Geoengineering with Chemical Weathering

One of the natural mechanisms responsible for stabilizing Earth's atmospheric CO2 concentrations, on geologic time scales, involves the weathering of soil-based silicate minerals. As CO2 concentrations increase in the atmosphere, climate warming affects weather patterns that, in turn, enhance the weathering of these minerals. Alkaline metal ions (calcium, magnesium) are freed from the soil and are washed into surface water bodies and the ocean, where they can participate in the sequestration of dissolved CO2. Technologies are under development that would use this chemical approach to capturing CO2 at the point of emission, particularly at power generation facilities [54]. One form of geoengineering that has been proposed involves the expansion of this approach to draw CO2 directly from the atmosphere.

Mitigation options: Various alkaline materials (sodium hydroxide, amines), have been proposed for use in wet-scrubbing devices (artificial trees) that would be deployed on a massive scale or spread over agricultural fields and in forests (oliv-ine), for CO2 direct capture [55-57]. Cost estimates for the application of this technique vary widely, depending upon the chosen technology, estimated capital costs and operating assumptions, from $15/t- CO2 for olivine application, $30/t-CO2 for one sodium hydroxide-based wet scrubbing approach, to as much as $500/t-CO2, using other currently available technologies [56, 57].

Feasibility: Deployment on the scales needed to meaningfully affect atmospheric CO2 concentrations appears to be decades away. Critics have argued that the energy demands for operating the proposed scrubbers will substantially reduce the net carbon sequestered [58] . For efficient high volume-high applications, it appears that scrubbing technologies are not yet in reach.

Co-benefits and undesirable consequences: Assuming stable depositories for the products of the scrubbing process could be found, this method may have fewer potentially negative environmental consequences than other geoengineering approaches. Any impacts related to the long-term storage of the carbon sequestered would depend upon the chemical form in which the CO2 is sequestered, as well as where and how that material is stored. As with windmills for energy production, however, the installations used for processing the atmosphere may be viewed as having a negative visual impact on the landscape.

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