Short-Lived Climate Forcers

Transcription

1 em feature by Erika Sasser and Linda Chappell, U.S. Environmental Protection Agency How Air Pollution Affects Climate: Short-Lived Climate Forcers While long-lived greenhouse gases particularly carbon dioxide (CO 2 ) continue to occupy center stage in international climate negotiations, another group of pollutants has been gaining attention among scientists and policy-makers: the so-called short-lived climate forcers, including black carbon and ozone. These pollutants have been regulated in the United States for decades due to their impacts on air quality, but their impacts on climate have only recently become a hot topic. Why so much interest? The answer relates to the specific characteristics of these pollutants and their behavior in the atmosphere. Unlike CO2 and other long-lived climate gases, which remain in the earth s atmosphere and continue to affect climate for hundreds or even thousands of years after they are emitted, short-lived climate forcers remain in the atmosphere for much shorter periods sometimes as little as a few days. As a result, emissions controls affecting short-lived forcers have the potential to provide climate benefits sooner. The potential for reductions in these pollutants to produce near-term climate benefits has excited widespread interest, especially as international action on the long-lived greenhouse gases (GHGs) has proven elusive. This article explores some of the key characteristics of short-lived climate forcers, including their relatively short life spans and their regional nature. It describes some of the challenges inherent in managing them as both climate pollutants and air pollutants, including the need to be more careful in designing mitigation strategies. Finally, it describes the latest approaches for estimating the value to society of reductions in climate pollutants, based on avoided climate damages. What Are Short-Lived Climate Forcers? Any pollutant that affects the earth s energy balance, including GHGs and other gases and particles, is a climate forcing pollutant. The earth s energy balance is determined by the amount of incoming solar radiation versus the amount of exiting infrared radiation from the earth s surface. Pollutants can affect both incoming and outgoing radiation: they can absorb incoming light (e.g., black carbon), they can scatter or reflect incoming light (e.g., sulfates), and/or they can trap outgoing infrared radiation (e.g., all GHGs). The term radiative forcing is used to refer to the impact of a pollutant on the balance between incoming and outgoing radiation. Radiative forcing is typically used to express changes relative to 1750 (preindustrial) conditions and is measured in watts per square meter (W/m 2 ). There is a direct relationship between radiative forcing and global temperature: positive radiative forcing is associated with warming, whereas negative radiative forcing leads to cooling. 8 em april 2011 awma.org

2 A short-lived climate forcer (SLCF) is simply a pollutant that influences radiative forcing, but has a significantly shorter life span than long-lived GHGs. Thus, the term is a relative one. The lifetime of pollutants within the short-lived basket varies from hours to decades. Typically, the term includes black carbon and ozone, which remain in the atmosphere for hours to weeks; other pollutants, such as hydrofluorocarbons (HFCs) and methane, which have intermediate atmospheric lifetimes of a decade or more, can also be considered short-lived relative to long-lived GHGs like CO2, sulfur hexafluoride (SF6) or halogenated compounds (e.g., perfluorocarbons, or PFCs), which remain in the atmosphere for centuries or millennia. In general, all atmospheric particles affect climate. This includes both direct effects (i.e., absorption/ reflection) and indirect effects (i.e., impacts on clouds and precipitation). The direct effects of different types of particles depend on their size, chemical composition, and location. Black carbon, for example, strongly absorbs light. In addition, when deposited on ice and snow, black carbon darkens the underlying surface, reducing reflection of light back to space. For this reason, black carbon over snow and ice is thought to exert particularly strong direct radiative forcing at the local/regional scale. Other types of particles scatter light and are associated with negative direct radiative forcing (i.e., cooling). This includes sulfates and nitrates, which are formed in the atmosphere following the emission of gases, and directly emitted