As the ocean absorbs the excess CO2 produced by the burning of fossil fuels, several methods for mitigating ocean CO2 absorption, called marine carbon dioxide removal or mCDR have surfaced and been funded. However, a recent study shows that, as with any human-designed solution to climate change, a failure to think in terms of ecosystems rather than symptoms can lead to even bigger problems.
Researchers led by Prof. Dr. Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research Kiel noted that amount of dissolved molecular oxygen in the ocean is 160 times less than the amount of oxygen in the atmosphere and has decreased by about 2% over the last five decades. But the amount of CO2 in the ocean is about 40 times more than that in the atmosphere. About 15% of the observed oxygen loss can be explained by the warming-driven loss of solubility. The remaining and dominant fraction is thought to be associated with warming-induced changes in ocean circulation, and more than 70% of the ocean’s oxygen loss appears to have occurred at depths below 1200 meters.
A large percentage of the ocean is losing access to sunlight, driving lower oxygen levels throughout. As the oceans grow warmer, scientists warn that the deeper parts of our oceans will become much less efficient at circulating oxygen.
Oxygen and CO2 are closely linked. The same plants that create oxygen both on land and in the ocean release carbon dioxide when they die and begin to decay. For that reason the mCDR approaches that just increase the amount of biomass that aims to increase oxygen production, but that allows that biomass to sink to the bottom and decay actually reduces the overall oxygen in the water.
However, large-scale macroalgae farming where the biomass is harvested–removed from the ocean–results in an overall increase in oceanic oxygen levels. So early results show that, for example, seaweed farming is a viable choice for increasing oxygen levels in the oceans.
“Model results suggest that if deployed at sufficient scale, this approach could even reverse past oxygen losses—providing up to 10 times more oxygen than has been lost due to climate change within a century.” The bad news is that removal of nutrients could negatively impact biological productivity in the ocean. In other words, plants serve as a food source for everything up the food chain.
Geochemical mCDR approaches that do not involve nutrient input—such as ocean alkalinity enhancement through the addition of alkaline substances based on limestone—appear to have minimal effects on ocean oxygen levels and are comparable to simply reducing CO2 emissions.
The researchers urge anyone promoting mCDR solutions to take into account their impact on ocean oxygen.
