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Tackling climate change: what is the impact on air pollution?
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Martin Williams*
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This article addresses the impact of climate change policies on air pollution. It notes firstly that while the co-benefits of climate change to air pollution are potentially large, institutionally the links have yet to be made in a substantive way in international treaties and agreements. The article discusses economic studies that quantify and monetize the synergistic benefits to health and the environment arising from improved air quality in pursuit of climate change objectives. This article also discusses the areas where antagonisms might arise necessitating trade-off decisions on the part of policymakers, notably in the use of biomass and diesel vehicles. It also discusses the merits of CCS for both climate change and air quality.
environment, particularly from nitrogen inputs, are still an important problem that has yet to be solved. There are also signs that in many places in the developed world existing policies are not reducing air pollution levels as quickly as might originally have been envisaged. Moreover, air pollutant levels in the developing world are likely to increase in the future. There are thus good reasons to maintain pressure on air pollution, and also very good reasons for continuing to address air pollution issues in a coordinated way with mitigation measures to combat climate change. Unfortunately there has been little recognition of the air quality co-benefits of climate change policies, although these could be large. This review article explores some of the reasons for this and also discusses the synergies and trade-offs between policies addressing climate change and air pollution. GHGs share common sources with many of the most important air pollutants and measures taken to mitigate climate change will, in many cases, result in reductions in air pollutants with concomitant improvements in air quality, public health and ecosystem status. However, some measures that benefit climate change can cause increases in emissions of air pollutants and vice versa. A schematic diagram summarizing this is shown in Figure 1 and relevant aspects of this diagram will be discussed in the sections of this article below.
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Climate change is one of the most significant environmental issues facing humanity at the present time. There are major concerns over the increase in global temperatures and the concomitant impacts on weather, extreme events and the spread of disease. Perhaps unsurprisingly, climate change has also become the single most important environmental issue to engage governments across the world. Progress in achieving a worldwide consensus may have been slow, but the political profile is nonetheless high. In the developed world it is broadly the case that, of all environmental issues, it is climate change that dominates the political and media agendas, while air pollution is, generally, accorded a lower priority. Evidence of this is difficult to quantify but a clear example is that presidents and prime ministers tend to attend climate change negotiations-at least for some of the time – with associated high-profile media coverage. Air quality issues, while not absent from media coverage, tend not to attract the same level of interest. The reasons for this are possibly that reductions in air pollution have been achieved over the past decades and policies are in place that should help continue this improvement. Air pollution is, thus, often seen as a problem solved. However, evidence on the health effects of air pollution continues to emerge and impacts on the wider
*Environmental Research Group, King’s College London, Franklin Wilkins Building, 150 Stamford Street, London, SE1 9NH, UK E-mail:
[email protected]
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10.4155/CMT.12.49 © 2012 Future Science Ltd
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AQ benefit
Energy efficiency Demand management Flue gas desulfurization
Nuclear
Three-way catalysts – petrol
Wind, solar and tidal
Particulate filters – diesel
Nitrogen efficiency Hybrids, LZEVs
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Increase in ‘uncontrolled’ diesel Biofuels
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Uncontrolled coal and oil fossil fuels in stationary and mobile sources
Combined heat and power?
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Buying credits overseas
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Figure 1. Synergies and trade-offs from policies and technologies to address climate change and air pollution. AQ: Air quality; CC: Climate change; LZEVs: Low- and zero-emission vehicles. Reproduced with permission from [34].
