IPCC Expert Meeting on Geoengineering

Annex 3: Keynote Abstracts
Keynote III-4: The Role of Different Geoengineering Options in Long-Term
Responses to Climate Change
Jason J Blackstock 1 and Ken Caldeira 2
1
Institute for Science, Innovation and Society, Oxford University, UK and International Institute for Applied Systems Analysis, Austria
2
Department of Global Ecology, Carnegie Institution for Science, USA
While proposed geoengineering techniques offer new potential response options to addressing climate change, the
relationship between these techniques and more familiar response option – specifically mitigation and adaption – has thus
far been poorly explored. Particularly over decadal time scales, different response options (or portfolios of options) are likely
to generate significant differences in climatic and societal outcomes. This talk overviews some relevant technical
characteristics of currently proposed geoengineering techniques, and contrasts them with the characteristics of the climate
challenge and prominent mitigation and adaption response options. Because Carbon Geoengineering (also, Carbon Dioxide
Removal or CDR) proposals differ dramatically from Solar Geoengineering (also Solar Radiation Management or SRM)
proposals they are treated separately throughout.
The long-term nature of the anthropogenic climate change problem are associated with long-lived radiatively active
substances added to the atmosphere (including CO 2; Archer et al., 2009) as well as long time scales associated with
transformation of global systems of energy production and use and patterns of land use (Davis et al., 2010). Discussions
focused on CO 2 may be extended to consider other anthropogenic emissions of radiatively active substances. In the <strong>IPCC</strong>
AR4, the term “long term” was used to refer to the period after year 2030 (see for example <strong>IPCC</strong> AR4, Working Group III,
Summary for Policy Makers, Section D; <strong>IPCC</strong>, 2007). That usage will be adopted here.
Proposed CDR techniques attempt to reverse the effects of CO 2 emissions by diminishing atmospheric CO 2 concentrations.
From the perspective of the climate system, if CO 2 were removed from the atmosphere and then isolated from the
atmosphere forever, this would closely approximate the effect of an avoided emission. An important issue is the amount of
time that the CO 2 would remain isolated from the atmosphere. This issue of permanence of storage has been discussed
within the <strong>IPCC</strong> both in reference to Carbon Capture and Storage (<strong>IPCC</strong>, 2005) and with regard to land use practices (<strong>IPCC</strong>,
2000). Permanence (or longevity) of storage is an important issue that is relevant to the potential of various CDR
approaches to reduce long-term risks associated with climate change. Impermanent storage is largely equivalent to a
delayed emission (Herzog et al., 2003). Delayed emission can be valuable in slowing rates, and in some cases maximum
amounts, of warming. Issues of impermanence of storage are most commonly raised with respect to various biologicallyoriented
strategies to remove CO 2 from the atmosphere, such as ocean iron fertilization or proposals to increase organic
carbon storage in plants or soils. Impermanence of storage also raises issues of intergenerational equity and transfers of
responsibility. Most of these issues already have been addressed by the <strong>IPCC</strong> in other contexts. Furthermore, after removal
of excess CO 2 from the atmosphere, excess CO 2 will tend to continue degassing from the oceans and land biosphere (Cao
and Caldeira, 2010); thus, the long lifetime of CO 2 in these reservoirs indicates a long-term commitment to CO 2 removal if
atmospheric CO 2 concentrations are to be maintained at very low levels.
If it were possible to deploy CDR techniques with permanence of capture at sufficient scales to reduce net atmospheric
emissions or even draw down atmospheric concentrations, from the perspective of the climate system, this could effectively
reduce anthropogenic climatic change. However, according the 2009 report of the UK Royal Society (The Royal Society,
2009) and 2011 report of the American Physical Society (Socolow et al., 2011), while there may be some relatively low-cost
CDR options that can be deployed at scales that are small relative to that of global emissions, deployment at the scale of
global emissions is likely to be more costly than emissions avoidance options.
SRM techniques aim to diminish the amount of climatic change by reducing the solar energy absorbed by the atmosphere,
surface and/or oceans, thereby reducing global warming. Because some proposed SRM techniques (specifically
stratospheric aerosols and cloud whitening) appear to offer the technical capacity to reduce globally absorbed solar
radiation on the order of 1% (or more) within a few months, and for deployment costs on the order to $1B to $10B
(McClellan et al., 2012)these techniques provide potentially fast leverage over the global climate. However, SRM
techniques would not address non-climate effects increased atmospheric CO 2, such as ocean acidification (Matthews et al.,
2009). Moreover, because SRM alters a different component than CO 2 of the energy flow driving the climate system
(shortwave for SRM versus longwave for CO 2), the climatic conditions produced by offsetting CO 2-induced global warming
with SRM would not be equivalent to climatic conditions under lower atmospheric CO 2 concentrations; differences in
various climatic conditions, specifically including the hydrologic cycle, are expected. Moreover, were an SRM deployment to
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