Scientific understanding behind the development of integrated co … · 2020. 2. 13. · issues for consideration at the meeting. ... removed from the atmosphere by deposition back

Introduction

The aim of the first session of the conference is toreview the current scientific understanding of the lin-kages between air pollution and climate changeunderpinning the formulation of integrated co-benefitsstrategies.

As background for this discussion, this note brieflysummarises the key areas where air pollutants,greenhouse gases and climate change interact. Thisis preceded by an overview of current trends inatmospheric emission and followed by a brief outlineof methodological approaches to quantifying co-bene-fits. The conclusion and discussion items presentedmake suggestions on some possible key scientificissues for consideration at the meeting.

In the development of this background paperthere was heavy reliance on some key documents.These are Chapter 2 of the Global EnvironmentOutlook, GEO 4, called "Atmosphere" published byUNEP (Kuylenstierna et al, 2007), the review by theUK Air Quality Expert Group called "Air Quality andClimate change: a UK Perspective" published by UKDEFRA (AQEG 2007) and the review by theEuropean Environment Agency on "Air Pollution andClimate Change Policies in Europe" (EEA 2004).

In many cases greenhouse gases (GHGs such asCO2, CH4, N2O) and air pollutants (such as SO2,NOx, PM, O3 VOCs) share the same emission sources and are affected by the same driving forces.Policies and measures that affect one emission will

affect the other in some way, either increasing ordecreasing its emission. Importantly, once emittedinto the atmosphere some substances traditionallytermed "air pollutants" also increase radiative forcing,thus contributing to global warming. This includesboth primary pollutants such as black carbon andsecondary pollutants such as ozone. Other pollutantslead to cooling of the atmosphere (such as sulphateaerosols). Changes to climate such as shifts in rainfallpatterns also arise from the presence of particulatematter in the atmosphere (the "Atmospheric BrownCloud" effect – Ramanathan et al., 2002), alongsidethe climatic changes induced by global warming. Theimpacts of climate change and air pollution togetherwith elevated carbon dioxide concentrations ofteninteract, in some cases augmenting each other and inothers reducing overall impacts.

1. Atmospheric Emissions:

driving forces and general trends

Human activities emit a wide range of substancesinto the atmosphere; these substances have differentbehaviour and effects (AQEG, 2007). Localisedpeaks of higher concentration may occur close to theorigin of emissions of importance, for example, inurban areas for air quality and possible health effects.Chemical reactions occur in the atmosphere thattransforms some pollutants as they disperse leadingto secondary pollutants with additional consequences.

POLLUTION ATMOSPHÉRIQUE - NUMÉRO SPÉCIAL - AVRIL 2009 55

* [email protected]

Scientific understanding

behind the development

of integrated co-benefits strategies

Les connaissances scientifiques

sous-jacentes aux stratégies

visant un double bénéfice

J KUYLENSTIERNA*, HM SEIP*, K HICKS*, J CLARK*, R MILLS*

Background paper for the Global Atmospheric Pollution Forum Stockholm Conference on

"Air Pollution and Climate Change: Developing a Framework for Integrated Co-benefits Strategies".

Document préparatoire à la conférence du GAPF à Stockholm sur

« Pollution Atmosphérique et changement climatique :

développer un cadre pour des stratégies intégrées visant des co-bénéfices ».

56 POLLUTION ATMOSPHÉRIQUE - NUMÉRO SPÉCIAL - AVRIL 2009

Examples include gases such as O3 and fine particu-late matter (PM) comprising substances such as sulphates and nitrates. Pollutants may also be removed from the atmosphere by deposition back tothe surface, either directly from surface air, or follo-wing absorption into and precipitation in rain.Chemical reactions can occur on a time scale of daysand pollutants may travel a few thousand kilometres,so that problems such as acidic deposition or eutro-phication (e.g. excess nitrogen in sensitive eco -systems) are transboundary/continental in scale.Unreactive gases however, may persist for tens oreven hundreds of years, mixing globally and penetra-ting the whole troposphere. This leads to accumula-tion and globally increased concentrations with a slowresponse to reduction of emissions. Both gases andaerosols can contribute to climate change, throughtheir effect on the balance of incoming and outgoingradiation to and from the Earth and its atmosphere.This effect can be either direct, as is the case fordirectly emitted greenhouse gases (GHGs) such asCO2 and CH4 and emitted particles (e.g. black carbon), or indirect, through the formation andconcentration of GHGs (e.g. ozone) and particles(e.g. sulphate, nitrate and ammonium aerosol) viaatmospheric chemistry. This range of behaviour isillustrated for some common emitted substances inTable 1.

The linkages between sources of pollutants andgreenhouse gases and their effects on receptors are

complex as illustrated in Figure 1. Although not illus-trated in Figure 1 the same source sectors give rise toCO2 and N2O affecting climate change.

Atmospheric composition is affected by virtually allhuman activities. Population increases, incomegrowth and the global liberalization of trade in goodsand services all stimulate an increase in energy andtransport demand. These are drivers of emissions ofsubstances into the atmosphere. Significant down-ward pressure on emissions has come from increasesin efficiency and/or from implementation of new orimproved technology. These drivers are common toboth air pollutants and greenhouse gases.

The increasing population on the planet contri -butes to the scale of activity but, of even greaterimportance, the continuing expansion of the globaleconomy has led to massive increases in productionand consumption, indirectly or directly causing emis-sions to the atmosphere. In addition, urban popula-tions have risen to include half of humanity(Kuylenstierna et al, 2007).

In most cases the same sectors and processesare responsible for releasing emissions of both airpollutants and greenhouse gases. The major sectorsare transport (road, rail, aircraft and shipping), electri-city and energy supply, major and small industrial sectors, household emissions and land use changeand practices. Figure 2a shows the emission of diffe-rent GHGs in 1990 and 2004 and Figure 2b the sec-toral breakdown of GHG emissions.

