cbd-ts-66-en

Part I: Impacts of Climate-related Geoengineering on Biological Diversity
is therefore unsurprising that different regional results are provided by different global climate models: as intercomparison
exercises for climate models (without SRM) have demonstrated,181 relatively small differences in
model structure, parameterisations and start-up conditions can generate a diversity of regional climate projections
for the same emission scenarios. In some regions, the results of several models converge, increasing confidence
that regional projections are correct—although that may be because they may all share the same deficiency (e.g.
omitting a feedback factor, due to inadequate knowledge of the processes involved). However, in other regions,
there is either less or no agreement.
Inter-comparisons between models that include SRM simulations are currently underway,182, 183 and preliminary
results from one four-model experiment (based on quadrupling CO2 and uniform dimming) have recently been
published,184 broadly confirming the main conclusions discussed above. Recognising that full climate restoration is
unlikely to be achievable (even with sophisticated application of non-uniform dimming, targeting specific regions185),
recent papers have proposed SRM approaches that might nevertheless minimize the overall effects of adverse
impacts,186 based on different social objectives—egalitarian, utilitarian or ecocentric,187 albeit at a relative crude level.
However applied, SRM would introduce a new dynamic between the heating effects of greenhouse gases and the
cooling effects of sunlight reduction. There are no known palaeo-precedents for the radiative impacts of high
greenhouse gases to be balanced by reduced light quantity; thus the stability of that combination is uncertain,
and it is not clear what specific environmental challenges an “SRM world” would present to particular species and
ecosystems, either on a short-term or long-term basis, as discussed below in greater detail.
4.1.2 Projected impacts of uniform dimming on hydrological and nutrient cycles
As noted above, modelling to date has mostly focussed on the global and regional temperature and precipitation
changes likely to result from idealised SRM deployment compared to a high CO2 world. However, for biodiversity,
ecosystems and their services, relative precipitation changes and temporal patterns in precipitation delivery are
much more important than absolute amounts. Thus, for an arid region, a change in quantity or timing of around
5–10 cm yr-1 could be critical, yet such a change would be insignificant for areas annually receiving several metres
of precipitation. Furthermore, precipitation minus evaporation (P-E) is a much more useful metric of biologicallyavailable
water than is precipitation alone,188 with soil moisture the key hydrological variable for healthy terrestrial
ecosystems. Additional relevant processes affecting soil and plant water loss (evapotranspiration) include total
insolation, the response of plants to increased CO2 (affecting stomatal opening), and the different projected changes
in the distribution of different ecosystems in response to different emission scenarios.
The combined response of these processes to SRM-induced solar dimming is currently highly uncertain, and there
is likely to be considerable regional variability. Whilst it has been calculated that SRM might be able to reduce the
overall P-E change due to global warming by ~75%189 (compared to doubled CO2), soil moisture in the tropics
under SRM is still likely to be significantly less than at present.190, 191 Changes in P-E and soil moisture have major
181 IPCC (2007b).
182 Kravitz et al. (2011).
183 Robock et al. (2011).
184 Schmidt et al. (2012).
185 Even if such sophistication were possible, we would never know exactly how the naturally-dynamic climate system would have
evolved in the absence of SRM; e.g. as a consequence of natural decadal variability in climatic processes, that may partly be due to
highly uncertain changes in the ‘solar constant’.
186 Ban-Weiss &.Caldeira (2010).
187 Moreno-Cruz et al. (2012).
188 Running & Kimball (2006).
189 Ban-Weiss &.Caldeira (2010).
190 Bala et al. (2008).
191 Matthews & Caldeira (2007).
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