How much carbon dioxide can be removed from the atmosphere if the mineral olivine is increasingly decomposed on land?

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:

1. 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.
2. Large-scale weathering of olivine on land leads to a significant increase in pH values in rivers (river alkalinisation in contrast to ocean acidification).
3. 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.”

Background: Climate protection and geoengineering

In view of the failure of the UN Climate Conference in Copenhagen in December 2009, there are no binding rules for climate protection for the period as of 2012 after expiration of the Kyoto Protocol. The only result of this UN conference was the Copenhagen Accord, a non-binding political paper under international law that takes note of the objective of limiting global warming to less than 2°C compared to the preindustrial level.

The continued rise in CO₂ emissions from fossil fuels and deforestation reached a level of more than 10 Pg C yr^-1 in 2008. This puts the emissions at the top end of the proposed emissions on which the future scenarios of the World Climate Report (IPCC Report) are based. Only last year the annual mean atmospheric CO₂ concentration rose to 387 ppmv and is thus more than 100 ppmv above the preindustrial level of around 280 ppmv. It can therefore be assumed that, given the insignificant change in global user behaviour regarding energy consumption to date, the objective of keeping global warming below 2°C cannot be achieved.

“Reduction of greenhouse gas emissions to achieve climate protection goals is regarded as the ideal solution and related adjustment measures still appear acceptable to many people,” assess the authors of the study. “The mention of geoengineering triggers apprehension among the population, however: How can humanity claim to want to change global climate? In reality we have already altered the world climate through the constant emission of greenhouse gases and continue to alter it. However, this was not our objective, but a ‘side effect’ of our enormous energy consumption, which is largely still fed by fossil energy sources (coal, oil, gas).”

The term geoengineering (or also climate engineering) encompasses a wide variety of methods whose large-scale application is supposed to influence the global climate. A distinction is made between two groups of methods. The so-called solar radiation management aims at reducing the energy radiation of the sun onto the Earth’s surface, e.g. by means of mirrors in space or the injection of tiny particles, called aerosols, in the stratosphere, the Earth’s atmosphere above an altitude of 10 km. The methods for carbon dioxide removal (CDR), on the other hand, attempt to remove CO₂ from the atmosphere and transfer it to other reservoirs on a long-term basis, for instance as charcoal in the ground.

Reference: Peter Köhler, Jens Hartmann, and Dieter A. Wolf-Gladrow: Geoengineering potential of artificially enhanced silicate weathering of olivine. doi:10.1073/pnas.1000545107.

Notes for Editors: Your contact persons at the Alfred Wegener Institute are Dr Peter Köhler (tel. +49 (0)471 4831-1687, e-mail: Peter.Koehler@awi.de), Prof. Dieter Wolf-Gladrow (tel. +49 (0)471 4831-1824, e-mail: Dieter.Wolf-Gladrow@awi.de) as well as Folke Mehrtens, Department of Communications and Media Relations (tel. +49 (0)471 4831-2007, e-mail: Folke.Mehrtens@awi.de).

Your contact person at KlimaCampus, University of Hamburg, is Prof. Jens Hartmann (tel. +49 (0)40 42838-6686, e-mail: jens.hartmann@zmaw.de).

The Alfred Wegener Institute conducts research in the Arctic, Antarctic and oceans of the high and mid latitudes. It coordinates polar research in Germany and provides major infrastructure to the international scientific community, such as the research icebreaker Polarstern and stations in the Arctic and Antarctica. The Alfred Wegener Institute is one of the sixteen research centres of the Helmholtz Association, the largest scientific organisation in Germany.

KlimaCampus is a network of climate research experts in Hamburg established in 2007. It comprises 18 university-affiliated institutes, non-university partners like the Max Planck Institute for Meteorology, the Institute of Coastal Research of the Helmholtz Centre in Geesthacht and the German Computing Centre for Climate Research. The nucleus of KlimaCampus is the Cluster of Excellence Integrated Climate System Analysis and Prediction (CliSAP) of the University of Hamburg.