The co-benefits of climate change policies & air quality It is clear that measures to reduce emissions of long-lived GHGs in the Kyoto Protocol, particularly CO2 and CH4, could also result in reductions of emissions of air pollutants. While air quality issues have been included to some extent in climate change discussions within UNFCCC, there has been no delivery of strategies to deliver optimal outcomes for both climate change and air quality. There are many reasons for this, not least that the climate negotiations within the UNFCCC are complex and additional issues Key term such as air quality are often seen as a Integrated assessment model: distraction. There are other reasons Mathematical model that incorporates and these have been discussed [1] . atmospheric transport and chemistry These authors cited the main reawith abatement/mitigation costs for all son as being the fact that climate sources to arrive at an optimized pathway to a prescribed environmental policy assessment was generally target. concerned with cost-minimization
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of achieving a predetermined target for GHG reductions, rather than through a comparison of overall costs and benefits. Inclusion of air pollution co-benefits could result in a lower cost of achieving the same GHG target or could mean that for the same mitigation costs, a more stringent GHG target could be achieved. Nemet et al surveyed 37 peer-reviewed papers of air quality co-benefits in climate studies and found that the range of benefits, across both developed and developing countries, was US$2–128/tCO2 at constant 2008 dollars [1] . This is broadly of the same order of magnitude as damage costs of carbon. Valuing carbon is complex but as an illustration, ‘damage’ costs associated with carbon have been quoted by the UK Department for Energy and Climate Change as GB£27–62 from 2008 to 2050, or $43–98 at current exchange rates [2] . There are scientific as well as political challenges to integrating air pollution and climate change policies, not least the fact that air pollutants in general have much shorter
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interactions and synergies (GAINS), developed at the International Institute for Applied Systems Analysis in Austria. These models calculate optimal cost pathways to achieve specified environmental targets. Originally developed in Europe, GAINS is at present unable to employ spatial resolution better than 25 × 25 km grid squares. The European Environmental Agency used the model to study the ancillary air pollution benefits in Europe of three scenarios, the most ambitious of which reduced CO2-e emissions by 40% by 2030 on a 1990 base [9] . The work concluded that in the baseline scenario there would be 311,000 premature deaths in Europe from particulate matter (PM2.5) and ozone exposures, but that these would reduce to 288,000 in the ambitious climate scenario. For three climate scenarios tested in the UK, compared with a baseline of 6.9 months loss of life expectancy in 2000, the ambitious climate scenario gave a loss of 2.2 months, the scenario aimed solely at air quality targets in future showed a loss of 3.4 months, but the less ambitious climate scenario showed a smaller improvement of 4.8 months. This early work highlights the importance of optimised scenarios for public health improvements. Another study, also using the RAINS model, assessed the monetary benefits for Europe as a whole [10] . For the policies then in place they reported the total benefits of implementing Kyoto policies, in terms of reduced costs of air pollution abatement, as €2.5–7 billion. A more recent study performed a global assessment of the local air quality implications of meeting climate change goals in 2050 [11] . Their conclusion was that significant health benefits could arise and might be an added incentive for developing countries to pursue climate policies. These local air quality health assessments were carried out in a rudimentary manner at an even coarser level than the RAINS model. Efficiency gains from integrating health and crop damage costs and climate effects were demonstrated in a study incorporating climate impacts into an assessment of air pollution policies in the EU region [12] . In the southern hemisphere, a study in South Africa made the case for integrating air quality and climate change policies using the city of Durban as a case study [13] . A recent study illustrated the trade-offs involved in reducing air pollutants such as SO2, which act to cool the Earth’s climate [14] . They suggested that in formulating policies targeted at climate, reductions in SO2 and NOx (another important air pollutant) should be ignored, although such reductions would bring health and environmental benefits. A further discussion of the trade-offs between climate and air quality policies is given below. A paper specifically addressing the implications of climate policies for the UK was published by the
