Can warming particles enter global climate discussions?

Tami C Bond

Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

Received 17 April 2007
Accepted 14 August 2007
Published 21 December 2007

Contents

• 1. Introduction
• 2. Will carbon particles soon be addressed by local regulations?
  • 2.1. Major sectors
  • 2.2. Large sub-sectoral contributions
• 3. Do carbon particles have primarily local effects?
• 4. Are aerosols too complex to address in discussions about climate mitigation?
  • 4.1. Simple metrics
  • 4.2. Impacts of an emission increment
  • 4.3. Global warming potential and its relationship to simple metrics
• 5. Is our knowledge about aerosols too uncertain to quantify actions?
  • 5.1. Two-box model
  • 5.2. Conclusions despite uncertainty
  • 5.3. What missing knowledge is important?
• 6. Conclusion
• Acknowledgments
• References

1. Introduction

Activities that produce greenhouse gases also result in aerosols (suspended particles), ozone, and carbon monoxide. Although their atmospheric lifetimes are far shorter than those of greenhouse gases, these species alter the Earth's radiative balance, either directly or through atmospheric chemistry. Most aerosols, including sulfates and the light-colored organic carbon, offset and mask the warming effects of greenhouse gases by reflecting light. The dark aerosol known as black carbon absorbs light and warms the atmosphere. The perturbations caused by both greenhouse gases and aerosols are often quantified in terms of radiative forcing—the change in energy balance at the top of the troposphere. For some combustion sources, short-lived species contribute to or offset a portion of the radiative forcing. In this paper I focus on black carbon and its potential contribution.

There has been no shortage of scientific discussions on the role of aerosols in climate policy. West et al (1997) averred that climate policy could not ignore aerosol emissions. The `alternative scenario' of Hansen et al (2000) included both short-lived species and traditional greenhouse gases in a climate management portfolio. Holloway et al (2003) suggested that agreements focusing on transboundary pollution could address short-lived species. Rypdal et al (2005) outlined the scientific and political hurdles to be overcome for inclusion of such species in climate accords. Bond and Sun (2005) suggested that addressing black carbon could be an economical way to reduce climate impact for some sources. Hansen's counsel to consider black carbon and ozone in climate mitigation has been echoed by others, most recently the Scientific Expert Group on Climate Change (SEG 2007). Such actions are promising, not only because they could be economical, but also because they could reduce climate forcing rapidly, buying time to develop greenhouse-gas reductions.

The vast majority of organizations responsible for quantifying greenhouse-gas emission reductions and selecting mitigation projects are either unaware of the forcing contributions of other species or they disregard these contributions. There are practical reasons: no credit is given for such species under the present Kyoto Protocol, and thus no reward accrues for including the additional complexity. Ignoring a portion of radiative forcing, however, can result in misinformation about the net benefits of chosen actions. For example, some CO₂ mitigation measures also reduce cooling aerosols, and would reduce positive radiative forcing less than expected (e.g. Smith et al 2000a, Berntsen et al 2006). Smith et al (2000b) observed that minimizing traditional greenhouse gases (CO₂, methane, and nitrous oxide) by choosing renewably harvested biofuel instead of fossil fuels could actually increase total radiative forcing if the products of incomplete combustion were counted. `The policy implications', they wrote, `... are profound'. Jacobson (2002) reported a similar dissonance in choosing vehicle fuels: diesel vehicles produce less carbon dioxide, but may cause more positive radiative forcing if black carbon particles are counted.

In summary, scientists agree that aerosols and other short-lived species affect climate. Some go as far as to say that they are worth considering in climate mitigation strategies. However, in current practice for evaluating actions and making decisions, their effects are ignored. What causes this gap? In this paper I examine four conceptual barriers to considering and quantifying carbon particles in global agreements. These obstacles are often raised in policy-relevant discussions, but are not addressed in the scientific literature, which tends to focus on detailed aerosol physics and climate response.

2. Will carbon particles soon be addressed by local regulations?

If local regulations are the most effective way of addressing carbon particles, including those species in global agreements may be unwise. Here, I identify major emitters, and discuss likely reductions in the light of history or upcoming regulations. I rely on a global emission inventory in which emissions are estimated by combining fuel use, technology-specific emission factors, and technology differences between regions (Bond et al 2004). Data used here are updated from the earlier work with year-2000 energy data and the biofuel tabulation of Fernandes et al (2007). Global emission inventories are notoriously uncertain; knowledge including real-world emission factors, combustion types, and total fuel quantities in some sectors is lacking. The presentation should be taken as qualitative rather than definitive.

