Concrete, quantitative goals for limiting the magnitude of climate change offer the benefit of allowing all parties involved to have a common sense of purpose and a clear metric against which to measure progress. Finding agreement on quantitative
ecosystems.e Other post-emission approaches involve removing CO2 from the air though chemical processes, but as with conventional carbon capture and storage from point emission sources, direct air capture schemes require reliable geological repositories for the removed CO2. In general, most CO2 removal approaches seem to pose fewer ancillary risks than SRM approaches, but they appear likely to be expensive, and because they would have only a gradual effect on atmospheric GHG concentrations, they would not have the potential to produce substantial cooling quickly.
Because some forms of geoengineering would have consequences that span national boundaries, international legal frameworks are needed to govern the development and possible deployment of these options. Such frameworks need to include a clear definition of the “climate emergency” that would trigger deployment of large-scale SRM, and criteria for whether, when, and how SRM (and some versions of post-emission GHG management) would be tested —recognizing that even the act of field testing may create international tensions. More fundamentally, intentional alteration of the Earth’s environment via geoengineering raises significant ethical issues, including the distribution of risks among population groups in both present and future generations, as well as challenging questions of public perceptions and acceptability.f
In conclusion, geoengineering approaches may conceivably have a role to play in future climate risk management strategies, particularly if efforts to reduce global GHG emissions are unsuccessful or if the impacts of climate change are unexpectedly severe. At present however, the costs, benefits, and risks of many geoengineering approaches are not well understood. In the committee’s judgment, it would therefore be imprudent to use certain geoengineering approaches (in particular, SRM and ocean fertilization strategies) to manipulate the Earth’s environment in the near future, and it would be unwise to assume they will be attractive options even in the more distant future. We recommend instead a program of research to better understand the potential effects of different geoengineering options and efforts to address the international governance issues raised by many geoengineering proposals.
aSee, e.g., Royal Society, Geoengineering the Climate: Science, Governance, and Uncertainty, RS policy document 10/09 (London: The Royal Society, 2009); American Geophysical Union, Geoengineering the Climate System. A Position Statement of the American Geophysical Union (Adopted by the AGU Council on 13 December 2009); American Meteorological Society, Geoengineering the Climate System. A Policy Statement of the American Meteorological Society (adopted by the AMS Council on 20 July 2009).
bG. C. Hegerl and S. Solomon, “Risks of climate engineering” (Science 325:955-956, 2009, doi: 10.1126/science.1178530).
cSee NRC, Advancing the Science, Chapter 15 for additional discussion of proposed SRM approaches, including the research needed to better understand their potential efficacy and risks.
dSee NRC, Advancing the Science and Limiting the Magnitude for further discussion and references.
eK. O. Buesseler, S. C. Doney, D. M. Karl, P. W. Boyd, K. Caldeira, F. Chai, K. H. Coale, H. J. W. De Baar, P. G. Falkowski, K. S. Johnson, R. S. Lampitt, A. F. Michaels, S. W. A. Naqvi, V. Smetacek, S. Takeda, and A. J. Watson, “Environment: Ocean iron fertilization—Moving forward in a sea of uncertainty” (Science 319:162, 2008).
fNRC, Advancing the Science.