Artificially accelerated weathering of the mineral olivine may increasingly remove carbon dioxide (CO2) from the atmosphere and counteract ocean acidification. Around a ton of olivine dissolved in water would be necessary for each ton of CO2 that could be transferred from the atmosphere to the ocean using this method.

This is the result of model calculations published by researchers of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association and KlimaCampus of the University of Hamburg in the journal PNAS (Proceedings of the National Academy of Sciences of the United States of America). However, the method is presumably not suitable as a measure for completely neutralising present-day greenhouse gas emissions.

In view of the continued increase in CO2 emissions and the related changes in the concentration of the greenhouse gas in the atmosphere and ocean, consideration is being given to how CO2 can be removed from the atmosphere. A method proposed several years ago, but not really looked into to date, is based on the naturally occurring chemical weathering of olivine, a widespread mineral on the Earth whose dissolution products are deposited in the ocean via rivers after degradation on land. "Using our model calculations, we wanted to theoretically examine whether artificial acceleration of these natural weathering processes might, in fact, represent an effective means of countering climate change," the first author, Dr. Peter Köhler from the Alfred Wegener Institute, explains the objective of the study.

"The Alfred Wegener Institute has neither the intention nor an interest in paving the way for commercial implementation of geoengineering measures through this study. However, it makes an important contribution to improving the scientific database on geoengineering methods," comments Prof. Karin Lochte, Director of the Alfred Wegener Institute. International bodies and organisations such as, most recently, the Biodiversity Conference in Nagoya, Japan demand better basic information so they can better assess the effectiveness and associated risks of geoengineering measures with respect to the environment and biodiversity.

Olivine is a mineral containing silicate but no carbon. It is with 90%the main component of dunite, a widespread rock. During the chemical weathering process of olivine CO2 is removed from the atmosphere. Atmospheric CO2 reacts with rainwater to form carbonic acid. In the end this carbonic acid chemically attacks the olivine on its surface and dissolves it. The reaction products are silicic acid, magnesium ions and bicarbonate. Accounting for 90%, the latter is the predominant chemical form in which dissolved CO2 exists in the ocean. The dissolved substances are transported to the sea via rivers. This consequently increases oceanic alkalinity (buffer capacity with respect to changes in the pH value) and leads to pronounced uptake of CO2 by the ocean surface. At the same time this process causes an increase in the pH value, which counteracts the ocean acidification.

The concept of geoengineering by means of olivine involves the acceleration of this natural weathering process. The greater the reaction area of a mineral (e.g. its surface), the faster it can weather, i.e. dissolve. Heat additionally boosts this process. "That is why the approach specifies spreading finely ground olivine powder on acid soil as far as possible (lower pH value) in warm and humid regions," Köhler explains the initial prerequisite for the model calculation. Grinding olivine minerals enlarges the reaction area with water and thus increases the potential for the dissolution of as much olivine as possible in the shortest time possible. In this way optimal conditions were created for accelerated weathering of olivine (and at the same time CO2 removal from the atmosphere).

The theoretical study conducted by Köhler and his co-authors, Prof. Jens Hartmann (KlimaCampus, University of Hamburg) and Prof. Dieter Wolf-Gladrow (Alfred Wegener Institute), evaluates the possible consequences and potential for transferring atmospheric CO2 to the ocean as bicarbonate in connection with application of such a method in the catchment areas of large rivers in the tropics. The three most important findings are:

  • Dissolution of about one ton of olivine is necessary for every ton of CO2 that could be shifted from the atmosphere to the ocean through this method.
  • Large-scale weathering of olivine on land leads to a significant increase in pH values in rivers (river alkalinisation in contrast to ocean acidification).
  • The finite solubility of silicic acid (a byproduct of olivine weathering) will additionally restrict the potential of the olivine geoengineering method. For this reason the maximum potential for transferring atmospheric CO2 using this method is estimated at one petagram of carbon a year (Pg C yr-1; 1 petagram = 1015 g).

"Olivine weathering on land may be a method for transferring atmospheric carbon dioxide to the ocean," states Köhler. "However, we would first have to examine precisely how the local pH value changes predicted by us impact river ecosystems and adjacent habitats." The solubility limits of silicic acid, moreover, mean that the CO2 reduction potential of artificially increased olivine weathering is one order of magnitude below present-day anthropogenic CO2 emissions. As a comparison, the potential transfer of up to 1 Pg C per year by means of the proposed method contrasts with current CO2 emissions of more than 10 Pg C per year.

"Therefore, it doesn't appear possible to neutralise present and future greenhouse-gas emissions using the proposed method," Köhler adds. In his opinion, however, the method could make a contribution to stabilisation and reduction of the atmospheric CO2 concentration in connection with other methods. Furthermore, it would counteract the current trend toward ocean acidification. Köhler also points out that large amounts of olivine would have to be extracted, transported and ground for large-scale application of the method: "The amount of additionally weathered olivine necessary to achieve this CO2 transfer is on the same order of magnitude as present-day coal mining."

Source: AWI