2.1. Major sectors

Figure 1(a) shows per-capita black carbon emissions from energy-related combustion (which excludes open vegetative burning) in 17 world regions. Figure 1(b) summarizes global emissions of both black and organic carbon, including the contribution of open burning. Colors indicate the economic sector producing the emissions: purple is electricity generation or industry, orange is transport, and blue is residential or commercial. There are a few notable differences among regions. Where coal is burned in the residential sector, it boosts per-capita total emissions. `Other' residential emissions are usually middle-distillate fuel burned either in generators or in boilers. High industrial emissions in East Asia include the contribution of coke-making for the steel industry. North America probably has higher emissions from transport because of the longer distances required to convey goods. The figures illustrate the following points:

Local control is typically invoked when pollutant concentrations are high enough to affect the health of the average population. Dispersed sources tend to receive this attention last. Only one of the major black carbon sources—on-road diesel emissions—has a long history of regulation. Rules for off-road diesel emissions are now on the horizon in the United States and Europe, but attention to this occurred much more slowly, and we may expect other countries to follow similar trajectories. Few policies addressing residential solid fuels are in place, although some countries have programs to ameliorate the important health effects of indoor cooking. Thus, some local regulations may reduce near-term radiative forcing by warming aerosols, but other major sectors remain largely untouched.

2.2. Large sub-sectoral contributions

Sectoral averages like those in figure 1 can bury the large contribution of small fractions. As an example, figure 2 shows the contribution of `superemitters'—poorly tuned vehicles—to total emissions. In one scenario, vehicles are relatively clean and superemitters comprise 5% of the population. In the other scenario, normal vehicles have higher emissions and poorly tuned vehicles form a larger fraction of the population, namely 20%. Superemitting vehicles might contribute nearly half of the vehicular emissions in either case, as is common to many locations (Zhang et al 1995).

Figure 3 demonstrates heterogeneity for four of the major emitting sectors, again drawing data from the year-2000 emission inventory depicted in figure 1. For clarity, only central values are shown here. Note that the substantial variability among apparently identical sources is not represented; for example, superemitters are identified separately, but differing levels of repair among `average' vehicles are not. A local inventory which identified more separate sources would result in smoother curves.

When sources within a sector are dissimilar, emissions could be either easier or harder to mitigate than those from a homogeneous sector. In a sector with little variation, cleaning up any source yields similar results, actions need not target specific emitters, and policies may be easier to formulate. Residential biofuel is an example of a relatively homogeneous sector. Most of the emissions come from sources with similar characteristics. Europe, where biofuel is burned in apartment building stokers as well as small stoves, is less homogeneous. Both residential and industrial coal sectors are heterogeneous. High-emitting fractions are composed of coal for individual households in the residential sector, and small boilers and coking coal in the industrial sector. Emissions from heterogeneous sectors might be attractive targets for mitigation if high emitters can be identified and addressed at a lower cost than other sources in the sector. However, they might be harder to target if high emitters evade detection.

Air-quality rules have certainly addressed some prevalent sources such as power plants and current-model vehicles, producing the high, relatively clean tails in figure 3. If the remaining sources are outside areas of high population density, urban management may miss them. There are also key differences between management of urban air quality and management of global or regional atmospheric health. In urban management, polluters are often penalized, discouraging high polluters from coming forward. Mechanisms that protect regional air quality (such as those addressing acid rain) or reduce climate impacts (such as the clean development mechanism) may place a value on emissions, providing incentives for polluters to identify themselves.

I set out to answer the question of whether local regulations would soon ameliorate the emissions that affect climate. At least three major sources—residential solid fuels, off-road diesel, and open vegetative burning—are not addressed in countries' first air-quality regulations. Important fractions of the other two sectors—industrial coal and on-road diesel vehicles—may be missed as well. The assumption that urban management will reliably address these sources in the near future is questionable at best.

