Enhanced Weathering In The Ocean
About a quarter of the CO2 emitted by burning fossil fuels is absorbed by the oceans in natural processes. This leads to the formation of carbonic acid and is responsible for the increasing acidification of the oceans. Ocean acidification can have far reaching adverse effects on food chains and biodiversity; it also increasingly causes the dissolution of calcareous sediments and organisms such as corals. This is countered by natural weathering, in which minerals are released as certain rocks break down. These minerals are washed from land to sea, where they react with carbon that becomes permanently captured in the form of bicarbonate and carbonate ions. This changes the chemical properties of sea water, making it more alkaline. When this process takes place in surface water in contact with the atmosphere, the CO2 removed from the water by weathering can be replaced by CO2 from the atmosphere.
In this way, enhanced weathering, which has already been discussed above under terrestrial methods, is also being considered as an oceanic application of CDR. Alkaline substances such as powdered silicate or carbonate rock are directly introduced into the surface waters of the oceans in order to chemically bind CO2. The material can be mined or industrially produced on land, shipped out to sea and spread in the water. Use of this method would not only enable the oceans to absorb more CO2. It would also have the positive side effect of counteracting ocean acidification.
In principle, there are enough minerals worldwide to bind all CO2 emissions. However, these minerals would have to be mined and ground to a fine powder or chemically modified in an industrial process so that they dissolve quickly in water and do not sink into the depths before reacting with CO2. Recently, new studies have estimated the potential of enhanced weathering. The outcome was that if work on establishing this CDR method were to begin immediately, between ten million and five billion tonnes of CO2 could be removed from the atmosphere per year from 2050 onward.
For enhanced weathering to have a global effect, the quantity of minerals needed would mean establishing a completely new large-scale mining industry or a large-scale mineral production industry. This is because compensating global CO2 emissions would require the spreading of minerals in quantities comparable to the volume of coal mined today. The fine mineral powder would also either have to be dissolved in seawater at land-based plants and subsequently discharged into the sea or carried out to sea by large cargo vessels. In total, this CDR method would be costly and energy-intensive and could also have major environmental impacts on land.
Application readiness and research needs
The potential effectiveness of enhanced weathering on a global scale, its cost and whether it is viable in energy terms is currently being investigated by modelling and, in part, in small-scale laboratory experiments. Data from larger, field experiments covering a restricted geographical area would be needed in order to make the modelling more precise. This would make it possible to more accurately determine how much additional CO2 would be absorbed by the mineral enrichment of seawater. At the same time, knowledge could be gained about the effects of increased mineral concentrations on marine life. Some minerals contain iron, which has a fertilising effect in the sea, but may also carry toxic impurities that could have unintended side effects on marine ecosystems.