doi: 10.1098/rsta.2008.0131 , 4007-4037 366 2008 Phil. Trans. R. Soc. A Chih-Chieh (Jack) Chen, Georgiy L Stenchikov and Rolando R Garcia Philip J Rasch, Simone Tilmes, Richard P Turco, Alan Robock, Luke Oman, stratospheric sulphate aerosols An overview of geoengineering of climate using References l.html#ref-list-1 http://rsta.royalsocietypublishing.org/content/366/1882/4007.ful This article cites 60 articles, 6 of which can be accessed free Rapid response 1882/4007 http://rsta.royalsocietypublishing.org/letters/submit/roypta 366/ Respond to this article Subject collections (45 articles) climatology collections Articles on similar topics can be found in the following Email alerting service here in the box at the top right-hand corner of the article or click Receive free email alerts when new articles cite this article - sign up http://rsta.royalsocietypublishing.org/subscriptions go to: Phil. Trans. R. Soc. A To subscribe to This journal is �� 2008 The Royal Society on 21 November 2009 rsta.royalsocietypublishing.org Downloaded from

the surface, and this may influence ecosystems. The impact of geoengineering on these components of the Earth system has not yet been studied. Representations for the formation, evolution and removal of aerosol and distribution of particle size are still very crude, and more work will be needed to gain confidence in our understanding of the deliberate production of this class of aerosols and their role in the climate system. Keywords: climate change geoengineering sulphate aerosols global warming 1. Introduction The concept of ���geoengineering��� (the deliberate change of the Earth���s climate by mankind Keith 2000) has been considered at least as far back as the 1830s with J. P. Espy���s suggestion (Fleming 1990) of lighting huge fires that would stimulate convective updrafts and change rain intensity and frequency of occurrence. Geoengineering has been considered for many reasons since then, ranging from making polar latitudes habitable to changing precipitation patterns. There is increasing concern by scientists and society in general that energy system transformation is proceeding too slowly to avoid the risk of dangerous climate change from humankind���s release of radiatively important atmospheric constituents (particularly CO2). The assessment by the Intergovernmental Panel on Climate Change (IPCC 2007a) shows that unambiguous indicators of human- induced climate change are increasingly evident, and there has been little societal response to the scientific consensus that reductions must take place soon to avoid large and undesirable impacts. To reduce carbon dioxide emissions soon enough to avoid large and undesirable impacts requires a near-term revolutionary transformation of energy and transportation systems throughout the world (Hoffert et al. 1998). The size of the transformation, the lack of effective societal response and the inertia to changing our energy infrastructure motivate the exploration of other strategies to mitigate some of the planetary warming. For this reason, geoengineering for the purpose of cooling the planet is receiving increasing attention. A broad overview to geoengineering can be found in the reviews of Keith (2000), WRMSR (2007), and the papers in this volume. The geoengineering paradigm is not without its own perils (Robock 2008). Some of the uncertainties and consequences of geoengineering by stratospheric aerosols are discussed in this paper. This study describes an approach to cooling the planet, which goes back to the mid-1970s, when Budyko (1974) suggested that, if global warming ever became a serious threat, we could counter it with airplane flights in the stratosphere, burning sulphur to make aerosols that would reflect sunlight away. The aerosols would increase the planetary albedo and cool the planet, ameliorating some (but as discussed below, not all) of the effects of increasing CO2 concentrations. The aerosols are chosen/designed to reside in the stratosphere because it is remote, and they will have a much longer residence time than tropospheric aerosols that are rapidly scavenged. The longer lifetime means that a few aerosols need be delivered per unit time to achieve a given aerosol burden, and that the aerosols will disperse and act to force the climate system over a larger area. P. J. Rasch et al. 4008 Phil. Trans. R. Soc. A (2008) on 21 November 2009 rsta.royalsocietypublishing.org Downloaded from