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Table 1.Contrasting characteristics of some common pollutants (source: AQEG, 2007).

Main Lifetime Potential effects

Pollutant anthropogenic in the AQ/health effects Acid deposition/ tropospheric Radiative Oxidising capacitysources atmosphere eutrophication Ozone acid forcing/climate of atmosphere

SO2 (→SO24–) Fossil fuel ~ days SO2 & SO2

4– aerosol Add deposition SO2

4– short-term

combustion cooling

NOx (NO + NO2) Stationary ~ days NO2 & NO3– aerosol Add deposition and ✓ NOx indirect effect ✓

(→ NO3–) combustion and eutrophication on CH4 and O3

transport NO3– short-term cooling

NH3 (→NH4+) Agriculture ~ days (NH4

+ aerosol) Add deposition and NH4+ short-termeutrophication cooling

N2O Soils, biomass > 100 years Warming

CO2 Combustion 50-200 years Warming

CH4 Fossil fuel, 12 years ✓ Warming ✓agriculture, landfills (adjustement time)

CO Traffic ~ 1 month Yes ✓ Indirect effect on ✓CH4 and O3

VOCs Fuel combustion, Varies by compound Some species ✓ Indirect effect on ✓solvents, traffic CH4 and O3

Primary particles Combustion, traffic ~ days Yes in combination Short-term warmingPM10/PM2.5 and grinding/dusty with secondary PM: and cooling

processes SO24–, NO3

–+,

organics, etc.


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Figure 1.Examples of some of the potential linkages between regional air quality and climate change issues (from AQEG 2007, after

Grennfelt et al. (1994)).

Sources

Energy

Farming

Industry

Traffic

Wastes

Compounds

SO2

NH3

NOx

VOC

CO

CH4

Effects

Acidification

Eutrophication

Fine particles

Regional O3

Trospopheric O3

Receptors

Groundwater

Lakes

Terrestrialecosystems and soils

Marine environment

Agricultural cropsand forests

Climate

Health

Figure 2a.GHG emissions by sector in 1990 and 2004 (Source: Barker et al., 2007).

Figure 2b.GHG emissions by sector in 2004 (Source: Barker et al., 2007).

Gt

CO

2–eq

.

14

12

10

8

6

4

2

0

F-gases

N2O

CH4

CO2

Energysupply

(1)

Energy supply (1) 25,9%

Transport(2)

Transport (2) 13,1%

Residential& commercialbuildings (3)

Residential and commercial buildings (3) 7,9%

Industry(4)

Industry (4) 19,4%

Agriculture(5)

Agriculture (5) 13,5%

LULUCF/Forestry

(6)

Forestry (6) 17,4%

Waste andwastewater

(7)

Waste and wastewater (7) 2,8%

1990-2004 1990-2004 1990-2004 1990-2004 1990-2004 1990-2004 1990-2004


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Figure 3.The relative importance for global emissions for selected pollutants (non-methane volatile organic compounds, black carbon,

sulphur dioxide and nitrogen oxides) of different sectors and fuel types for the year 2000.(Source redrawn from EDGAR data based on Olivier et al., 2005 and Bond et al. (2004).

Waste 2%

NMVOC BC

SO2NOx

Fossil fuel Large stationarycombustion 23%

Biomass Large

stationary2%

Fossil fuel Small

stationaryCombustion

1%Biomass

Small stationarycombustion

16%Road transport 20%

Non-road transport 3%

Industrialprocesses

16%

Biomass burning 17%

Fossil fuel Large stationary combustion

29%Fossil fuel

Large stationary combustion

64%Biomass

Large stationary

combustion1%

Biomass Large stationary combustion0%

Fossil fuel Small

stationarycombustion

2%

Fossil fuel Small stationarycombustion 5%

Biomass Small stationarycombustion 5%

Biomass Small stationarycombustion 2%

Road transport 22%

Road transport 2%



Industrialprocesses 5%

Industrialprocesses 19%Biomass burning 23%

Biomass burning 2%

Fossil fuel Large stationary combustion 3%

Biomass Large stationary combustion 2%

Fossil fuel Small stationary

combustion 15%

Biomass Small stationary

combustion22%

Road transport 14%


Biomass burning 39%

Figure 4.Relative development of income, population, energy and CO2 emissions, 1970-2004 (IPCC 2007b).

3,0

2,8

2,6

2,4

2,2

2,0

1,8

1,6

1,4

1,2

1,0

0,8

0,6

0,4

Index 1970 = 1Income (GDP-ppp)

Energy (TPES)CO2 emissionsIncome per capital(GDP-ppp/cap)

Population

Carbon intensity (CO2/TPES)Energy intensity(TPES/GDP-ppp)

Emission intensity(CO2/GDP-ppp)

1970 1975 1980 1985 1990 1995 2000


Figure 3 shows the importance of different sectorsand fuels for air pollutant emission, globally. Clearlythe main sources of GHGs and air pollutants are thesame: fossil fuel combustion in the energy sector andin transport, industrial processes, land use change/practices and other biomass burning.

1.1. Emissions from energy use

Energy use, especially from the use of fossil fuel,is a driver of most air pollutants and GHG emissions.This has been partly decoupled from the growth ofGDP (see Figure 4), due to increased efficiency inenergy and electricity production, improved produc-tion processes and a reduction in material intensity,but it should also be noted that the declining trends inenergy intensity seem to have flattened out during thelast years covered in Figure 4.