organic carbon particles. The indirect effects of particles on climate include impacts on cloud formation and lifetime, and precipitation patterns. These indirect effects are generally understood to be cooling. While emerging evidence suggests that the direct (i.e., warming) effects of black carbon likely predominate, there remains significant scientific uncertainty around this issue. Furthermore, since black carbon is generally co-emitted with cooling pollutants such as sulfate, nitrate, and/or organic carbon, it can be quite challenging to disentangle the net impact of emissions from a particular source. Figure 1 illustrates currently estimated radiative forcing for ozone, methane, black carbon, and other aerosols, as compared to CO2. The uncertainty bands in the figure illustrate the range of estimates regarding the magnitude of radiative forcing and the current lack of consensus within this range. A great deal of work, including several of the large-scale assessments discussed in this issue of EM, is currently directed at providing more precise estimates of radiative forcing for these pollutants. In some cases, the pollutants may Key Climate Forcers Black Carbon (BC) describes very small, carbon-based particles, ranging in size from 0.1 to 1.0 micrometers, emitted as a result of incomplete combustion of fossil fuels, biofuels, and biomass. BC is very effective at absorbing solar radiation. The atmospheric lifetime of BC ranges from days to weeks; it can have longer-term impacts if deposited on snow and ice, where it affects the earth s albedo, or reflectivity. Ozone is a gas composed of three oxygen atoms. It is formed throughout the lower part of the earth s atmosphere through a series of chemical reactions involving sunlight and precursor pollutants, including volatile organic compounds (VOCs), nitrogen oxides (NOx), methane (CH4), and carbon monoxide (CO). Ozone lifetime varies based on location and atmospheric conditions, but generally ranges from hours to weeks. It is the third most important GHG after CO2 and CH4. Methane (CH4) is a gas emitted from a variety of anthropogenic and natural sources, including fossil fuel production, agriculture, wetlands, biomass burning, and waste management. Methane is both a GHG and a precursor to ozone. It has an atmospheric lifetime of about 10 years. awma.org april 2011 em 9

3 Global Average Radiative Forcing in 2005 compared to 1750 (W/m2) Carbon Dioxide Methane Tropospheric Ozone Black Carbon Figure 1. Net radiative forcing of short-lived climate forcers as compared to CO 2. Notes: Colored bars indicate the best estimate; gray lines show 90% confidence interval, reflecting uncertainty. Source: Forster, P.; Ramaswamy, V.; Artaxo, P.; Berntsen, T.; Betts, R.; Fahey, D.W.; Haywood, J.; Lean, J.; Lowe, D.C.; Myhre, G.; Nganga, J.; Prinn, R.; Raga, G.; Schulz, M.; Van Dorland, R. Changes in Atmospheric Constituents and in Radiative Forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller, Eds.; Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA, Black Carbon on Snow Organic Carbon Sulfates Nitrates Indirect Effects (All Aerosols) exhibit both warming and cooling influences (e.g., direct vs. indirect effects of black carbon), or may have precursors that produce a net cooling impact (e.g., nitrogen oxides for ozone). Both black carbon and ozone are involved in complex atmospheric chemistry affecting formation, processing, transport, and environmental fate. For example, emissions mixtures containing black carbon particles are rapidly processed in the atmosphere, changing both the chemistry of the mixture and its climate-relevant properties. Similarly, ozone chemistry involves a constant series of reactions among a range of precursors including methane, which is a GHG in its own right. Ozone is constantly being formed and destroyed, and net impacts on climate are highly dependent on which precursor species nitrogen oxides (NOx), methane, carbon monoxide, and non-methane volatile organic compounds (VOCs) are present, and in what amounts. This complex chemistry is challenging to capture in computer models, and can lead to significant temporal and spatial variability in SLCF concentrations. One result of the variability in SLCF concentrations in time and space is a more regional pattern of climate impacts, particularly in the case of black carbon. Whereas long-lived GHGs like CO2 are well-mixed in the atmosphere and exhibit relatively homogenous concentrations globally, black carbon is more regionally concentrated, with significant variation among regions. This means that black carbon s impact on the climate system both through absorption of light and through impacts on clouds and precipitation comes via more concentrated regional pulses. This provides challenges for measuring the net impact of black carbon on climate at the global level. Another important feature of SLCFs is their ability to influence climate via mechanisms besides just temperature. Ozone, for example, damages vegetation and restricts plant growth, reducing primary productivity. 