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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). 50
Part I: Impacts of Climate-related Geoengineering on Biological Diversity Analyses of effects of large-scale, aerosol-based SRM on marine photosynthesis have not been carried out; however, primary production in the upper ocean is closely linked to the depth of light penetration, that is greatest for direct sunlight.229 Thus, ocean productivity could be expected to decrease under SRM in comparison to present-day values. However, the comparison to unmitigated climate change is not straightforward, since many other factors would also then be involved. The potential effects on animals of the (relatively small) changes in total solar irradiance and its direct/diffuse ratio that would result from SRM using stratospheric aerosols have yet to be investigated. Bees and other insects that use polarized light for navigation may be particularly sensitive; whilst they are still able to detect celestial polarization patterns under cloudy skies,230 year-to-year variability in early summer sunshine can have a significant effect on honey production.231 4.2.2 Potential impacts on biodiversity of cloud brightening Cloud brightening involves increasing the concentration of cloud-condensation nuclei (CCN) in the troposphere (lower atmosphere), to increase the reflection back to space of short-wave solar radiation.232 The technique is effectively limited to ocean areas,233, 234 particularly the southern hemisphere, where CCN abundance is naturally low. Whilst deployment locations could (in theory) be chosen to spatially maximize beneficial effects,235 the large-scale application of this technique seems likely to cause strong regional or local atmospheric and oceanic perturbations236 with potentially significant impacts on terrestrial and marine biodiversity and ecosystems. Cloud brightening, if effective, could be expected to reduce local radiative forcing by up to 40 W m-2 in tropical areas. Persistent local/regional cooling on that scale could affect regional weather systems of high ecological and societal importance, such as the West African Monsoon and the El Niño Southern Oscillation. These complex systems, and their year-to-year variability, are not well-represented in current global climate models; comparisons with future, unmitigated climate change scenarios are therefore highly uncertain. A reduction in solar radiation at the ocean surface would be expected to reduce global evaporation and hence precipitation elsewhere.237 Increased numbers of cloud droplets could also suppress precipitation.238 An idealised model that assumed that cloud droplet size could be reduced uniformly over all the global ocean has indicated that such an “intervention” could counteract most of the temperature and precipitation changes caused by doubling CO2, although with an (unexpected) slight residual increase in precipitation over land, compared to the pre-industrial climate, for double CO2 plus CCN increase.239 This model is, however, unrealistic in many of its assumptions. In addition to these uncertain local, regional and global effects of cloud brightening (with potential for both positive or negative effects on terrestrial biodiversity), the relatively dramatic changes in light intensity and temperature near to the sites of deployment are also likely to affect ocean productivity. Increases in primary production are, however, more likely than decreases, since the ocean areas most suitable for cloud brightening deployment are mostly strongly stratified, with photosynthesis constrained by nutrient availability rather than lack of light. Strong 229 Morel (1991). 230 Pomozi et al. (2001). 231 Holmes (2002). 232 Latham (1990). 233 Latham et al. (2008). 234 Latham et al. (2012). 235 Rasch et al. (2009). 236 Jones et al. (2010). 237 Vaughan & Lenton (2011). 238 Albrecht (1989). 239 Bala et al. (2010). 51
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