Modeling Indicates Volcanic Ash Could Be A Powerful Weapon Against Global Heating

Volcanic material, known as tephra, spills down the slope of Soufrierre Hills volcano, Montserrat. Martin Palmer

With concentrations of carbon dioxide in the atmosphere and oceans already exceeding safe levels, there is a desperate need to find an affordable way to remove carbon dioxide as well as to slow the rate we are adding to the problem. A study suggests depositing volcanic ash at sea could be the answer. However, the authors warn we need more testing, both to confirm it works as anticipated and to rule out unanticipated damage.

Earth has been through many cycles of changing CO2 levels, although never as fast as the one humanity has created. When forces such as volcanoes fill the air with carbon dioxide, some is removed relatively quickly through increased plant growth. However, as forests rot or burn, much of that carbon returns to the atmosphere, minimizing long-term reductions.

Over longer periods of time, geological forces take over the removal, with minerals reacting with the carbon dioxide and trapping much of it on the seafloor. The process normally runs far too slowly to get us out of our current predicament, but scientists have started to wonder if there might be ways to speed it up. Professor Martin Palmer of the University of Southampton thinks he might have found the answer in volcanic ash.

Inevitably, some volcanic ash falls on the ocean, where it settles. The ash induces a range of effects such as ocean fertilization and the chemical changes that lead to more deposition of organic chemistry and calcium carbonate (chalk) on the seafloor. When oceanic carbon dioxide levels fall, they absorb more from the atmosphere to compensate. Compared to other geological forms of carbon removal, like the weathering of rocks, this occurs relatively rapidly.

Palmer has tried to model the likely costs and benefits of digging tephra (fragmentary volcanic material) from island volcanoes for disposal at sea. At first sight, the return might look a little measly – just 55 kilograms (121 pounds) of CO2 sequestered for every tonne of volcanic ash dumped, but the idea has many advantages.

“Tephra addition is simply an augmentation of a natural Earth process, it is a low technology approach that requires few developments, and it may sequester carbon for thousands of years," writes Palmer in Anthropocene.

Crucially, volcanic ash is abundant and cheap, with the costs primarily in crushing and transportation. For the vast quantities available close to oceans, Palmer and co-authors estimate a price of about $55 per tonne of CO2 sequestered. In certain locations, timing the distribution of tephra to follow seasonal upwellings might improve the sequestering cost further.

That’s 10 times lower than a lot of other drawdown options under consideration. If their figure is right, and the authors acknowledge experimental confirmation is needed, it would still be more expensive than cutting back emissions in the first place. On the other hand, ash dumping could be an affordable, if far from cheap, option for removing what has already been emitted once the world wakes up to the danger we are in.

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