Methods to mitigate global warming have been ineffective thus far. For this reason, geoengineering methods to combat climate change have become a topic of much interest. Because the ocean holds a significant amount of anthropogenic carbon, deep ocean carbon sequestration is proposed to be a long-term solution to reducing the amount of carbon accumulation in the atmosphere. However, this solution also enhances ocean acidification at the seafloor. The authors study the effectiveness and side effects of CO2injection at various locations using an Earth model system. They compare the effects at the injection sites to the effects that would occur without using this mitigation method at those sites. The authors conclude that sequestration of CO2 was more effective under climate change and with larger overall emission, but poorly chosen sites that are shallow and or less accessible to the ocean can exacerbate future climate change. There are also many obstacles to using this method: a lack of public acceptability, costly and under-developed technologies for ocean CO2 storage, and a lack of complete evaluation of the benefits and consequences. The authors conclude that more thorough research is needed before the method is employed. —Michela Isono
Ridgwell, A., Rodengen, T., Kohfeld, K. 2011. Geographical Variations in the Effectiveness and side Effects of Deep Ocean Carbon Sequestration. Geophysical Research Letters 38, doi:10.1029.
The rising accumulation of carbon in the atmosphere has proven to affect the planet detrimentally . Methods to mitigate these effects have therefore been proposed and studied. Because the ocean holds a significant amount of carbon, deep ocean carbon sequestration is a specific technique of geoengineering. This method injects liquefied CO2thousands of meters deep into the ocean, where the carbon would sink and be stored. The geologic storage of CO2 in the ocean serves to prevent CO2 from entering the atmosphere and perpetuating the effects of global warming.
Methods: A low resolution Earth system model (GENIE) is used to represent ocean circulation and carbon cycling. Five models are used in total: a bathymetry of the Earth system model is used to track measurements of ocean depth; an observation model based on data-estimated observations from other studies; a control model based on a 10,000 year spin-up under pre-industrial boundary conditions which is continued to year 2010 where levels of atmospheric CO2are based on historical data; a model based on model-estimated distributions of water-column integrated anthropogenic CO2 inventory for year 1994; and an experimental model that incorporate SRES emission scenarios for years 2100 and 2000 where 10% of the emissions are directed towards the ocean and the other 90% enter the atmosphere. Seven location points for injection were used: Bay of Biscay, New York, Rio de Janerio, San Francisco, Tokyo, Jakarta, and Bombay. These points represented the Antlantic Ocean, the Pacific Ocean, and the Indian Ocean. The injected CO2, once dissolved, is referred to as Dissolved Inorganic Carbon (DIC).
Results and Discussion: CO2 that is injected at the ocean floor instead of being released into the atmosphere man interact with CO2 taken up at the ocean surface. Sequestration efficiency is therefore considered in the context of how much CO2 would invade the ocean from the atmosphere. In the San Francisco location, DIC extended outward from the injection point but there was also a reduced DIC in the North Atlantic because less CO2was taken up from the atmosphere. Higher DIC concentrations were found in the Pacific, but there was also a reduction in the amount of calcium carbonate saturation and an increase in the amount of seafloor area that had unsaturated conditions. However, in the Atlantic, the reduction of atmospheric CO2increased the amount of seafloor area that had saturated conditions.
Data regarding the variation in effectiveness of CO2 depletion and relative mitigation of the surface ocean acidification as a function of time, injection depth, and ocean sector demonstrate that carbon sequestration can fail to work. The Pacific and Indian Ocean point of injection sites were more likely to fail and result in a negative sequestration at year 3000 compared to unmitigated atmospheric CO2 release. However, the Atlantic injection sites did not have negative sequestration even though there were more shallow and intermediate ocean depth levels within this location.
Sequestration efficiency was mapped to visualize the retention of injected CO2 in the entire ocean for the release at each grid point. The relative efficiency of carbon sequestration in percent (RE) at the beginning of the time period and located away from shallow continental margins was more or less the same at over 70%. However, later in the millennium, many inter-basin gradients in CO2 retention developed and RE approached zero because the carbon equilibrium was reestablished between the ocean and atmosphere.
In locations where climate change was strongly prevalent, RE was enhanced. In this case, carbon mitigation led to lower CO2 in the atmosphere and decreased the temperature of ocean surfaces. This increased the solubility of CO2 and improved CO2 uptake at the ocean surface. RE was higher for greater emissions. This means that carbon buffering is reduced when more CO2 is released and absorbed by the ocean surface. Thus, the results indicated that CO2 injection improved the ability for uptake from the atmosphere. Locations that included shallow sites and sites that are not well connected to the entire ocean exhibited an RE < 0.0.
The choice of CO2injection site was also analyzed based on levels of under saturated waters. At year 2100, there was little changed by injection in the NW Pacific compared to the unmitigated case. Injections in the SE Pacific and S Atlantic experienced a 10% additional increase in the seafloor area that was under saturated. By the end of the millennium, injection led to less than a 2% increase in additional under saturated seafloor area.
Conclusion: Sequestration of CO2was more effective under climate change and with larger overall emissions. For higher emissions, the naturally occurring CO2 buffer of ocean surface waters is depleted faster. Overall, RE is better than 70% by year 2100 and in certain places can stay above 50% by year 3000. Poorly chosen sites that are shallow and or less accessible to the ocean can exacerbate future climate change. Injection in the deep NW Pacific (a high efficiency site) minimizes the exacerbation of under saturated seafloor conditions. There are many practical constraints that would limit the use of this geoengineering technique such as negative environmental impacts, harmful effects on organisms, and the method’s effect on other associated biotic impacts still need further research and analysis.