Sulphate aerosols are always found in the stratosphere. Low background concentrations arise due to transport from the troposphere of natural and anthropogenic sulphur-bearing compounds. Occasionally much higher concen- trations arise from volcanic eruptions, resulting in a temporary cooling of the Earth system (Robock 2000), which disappears as the aerosol is flushed from the atmosphere. The volcanic injection of sulphate aerosol thus serves as a natural analogue to the geoengineering aerosol. The analogy is not perfect because the volcanic aerosol is flushed within a few years, and the climate system does not respond in the same way as it would if the particles were continually replenished, as they would be in a geoengineering effort. Perturbations to the system that might become evident with constant forcing disappear as the forcing disappears. This study reviews the state of understanding about geoengineering by sulphate aerosols as of early 2008. We review the published literature, introduce some new material and summarize some very recent results that are presented in detail in the submitted articles at the time of the writing of this paper. In our summary we also try to identify areas where more research is needed. Since the paper by Budyko (1974), the ideas generated there have received occasional attention in discussions about geoengineering (e.g. NAS92 1992 Turco 1995 Govindasamy & Caldeira 2000, 2003 Govindasamy et al. 2002 Crutzen 2006 Wigley 2006 Matthews & Caldeira 2007). There are also legal, moral, ethical, financial and international political issues associated with a manipulation of our environment. Commentaries (Bengtsson 2006 Cicerone 2006 Kiehl 2006 Lawrence 2006 MacCracken 2006) to Crutzen 0�� 90�� evaporation tropical pipe transport sedimentation tropopause fold aircraft sulfur and soot transport of source gases cloud scavenging tropopause cloud processes nucleation condensation and coagulation tropical stratospheric reservoir polar vortex nucleation PSCs removal ~17 km ~8 km Figure 1. A schematic of the processes that influence the life cycle of stratospheric aerosols (adapted with permission from SPARC 2006). 4009 Review. Geoengineering by sulphate aerosols Phil. Trans. R. Soc. A (2008) on 21 November 2009 rsta.royalsocietypublishing.org Downloaded from

(2006) address some of these issues and remind us that this approach does not treat all the consequences of higher CO2 concentrations (such as ocean acidification others are discussed in Robock 2008). Recently, climate modellers have begun efforts to provide more quantitative assessments of the complexities of geoengineering by sulphate aerosols and the consequences to the climate system (Rasch et al. 2008 Tilmes et al. 2008, submitted Robock et al. 2008). 2. An overview of stratospheric aerosols in the Earth system (a ) General considerations Sulphate aerosols are an important component of the Earth system in the troposphere and stratosphere. Because sulphate aerosols play a critical role in the chemistry of the lower stratosphere and occasionally, following a volcanic eruption, in the radiative budget of the Earth by reducing the incoming solar energy reaching the Earth surface, they have been studied for many years. A comprehensive discussion of the processes that govern the stratospheric sulphur cycle can be found in the recent assessment of stratosphere aerosols (SPARC 2006). Figure 1, taken from that report, indicates some of the processes that are important in that region. Sulphate aerosols play additional roles in the troposphere (IPCC (2007a) and references therein). As in the stratosphere they act to reflect incoming solar energy (the ���aerosol direct effect���), but also act as cloud condensation nuclei, influencing the size of cloud droplets and the persistence or lifetime of clouds (the ���aerosol indirect effect���) and thus the reflectivity of clouds. Althoughourfocusisonstratosphericaerosols,onecannotignorethetroposphere, and so we include a brief discussion of some aspects of the tropospheric sulphur cycle also. A very rough budget describing the sources, sinks and transformation pathways1 during volcanically quiescent times is displayed in figure 2. Sources, sinks and burdens for sulphur species are much larger in the troposphere than in the stratosphere. The sources of the aerosol precursors are natural