3. Do carbon particles have primarily local effects?

The global spread of carbon particles has been modeled for over a decade (Haywood et al 1997, Penner et al 1998). Bergin et al (2005) review models and measurements that demonstrate long-range transport of aerosols. Koch et al (2007) found that 65–90% of industrial aerosol is transported beyond the boundaries of emitting regions. However, because aerosols have short atmospheric lifetimes, high concentrations of primary (directly emitted) species occur near sources. Contour maps showing such `hot spots' are sometimes used to demonstrate that carbon particles have only local effects. This is a misconception. High near-source concentrations do not imply that distant effects are negligible.

I first illustrate this concept using single-source groups in selected countries. These groups included biofuel emissions in Honduras and Tamil Nadu (India), and diesel emissions in Thailand. These groups were chosen to depict relatively small areas and to examine different geographic and meteorological conditions. I then used each emission distribution as an input to the Community Atmosphere Model (CAM), version 3, developed at the National Center for Atmospheric Research (Collins et al 2006) to infer the direct radiative effect of each source. The model has 26 vertical levels and a horizontal resolution of 1.9° × 2.5°.

Figure 4(a) shows annually averaged forcing for one of the single-source global model runs, while figure 4(b) summarizes all the simulations. Both black and organic carbon are included in the model; the high black carbon fraction in these emissions results in positive net forcing. As is typical for primary particles, forcing appears to be intensely concentrated around the emission region, in a hot spot covering about 0.1% of the Earth's surface.

Next, I sorted the grid boxes of the global model according to radiative forcing in each box. Their cumulative contribution to the global average is shown in figure 4(b). The long, high tail in the figure roughly corresponds to the hot spot in figure 4(a), and contributes less than 10% to the calculated forcing. The total contribution and maximum forcing differ seasonally, but the general trend is consistent. Most of the impact occurs over an area about 40 times greater than the apparent hot spot. The hot-spot figure is misleading: any source or urban area could be presented in this way, drawing attention to local high concentrations but ignoring most of the forcing. Primary particles do have local impacts, but those are a fraction of their contribution to global climate change.

4. Are aerosols too complex to address in discussions about climate mitigation?

4.1. Simple metrics

The possibility of quantifying how individual actions affect radiative forcing implies that this analysis could be accomplished by non-scientists. In fact, the complexity is daunting. Regional meteorology, particle microphysics, and chemical reactions contribute to uncertainties. How can practitioners without a detailed knowledge of aerosol science move forward in the face of this complexity?

I suggest that scientists can condense their hard-won understanding into tractable metrics, as shown in figure 5. The pathway along the bottom of the figure is similar to that presented by Fuglestvedt et al (2003). Here, I have added the simple metrics needed to express the relationships between calculation steps. These are emission factor (which is already widely used), lifetime, normalized forcing, and climate sensitivity. The idea of using such metrics is not new. Both the Intergovernmental Panel on Climate Change (Ramaswamy et al 2001) and Schulz et al (2006) compared model outputs using normalized forcing, while Textor et al (2006) compared aerosol lifetimes.

This is not a call to oversimplify the aerosol system. Aerosol lifetime is an easily understood concept, but complex simulations of transport and chemistry are required to calculate it, and intensive studies combining field measurements with targeted models are needed to confirm it. Simplified metrics encapsulate, not bypass, the results of complex studies.

4.2. Impacts of an emission increment

A comment on incremental versus total forcing is in order. Significant efforts have been expended to quantify total aerosol forcing, including several major field measurement programs and satellite missions. This effort greatly advances present-day understanding of the atmospheric chemical system. Incremental forcing may be thought of as , the change in forcing with respect to a change in emissions. If forcing is linear in emissions, this quantity is a constant.

For two critical policy-relevant questions, incremental forcing is more important than total aerosol forcing. The first question is the value of climate sensitivity, or temperature response to unit forcing. This sensitivity can be inferred by comparing modeled climate forcing with the temperature record over several decades (e.g. Andronova and Schlesinger 2001). Knowledge of total present-day aerosol forcing is insufficient for this comparison. Of greater importance is identifying how forcing has changed over time. The second question is what future climate one might expect, given an emission pathway. Observations or models of present-day aerosol forcing cannot provide this answer, either. Both climate sensitivity and future climate require the estimation of (1) emission time trends and (2) changes in forcing that correspond to these emission changes. Present-day forcing constrains these only indirectly.