The global primary energy supply has increasedby 4 per cent/year between 1987 and 2004 (from IEA2007 in GEO Data Portal) and fossil fuel still supplyover 80 per cent of our energy needs (Figure 5). Thedeveloped world is still the main per capita user offossil fuel and the main emitter of greenhouse gases.However, in many developing countries the use oflong-lived, outdated and polluting technology exposesvulnerable communities to the adverse health effectscaused by air pollution and at the same time this inef-ficiency leads to high emissions of GHG per unit ofproduction.

Atmospheric emissions from large stationary sources in developed countries have been reducedby using cleaner fuels, end-of-pipe controls, relo -cating or shutting down high-emitting sources andpromoting more efficient energy use. In many develo-ping countries such measures have not been fullyimplemented, but if implemented have the potential torapidly reduce emissions. Industrial sources that useobsolete technology, lack emission controls and arenot subject to effective enforcement measures, contri-bute significantly to the emission load.

Among the factors that define the level of emis-sions from the energy sector are fuel quality, techno-logy, emission control measures, and operation andmaintenance practices. Energy efficiency improve-ments and energy conservation are given high priorityin the energy development strategies of many coun-tries, including developing countries. High efficiencyand clean technology will be crucial to achieve a low-emission development path for both GHGs andtraditional air pollutants, combined with security ofsupply.

1.2. Transport sector emissions

The high growth in passenger car sales revealsthat people put a high preference on car ownership asthey become more affluent. Atmospheric emissionsfrom the transport sector depend upon several factors, such as vehicle fleet size, age, technology,fuel quality, vehicle kilometres travelled and drivingmodes. Shifting from public transport systems to private car use increases congestion and atmo -spheric emissions. Poor urban land-use planning,which leads to high levels of urban sprawl (spreadingthe urban population over a larger area), results inmore car travel and higher energy consumption. Roadtransport in Europe (EU27) is responsible for 40% ofthe NOx, 18% of the total PM2.5 emission and 18% ofthe VOC emission (EEA 2004). At the same time roadtransport in the EU15 is responsible for 19% of thetotal GHG emission (EEA 2004).

Air transport is one the fastest rising transportmodes, with an 80 per cent increase in kilometresflown between 1990 and 2003 (UNSD 2007 in GEOData Portal). This dramatic increase was driven bygrowing affluence, more airports, the rise in low-costairlines and the promotion of overseas tourism.Economic efficiency is driving improvements inenergy efficiency, and new commercial aircraft areclaimed to use up to 20 per cent less fuel than thosesold 10 years ago (IATA 2007). The aviation industry

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Figure 5.Total Primary Energy supply by energy source (source IEA 2007 in GEO Data Portal).

20041987 Crude, NLG and feedstocks (crude oil)

Coal and coal products

Natural gas

Combustible renewables and waste

Nuclear

Hydro

Geothermal

Solar/Winf/Other

0 500 1 000 1 500 2 000 2 500 3 000 3 500 4 000 4 500Million tonnes oil equivalentNote: NLG = Natural liquiefield gas


gives rise to both air pollutants and GHGs. The prin-cipal aviation pollutants include NOx, SO2, CO, PMand VOCs, and the main GHGs are CO2, CH4 andO3. Aviation emissions account for around 3.5 percent of man’s contribution to global warming from fossil fuel use. By 2050, this percentage could grow tobetween 4 per cent and 15 per cent (IPCC, 1999). Inaddition, the impact of all aircraft emissions at altitudeis three times more damaging than CO2 emitted atground level.

Shipping has also grown remarkably over recentdecades, mirroring the increase in global trade. It hasrisen from 4 billion tonnes in 1990 to 7.1 billion tonnestotal goods loaded in 2005 (UNCTAD 2006).Improvements in the environmental performance ofthe shipping industry have been less pronouncedthan for air transport. The pollutant emissions fromshipping are regionally very significant especiallyclose to key shipping lanes and in ports across theworld. Globally shipping is responsible for 56% of sulphur emissions from the transport sector[Fugelstvedt et al, 2008]. The application of pollutioncontrol equipment in shipping has been lacking, causing high levels of emission. The cooling causedby SO2 emissions from ships probably more thancompensates for the warming caused by GHG emis-sions (Figure 6).

1.3. Industrial processes

Manufacturing processes can also cause directemissions, such as CO2, SO2 and particulate matterfrom steel and cement production, SO2 from copper,lead, nickel and zinc production, NOx from nitric acidproduction, CFCs from refrigeration and air condi -tioning, SF6 from electricity equipment use, and perfluorocarbons (PFCs) from the electronic industryand aluminium production. CFCs, SF6 and PFCs areimportant GHGs. The emissions depend mainly onthe fuel used and the pollutants or the technologyapplied. CFC are not Kyoto GHGs as they are coveredby the Montreal Protocol on stratospheric ozonedepletion but HFCs are covered by the IPCC (2006)guidelines.

1.4. Biomass and land use practices

In rural areas, customary land-use practices alsodrive atmospheric emissions. The clearance of forestedland, and its subsequent use for cattle and crop pro-duction, releases carbon stored in the trees and soils,and depletes its potential as a CO2 sink. It may alsoincrease methane, ammonia and nitrogen oxide emis-sions. Deforestation is known to have contributedabout 20 per cent to annual atmospheric emissions ofCO2 during the 1990s [IPCC 2007 WG1]. Normalagricultural land-use practices, such as burning cropresidues and other intentional fires, increase emis-sions of CO2, particulate matter and other pollutants.Wildfires and forest fires used for land clearance alsorelease very high levels of particulates.

