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Researchers analyze “rock dissolving” method of geoengineering

Thu, 01/24/2013 - 10:59am

The benefits and side effects of dissolving particles in our ocean’s surfaces to increase the marine uptake of carbon dioxide, and therefore reduce the excess amount of it in the atmosphere, have been analyzed in a new study.

The study, published in Environmental Research Letters, assesses the impact of dissolving the naturally occurring mineral olivine and calculates how effective this approach would be in reducing atmospheric carbon dioxide.

The researchers, from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, calculate that if three gigatons of olivine were deposited into the oceans each year, it could compensate for only around 9% of present -day anthropogenic carbon dioxide emissions.

This long-discussed ‘quick fix’ method of geoengineering is not without environmental drawbacks; the particles would have to be ground down to very small sizes (around one micrometer) in order to be effective. The grinding process would consume energy and therefore emit varying amounts of carbon dioxide depending on the sort of power plants used to provide the energy.

Lead author of the study Peter Köhler says, “Our literature-based estimates on the energy costs of grinding olivine to such a small size suggest that with present-day technology, around 30% of the carbon dioxide taken out of the atmosphere and absorbed by the oceans would be re-emitted by the grinding process.”

The researchers used a computer model to assess the impact of six different olivine dissolution scenarios. Olivine is an abundant magnesium-silicate found beneath the Earth’s surface that weathers quickly when exposed to water and air—in its natural environment it is dissolved by carbonic acid which is formed from carbon dioxide out of the atmosphere and rain water.

If olivine is distributed onto the ocean’s surface, it begins to dissolve and subsequently increases the alkalinity of the water. This raises the uptake capacity of the ocean for carbon dioxide, which is taken up via gas exchange from the atmosphere.

According to the study, 92% of the carbon dioxide taken up by the oceans would be caused by changes in the chemical make-up of the water, while the remaining uptake would be down to changes in marine life through a process known as ocean fertilisation.

Ocean fertilization involves providing phytoplankton with essential nutrients to encourage its growth. The increased numbers of phytoplankton use carbon dioxide to grow, and then when it dies it sinks to the ocean floor taking the carbon dioxide with it.

“In our study we only examined the effects of silicate in olivine. Silicate is a limiting nutrient for diatoms—a specific class of phytoplankton. We simulated with our model that the added input of silicate would shift the species composition within phytoplankton towards diatoms.

“It is likely that iron and other trace metals will also impact marine life if olivine is used on a large scale. Therefore, this approach can also be considered as an ocean fertilisation experiment and these impacts should be taken into consideration when assessing the pros and cons of olivine dissolution,” continues Köhler.

The researchers also investigated whether the deposition of olivine could counteract the problem of ocean acidification, which continues to have a profound effect on marine life. They calculate that about 40 gigatons of olivine would need to be dissolved annually to fully counteract today’s anthropogenic carbon dioxide emissions.

“If this method of geoengineering was deployed, we would need an industry the size of the present day coal industry to obtain the necessary amounts of olivine. To distribute this, we estimate that 100 dedicated large ships with a commitment to distribute one gigaton of olivine per year would be needed.

“Taking all our conclusions together—mainly the energy costs of the processing line and the projected potential impact on marine biology—we assess this approach as rather inefficient. It certainly is not a simple solution against the global warming problem.” says Köhler.

Source: Institute of Physics

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