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lifetimes in the atmosphere (typically days to months) than GHGs do – the lifetime of CO2, for example, is of the order of 100 years. The scientific and political difficulties have been discussed [3] . Political, economic and trade issues also play a very important role in climate negotiations, particularly in the developing world. One argument for incorporating ancillary benefits, such as those resulting from improved air quality, is that overall solutions and policies for climate and health might be more readily agreed in both developing and developed countries. A modeling study of international negotiations in this context has been given using the well-known ‘prisoner’s dilemma’ approach from game theory [4] . Measures to mitigate climate change will involve the reduction of emissions of CO2 and other GHGs, and most of these measures will at the same time reduce air pollution. However, some measures to mitigate climate change could worsen air quality, and vice versa. At the present time, for example, the EU has a policy calling for a reduction of 20% in GHGs by 2020, which could increase to 30% given commitments from other countries around the world. At this level of ambition, the potential for air quality co-benefits to be eroded by possible disbenefits is greater than it potentially is if the reductions in GHGs in climate policies are larger. At low levels of ambition (~20% reduction in GHG emissions) there is the potential for targets to be met using techniques/fuels, which potentially do not improve air quality or could even make it worse, such as biomass [5] . With large reductions in GHG emissions, such as the 80% in the UK Climate Change Act, there is the potential for significant energy and transport infrastructure changes with concomitant air quality improvements [6] . A recent policy document from the Department for the Environment, Food and Rural Affairs in the UK pointed out the potential benefits of aligning climate and air pollution policies [7] . There has been little research directed to the explicit assessment of the cobenefits of climate change policies, either in the UK or elsewhere, although work is now beginning to appear and this is summarized below. The review of the economics of climate change by Stern made brief mention of the monetary co-benefits of climate policies, but did not give details nor specifically mention air quality [8] . A more recent economic analysis concluded that the co-benefits could be on average $44/tCO2 [1] . Some earlier studies assessed the co-benefits of air quality and climate policies at a regional level in Europe at a relatively coarse spatial scale (generally 25–50 km grids). Several of these studies are based on a so-called ‘integrated assessment model’ – regional air pollution information and simulation (RAINS) – with the extension to include GHGs known as GHG and air pollution
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motor vehicles (160 disability-adjusted life-years in London and 1696 in Delhi). Other studies in the same series investigated the public health benefits of reductions in GHGs with measures in other sectors, specifically: the benefits of increased energy efficiency in the UK housing stock [17] ; low-carbon electricity generation [18] ; and short-lived climate forcers [19] . Space precludes a detailed discussion of the findings but this series represents the first concerted attempt to assess the health benefits arising from GHG reductions. However, it is not only in the developed world that such potential conflicts and synergies arise. A recent study of potential future power generation in Brazil, for example, analyzed the possible conflicts for both local air quality and GHG emissions from choices over future power systems [20] . The issues in Brazil are particularly interesting in that the dominant fraction of current power generation comes from hydroelectricity so that there are potentially difficult choices for the future between fuels which are ‘clean’ either in an air quality sense but potentially higher in carbon emissions, or vice versa. Most of the above studies have estimated health and air quality co-benefits from measures directed at GHG emissions that happen to share the same sources as some air pollutants. A more focused approach would be to address the so-called short-lived climate forcers specifically, recognizing from the outset that benefits should accrue to both climate and air quality. A major recent assessment of the worldwide air quality and climate benefits from reductions in two of the most important short lived climate forcers, black carbon and ozone, has been published [21] . The study selected 16 measures to reduce black carbon emissions and those of CH4, the most important ozone precursor on a global scale [21] . This work showed that reducing these air pollutants could have major health benefits, particularly in Asia, as well as mitigating near-term climate change (~40 years). Globally, the health benefits were estimated at 2.4 million premature deaths avoided (range of 0.7 to 4.6 million), largely from reductions in PM, and crop losses of 52 million tonnes being avoided (range of 30 to 140 million) from reductions in tropo spheric ozone. The climate benefit was estimated at 0.5°C (range of 0.2 to 0.7°C). An important feature of this study was the fact that all the measures to reduce emissions have already been applied somewhere in the world and all use only existing technologies; therefore, in principle, they could be applied immediately. Moreover, some of the measures to prevent CH4 emissions and, hence, loss of potentially valuable products such as oil and gas, can actually save money rather than imposing costs.