Normal agricultural practice leads to emission ofvarious pollutants and greenhouse gases such asCO2, particulate matter, NH3, NOx, SOx, and VOCs.Black carbon emitted, for example, from vegetationburning contributes to global warming, but the organicor white carbon leads to surface cooling. An equiva-lent amount as the CO2 emitted would be absorbedduring the growth of crops in the following year.Therefore, this regular burning is mainly an air pollu-tant and aerosol issue and contributes to the "River ofSmoke" across southern Africa. Land use change isanother matter. Deforestation changing forests tograsslands leads to a large proportion of the CO2emitted annually to the atmosphere. It is also respon-sible for high levels of pollution in many areas in Asia,Africa and Latin America. Examples are the plumesfrom the burning of Amazonian rainforest across largeswathes of South America and the Indonesian forestfires leading to transboundary pollution by particulatematter in SE Asia, itself giving rise to significant healthimpacts many hundreds of kilometres away from thefires themselves.

1.5. Domestic sector

The domestic sector is responsible for much of thedemand for goods and services such as electricitythat drive up emissions of both GHGs and air pollu-tants. Practices in the home also give rise to emis-sions and indoor air pollution. This is a particular issuein poor households of Africa and Asia where biomassand/or coal are used for cooking, often using primitivemeans and with poor ventilation. This indoor air pollu-tion is particularly important for health impacts onwomen and children in each house but also can leadto significant levels of outdoor air pollution. The blackcarbon emitted from these sources is significant inpart of the world such as S Asia. Burning the biomassalso gives rise to organic carbon emissions whichhave a cooling effect and the net impact will bedependant on relative emissions and concentrationsin the atmosphere.

1.6. Increasing urbanisation

Emissions in densely populated areas tend to behigher due to the total level of emission-related acti-

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Figure 6.The net radiative forcing in 2000 from different transport

sub-sectors globally. The negative forcing from shipping is areflection of the sulphur emissions (Fuglestvedt et al 2008).

250

200

150

100

50

0

– 50

– 100

– 150

Net

forc

ing

(mW

/m2 )

Road Shipping Aviation Rail Rail indirect


vity, even though the per capita emissions are reducedby higher efficiency and shorter travel distances usingpersonal transport. In combination with low dispersionconditions, this results in exposure of large popula-tions to poor air quality. Urbanization, seen in suchforms as urban population growth in Latin America,Asia and Africa, and urban sprawl in North Americaand Europe, is continuing as a result of a combinationof social and economic drivers. Urban areas concen-trate energy demands for transport, heating, cooking,air conditioning, lighting and housing. Moreover, citiescreate heat islands that alter regional meteorologicalconditions and affect atmospheric chemistry and climate.

1.7. Technology development

Technological innovation, coupled with technologytransfer and deployment, is essential for reducingemissions. A broad portfolio of technologies is neces-sary, as no single technology will be adequate toachieve the desired level of emissions. Desul -phurization technologies, low nitrogen combustorsand end-of-pipe particulate capture devices areexamples of technologies that have contributed consi-derably to SO2, NOx and PM emission reduction.However, some technologies, such as flue gas desul-phurisation, while effectively reducing sulphur emis-sions by more than 90%, also lead to increased CO2emissions per unit electricity produced. A further factor to consider is that reduced sulphur emissionsalso result in a reduction in the cooling effect of sulphate aerosol formation. A number of technologiesmay play key roles in reducing GHG emissions. Theyinclude those for improved energy efficiency, rene -wable energy, integrated gasification combined cycle(IGCC), clean coal, nuclear power and carbonsequestration.

2. Greenhouse gases

and air pollutants in the atmosphere

The greatest direct human pressure on the climatesystem arises from the emission of greenhousegases, chief of which is CO2, mainly originating fromfossil fuel consumption. The unprecedented recentrise has resulted in a current level of 380 parts permillion, much higher than the pre-industrial (18th cen-tury) level of 280 ppm. There has also been a sharprise in the amount of methane, another major green-house gas, with an atmospheric level 150 per centabove that of the 19th century [Siegenthaler et al.,2005, Spahni et al., 2005]. Examination of ice coreshas revealed that levels of CO2 and methane are nowfar outside their ranges of natural variability over thepreceding 500 000 years [Siegenthaler et al., 2005].The potential negative impact of different substanceson the global climate is measured by its GlobalWarming Potential (GWP). These are defined as theratio of the time integrated radiative forcing from theinstantaneous release of 1 kg of a trace substance

relative to that of a reference gas normally CO2[Ramaswamy et al. 2001 quoted in AQEG 2007].Examples of GWP over a 100 year time horizon are 1for CO2 (by definition), 23 for methane and 296 fornitrous oxide (as quoted in AQEG 2007).

Once emitted, some atmospheric pollutants affectthe planet’s heat balance contributing to radiative forcing in direct and indirect ways [EEA 2004]. Theyinclude industrial gases, such as sulphur hexafluo-ride, hydrofluorocarbons and perfluorocarbons; seve-ral ozone-depleting gases that are regulated underthe Montreal Protocol; tropospheric ozone; nitrousoxide; particulates; and sulphur- and carbon-basedaerosols from burning fossil fuels and biomass.

Aerosols, small particles suspended in the air,play a substantial role in the radiation balance of theearth, mostly through scattering and absorption pro-cess [EEA 2004]. They produce brighter clouds thatare less efficient at releasing precipitation and lead tolarge reductions in the amount of solar irradiance reaching the Earth’s surface, a correspondingincrease in solar heating of the atmosphere, changesin the atmospheric temperature structure, suppres-sion of rainfall, and less efficient removal of pollutants.This has direct implications to availability and qualityof freshwater [Ramantahan et al., 2001]. Sulphateaerosols, formed from the oxidation of SO2 in theatmosphere, reflect radiation from the sun and giverise to a cooling effect, masking much of the currentwarming effect. Organic aerosols also cool but blackcarbon heats the atmosphere through absorption ofradiation [EEA 2004]. Combustion processes are thelargest source for black carbon [Penner et al., 1993 inEEA 2004] whilst organic carbon is the single mostimportant component of biomass burning aerosols[Andreae et al., 1988 in EEA 2004]. As elemental carbon aerosols (soot or "black carbon") contribute toglobal warming while also contributing to local air pollution, removing such pollutants will be beneficialboth with respect to climate change and healtheffects.

