cbd-ts-66-en

Part I: Impacts of Climate-related Geoengineering on Biological Diversity
Ozone depletion and increased UV radiation
If stratospheric sulphate injection were to be used for SRM, there is evidence that this could result in increased
ozone depletion, primarily in polar regions in spring. This effect was observed221 after the 1991 Mount Pinatubo
eruption. However, the consequences of decreased ozone (in terms of allowing additional ultra violet (UV) radiation
to reach the Earth’s surface) could be at least partly offset by UV scattering and attenuation by the sulphate aerosol
itself. If surface UV were to significantly increase, some species would be affected more than others. Certain plants
possess a protective layer on the upper surface of their leaves, making them less susceptible to UV damage. The
ecological effects of any increased UV radiation will also depend on which spectral form (UVA, UVB and UVC)
is most affected.
An additional uncertainty is the appropriate comparison to be made with regard to future conditions, since
projections of stratospheric ozone in 50–100 years time are subject to assumptions regarding societal behaviour
(the future effectiveness of the Montreal Protocol, or other measures that might be introduced), as well as climateinduced
changes in atmospheric chemistry.222
Changes in the nature and amount of light reaching ecosystems
Stratospheric aerosols would decrease the amount of photosynthetically active radiation (PAR) reaching the Earth;
they would also increase the amount of diffuse (as opposed to direct) short-wave solar irradiation. For terrestrial
ecosystems, these processes would have opposing ecological effects, with the net impact likely to differ between
species and between ecosystems. The net impact may also depend on the percentage reduction in PAR, and the
absolute levels under current conditions; these vary latitudinally and are also subject to spatial variability in cloud
cover.
Thus, the net efficiency of carbon fixation by a forest canopy is increased when light is distributed more uniformly
throughout the canopy, as occurs with diffuse light. Diffuse light penetrates the canopy more effectively than
direct radiation because direct light saturates upper sunlit leaves but does not reach shaded, lower leaves. The
negative effects of a (small) reduction in total PAR might be less than the positive effects of the increase in diffuse
radiation giving a net improvement in photosynthetic efficiency, hence an overall increase in terrestrial primary
production. Crop species may also benefit223 although inter-species differences are likely, as a function of canopy
structure. There may also be additional hydrological effects driven by the effects of the diffuse/direct ratio on
evapotranspiration.224 There is evidence for such responses following the Mount Pinatubo eruption,225 and during
the “global dimming” period (1950–1980).226, 227
However, the magnitude and nature of such effects on biodiversity are currently not well understood, and their
wider ecological significance is uncertain. Even if gross primary production (GPP) were to increase, GPP is not
necessarily a good proxy for biodiversity: increases in GPP could be due to a few plant species thriving in more diffuse
light. Furthermore, for ecosystems where total light availability is the major growth-limiting factor, the negative
impacts of total radiation decrease could be greater than any benefits provided by the increase in diffuse radiation.
A further complication is that while diffuse light is better at penetrating a multi-layered canopy, sunflecks (bursts
of strong light which penetrate the canopy and reach ground level) would be less intense with diffuse light as
opposed to direct light.228
221 Tilmes et al. (2008).
222 McKenzie et al. (2011).
223 Zheng et al. (2011).
224 Oliveira Pet al. (2011).
225 Gu et al. (2003).
226 Mercado et al. (2009).
227 Wild (2009).
228 Montagnini & Jordan (2005).
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