbacks. One of these drawbacks was the common belief that engineering the climate would not have any beneficial effects on ocean acidification, which is a negative component of climate change. The writers of this paper, however, proposed that geoengineering could have a beneficial impact on ocean acidification and offset some of the impacts that greenhouse gas emissions have had on our planet’s oceans, specifically pH levels. Aquatic organisms that rely on shells for survival can only build these shells in waters with higher aragonite saturation values, and as the ocean becomes more acidic, the aragonite saturation levels go down, and these organisms cannot survive. Climate engineering could potentially slow the ocean’s current pH decreases, which would ideally slow the rapid reduction of aragonite saturation in the oceans (Matthews et al, 2009). But, although simulations from this experiment do show an increase in oceanic pH values, but not a significant enough increase to stop the rapid decline in aragonite saturation levels. — Ellie Pickrell
Matthews, H., Cao, L., Caldeira, K., 2009. Sensitivity of ocean acidification to geoengineered climate stabilization. Geophysical Research Letters, 36.

 Matthews et al. conducted a series of experiments on an earth system model that resembled a world exposed to climate engineering. They performed five simulations which all began at a preindustrial climate equilibrium, and compared the model’s results at what represented conditions in the year 2100. The control group, A2, lacked climate engineering and consisted of prescribed SRES CO2 emissions. The next simulation, A2+eng consisted of prescribed CO2 emissions and climate engineering, which began after 2010. Next was the A2A+ eng, which consisted of the same CO2 emissions as simulation A2, but again was exposed to climate engineering after 2010. The first three simulations were representing a world with an active biosphere, which means that the land biosphere was exchanging carbon with the atmosphere, i.e. carbon sinks. The fourth simulation, A2nb, consisted of a neutral biosphere (the land biosphere does not exchange carbon with the atmosphere after 2010), prescribed CO2 emissions, and no geoengineering. The final simulation, A2nb+eng was the same as the previous simulation, but was exposed to climate engineering.
     After all of the simulations were tested, Matthews et al. compared the results of the pH and the aragonite tests between the different scenarios. In both the A2 and A2+ eng simulations, pH values were reduced (7.6 and 7.85), compared to the control group with a pH value of 8.05, and aragonite concentrations of 1.85. A model with climate engineering showed a slightly higher pH value at the 2100 mark in comparison to the A2 non-engineered simulation, but a lower aragonite saturation value (1.80 to 1.90). Climate engineering was also effective at reducing the average atmospheric temperatures, as well as lower atmospheric CO2 concentrations, due to an increase in carbon uptake by natural carbon sinks as a result of the cooler temperatures.
     In the A2A+eng simulation, the change in ocean pH was smaller and was extremely close to the control simulation’s pH, but the aragonite saturation decreased more rapidly as a result of climate engineering. An increase in dissolved inorganic carbon and colder temperatures lead to aragonite saturation values that were 9% lower than the values in the A2 simulation (from 1.72 in A2 to 1.58 in A2A+eng), because colder temperatures lead to slightly higher pH values, but result in lower aragonite saturation values.
     Next, A2nb+eng and A2nb were compared. In A2nb+eng, surface temperatures were colder, and ocean dissolved inorganic carbon values were higher than in A2nb, which created unaffected pH values and a further decrease in aragonite saturation relative to the non-engineered simulation. The A2nb+eng simulation had a pH value of 7.75, and aragonite saturation values of 1.7, while the A2nb simulation had a pH value of 7.25 and aragonite saturation values of 1.85.
     These effects and results are dependent on the enhanced accumulation of carbon in the land biosphere. Without this accumulation of carbon, climate engineering will have little effect on ocean pH levels, which would then lead to accelerated declines in aragonite saturation. 

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