The aerosol direct radiative effect has beenapproximately quantified for sulphate aerosol, biomass burning aerosol, fossil-fuel organic and fossil-fuel BC aerosols, and mineral dust [Penner et al., 2001]. In addition, the radiative forcing due tonitrate aerosol has recently been quantified, as hasthe effect of BC deposition onto snow surfaces.However, considerable uncertainty still exists withregard to the magnitude (and even sometimes thesign) of the radiative forcing due to aerosols. IPCC in2007 reported that the anthropogenic contributions toaerosols together produced a net cooling effect(Figure 7) and observations and model calculationshave shown that the aerosols in the atmosphere aredelaying the global warming expected from theincrease in greenhouse gases.

Whilst most studies have characterised carbonfrom biomass burning as "black" or "organic", analysisof aerosols over Asia suggest that some particlesbelong to a category of "brown carbon" which makes

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a contribution to radiative forcing [Duncan et al., 2008]which is a ubiquitous and previously unidentified component of the organic aerosol which has recentlycome in to the forefront of atmospheric research[Lukács et al., 2007]. This highlights the uncertaintiesconcerning the aerosol impact on radiative forcing.This uncertainty is clear in the debate concerningsolid fuel use in developing countries, which is amajor source of carbonaceous aerosols and other airpollutants affecting climate.

The radiative forcing attributable to household fuelcombustion in Asia has been quantified by Aunan et al.(in press) in terms of current global annual meanradiative forcing and future global integrated radiativeforcing for a one-year pulse of emissions (2000) overtwo time horizons (100 and 20 yr). They find that theradiative forcing from the Kyoto gases, CO2 and CH4,is probably counteracted by co-emitted species.

Despite the significant emissions of black carbonaerosols (BC), short-lived (non-Kyoto) componentsfrom household fuel use overall seem to exert a netnegative radiative forcing because of the stronginfluence of reflective aerosols. Net radiative forcing(current and future integrated) from regional biomassburning is close to zero if the issue of land use changeand forestry is not accounted for, since the positiveand negative radiative forcing values essentially cancel each other out. For household fossil fuels thenet radiative forcing from short-lived species is nearzero, thus reduction of household coal burning willhave little short-term impact on global climate, but willmitigate global warming over time through CO2 andCH4 reduction (Aunan et al. in press).

Ozone formation is affected by the climate, espe-cially temperature, as well as irradience levels, both ofwhich are affected by global climate change. Ozone is

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Figure 7. Summary of the principal components of the radiative forcing of climate change (Source: Forster et al., 2007).

Radiative forcing terms

– 2 – 1 0 1 2

Radiative forcing (watts per square metre)

Hu

man

act

ivit

ies

Nat

ura

lp

roce

sses

Long-lived greenhousegases

Ozone

Totalaerosol

Stratospheric (– 0.05)

Halocarbons

CO2

CH4

Tropospheric

Black carbon on snow

0.01

Land use

Stratospheric water vapour

Surface albedo

Direct effect

Cloud albedoeffect

Linear contrails

Solar irradiance

Total net human activities

N2O


a potent pollutant affecting vegetation, crop yields andhuman health and it is also the third most importantgreenhouse gas (see Figure 7). Methane is an impor-tant compound for the formation of ozone and, ofcourse is the second most important greenhouse gas.Any reduction in ozone concentrations will result inreduced pollutant impact and warming.

Many air pollutants can be seen as short-lived"greenhouse gases" (or rather compounds) andcontrol of these compounds will therefore result in afaster response of the climate system than the controlof traditional greenhouse gases that tend to be long-lived (e.g. about 20% of CO2 remains in the atmo-sphere after a millennium [IPCC 2007; WGI].However, the net radiative effects of air pollutants arehard to estimate as they tend to vary greatly in spaceand time [EEA 2004].

The interrelations between pollutant gases andGHGs in the atmosphere tend to be quite complexwith, for example, an increase in NOx concentrationleading to a decreased lifetime of CH4 and HFCs (viaOH) and thus to reduced radiative forcing [EEA 2004].At the same time the increased NOx will lead toincreased ozone formation, leading to increasedradiative forcing. Increased NOx will lead to increasedN deposition fertilising vegetation growth and thusabsorbing more CO2 and sequestering more carbon,but once in the nitrogen cycle of the ecosystem, mayin some circumstances be remitted as N2O, a potentgreenhouse gas [EEA 2004].

The global system response to increased radiativeforcing will be experienced through changes in surface and atmospheric temperatures, changes inwinds and the global circulation, clouds and precipita-tion, ice cover and ocean currents. The changingweather patterns brought about by climate change willalter the chemical transformation and transport of pollutants in the atmosphere. This in turn will lead tochanges in the exposure of sensitive receptors suchas people, ecosystems, crops and man-made mate-rials. For example, it is highly unlikely that the frequency of wintertime pollution events in the UK willremain unchanged as a result of climate change, asthe likelihood of stable, calm and cold conditionsbecome less likely [AQEG, 2007]. It is also highly unli-kely that the frequency of summertime pollutionevents in Europe will also remain unchanged as aresult of climate change. The intense O3 episodes inthe UK during 1976 and in France and Switzerlandduring 2003 were brought on by sustained high temperature and drought conditions. Such conditionsfavour O3 production and minimise O3 destructionthrough the uptake by vegetation at the earth’s surface. These conditions are anticipated to becomemore frequent in some parts of the world as a resultof global warming. Climate change may well makelocal and regional O3 air quality goals become moredifficult to achieve in the future [Anderson et al.,quoted in AQEG 2007].

