| Summary: | Fossil fuel CO2 emissions result in climate change and ocean acidification. Enhanced weathering or artificial ocean alkalinization (AOA) has been proposed as a geoengineering method to mitigate further increase of atmospheric CO2 and decrease of ocean pH. A variant of AOA involves reacting carbonates and adding the dissolved materials into the upper ocean. The net effect of this approach is to increase ocean alkalinity, thereby increasing the oceanic capacity to store fossil fuel CO2. Another effect of adding alkalinity would be to drive seawater to higher pH values and thus counteract the ongoing ocean acidification. We test implications of AOA for marine carbon cycle using the global ocean biogeochemical model HAMOCC. In our model scenarios we add alkalinity in the amounts proportional to fossil fuel emissions. We show that large-scale AOA scenarios in which large amounts of alkalinity are added would be necessary to avoid a significant increase in atmospheric CO2 and to hold the global seawater pH close to today’s value. Even a short-term AOA would have long-lasting effects on seawater chemistry. When AOA stops, atmospheric CO2 (and pH) reverts back to rising (decreasing) at the rate determined by the fossil fuel CO2 emissions growth, but the effect of AOA is permanent. Hence, in contrast to SRM, effects of AOA on seawater chemistry and atmospheric CO2 retain after stopping alkalinity addition; AOA would not involve a long-term commitment.
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