1 This leads to significant impacts on crops and ecosystems, and may limit CO2 sequestration. 2 Methane, by contributing to ozone formation, is indirectly linked to these impacts. Black carbon is associated with a whole array of climate impacts. Studies have specifically linked black carbon to the melting of ice and snow 3-5 and glacial retreat, 6 to surface dimming, 7 and to the formation of Atmospheric Brown Clouds, disruption of the Asian monsoon, and altered precipitation patterns. 8,9 The impacts on snow and ice provide a significant climate feedback: as these reflective surfaces shrink, the amount of incoming solar radiation scattered back to space decreases and more energy is absorbed into the earth s system. 10 em april 2011 awma.org

4 Bounding the Role of Black Carbon in the Climate System A Summary Assessment by Tami C. Bond, University of Illinois Will reducing black carbon emissions affect climate change in the right direction? This question has been the focus of an assessment called, Bounding the Role of Black Carbon in the Climate System, conducted by about 30 scientists over the last two years. The watchwords of Bounding authors became comprehensive, quantitative, and diagnostic. Comprehensive meant considering all possible effects, so in addition to direct forcing (i.e., interaction between particles and sunlight), the paper also evaluates snow deposition and interactions with liquid and ice clouds. To be quantitative, we sought to first identify the total impact of black carbon, along with uncertainties, and then translate it into an impact per emission: a dose-response curve for the atmosphere. The goal of comprehensiveness also required summarizing all studies on each topic, rather than selecting just one or two. These studies disagree, but cataloguing a wide range of estimates is insufficient. Explaining the sources of disagreement the diagnostic treatment is important for gaining confidence in our current understanding and for prioritizing scientific inquiry. The evidence indicates high confidence that black carbon has positive climate forcing, even when cloud changes are considered. However, the highest published estimates of direct forcing, near 1 W/m 2, are unlikely. Models generally predict black carbon concentrations well, except in parts of Asia and South America. Clearly, the issues of air quality and climate change are intricately linked. Air quality managers control sources, not species. The major sources of atmospheric black carbon are diesel engines, residential solid fuels, small industry, and open biomass burning. To the best of our knowledge, power plants play very little role. None of these sources emits black carbon in isolation. Co-emitted organic carbon and sulfur also affect solar radiation and clouds, resulting in negative rather than positive forcing. Emitted gases affect the budgets of tropospheric ozone and methane. To be comprehensive with regard to species, we estimated the total climate forcing by sources that have large black carbon emissions, known as BC-rich source categories. Direct forcing by most BC-rich sources appears positive, even considering the co-emitted aerosols, so control of most BC-rich sources is highly likely to reduce direct positive forcing. Exceptions are sources with high sulfur emissions, like small coal-fired boilers. There remains an important risk that cloud responses and other changes will counteract the direct-forcing benefit, a risk that is greater for sources with larger organic carbon or sulfate emissions. For sources with large emissions of organic matter residential solid fuel and open biomass burning cloud changes appear to overwhelm positive direct forcing. These sources produce about two-thirds of black carbon emissions. High-NO x sources, like diesel engines, also produce negative forcing by co-emissions from nitrate formation immediately after emission, and from reduced methane in the decades following emission. Therefore, modeled climate benefits of reducing BC-rich sources depend critically on how the model represents cloud effects