3. Interactions between climate change and air pollution on impacts

Crops and vegetation

Ozone (O3) is the main atmospheric pollutantaffecting crop yields globally [Emberson et al., 2003]and is also the third most important GHG [IPCC,2007]. Economic losses resulting from O3 inducedcrop yield losses have been estimated for differentregions around the globe: for the US, economic losses of $3 billion per year were estimated for 9 arablecrops [Adams et al., 1989]; in Europe estimates werein the order of $8 billion for 20 arable crops [Hollandet al., 2000] and in East Asia, losses of US$ 5 billionbased on 4 crops have been estimated [Wang &Mauzerall, 2004]. These studies emphasise the current-day scale of economic losses due to O3, astrong indication that efforts to control O3 precusorswill have immediate economic benefits through impro-ved crop productivity as well as reducing the role thatO3 plays as a GHG.

The fact that crop productivity is threatened fromboth air pollutant and climate change provides anopportunity to investigate potential co-benefits thatmay be realised from ozone concentration reductionsin relation to climatic changes and identify the complex interactions between these stresses in rela-tion to crop impacts. In an attempt to define the scopeof these interactions it is useful to categorise the pos-sible effects according to direct (resulting from contactwith the pollutant) and indirect (resulting from envi-ronmental change) effects. For example, elevatedCO2 and O3 directly affect crop productivity throughimpacts on photosynthesis; indirect impacts canmanifest themselves through shifts in nutrient cycling,crop–weed interactions, insect pest occurrence, andplant diseases [Fuhrer, 2003]. In terms of directeffects, some crops are expected to benefit from ele-vated CO2 through the "CO2 fertilization" effect andassociated gains in photosynthetic efficiency. Incontrast, elevated O3 will reduce photosynthesis. Incombination the situation is complicated by the factthat elevated CO2 leads to a reduction in stomatalconductance and hence gas exchange, resulting in areduced O3 dose and subsequent O3 damage.Evidence also suggests that many of the beneficialeffects of elevated CO2 alone may be lost in a warmerclimate due to factors such as accelerated plant deve-lopment, decreased water- and nitrogen- use effi-ciency. As such, it tends to be assumed thatagroecosystem responses to climate change will bedominated by those caused directly or indirectly byshifts in climate rather than elevated CO2 concentra-tions. Such changes in environmental systems willalso affect O3 influence both by altering physiologi-cally-determined O3 dose as well as the consequencesof such dose resulting in O3 damage. For example,O3 is a photo-chemical pollutant and as such tends tobe associated with hot dry conditions, conditions thatalso lead to reduced soil water and drought whichwould limit O3 uptake but also affect water use effi-

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ciency. O3 has also been shown to affect plant susceptibility to pests and diseases which will also bemediated by climate change.

As well as indirect and direct effects there are alsofeedback effects that are important to consider.A recent study by Sitch and others (2007) estimatedthe impact of projected changes in O3 levels on theland-carbon sink. The study found a significant sup-pression of the global terrestrial carbon sequestrationas increases in O3 concentrations decreased plantproductivity. In consequence, more CO2 accumulatesin the atmosphere suggesting that indirect radiativeforcing by O3 effects on plants could contribute moreto global warming than the direct radiative forcing dueto tropospheric O3 increases. A similar study wasconducted for China for grassland ecosystems [Renand others 2007] found that surface O3 (in combina-tion with climate and CO2) played a significant role inlimiting net primary productivity (NPP), and that animprovement in air quality could significantly increaseproductivity and carbon storage in China’s grasslandecosystems.

3.1. Health

Health is impacted by both climate change and airpollutants. The WHO has estimated that warming andprecipitation trends due to anthropogenic climatechange over the past 30 years already claim over150,000 lives annually. Many prevalent human diseasesare linked to climate fluctuations, from cardio-vascularmortality and respiratory illnesses due to heatwaves,to altered transmission of infectious diseases andmalnutrition due to crop failures [Patz et al., 2005].Climate change may affect exposures to air pollutantsby affecting weather, anthropogenic and biogenicemissions, and by changing the distribution of air-borne allergens [Bernard et al., 2001].

The most important pollutant affecting humanhealth is particulate matter (PM). Airborne PM is acomplex mixture of particles with components havingdiverse chemical and physical characteristics.Particles are generally classified by their aerodynamicdiameter since size is a critical determinant of the siteof deposition within the respiratory tract. PM10 areparticles less than 10 microns in aerodynamic diameter.They can further be divided into coarse particles (from2.5 to 10 μm), fine particles (PM2.5, less than 2.5 μm)and ultrafine (UF) particles (particles of diameter lessthan 0.1 μm). PM10 includes inhalable particles whichcan penetrate to the thoracic region. PM2.5 has highprobability of deposition in the airways and alveoli.Particles with different aerodynamic diameters differin their overall contributions to airborne particle massand in their origin, physical characteristics, chemicalcomposition, and health effects. WHO estimates thatoutdoor air pollution of particulate matter leads toapproximately 800,000 premature deaths per yearand indoor air pollution to about double that number.The economic cost of these health impacts is consi-derable. According to a World Bank report [WorldBank 2007] using conservative estimates, the econo-

mic burden of premature mortality and morbidityassociated with air pollution in China was 157.3 billionYuan (approx $23 billion) in 2003, or 1.16 percent ofGDP. This assumes that premature deaths are valuedusing the present value of per capita GDP over theremainder of the individual’s lifetime. If a prematuredeath is valued using a value reflecting people’s willingness to pay to avoid mortality risks, the damagesassociated with air pollution are 3.8 percent of GDP.In Europe, the EU has assessed that there would becost savings due to health benefits from alternativescenarios, outlining different ambition levels for thereduction of ozone and fine particles in 2020, rangingfrom €37 to €160 billion/year [AEAT, 2005].