and anthropogenic sulphur-bearing reduced gases (DMS, dimethyl sulphide SO2, sulphur dioxide H2S, hydrogen sulphide OCS, carbonyl sulphide). These precursor gases are gradually oxidized (through both gaseous and aqueous reactions) to end products involving the sulphate anion (SO4K) 2 in combination with various other cations. In the troposphere where there is sufficient ammonia, most of the aerosols exist in the form of mixtures of ammonium sulphate ((NH4)2SO4) and bisulphate ((NH4)HSO4). The stratospheric sulphur-bearing gases oxidize (primarily via reactions with the OH radical) to SO2, which is then further oxidized to gaseous H2SO4. Stratospheric sulphate aerosols exist in the form of mixtures of condensed sulphuric acid (H2SO4), water and, under some circumstances, hydrates with nitric acid (HNO3). 1 Sulphur emissions and burdens are frequently expressed in differing units. They are sometimes specified with respect to their molecular weight. Elsewhere they are specified according to the equivalent weight of sulphur. They may be readily converted by multiplying by the ratio of molecular weights of the species of interest. We use only units of S in this paper, and have converted all references in other papers to these units. Also, in the stratosphere, we have assumed that the sulphate binds with water in a ratio of 75/25 H2SO4/water to form particles. Hence 3 Tg SO4K 2 Z 2 Tg SO2 Z 1 Tg Sz4 Tg aerosol particles: P. J. Rasch et al. 4010 Phil. Trans. R. Soc. A (2008) on 21 November 2009 rsta.royalsocietypublishing.org Downloaded from

Although the OCS source is relatively small compared with other species, owing to its relative stability, it is the dominant sulphur-bearing species in the atmosphere. Oxidation of OCS is a relatively small contributor to the radiatively active sulphate aerosol in the troposphere, but it plays a larger role in the stratosphere where it contributes perhaps half the sulphur during volcanically quiescent conditions. Some sulphur also enters the stratosphere as SO2 and as sulphate aerosol particles. The reduced sulphur species oxidize there and form sulphuric acid gas. The H2SO4 vapour partial pressure in the stratosphere���almost always determined by photochemical reactions���is generally supersaturated, and typically highly super- saturated, over its binary H2O���H2SO4 solution droplets. The particles form and grow through vapour deposition, depending on the ambient temperature and concentrations of H2O and H2SO4. These aerosol particles are then transported by winds (as are their precursors). Above the lower stratosphere, the particles can evaporate, and in the gaseous form the sulphuric acid can be photolysed to SO2, where it can be transported as a gas, and may again oxidize and condense in some other part of the stratosphere. Vapour deposition is the main growth mechanism in the ambient stratosphere, and in volcanic clouds, over time. Because sources and sinks of aerosols are so much stronger in the troposphere, the lifetime of sulphate aerosol particles in the troposphere is a few days, while that of stratospheric aerosol is a year or so. This explains the relatively smooth spatial distribution of sulphate aerosol and resultant aerosol forcing in the stratosphere, and much smaller spatial scales associated with tropospheric aerosol. The net source1 of sulphur to the stratosphere is believed to be of the order of 0.1 Tg S yrK1 during volcanically quiescent conditions. A volcanic eruption completelyalters the balanceofterms in the stratosphere. For example,the eruption 0.0001 (13 days) 0.06 (1.4 days) 0.3 (10 years) 0.01 (30 days) 0.2 (2 years) 2 (1 year) 0.6 (5 days) 0.4 (2 days) CS2,H2S,DMS OCS SO2 SO4= surface tropopause 15 2 65 1 0.004 0.03 0.03 0.06 0.1 (sed) 0.03 0.06 32 50 51 (scav) 2 15 Figure 2. A very rough budget (approx. 1 digit of accuracy) for most of the major atmospheric sulphur species during volcanically quiescent situations, following Rasch et al. (2000), SPARC (2006) and Montzka et al. (2007). Numbers inside boxes indicate species burden in units of Tg S, and approximate lifetime against the strongest source or sink. Numbers beside arrows indicate net source or sinks (transformation, transport, emissions, and deposition processes) in Tg S yrK1. 4011 Review. Geoengineering by sulphate aerosols Phil. Trans. R. Soc. A (2008) on 21 November 2009 rsta.royalsocietypublishing.org Downloaded from