Reconsidering deep ocean carbon disposal

There is an interesting commentary by oceanographer Ralph Keeling in the December issue of Nature Geoscience, in which the author argues that deep ocean disposal of carbon dioxide has been dismissed prematurely by environmentalists and policymakers.

Carbon sequestration by injection into the deep ocean (source: Lawrence Berkeley National Laboratory)
Carbon sequestration by injection into the deep ocean (source: Lawrence Berkeley National Laboratory)

Discussion of carbon storage tends to focus on the pumping of carbon dioxide into stable rock formations underground. Ocean disposal involves injecting the gas into seawater at depths of more a kilometre, where owing to the extremely high ambient pressure it will exist as a dense liquid.

The world’s oceans already act as a massive natural carbon sink, absorbing billions of tonnes of carbon dioxide each year. As we raise the atmospheric level of this greenhouse gas, the ability of the oceans to absorb increases. Much of the excess carbon generated from burning fossil fuels will eventually be taken up by the oceans, and it’s just a question of the amounts and timescales involved. Around one fifth of any carbon dioxide deposited in the deep ocean will leak into the atmosphere over decades to centuries; most of it will be retained by the sea indefinitely.

Deep ocean disposal of carbon dioxide has been damned by environmentalists, owing to the potential for damage to marine ecosystems. Such concerns led to field tests off the coasts of Hawaii and Norway being halted, and a few years ago this approach to carbon sequestration was banned under an amendment to the London Convention on the prevention of marine pollution.

It is a brave man who in these circumstances argues for the deep ocean disposal of carbon dioxide. Keeling points out that the current natural uptake of the gas is eight billion tonnes per year, and the rate is likely to double during this century. The field tests referred to above involved just 60 and five tonnes, respectively, yet they were stopped on environmental grounds.

Keeling accepts that carbon dioxide-rich water will harm marine organisms, but argues that the impacts would most likely be local to the injection site. Other scientists, Brad Seibel among them, strongly disagree. Keeling says that wider effects would be on a scale similar to those due to rising levels of atmospheric carbon dioxide on the deep ocean. He adds that local and global impacts could be mitigated by combining the injected carbon dioxide with dissolved carbonate minerals. This would help lock up the carbon in stable compounds.

Ocean disposal is, says Keeling, less attractive than below-ground sequestration, but suitable geological reservoirs are far from universal, and in Europe there is considerable popular opposition to the underground disposal of carbon dioxide.

The argument comes down to a balance between benefits and costs:

“Like most mitigation options, direct ocean CO2 disposal involves offsetting global environmental benefits against localized environmental harm. Some negative local impacts are inevitable if we are to halt greenhouse warming. Instead of condemning any proposal that involves harm in some location, the impacts of various options for decarbonizing energy supply, such as the clearing of land for solar power, damming of rivers for hydroelectric power or disturbance to deep-ocean chemistry should therefore be weighed against each other — and against the even greater environmental destruction expected from continued global warming.”

Keeling criticises approaches that focus exclusively on local environmental concerns:

“[I]f all stakeholders succeed in protecting their preferred ecosystem, they are all worse off. Progress is only possible through a give-and-take approach that seeks the optimum trade-off between local harm and global benefit.”

This “tragedy of the commons” argument is persuasive in moral terms, but it fails to account for politics and psychology. Surely Keeling can see this, in which case his article could be viewed as a provocative yet constructive contribution to an worthwhile debate. The current R&D focus is on underground carbon sequestration, and deep ocean disposal is unlikely to get a serious look-in.

Further reading

Ralph F Keeling, “Triage in the greenhouse”, Nature Geoscience 2, 820 (2009)