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present author [6] . This considered the implications for what was then Urban background location: Site in an a hypothetical UK climate change urban area not directly influenced by very close pollution sources, as target of 60% reduction in CO2-e contrasted with a roadside site, close to by 2050. The study showed that a busy road. impacts of air pollution on morDisability-adjusted life years: Years of tality and morbidity in London in life lost associated with exposure to a 2050 could be approximately halved pollutant also incorporating years lost due to the induced disability. compared with ‘business-as-usual’. Current estimates of the impact of PM2.5 on mortality in the UK suggest that annual average PM 2.5 concentrations are associated with 29,000 premature deaths in the UK, with a loss of life expectancy of 3–4 months across the whole population of Scotland and Northern Ireland, and 6–7 months over the population of England and Wales [15] . With business-as-usual, that is, no specific measures to address climate change urban background, concentrations of PM2.5 in London were estimated to reduce by 24–36% in 2050 relative to 2004. However, hypothetical measures to achieve a reduction of CO2-e of 60% by 2050 on a 1990 base resulted in an estimated further reduction in 2050 of 55% beyond the ‘business-as-usual’ case at urban background locations. If these reductions were repeated across the UK this could result in the business-as-usual policies reducing the number of premature deaths by approximately 8700 compared with the present case (assuming the relationship between PM2.5 and mortality continued to hold and that baseline mortality rates remained constant). The measures to achieve a 60% reduction in CO2-e could then reduce this figure by approximately 11,000 further premature deaths. Since that paper, an even more ambitious target of 80% reduction by 2050 is now in UK law in the Climate Change Act of 2008. A series of papers in The Lancet in 2009 addressed climate change issues. One paper studied the implications for health of a low-carbon vehicle fleet in London and Delhi [16] . This work studied several scenarios out to 2030, none as ambitious as those discussed in the previous paragraph [6] . The ‘low-carbon’ transport scenario incorporated a reduction of 35% in CO2 emissions in London from road transport on 1990 levels. In Delhi, the authors assumed a low-carbon scenario with a rise of 447% by 2030 compared with a rise of 526% in the business-as-usual case. This marked difference in future emissions is a stark illustration of the problem of reducing emissions of GHGs and air pollutants in the developing world. The authors found that, in both cities, reduction in CO2 emissions through an increase in active travel and less use of motor vehicles had larger health benefits per million population (7332 disabilityadjusted life years in London and 12,516 in Delhi in 1 year) than from the increased use of lower-emission Key terms
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harmful air pollutants – fine particles (PM2.5 and PM10, considered to be strongly associated with premature mortality) and potential carcinogens such as polycyclic aromatic hydrocarbons. Consequently, the concern is that if such fuels replaced fuels that are ‘cleaner’ in the air quality sense (e.g., gas) then air quality could worsen in the pursuit of targets for GHGs. Of particular concern are smaller, less efficient combustion systems at the residential or small commercial scale, where combustion and abatement may be less efficient than in large power stations. Although, even in the latter case, if biomass use were to prevent wider deployment of other ‘zero-carbon’ methods of energy generation, such as wind, solar, tidal or nuclear power, then air quality may still not improve in an optimal sense. Studies of the air quality impacts of biomass have been reported using specific chemicals as tracers, notably levoglucosan [25] . Other studies have attempted to quantify the emissions of a range of harmful species including polycyclic aromatic hydrocarbons, as well as PM from wood boilers [5] . There are emerging signs that some governments are beginning to recognize that there can be large cobenefits between policies addressing climate change and air quality, but also that there can be tensions and tradeoffs. In Scotland, the Scottish Executive published a study on the potential impact of wood-burning biomass boiler use in Scotland and concluded that, while the additional impacts were relatively small (but dependent on emission factors that are uncertain), biomass use could result in exceedances of EU obligations for PM10 and PM2.5 [26] . Guidance was also given to local authorities in considering planning applications for new biomass installations. Following the adoption of the Climate Change Act in 2008, HM Government in the UK published the Low Carbon Transition Plan, which concluded that while some policies would result in large improvements in air quality, the adverse health effects from an unmanaged major uptake of biomass (wood) in the residential sector would outweigh the air quality/ health benefits from all the changes in other sectors, at a net air quality cost rising to £2.6 billion in 2022 [27] . Very recently, the UK Department for Energy and Climate Change published The Carbon Plan, which sets out options for achieving the 2050 target, based on the macroeconomic model MARKAL [28] . There are many potentially conflicting policies and trade-offs, and the report set out three illustrative scenarios, some of which show wide ranges of air quality benefits and disbenefits. The three illustrative pathways are: higher renewable, more energy efficiency; higher CCS, more bioenergy; and higher nuclear, less energy efficiency. Damage costs arising from air pollution were calculated by simply assigning a cost per tonne to total UK emissions of