Incremental forcing may also be used to answer the question: what changes in climate might be expected from any particular action? Such an appraisal could support decisions for mitigating climate effects. Assuming that aerosol physics, chemistry, and optics are known within desired uncertainty, the radiative effects of a single action can be estimated without knowing total aerosol forcing.

4.3. Global warming potential and its relationship to simple metrics

Under the Kyoto Protocol, actions are evaluated with the 100-year global warming potential (GWP), an incremental measure. The GWP is an accepted simple metric that can be used in policy discussions, and that incorporates some of the metrics outlined in figure 5. It is used to compare the radiative effects of different species by comparing the warming by some substance of interest with that of carbon dioxide:

where τ is the time frame of interest, a is the radiative forcing per mass, r is the atmospheric burden after a unit-mass pulse injection, and the subscripts s and CO₂ refer to the substance of interest and CO₂, respectively. The equation relates directly to the simple metrics in figure 5. Lifetime is another way of representing the time-dependent burden r(t), and a is normalized forcing.

Table 1 summarizes metrics that have been used to evaluate warming. While some differ from the definition in equation (1), they are all of similar magnitude. Values from Jacobson (2005) and Berntsen et al (2006) are lower than the other two by factors of three to four. The simple metrics outlined in section 3 can provide insight into such differences. For example, Bond and Sun (2005) showed that normalized forcing calculated by Jacobson (2002) was half that of other studies. Berntsen et al (2006) used lower absorption and normalized forcing, because they ignored black carbon mixing with other species.

The global warming potential is not perfect, and has been scrutinized extensively elsewhere (e.g. Harvey 1993, Smith and Wigley 2000, Fuglestvedt et al 2003). It is controversial for short-lived species, for which the temperature response to a pulse change differs from the response to a step change (Shine et al 2005). Prather (2002) showed that atmospheric burdens normalized to total emission were identical for either an emission pulse or sustained emissions. (This fact probably explains the similarity between the Bond and Sun (2005) and Hansen et al (2007) values.) Even if a measure other than the global warming potential becomes accepted, lifetime and normalized forcing will greatly affect it, and will probably be used to as direct inputs to the calculation.

In summary, a few scientists are estimating global warming potentials, and even more use lifetime and normalized forcing for model comparisons. The latter measures can also communicate the implications of improved understanding of aerosol science. For example, Stier et al (2006) showed that interactions between different aerosol species affect aerosol lifetime. The relevant contribution of aerosols, then, is not too complicated to communicate using simple metrics.

5. Is our knowledge about aerosols too uncertain to quantify actions?

Even if simple metrics can be accepted, many gaps in our understanding of aerosols remain. Uncertainty in the physics, chemistry, and optics of these particles is often given as a reason to ignore their climatic impact. Here, I employ aerosol lifetime and normalized forcing from three-dimensional model studies to constrain a simple model, and then use that model to examine these uncertainties.

5.1. Two-box model

I developed a simple, time-dependent model with two boxes: a `surface' box into which emissions are injected and an `aloft' box. I chose these two boxes because models show that forcing by an absorbing aerosol is quite sensitive to the amount of aerosol above the clouds. Table 2 summarizes the inputs to this model, their uncertainties, and the source of data.

The amount of black and organic carbon from burning 1 kg of different fuels was released into the surface box. The total removal rate from the system was set using aerosol lifetimes from AEROCOM, a recent aerosol model intercomparison effort (Textor et al 2006). Model variability is not a complete measure of uncertainty, because many models have similar parameterizations. I increased the uncertainty in overall lifetime above that reported by AEROCOM using two sources: my own runs of CAM in which wet removal parameters were varied, and results from similar published sensitivity studies (Koch 2000, Cooke et al 2002). I estimated the difference between the total removal rate and the removal rate in the surface box by comparison with a `normal' CAM run and a run in which convection was turned off, preventing aerosol from being transported aloft. Lifetime near the surface was about 1 day shorter than the lifetime in the entire system. Finally, removal rates from the box aloft were set so that the fraction of mass aloft corresponded to that from AEROCOM models.

