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Part I: Impacts of Climate-related Geoengineering on Biological Diversity
change mitigation as well as other purposes, and involve minimal use of novel technologies. For similar reasons,
there is debate over whether biomass-based carbon should be included. However, for the sake of completeness, all
of these approaches are discussed in this report without prejudice to any subsequent discussions within the CBD
on definitions or policy on geoengineering.
Scope in terms of the scale of the response
The natural balance of plant growth and decomposition in the terrestrial biosphere currently results in a net uptake
of about 2.6 Gt C/yr (gigatonnes of carbon) from the atmosphere, although this is partially offset by emissions of
about 0.9 Gt C/yr from tropical deforestation and other land use changes. In comparison, the current CO2 release
rate from fossil fuel burning alone is about 9.1 Gt C/yr;47 so to have a significant positive impact, one or more
CDR interventions would need to remove from the atmosphere several Gt C/yr, maintained over decades and
more probably centuries. It is very unlikely that such approaches could be deployed on a large enough scale to alter
the climate quickly; thus they would be of little help if there was a need for “emergency action” to cool the planet.
2.2.3 Comparison between SRM and CDR techniques
Although described above separately, it is possible that, if geoengineering were to be undertaken, a combination
of SRM and CDR techniques could be used, alongside mitigation through emission reductions, with the objective
of off-setting at least some of the impacts of changes to the climate system from past or ongoing emissions. While
SRM and CDR interventions would both have global effects, since climate operates on a global scale, some of the
proposed SRM interventions (such as changing cloud or land surface albedo) could result in strong hemispheric or
regional disparities, likely to change the frequency of extreme events and the behaviour of major weather systems,
e.g. the South East Asia monsoon. Under conditions of rapid climate change, the unequivocal separation of impact
causality between those arising from the SRM intervention and those that would have happened anyway would
probably not be possible. Likewise, CDR techniques will ultimately reduce global CO2 concentrations but might
also involve regional effects, e.g. if removal is strongly hemispherically biased. Furthermore, climatic conditions
for a particular atmospheric CO2 level are likely to be different according to whether global CO2 is increasing or
decreasing.48
In general, SRM can have a relatively rapid impact on the radiation budget once deployed, whereas the effects of
many of the CDR processes are relatively slow. Furthermore, while approaches using SRM have the potential to
offset the radiative effects of all greenhouse gases, they do not directly alleviate other consequences of changes in
atmospheric chemistry, such as ocean acidification. In contrast, CDR techniques do address changes in atmospheric
CO2 concentrations, but they do not address the radiative effects of increased atmospheric concentrations of
other greenhouse gases (e.g., methane, nitrous oxide, tropospheric ozone, and halocarbons) and black carbon.
Whilst CDR techniques would reduce or slow surface ocean acidification to some degree (in relation to their
overall effectiveness), that benefit could be compromised if the carbon or CO2 removed from the atmosphere is
subsequently added elsewhere to the ocean.
The 2009 Royal Society report49 provided a generally well-regarded overview of the effectiveness, affordability safety
and timeliness of the main SRM and CDR techniques that have been proposed. Several other reviews have since
been published,50,51,52,53 including those giving quantitative comparisons of maximum potential effectiveness, in
47 Global Carbon Project (2011).
48 Chadwick et al. (2012).
49 The Royal Society (2009).
50 McLaren (2011).
51 Ginzky et al. (2011).
52 Rickels et al. (2011).
53 Gordon (2010).
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