The most important compounds in fine particulatematter (usefully characterised by PM less than2.5 μm) are sulphates and black carbon. Sulphatescool the atmosphere and black carbon warms it. FinePM gets deep into lungs and can cause pulmonaryand cardiovascular diseases, exacerbate asthmaattacks, increase workdays lost, and can cause pre-mature mortality of vulnerable people. There is greatpressure to reduce health impacts from particulatematter. This will imply a reduction in sulphate concen-trations and so weaken the cooling influence that is sofar reducing the full impact of global warming, thusproviding an additional impetus to reduce GHG emis-sions. For black carbon there is a double benefit interms of health improvement and reduced radiativeforcing.

Ozone also has important health impacts such asrespiratory symptoms, pulmonary function changes,increased airway responsiveness and airway inflam-mation, change of pulmonary defence mechanisms,disruption of the normal function of the airways andpulmonary immune system, increased daily mortality,independent of the effects of PM. Increased schooland work absenteeism, increased hospital or emer-gency room admissions for asthma, respiratory infec-tions, and exacerbation of chronic airway diseasesmay result. Epidemiological evidence of the chroniceffects of O3 is less conclusive.

3.2. Ecosystems

The main ecosystem impacts from traditional airpollution are related to biodiversity changes fromeutrophication by N deposition, acidification by S- andN- compounds (including ammonium) and impacts ofozone. There are also changes in net primary pro-ductivity of ecosystems by ozone concentrations andreductions in productivity and fish populations due tolake and stream acidification. Climate changes alsoaffect plant and animal distributions from changes intemperature, frosts, rainfall patterns and soil wateravailability.

It is likely that the impacts, particularly of nitrogenand climate change will interact in ways that will becomplex and difficult to predict. The increased nitrogen deposition enhances the growth-rates ofnitrogen-limited forests and this is thought to have led

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to increased uptake of CO2 in boreal forests[Schimel,1995 in AQEG 2007], but the long-termeffects of increased nitrogen deposition may be dele-terious, due to interactions with pathogens and pests,changes in soil nitrogen levels influencing mineraliza-tion and leaching of nitrogen from the soil, andrelease of soil carbon as CO2. There is therefore aneed for careful evaluation of the impact of enhancedN deposition on the strength of forests as a sink forCO2 [AQEG, 2007]. There is also a risk that theenhanced N deposition will lead to increased N2Oemission from soils, which is a very potent GHG[AQEG, 2007].

Regional acidification will also be influenced by climate change due to changes in weather patternsand intensity of rainfall [AQEG, 2007], due to changesin run-off and evaporation as well as N cycle changesin sensitive catchments and frequency of droughtevents.

Since CO2 dissolved in water gives rise to carbonicacid, increased CO2 levels will make the oceans moreacidic. The uptake of anthropogenic carbon since1750 has led an average decrease in pH of 0.1 units.Increasing atmospheric CO2 concentrations lead tofurther acidification. Projections based on SRES scenarios give a reduction in average global surfaceocean pH of between 0.14 and 0.35 units over the21st century. While the effects of observed ocean acidification on the marine biosphere are as yet undo-cumented, the progressive acidification of oceans isexpected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependentspecies [IPCC 2007: WGI SPM; WGII SPM].

3.3. Corrosion of materials

The action of sulphur dioxide, ozone and nitricacid on corrosion of materials such as steel and stonein cars, buildings and monuments is substantiallyaffected by temperature, rainfall and humidity andtherefore there is a clear interaction between air pollutant impacts and climate change on corrosion.

The EU Noah’s Ark project on global climatechange impact on built heritage and cultural land -scapes considers that climate change over the next100 years will likely have a range of direct and indirecteffects on the natural and material environment, inclu-ding the historic built environment. Important changeswill include alterations in temperature, precipitation,extreme climatic events, soil conditions, groundwaterand sea level. Some processes of building decay willbe accelerated or worsened by climate change, while

others will be delayed. For example, the likely reduc-tion in freeze-thaw cycles across much of Europe inthe future will lower the potential for frost shattering ofporous building stone. On the other hand, it is likelythat northern climates may have been so cold in pastcenturies that they experienced fewer freeze thawcycles, thus in the future freeze-thaw events may bemore frequent.

Pollution and climatic parameters have a directeffect on several materials independent of each otherbut it is also very common that pollutants and climateact together. In developing future policy for protectionof our cultural heritage, it is therefore important toconsider effects of pollution and global climatechange in a common framework.

Research to date has shown that the cost savingsof mitigation measures that reduce corrosion impactsare potentially very large and may compensate for aconsiderable proportion of abatement. For example,estimated annual cost savings associated with theimplementation of the Second Sulphur Protocol underthe Convention on Long-range Transboundary AirPollution for urban and rural areas in western andEastern Europe combined was in the order of 9,500Million USD [Cowell and Apsimon, 1996].

4. Methodological approaches

to quantify co-benefits

Various methodologies have been used to quan-tify the co-benefits of using policies, measures ortechnologies that reduce traditional air pollution andgreenhouse gases together. These essentially workout the reduced impacts from the implementation ofmeasures on emissions and quantify these in econo-mic terms to allow comparison between differentmeasures. This is a cost-benefit approach as outlinedin Figure 8. A cost effectiveness approach may alsobe used where it is advantageous to avoid costing theimpacts but where comparison of changes in theemissions and common impacts is enough. The mainimpacts investigated in these studies are related tohealth impacts by particulate matter (generally resul-ting in the largest economic benefits), crop yieldreductions by ozone (also resulting in high economiclosses) and to some extent the corrosion of materials.Ecosystem impacts are inherently difficult to assess,and assessment of economic consequences are par-ticularly difficult and have so far rarely been includedin a cost-benefit analyses.