Optical properties (scattering and absorption) per mass include the effects of microphysical parameters such as particle size. The treatment of scattering by organic carbon includes increases due to hygroscopic growth. Clear-sky forcing is calculated using the one-dimensional forcing equation given by Chýlek and Wong (1995), and the same relationship is used to determine forcing by aerosol above clouds. The average normalized forcing for uncoated black carbon predicted by the two-box model (1100 W g^–1) is similar to, but slightly lower than, models summarized by Ramaswamy et al (2001, 1200 W g^–1). I integrated forcing over the atmospheric lifetime of the particles to determine the change in energy balance resulting from each packet of emissions. This integrated forcing corresponds to the numerator in figure 1.

5.2. Conclusions despite uncertainty

Three-dimensional models are more accurate than this simple two-box model, assuming that their chemistry and physics are correct. However, the simpler model can be used in Monte Carlo simulations to examine the effect of uncertainties. Each model parameter was sampled from a normal distribution, except for the emission factor which was sampled from a lognormal distribution. These simulations address the question `what is the uncertainty in the climate benefit that would result from various actions?'. I examine this question for three of the major sources that appear in figure 1.

Figure 6 shows the probability distribution of integrated forcing for 250 simulations from each of three sources. The greatest forcing by far results from diesel superemitters, because the emission factor is the highest. Biofuel emissions have a larger fraction of organic carbon, which is more likely to offset the positive forcing by black carbon; that fact explains the higher peak near zero. Almost all of the cases show positive climate warming, even though the contribution of co-emitted organic carbon is considered. The probability of incurring physical regrets—that is, of changing climate in the wrong direction—is nearly zero (about 3% for biofuel and less for the other sources).

Forcing by carbon dioxide from 1 kg of the same fuel, integrated over 20 and 100 years, is shown on each panel. Thus, it is quite certain that carbonaceous aerosol forcing is a substantial fraction of CO₂ forcing for these sources. For biofuel emissions, aerosol forcing is greater than 20-year CO₂ forcing in 50% of the runs, and greater than 100-year CO₂ forcing in 15%. Aerosols emitted from modern diesel have more forcing than CO₂ over 20 years in 10% of the cases, while superemitter emissions are greater than 20-year CO₂ forcing 90% of the time. Aerosol forcing exceeds 25% of the 100-year CO₂ forcing in 20%, 90% and 55% of the cases for modern diesels, superemitting diesels and biofuel for cooking, respectively. Despite the uncertainty, that fraction is large enough that ignoring it would be short-sighted.

5.3. What missing knowledge is important?

Placing a value on the climatic effects of individual actions is one effort that could be hampered by the amount of uncertainty. An organization that supports actions to reduce climatic impact may be uncomfortable with such large uncertainty in the net benefit, and hence in the benefit-to-cost ratio.

Much of the uncertainty in figure 6 is caused by lack of knowledge about emission totals and black carbon fractions. High emission factors, allowed by the lognormal random sampling, cause much of the high tail in each figure. When the simulations were re-run without variability in emission quantities and properties, the coefficient of variation (standard deviation divided by the mean) was reduced from above 100% to 40–50%.

Of the model variability not attributable to emission uncertainties, about 60% of the squared variation is due to lifetime and 40% is due to normalized forcing. Aerosol vertical location in addition to optical properties affects the latter. Thus, a large fraction of the uncertainty in forcing results from aerosol transport and removal processes, which should receive at least as much study as aerosol optical properties.

Two important physical processes are not included in the two-box model. The first is the role of aerosol in brightening clouds (the `indirect' effect). Hansen et al (2007) suggest that this cooling reduces warming by carbonaceous aerosols, but does not eliminate it. The second effect is changes in snow albedo through deposition of absorbing aerosol. Flanner et al (2007) estimated that this snow forcing could increase forcing by 10–30%. Regional variability in transport and removal and surface albedo is not included here, either. These factors could be addressed in future work. Nonlinearities due to interactions between different aerosol species (Stier et al 2006) could also be incorporated.