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Trade-offs & tensions between climate & air quality policies A classic example of the disbenefits of climate policies resulting in poorer air quality is afforded by diesel cars, the use of which has been encouraged by many governments on fuel economy and hence CO2 grounds. However, up to and including the Euro 5 level of regulation, compared with their petrol equivalents diesel cars emit considerably more particles and NOx. An analysis of the UK policy of encouraging the use of diesel cars estimated that the consequent increased emission of particles would be responsible for 20–300 deaths per year in the UK over the period 2000–2020 [22] . The Euro 6 regulations for diesel cars should in theory reduce the emissions of both particles and NOx considerably, but given past experience of real-world performance compared with regulatory limits [23] , research will be needed to check that the expected reductions are delivered in practice. Even so, Euro 6 diesel cars are still likely to be more polluting in an air quality sense than their petrol equivalents; however, optimal solutions for both climate change and air quality via hybrid, hydrogen and electric technologies are now becoming a realistic possibility. A further trade-off in removing harmful air pollutants from diesel exhausts arises because of the possible increase in fuel consumption incurred when an exhaust filter is fitted (as the back pressure increases in the exhaust more fuel must be burned to overcome it). The impact on climate of the increase in CO2 emissions is offset by the reduction in black carbon emissions, as black carbon is a powerful absorber of the sun’s radiation and a strong short-lived climate forcer. The ‘break-point’ of where the benefits to climate are balanced by the disbenefits of the CO2 increase has been studied [24] . The different lifetimes of black carbon (of the order of several days) and CO2 (of the order of 100 years) means that comparisons are difficult, but the authors investigated a wide range of time horizons so that policy choices could be made based on the time frame of the effect that one wishes to address. Climate benefit/disbenefit trade-offs were plotted as a function of time horizon and were dependent on the assumptions on the radiative effect of black carbon and the relative additional emissions of black carbon and CO2. An important area of concern regarding potential adverse impacts on air quality and public health concerns the likely increase in the use of biomass (chiefly wood) in combustion systems used for heating and power generation. Use of this fuel derived from recently grown trees can approach carbon-neutrality, although the carbon benefits will be offset to some extent by the emissions generated in the production and transport of useable biomass fuels. The problem for air quality arises because wood combustion can be a significant source of
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the main issue. Helpful guidance on the issue around CHP and air quality has been published [29] . The foregoing paragraphs and the title of this article are concerned with the impact of policies directed to mitigating climate change on air quality. However, there is one aspect of air quality policies that can potentially have a major impact on climate, which is therefore discussed here. Air pollution emissions of SO2 and NOx arise in large quantities from uncontrolled combustion sources such as power plants, industrial processes, road vehicles and residential sources. These two pollutants react in the atmosphere to form particles (aerosols) of sulfate and nitrate (usually present as the ammonium compounds), respectively. These aerosols, unlike black carbon particles, exert a cooling effect on the Earth’s climate. However, these pollutants are also damaging to health and the wider environment, and their emissions are being reduced significantly for these reasons. Policy instruments such as the UN Economic Commission for Europe (UNECE) Convention on Long Range Transboundary Air Pollution (CLRTAP), first signed in 1979, have been instrumental in achieving reductions of over 70% and approximately 40% in SO2 and NOx, respectively, since 1990 in Europe. Similar reductions have been achieved in North America, with beneficial consequences for public health and the wider environment. Moreover, new power stations in China are being fitted with abatement equipment to reduce SO2 emissions, for example; therefore, measures are also being put in place in the developing countries to reduce these harmful emissions. The problem, however, is that the burden of anthropogenic aerosols in the Earth’s atmosphere has been estimated to ‘mask’ a potential warming of approximately 1°C due to this cooling effect [30] . The effect of the anthropogenic sulfate, and to a lesser extent nitrate aerosols, has been cited as a contributor to the slowing down of the upward trend in global temperature observed in the 1940s–1980s (when emissions of SO2 and NOx were increasing rapidly in the developed world) and the increase in temperature subsequently (when emissions of SO2 and NOx began to reduce) [31] . These latter authors discuss the effects of reductions in SO2 and NOx emissions and the consequences of this ‘unmasking’ in terms of both the global temperature increase and its rate of change, which could both have consequences for climate impacts.
