A geoengineering strategy that involves changes to the thermal emissivity of the atmosphere would be constrained by the practical aspects of extracting GHGs at trace-level concentrations. Carbon dioxide is not the only GHG and, on a molecule-by-molecule basis, it is not the gas with the highest global warming potential (GWP). However, its atmospheric concentration makes it the most important of the anthropogenic GHGs. The anthropogenic greenhouse gases with higher GWPs, such as methane, nitrous oxide, and the hydrofluorocarbons, are present at concentrations that are lower by orders of magnitude  .
The capture of CO2 at its point of emission - typically at very large combustion sources such as coal-fired power plants - is a strategy under serious consideration by energy systems analysts as a means of reducing further GHG emissions, while allowing the continued use of fossil fuel (Princiotta, this book). However, many anthropogenic sources of CO2 exist that are not amenable to carbon capture and storage (CCS), such as fossil-fueled mobile sources, and those emissions related to forest clearing and agriculture. In addition to its limitations concerning the types of sources it can effectively control, carbon capture at the point of emission does not affect the existing excess concentrations of anthropogenic CO2 in the atmosphere. Several geoengineering proposals offer approaches for reducing overall atmospheric CO2 concentrations.
Carbon dioxide is readily removed from the atmosphere by natural chemical and biological processes. For example, naturally occurring alkaline minerals react with CO2 when exposed to the atmosphere to form carbonate compounds. Photosynthesis, the process by which green plants synthesize carbohydrates, is the natural route of biological CO2 sequestration. These processes seem suitably inexpensive and to have fewer negative environmental side effects than those involving the reduction of solar insolation. Nevertheless, CO2 exists at parts-per-million concentrations in the atmosphere, implying the need for very large scale processing of the atmosphere to ensure a meaningful reduction in its atmospheric concentrations.
Basic thermodynamics predicts, and geochemical models have shown, that removal of atmospheric CO2 by a terrestrial sink, such as tree farming or chemical weathering, will result in the out-gassing of CO2 from the oceans . Assuming humanity ultimately succeeds in eliminating its carbon emissions, the global oceans will require centuries to off-gas the excess dissolved CO2 before the ocean-atmospheric system begins to approach thermodynamic equilibrium  . Therefore, the success of any direct capture project is not only tied to the very long-term stability of the method used to store the captured carbon once it is removed from the atmosphere, but the capacity for storing quantities equivalent to current CO2 emissions, plus past emissions that have accumulated as dissolved carbon in the oceans.
The proposed techniques for direct capture, illustrated in Fig. 9.2, include altering the pH of the ocean surface, stimulating biological-photosynthetic sequestration at the ocean's surface or through reforestation or afforestation, or artificial chemical weathering projects that employ materials that react with and sequester CO2. Unlike
Fig. 9.2 Proposed approaches to the direct capture of CO2 from the atmosphere: enhancing phytoplankton growth; reducing ocean surface pH to increase CO2 dissolution; the construction of artificial "trees" that use alkaline chemical compounds to absorb atmospheric CO2, and; large-scale expansion of managed forests
Fig. 9.2 Proposed approaches to the direct capture of CO2 from the atmosphere: enhancing phytoplankton growth; reducing ocean surface pH to increase CO2 dissolution; the construction of artificial "trees" that use alkaline chemical compounds to absorb atmospheric CO2, and; large-scale expansion of managed forests the geoengineering strategies involving changes in solar flux, the costs to implement direct GHG capture methods relying on components of the natural cycle are not easily quantified. Further exploration is required to determine the capacity of the natural carbon sinks that are suitable for engineered enhancement.
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