6. Conclusion

Aerosols have acknowledged effects on climate, and reductions could contribute to mitigating those effects, especially for near-term climate change. However, aerosol forcing is ignored when evaluating actions and when making decisions about promising actions. Although warming aerosols are part of the global scientific picture, they are not yet considered in conventional portfolios for action to reduce climatic impact. Here, I have examined four barriers to such considerations:

Because of the uncertainties, more work remains for aerosol scientists. Emission characterization can reduce uncertainty in total forcing from individual actions to levels that may be acceptable, about 50%. Additional confidence requires a better understanding of atmospheric transport and removal processes as well as aerosol optics. Despite the uncertainties, some of the major objections to considering aerosols under hemispheric or global agreements are illusory, and vanish when examined closely.

Acknowledgments

I thank the Climate Office of the United States Environmental Protection Agency (purchase order 4W-3384-NAEX) and the National Aeronautics and Space Administration (grant NNG04GL91G) for support. This paper has not been subjected to EPA's required peer and policy review and therefore does not necessarily reflect the views of the Agency. No official endorsement should be inferred.

References

Andronova N G and Schlesinger M G 2001 Objective estimation of the probability density function for climate sensitivity J. Geophys. Res. 106 22605–11 
CrossRef

Bates T S et al 2006 Aerosol direct radiative effects over the northwest Atlantic, northwest Pacific, and North Indian Oceans: estimates based on in situ chemical and optical measurements and chemical transport modeling Atmos. Chem. Phys. 6 1657–732 
CrossRef

Bergin M S, West J J, Keating T J and Russell A G 2005 Regional atmospheric pollution and transboundary air quality management Annu. Rev. Environ. Resour. 30 1–37 
CrossRef

Berntsen T, Fuglestvedt J, Myhre G, Stordal F and Berglen T F 2006 Abatement of greenhouse gases: does location matter? J. Clim. 74 377–411 

Bond T C and Bergstrom R W 2006 Light absorption by carbonaceous particles: an investigative review Aerosol Sci. Technol. 40 27–67 
CrossRef

Bond T C, Habib G and Bergstrom R W 2006 Limitations in the enhancement of visible light absorption due to mixing state J. Geophys. Res. 111 D20211 
CrossRef

Bond T C, Streets D G, Yarber K F, Nelson S M, Woo J-H and Klimont Z 2004 A technology-based global inventory of black and organic carbon emissions from combustion J. Geophys. Res. 109 D14203 
CrossRef

Bond T C and Sun H 2005 Can reducing black carbon emissions counteract global warming? Environ. Sci. Technol. 39 5921–6 
CrossRefPubMed

Chýlek P and Wong J 1995 Effect of absorbing aerosols on global radiation budget Geophys. Res. Lett. 22 929–31 
CrossRef

Collins W D, Rasch P J, Boville B A, Hack J J, McCaa J R, Williamson D L and Briegleb B P 2006 The formulation and atmospheric simulation of the Community Atmosphere Model Version 3 (CAM3) J. Clim. 19 2144–61 
CrossRef

Cooke W F, Ramaswamy V and Kasibhatla P 2002 A general circulation model study of the global carbonaceous aerosol distribution J. Geophys. Res. 107 4279 
CrossRef

Fernandes S M, Trautmann N M, Streets D G, Roden C A and Bond T C 2007 Global biofuel use, 1850–2000 Glob. Biogeochem. Cyc. 21 GB2019 
CrossRef

Flanner M G, Zender C S, Randerson J T and Rasch P J 2007 Present-day climate forcing and response from black carbon in show J. Geophys. Res. 112 D11202 
CrossRef

Fuglestvedt J S, Berntsen T K, Godal O, Sausen R, Shine K P and Skodvin T 2003 Metrics of climate change: assessing radiative forcing and emission indices J. Clim. 58 267–331 

Hansen J, Sato M, Kharecha P, Russell G, Lea D W and Siddall M 2007 Climate change and trace gases Phil. Trans. R. Soc. Lond. A 365 1925–54 
CrossRef

Hansen J E, Sato M, Ruedy R, Lacis A and Oinas V 2000 Global warming in the twenty-first century: an alternative scenario Proc. Natl Acad. Sci. 97 9875–80 
CrossRefPubMed

Harvey L D D 1993 A guide to global warming potentials (GWPs) Energy Policy 21 24–34 
CrossRef

Haywood J M, Roberts D L, Slingo A, Edwards J M and Shine K P 1997 General circulation model calculations of the direct radiative forcing by anthropogenic sulfate and fossil-fuel soot aerosol J. Clim. 10 1562–77 
CrossRef