and, to a lesser extent, NO x chemistry. em The Links between SLCFs and Air Quality Ozone and black carbon (as part of fine particulate matter, or PM2.5) are also conventional air pollutants. These pollutants are associated with an array of adverse health and environmental effects, and are subject to national ambient air quality standards in the United States and many other countries. However, air quality regulations typically do not consider the effects of these pollutants on climate. Similarly, climate mitigation discussions have only recently begun to incorporate SLCFs: those efforts typically do not consider air quality impacts. For example, the Kyoto Protocol established under the United Nations Framework Convention on Climate Change (UNFCCC) considers only six major GHGs and compares them using a metric that considers only their impact on long-term climate. awma.org april 2011 em 11

5 CO2 Emissions BC Emissions Figure 2. Cause-effect chains for CO 2 and BC from emissions to damages. Source: Hartman, D. The Social Cost of Black Carbon A Scoping Document; Report for the National Network for Environmental Management Studies Fellows Program for the United States Environmental Protection Agency, 2010 [modified]. Atmospheric Concentrations Atmospheric Concentrations Deposition CO2 Fertilization Temperature Change Effects (Direct) Ocean Acidification Direct Health Visibility Temperature Change Effects (Direct & Indirect) Hydrological Pattern Shift Ecosystem CO2 Damages BC Damages Clearly, however, the issues of air quality and climate change are intricately linked. This suggests the need for integrated solutions. Reducing black carbon or ozone precursors for climate purposes will have strong co-benefits for public health and the environment. This can be a strong motivating factor for adopting SLCF mitigation strategies. However, putting an integrated approach into practice can be difficult as responsibilities for air quality and climate change are often separated within institutions. Furthermore, achieving the full suite of benefits depends on careful tailoring of control strategies to ensure reductions in pollutant species of concern. Not all strategies aimed at reducing ozone or PM2.5 will have net climate benefits: for example, reductions in sulfur dioxide (SO2) and NOx, two of the most common precursors of PM2.5 and ozone, are likely to have a net warming effect on climate. Therefore, climatefriendly control strategies must focus on reductions in constituents that are linked to warming (i.e., positive radiative forcing) and/or other climate impacts such as black carbon and methane. In the case of methane, there are clear win-win opportunities: methane reductions can help reduce background ozone concentrations, though the ozone response to methane emissions will require several decades to be fully realized given methane s relatively long atmospheric lifetime. To date, methane emissions in the United States have been reduced mainly through a variety of voluntary programs (see the discussion of methane initiatives in the accompanying article, U.S. and International Efforts to Address Black Carbon, Ozone, and Methane on page 20). The U.S. Environmental Protection Agency (EPA) does not directly regulate methane from stationary sources. However, a number of regulations requiring control of VOCs from stationary sources have provided significant reductions of methane as a co-benefit of control. This includes rules for landfill emissions, and controls on certain petroleum and chemical manufacturing processes. The linkages between SLCFs and air quality provide exciting opportunities for carefully designed control programs to provide both climate and air quality co-benefits. But realizing these benefits requires identifying the critical pollutants, analyzing their behavior in the atmosphere and their impacts on the climate system, and identifying control options that are both feasible and cost-effective. Given the variety among the pollutants considered SLCFs, this is a complex and difficult task. Valuing Reductions in SLCFs Evaluating the benefits and costs of potential mitigation measures for SLCFs requires valuation of the avoided damages associated with these reductions. Recently, EPA and other U.S. federal agencies have been involved in efforts to try to assign monetary values to reductions in climate pollutants such as CO2. Part of this benefit cost calculus involves estimating the value to society of avoiding climate damages. For example, the social cost of carbon (SCC) is an estimate of the long-term flow of monetized damages resulting from a 1-ton increase in CO2 emissions in a given year (or on the flip side, the benefit to society of reducing one ton of CO2). The SCC estimates are intended to include an array of impacts, such as changes in net agricultural productivity, effects on human health, property damages from increased flood risk, and changes in ecosystem services. EPA recently estimated the 12 em april 2011 awma.org