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pollutants, with no detailed analysis of health impacts or their relative Short-lived climate pollutant: Air distribution across the population. pollutant with atmospheric lifetime short compared with the Kyoto gases Using this approach, the report and which harms health and/or the estimated the impacts on health in environment and also exerts an 2050 from particulate pollution and influence on the Earth’s climate. found that for the ‘higher renewable/more energy efficiency’ and ‘higher nuclear/less energy efficiency’ scenarios there could be reductions in health impacts of 60–85% and 45–80% in 2050 compared with 2010, respectively. However for the ‘higher CCS/more bioenergy’ scenario, the report concluded that there was a wide range of outcomes, from a reduction of 80% to an increase in health impacts of 60%, in 2050 compared with 2010, respectively, the increase resulting from the increased use of biomass. The report did not state the reductions in these pollutants in absolute tonnage terms but in the damage–cost approach used in the study the total percentage emission reductions of PM, NOx and SO2 were directly proportional to the percentage changes in health impacts quoted above. A related issue, in the sense that it could involve biomass combustion systems, is the potential increase in combined heat and power (CHP) systems in urban areas. This is attractive to GHG emission reductions in that such systems can give significant increases in the efficiency of fuel use compared with separate distributed systems for heat and energy generation. In many countries where power generation is still located within urban areas this could also lead to improved air quality due to the increased efficiency of fuel use. However in some countries, notably the UK, the increase in such schemes could lead to significant amounts of power generation being returned to urban centers. This could reverse the trends, begun in the late 1950s and 1960s, to improve urban air quality from the notorious London ‘smogs’ by locating power stations with tall stacks to rural areas well away from population centers. The important issues here are the fuels used in the CHP installation and the level of abatement used, and the fuels that the CHP installation replaces. It is not possible to give other than general comments as each case will potentially be different, but if, for example, a new CHP plant fueled by biomass replaces dispersed systems relying on electricity (from a remote power station) then air quality could deteriorate. Likewise, if the plant replaces gas then it is also possible that air quality could get worse. On the other hand, if a gas-powered CHP plant replaces dispersed systems using coal or oil with little control or abatement then air quality could improve. In most applications the fuel used in the CHP plant is likely to be gas (when the pollutants of most concern will be NOx and NO2) or wood, where PM will Key term
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Low-carbon technologies At first sight, technologies for removing most of the carbon from energy and transport systems might seem to be beneficial for both climate change and air qualityhence their inclusion in the upper right hand quadrant in Figure 1. However, there are, in some cases, more detailed considerations which make the situation more
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Summary There are clearly major benefits to public health and impacts on ecosystems, including on food security, from coordinated policies to address climate change and air quality. These benefits are potentially larger the more ambitious is the climate change target of policy. Levels of ambition for the long-lived GHGs such as those in the Kyoto Protocol are currently approximately 20% reductions by 2020 compared with 1990 – the current EU target. At this level of ambition, there is more scope for the disbenefits of climate policies to adversely affect air quality. Although the G8 countries, for example, agreed to an aim of reducing their GHG emissions by 80% by 2050, the UK at present is unique in setting precisely this target (relative to a 1990 base year) in the law via the UK Climate Change Act of 2009. At this level of ambition, the changes required in energy and transport infrastructures become very significant with the potential to reduce both urban and rural air pollution by very large amounts, with consequent benefits for public health and the wider environment. However, there are potential risks and downsides, as discussed above, so that if the wrong policy choices are taken, the potential benefits could be foregone in the pursuit of climate targets alone. The need for close cooperation between policymakers dealing with air quality and climate change is therefore crucially important, not just at national levels but also at regional (e.g., within the EU and the UNECE CLRTAP) and global levels (e.g., within the UNFCCC and elsewhere). The recent announcement of the Climate and Clean Air Coalition to reduce short-lived climate pollutants (SLCPs) by the USA could potentially help to bridge this gap and generate a long-awaited dialogue between the two communities.
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complex – a good example is afforded by CCS. This is a technologically complex process involving the removal of carbon or CO2, pressurization and compression to facilitate transport and finally storage in underground reservoirs. All these processes involve the additional use of energy and can, thus, lead to increased emissions of air pollutants, as well as potentially offsetting some of the large reductions in CO2, which are feasible. There are three types of CCS systems currently under consideration for power generation: postcombustion CCS, whereby the CO2 is separated from the exhaust stream and captured using an amine-based solvent; precombustion CCS, whereby the original fossil fuel, for example coal, is converted into a gaseous fuel composed mainly of hydrogen and CO2 with the latter being subsequently separated (this technology is applicable to new plants rather than retrofit); and ‘oxyfuel’ CCS, which uses pure oxygen in the combustion of the fuel. The processes and impacts on air pollution emissions have recently been reviewed [32] . The pollutants that could potentially increase compared with a ‘no-CCS’ scenario are NOx and NH3, the latter particularly in the postcombustion option. The former emission increase arises from the additional fuel needed to power the additional processes and the latter from possible degradation of the amine based solvent used to capture CO2. Further emissions, which can be substantial, can arise from the transport and storage processes. Moreover, it has been suggested that in a postcombustion CCS process involving lignite fuelling there could be substantial CH4 emissions from mining the coal required to fuel the process of making the capture agent monoethanolamine [33] . Although on balance the European Environmental Agency review felt that CCS should be beneficial for both air quality and climate change, it would rank fairly low in the upper right quadrant of Figure 1 [32] . Another low-carbon technology of interest in this regard is the use of electric and hydrogen powered vehicles. Clearly, if the electricity or hydrogen is generated using ‘carbon-free’ methods such as ‘pure’ renewables – wind, solar or tidal power (but not necessarily biomass, as noted above) – or nuclear power, then there will be an air quality benefit along with the CO2 improvements. However, electric vehicles are already beginning to appear in service and measures have been introduced to encourage their take-up (the London congestion charge, for example, is not payable by electric vehicles). Depending on the relative rates of take-up of electric or hydrogen vehicles and that of zero-carbon power generation, there could be a short- to medium-term penalty for air quality. Quantifying this effect is not possible in a general way. As for CCS installations, the balance will depend on the circumstances of individual cases.
