| 英文摘要: | Elevated sea surface temperatures have been shown to cause mass coral bleaching1, 2, 3. Widespread bleaching, affecting >90% of global coral reefs and causing coral degradation, has been projected to occur by 2050 under all climate forcing pathways adopted by the IPCC for use within the Fifth Assessment Report4, 5. These pathways include an extremely ambitious pathway aimed to limit global mean temperature rise to 2 °C (ref. 6; Representative Concentration Pathway 2.6—RCP2.6), which assumes full participation in emissions reductions by all countries, and even the possibility of negative emissions7. The conclusions drawn from this body of work, which applied widely used algorithms to estimate coral bleaching8, are that we must either accept that the loss of a large percentage of the world’s coral reefs is inevitable, or consider technological solutions to buy those reefs time until atmospheric CO2 concentrations can be reduced. Here we analyse the potential for geoengineering, through stratospheric aerosol-based solar radiation management (SRM), to reduce the extent of global coral bleaching relative to ambitious climate mitigation. Exploring the common criticism of geoengineering—that ocean acidification and its impacts will continue unabated—we focus on the sensitivity of results to the aragonite saturation state dependence of bleaching. We do not, however, address the additional detrimental impacts of ocean acidification on processes such as coral calcification9, 10 that will further determine the benefit to corals of any SRM-based scenario. Despite the sensitivity of thermal bleaching thresholds to ocean acidification being uncertain11, 12, stabilizing radiative forcing at 2020 levels through SRM reduces the risk of global bleaching relative to RCP2.6 under all acidification–bleaching relationships analysed. Coral reefs are the iconic ecological communities of tropical seas, providing extensive ecosystem goods and services to around 500 million people13. However, coral reefs are under increasing pressure from anthropogenic climate change and in particular the effects of ocean warming1, 2. Coral bleaching has been observed to occur in response to a wide range of chemical and biological parameters, yet most evidence indicates that elevated sea surface temperatures (SSTs) are the dominant cause of both localized and mass bleaching events1. Elevated sea temperatures of only 1–2 °C above the average summer maximum increase the excess excitation energy associated with photoinhibition during photosynthesis, which causes the disintegration and expulsion of symbiotic zooxanthellae14. Mass coral bleaching can, although does not necessarily, result in extensive coral mortality2. Climate projections tell us that conditions causing bleaching at present will occur more frequently on coral reefs over the coming decades3, 4, 5. SRM could be achieved through the delivery of specific aerosols or aerosol precursors (in this study SO2) to the stratosphere, increasing the planetary albedo, cooling the planet and ameliorating the temperature rise resulting from increasing atmospheric CO2 concentrations15. Coral growth rates have previously been linked to volcanic and anthropogenic aerosol emissions16. Inadvertent SRM has therefore already been shown to influence coral reefs. A widely cited objection to SRM is that although it acts to ameliorate global warming, atmospheric CO2 concentrations continue to rise and ocean acidification continues unabated15. Important trade-offs therefore exist when considering the benefits of SRM to coral reefs—known to be sensitive to ocean acidification10—when compared with greenhouse gas mitigation scenarios. Aragonite saturation state (Ωarag) is a measure of the thermodynamic potential for the aragonite form of CaCO3 to form or dissolve and is strongly influenced by ocean acidification17. The possibility of a reduced Ωarag acting synergistically with high SSTs to drive coral bleaching at lower temperatures has been suggested11. As such, any benefits of SRM-based geoengineering of lower SSTs may be offset by the influence of ocean acidification on the thermal bleaching threshold. We explore this potential trade-off by projecting global coral bleaching under scenarios of mitigation and SRM and considering a liberal range of acidification–bleaching relationships. It should be noted that this is just one potential impact of ocean acidification, and although acidification effects on coral bleaching are highly uncertain11, 12, 18, negative impacts are also projected in relation to calcification and reproduction processes with higher confidence (for example, refs 9, 10). Bleaching is projected under the IPCC’s (Intergovernmental Panel on Climate Change’s) RCPs 2.6 and 4.5 (ref. 6), using ensembles of perturbed initial-condition simulations undertaken with the Hadley Centre Global Environmental Model version 2 (HadGEM2-ES) Earth system model19. In addition we project bleaching change under an SRM scenario (Fig. 1). The SRM simulation is identical to RCP 4.5 until 2020, and then injects sulphur dioxide into the stratosphere to stabilize global radiative forcing from 2020 until the end of the experiment in 2075. This SRM simulation represents a potentially realistic scenario similar to that of the Geoengineering Model Intercomparison Project (GeoMIP) G3 experiment20. The atmospheric concentration of CO2 associated with the SRM simulation is identical to RCP 4.5 (Fig. 1). These scenarios result in contrasting pathways of the bleaching-relevant parameters, tropical SST and Ωarag− (Fig. 1).
| http://www.nature.com/nclimate/journal/v5/n8/full/nclimate2655.html
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