Holloway T, Fiore A and Hastings M G 2003 Intercontinental transport of air pollution: will emerging science lead to a new hemispheric treaty? Environ. Sci. Technol. 37 4535–42 
CrossRefPubMed

Jacobson M Z 2002 Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming J. Geophys. Res. 107 4410 
CrossRef

Jacobson M Z 2005 Correction to `control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming' J. Geophys. Res. 110 D14105 
CrossRef

Koch D 2000 Transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM J. Geophys. Res. 106 20311–32 
CrossRef

Koch D, Bond T C, Streets D, Unger N and van der Werf G R 2007 Global impacts of aerosols from particular source regions and sectors J. Geophys. Res. 112 D02205 
CrossRef

Penner J E, Chuang C C and Grant K 1998 Climate forcing by carbonaceous and sulfate aerosols Clim. Dyn. 14 839–51 
CrossRef

Prather M 2002 Lifetimes of atmospheric species: integrating environmental impacts Geophys. Res. Lett. 29 2063 
CrossRef

Ramaswamy V et al 2001 Radiative forcing of climate change Climate Change 2001: The Scientific Basis Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press) chapter 6 

Rypdal K, Berntsen T, Fuglestvedt J S, Aunan K, Torvanger A, Stordal F, Pacyna J M and Nygaard L P 2005 Tropospheric ozone and aerosols in climate agreements: scientific and political challenges Environ. Sci. Policy 8 29–43 
CrossRef

Scientific Expert Group on Climate Change (SEG) 2007 Confronting Climate Change: Avoiding the Unmanageable and Managing the Unavoidable ed R Bierbaum, J P Holdren, M MacCracken, R H Moss and P H Raven (Research Triangle Park, NC: Sigma Xi) (Washington, DC: United Nations Foundation) p 144 (Prepared for the United Nations Commission on Sustainable Development) 

Schnaiter M, Horvath H, Möhler O, Naumann K-H, Saathoff H and Schöck O 2003 UV–VIS–NIR spectral optical properties of soot and soot-containing aerosols J. Aerosol Sci. 34 1421–44 
CrossRef

Schulz M et al 2006 Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations Atmos. Chem. Phys. Disc. 6 5225–46 
CrossRef

Shine K P, Fuglestvedt J S, Hailemariam K and Stuber N 2005 Alternatives to the global warming potential for comparing climate impacts of emissions of greenhouse gases Clim. Change 68 281–302 
CrossRef

Smith K R, Uma R, Kishore V V N, Zhang J, Joshi V and Khalil M A K 2000a Greenhouse implications of household stoves: an analysis for India Annu. Rev. Energy Environ. 25 741–63 
CrossRef

Smith S J and Wigley T M L 2000 Global warming potentials: 1. Climatic implications of emissions reductions Clim. Change 44 445–57 
CrossRef

Smith S J, Wigley T M L and Edmonds J 2000c A new route toward limiting climate change? Science 290 1109–10 
CrossRefPubMed

Smith S J, Wigley T M L, Nakićenović N and Raper S C B 2000b Climate implications of greenhouse gas emissions scenarios Technol. Forecast Soc. Change 65 195–204 
CrossRef

Stier P, Feichter J, Kloster S, Vignati E and Wilson J 2006 Emission-induced nonlinearities in the global aerosol system: results from the ECHAM4-HAM aerosol-climate model J. Clim. 19 3845–62 
CrossRef

Sun H, Biedermann L and Bond T C 2007 Color of brown carbon: a model for ultraviolet and visible light absorption by organic carbon aerosol Geophys. Res. Lett. 34 L17813 
CrossRef

Textor C et al 2006 Analysis and quantification of the diversities of aerosol life cycles within AeroCom Atmos. Chem. Phys. 6 1777–813 
CrossRef

West J J, Hope C and Lane S N 1997 Climate change and energy policy: the impacts and implications of aerosols Energy Policy 25 923–39 
CrossRef

Zhang Y, Stedman D H, Bishop G A, Guenther P L and Beaton S P 1995 Worldwide on-road vehicle exhaust emissions study by remote sensing Environ. Sci. Technol. 29 2286–94 
CrossRef