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Figure 8.A flow chart of co-benefit estimates

Project,Plan,Policy

Emissions Concentration ExposureQuantitative

benefitsValuation

Monetarybenefits


The evaluation of the co-benefits of measures toreduce both GHGs and air pollutants requires aninterdisciplinary approach including emission estima-tion, technology assessment, dispersion modelling,impact estimation and economic evaluation.

Analyses may be carried out by Bottom-up (B-U)or Top-down (T-D) methods. B-U methods are oftenused to analyse specific projects on a fairly smallscale while T-D models focus on the overall macro-economic effect of measures and are suited to analyse options such as carbon taxes or non-pricepolicies (market reforms, information, capacity building).

Figure 9 illustrates steps in a B-U analysis. To cal-culate the net benefits using a bottom up analysis, itis necessary to know: i. how the measures affect theemissions ii. how the exposure of humans and theenvironment changes iii. changes in the effects due tochanged exposure (exposure-response or dose-response functions) iv. size of non-emission effects v. the economic values of the changes vi. costs of themeasure.

There are large uncertainties in all steps, espe-cially in exposure-response functions and monetiza-tion. Exposure-response functions for mortality andmorbidity of PM in air have been developed frommany studies, but there are few studies of long-termeffects in developing countries. Monetization of theseeffects is mostly based on willingness-to-pay (WTP)studies. The number of such studies in developingcountries is unfortunately very low. The human capitalmethod, which is popular in the Chinese research andpolicy community, equates a statistical life lost topotential earnings lost by the diseased. This approachresults in lower values for health damage than theWTP approach.

Despite the uncertainties, cost benefit analysesgive useful information. As an example of the workthat has been undertaken Aunan et al. (2004) made acost-benefit analysis of six measures that wouldreduce emissions of CO2 and particles in ShanxiProvince, China. Only reduced health damage due toimproved air quality was included as co-benefit. Threeof the measures were found to have negative costsdue to improved fuel economy. Using the best esti-mates of health benefits, all six measures becamehighly profitable in a socio-economic sense. Evenwith a low estimate of health effects, the net costswere negative or nearly zero.

5. Conclusions

Given that tropospheric ozone contributes toradiative forcing, crop yield losses, health and corro-sion impacts, as well as reduction in net primary productivity, reducing the uptake of CO2 by vegeta-tion, there is a clear argument that all measures toreduce ozone will be a benefit for the global climate,crop yields and human health.

Methane is an ozone precursor as well as thesecond most important GHG. Therefore, reducingmethane emissions will have a beneficial effect onradiative forcing and ozone formation, which wouldbecome apparent within a fairly short time scale giventhe relatively short residence time of methane in theatmosphere.

Reducing another ozone precursor, NOx, withoutsupplementary reductions in CO, CH4 and VOCs, willnot have such clear benefits as this will lead to reduced OH in the atmosphere which may exacerbatethe build-up in global CH4 concentrations.

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Figure 9.The steps required in a bottom up analysis of co-benefits.

Control options

Cost of Control options

Implementation

1 Chemical fate model

2 Exposure assessment

Effect Effect … … Effect1 2 n

3 Dose-response assessment

5 Valuation of effects

6

Non-emission effects

Weightingof costs

and benefits

4


There are considerable uncertainties concerningthe impact of aerosols on net radiative forcing, but theimpacts on human health are pressing. There is the-refore intense pressure to reduce particulate matterconcentrations, including sulphate, black and organiccarbon components. The reduction of sulphate fromhuman health demands (and indeed from demandsfor reduced ecosystem acidification and corrosion ofmaterials) will mean that there will be a reduction inthe cooling effect that these have afforded the earth,thus putting increased pressure on the need for aba-tement of the GHGs.

The reduction in black carbon from some sources(e.g. diesel vehicles) will result in reductions in bothradiative forcing and human health impacts. Becauseof the short atmospheric lifetime of BC, the impact ofemission reductions is more or less instantaneous.This is therefore a very important consideration in theevaluation of the impact of diesel versus petrol carusage, which cannot be based on CO2 emissionsalone, and a holistic full life cycle perspective for allradiative forcing components must be taken intoaccount when evaluating competing technologies[AQEG, 2007].

There is uncertainty on the net effect on radiativeforcing from the burning of biomass due to thebalance of emissions of black carbon (warming effect)and organic carbon (cooling effect) from these sources. However, the imperative to reduce healthimpacts, especially of the poor will entail pressure toreduce emissions from these sources.

6. Discussion Points

This background paper makes clear the extensiveand profound scientific interactions between air pollu-tants and climate change. However, for the purposesof considering the long-term development of co-benefitstrategies, there remain a number of important issueson which discussion in the first session could focus:

• Is there now an overall consensus on the scale ofthe contribution of air pollutants to climate change?Is it sufficient for long-term policy development?

• Is there a general scientific consensus on whichpollutants we need to include for co-benefit policy purposes – ozone, methane, black carbon, sulphate,nitrate and ammonium aerosols and others?

• What are the critical uncertainties that must beaddressed?

• What should future research priorities be? Whatareas are high priority for research? Life cycle ana -lysis for biofuels, etc? Integration of short-term andlong-term effects? Common metrics between air pollutants and greenhouse gases? Others?

• What are the implications of these priorities forshort-term and long-term strategies integrating pollu-tion and climate change?

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