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Future perspective Although up until now there has been little linking of climate change and air quality management policies, there are good reasons to believe that this situation will improve in the future. An important consideration here is the emergence of interest in the so-called SLCPs, generated by recent scientific analyses and assessments of pollutants such as black carbon and ozone and the quantification of their potential benefits for climate change mitigation, the improvement of public health and food security [21] . At a policy and political level this interest has been taken up in the establishment of the Clean Air and Climate Coalition announced in February 2012 by the USA, with the intention of encouraging action to reduce emissions of the SLCPs in various parts of the world [101] . Additionally, for the first time in an international treaty, the UNECE CLRTAP has included black carbon, an important SLCP, in the recent revision of the Gothenburg Protocol to the convention. However,
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concerns over waste disposal and safety mean that the decisions are not straightforward. Equally, technologies such as CCS can potentially provide win–wins if the optimum types of technology prove to be feasible and practicable. Financial & competing interests disclosure The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.
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challenges still remain. Strategies for delivering large reductions (~80–90%) in the long-lived GHGs, which optimize the outcomes for air quality, have still to be developed. The recent formulation of the ‘Representative Concentration Pathways’ emission scenarios in the current work of the IPCC for its Fifth Assessment Report includes emissions of air pollutants as well as GHGs. However, these make assumptions about the implementation and effectiveness of air quality policies in the coming decades, which are inevitably uncertain. In the meantime, countries will be faced with potentially difficult choices [20] as they seek to achieve GHG targets and to maximize the benefits to public heath. Increases in nuclear power generation potentially offer win–win solutions to both air quality and climate change, but
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Executive summary
Air pollutants and GHGs arise from many of the same sources. There are, therefore, potential co-benefits to reducing emmisions related to both air quality and climate change.
For various reasons, however, international treaties and negotiations have not yet fully incorporated the co-benefits in their activities, nor have they necessarily sought optimal solutions for both air quality and climate change.
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Studies have shown that these co-benefits can be significant, and monetization studies suggest that in many cases the co-benefits for air quality and public health could be of a similar order to the damage costs from carbon.
A review of 37 studies by Nemet et al. showed a range of air quality benefits from US$2 to $128 per tonne of CO2 , compared with a damage
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cost of CO2 of $43–98 from 2008 to 2050 estimated by the UK Department of Energy and Climate Change. In general, the more ambitious are the climate change targets for GHG emissions, the larger are the potential benefits for air quality. Relatively small carbon reductions, of the order of 20–30%, could involve the increased use of biomass with negative effects on air pollution. Ambitious targets of the order of 80–90% reductions could result in significant changes to energy and transport infrastructures with potentially large improvements in air quality and health. There are, however, potential conflicts where climate policies could result in increased air pollution, and vice versa. Examples of the former are the use of biomass, diesel vehicles and, in some countries, the increased use of combined heat and power in urban areas. In these cases, although there are carbon benefits, there are also potentially increased emissions of harmful air pollutants. Air pollution measures resulting in the reduction of SO2 largely from power stations can result in a warming of the atmosphere while at the same time improving air pollution. CCS has been seen as an important future technology for mitigating climate change, and might have also been thought of as a win-win for air pollution too. However, depending on the design and concept of CCS used, emissions of some air pollutants could increase. Future developments could see the recent emerging interest in the so-called short lived climate pollutants stimulating interest in developing policies that optimize the benefits to climate and to air quality. The first steps in this regard have been taken by the establishment of the Clean Air and Climate Coalition by the US State Department, and by the incorporation of black carbon, an important short lived climate pollutant, in the recent revision of the Gothenburg Protocol under the UNECE Convention on Long Range Transboundary Air Pollution.
References 1
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