IPCC Expert Meeting on Geoengineering

Annex 4: Poster Abstracts
Geoengineering and Emissions Mitigation: Policy Context
Steven J. Smith
Joint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, MD, USA
The analysis presented in this poster considers the potential use of solar radiation management (SRM) techniques and the
context in which this climate response option might be considered. As noted by others (Wigley, 2006), SRM is a viable
option only if used in conjunction with emissions reductions. This is because SRM alone will not substantially reduce
impacts due to ocean acidification, SRM itself will cause ancillary climate changes, and use of SRM without emissions
mitigation would risk large climate changes if SRM were suddenly stopped. Only a long-term reduction in the net amount
of fossil carbon dioxide entering the atmosphere can stabilize the climate system.
Scenario analysis is used to examine the context for the potential use of geoengineering. Several points are demonstrated.
Emissions reductions, even under the most ambitious mitigation scenarios considered in the literature, cannot
ensure that temperature is below a given threshold at all times if the climate sensitivity is high. Emissions
reductions, however, can ultimately reduce long-term global temperature change below (or near) a 2 °C threshold
value by some point in the 22nd century (or earlier if the climate sensitivity is not too high), although the scale of
the necessary reductions may be large.
The situation where the climate sensitivity is high (or, equivalently, the sensitivity of some key impact is high), is
where forcing reduction through solar radiation management (SRM) techniques might be considered. SRM would
need to be used in addition to emissions mitigation in order to meet stated policy goals (e.g., Cancun
Agreements). SRM could potentially be used under these circumstances as a measure to reduce the magnitude of
global temperature changes and associated impacts over an interim period.
The primary determinant of the magnitude of potential SRM is the climate sensitivity. Delays in emissions
reduction also increase the magnitude of the SRM that would be needed to keep global temperatures under a
given threshold.
It is important to note that SRM will not exactly counter greenhouse gas forcing due to different spatial and spectral
characteristics. SRM will, therefore, cause climate changes itself. The character of these changes would need to be much
better understood before SRM could be deployed. It is important to examine the potential magnitude of these changes in
an appropriate context. SRM used as an adjunct to emissions mitigation could be of much smaller magnitude than many of
the scenarios considered in the literature (Wigley, 2006). Where scenarios in the literature have often considered SRM of
sufficient magnitude to counter a CO 2 doubling, the magnitude of SRM needed if used in conjunction with emissions
mitigation is generally much lower. SRM techniques are not understood well enough at present for application, and their
potential use raises numerous ethical, technological, scientific, and policy issues that will need to be addressed. Because
the goal of SRM is to reduce net impacts, the impacts due to the deployment of SRM measures would need to be smaller
than the impacts that would have occurred without SRM. Because a deterministic understanding of all relevant impacts is
unlikely to be realized, this comparison would need to be done on the basis of probabilistic and risk management
paradigms. In summary, the possible use of SRM should be considered in the context of substantial, long-term emission
reductions. The conditions under which SRM might be considered are the same conditions that require substantial
emissions reductions in order to meet stated policy goals. SRM does not substitute for emissions reductions and it would be
useful for analysis of SRM and its impacts to be done in this context. Policy measures to tie potential SRM use to long-term
emission reductions could also be considered.
References
Wigley T.M.L., 2006: A Combined Mitigation/Geoengineering Approach to Climate Stabilization. Science 314, 452–454.
(DOI: 10.1126/science.1131728).
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