Impacts of Stabilizing Atmospheric Carbon Dioxide Levels: The Role of the Oceans

The ocean has been recognized as the dominant sink for emissions of fossil fuel CO2 for over 70 years. But this is rarely considered in policy debates since no nation can claim this sink - it is in effect regarded as a universal public good. The quantities are indeed large, the rate is almost stable as long as the wind blows changing by about 0.3% per year as ocean chemical resistance grows, and the impacts are largely irreversible. The bare facts alone are remarkable: some 25-30% of the 31 billion tons of CO2 (6.2 billion tons per year; close to 1 million tons per hour) emitted in 2010 will be rapidly absorbed by the ocean. In the long (centuries) run some 85% of all CO2 emitted will be transferred to the ocean. The ocean has now accumulated a fossil fuel CO2 burden of over 550 billion tons fossil fuel CO2.

This is both blessing and bane for without the ocean sink for both heat and CO2 we would already face a climate crisis of epic proportions. But with the relentless uptake of CO2 we are significantly changing ocean pH and ocean acidification is now widely recognized as an environmental challenge. But how large a challenge? And how may we scale this against the desire to use the sub-sea oceanic reservoirs for industrial storage of captured CO2 with possible leakage, or even direct ocean injection.

The primary obstacle to CO2 capture and disposal is cost, now about $65-70 per ton CO2. The secondary obstacle is the legal status and penalties associated with environmental harm and/or leakage from a site. Objectively any environmental harm from leakage or disposal would be small and local compared to the massive invasion from above. For example if we were to take 1% of the oceanic uptake today as a target for direct oceanic storage of captured CO2 this would be ~10,000 tons per hour, or a 50,000 ton tanker or pipeline fill every five hours. Such numbers would strain industrial capacity and likely provoke both economic and environmental backlash. Yet 99% of CO2 entering the ocean would still be via the unregulated surface uptake.

It is thus very likely that oceanic CO2 levels will continue to increase, that pH will continue to decline, and that a largely unspoken world dependence on CO2 absorption by the ocean will continue. The impacts of this are under intensive investigation. There is little doubt that the decline in pH will impact shell formation in calcareous organisms and coral reefs, but new results suggest that the greater harm to the vast majority of marine life lies in ocean warming and declining O2, and that direct CO2 impacts are secondary.