6 SCC at $5 $65 per ton CO2 for 2010 in the benefits analysis for the final joint EPA and U.S. Department of Transportation rulemaking to establish light-duty vehicle GHG emission standards and corporate average fuel economy standards. 10 The wide range reflects alternative discount rates and a number of uncertainties regarding impacts. One of the limitations of the current SCC approach is that these estimates relate exclusively to CO2 emissions reductions and are not directly transferrable to other climate pollutants, including SLCFs, in part because of differences in temporal and spatial scales. Given that climate, health, and welfare impacts vary substantially between SLCFs and CO2, and also among SLCFs, alternative approaches may be needed to value the avoided damages from reducing short-lived forcers. As an example, Figure 2 illustrates the complexity involved in assessing the damages associated with black carbon relative to CO2. A great deal of additional work will be needed to establish a comprehensive cost-benefit framework for calculating avoided climate damages resulting from reductions in SLCFs. Conclusions Ongoing research within U.S. institutions and international bodies (as discussed elsewhere in this issue) will help clarify the precise impacts of shortlived forcers on climate, the availability of effective and cost-effective control opportunities, and the value of avoided damages (for health as well as Integrating Air Quality and Climate Power Generation by Drew Shindell, NASA Goddard Institute for Space Studies An integrated approach to improving air quality and mitigating climate change can offer a path to achieving these goals more efficiently. Consider a hypothetical comparison between a coal-fired power plant with no emissions controls, a coal-fired power plant with flue gas desulfurization, and a gas-fired power plant. For simplicity, let s assume that the only emissions are CO2 and SO2, and that the desulfurization is 100% efficient. Taking into account only air quality, the coal-fired power plant with flue gas desulfurization is just as good as the gas-fired power plant despite the fact that it emits far more CO2, hence causing more warming. Taking into account only climate change, the coal-fired power plant with no emissions controls is actually better than the coal-fired power plant with flue gas desulfurization as the SO2 emissions increase reflective aerosols. This is despite the fact that the SO2 emissions contribute to increased PM2.5, and hence, contribute to premature deaths. In contrast, considering both air quality and climate change leads to the conclusion that the gasfired plant is clearly the best choice, providing a win-win for both goals. em climate) associated with different mitigation strategies. Though not a substitute for CO2 reductions in the long term, controls on short-lived pollutants have the potential to slow the rate of global climate change, and to provide substantial regional climate and air quality benefits. em References 1. Air Quality Criteria for Ozone and Related Photochemical Oxidants (2006 Final); EPA/600/R-05/004aF-cF; U.S. Environmental Protection Agency, Washington, DC, Sitch, S.; Cox, P.M.; Collins, W.J.; Huntingford, C. Indirect Radiative Forcing of Climate Change through Ozone Effects on the Land-Carbon Sink; Nature 2007, 448, Hansen, J.; Nazarenko, L. Soot Climate Forcing via Snow and Ice Albedos; Proc. Natl. Acad. Sci. 2004, 101 (2), Flanner, M.G.; Zender, C.S.; Randerson, J.T.; Rasch, P.J. Present-Day Climate Forcing and Response from BC in Snow; J. Geophys. Res.-Atmos. 2007, 112 (D11). 5. Hadley, O.L.; Corrigan, C.E.; Kirchstetter, T.W.; Cliff, S.S.; Ramanathan, V. Measured Black Carbon Deposition on the Sierra Nevada Snow Pack and Implication for Snow Pack Retreat; Atmos. Chem. Phys. Dis. 2010, 10, Ramanathan, V.; Carmichael, G. Global and Regional Climate Changes due to Black Carbon; Nature Geoscience 2008, 1, Quinn, P.; Bates, T.S.; Baum, E.; Doubleday, N.; Fiore, A.M.; Flanner, M.; Fridlind, A.; Garrett, T.J.; Koch, D.; Menon, S.; Shindell, D.; Stohl, A.; Warren, S.G. Short-Lived Pollutants in the Arctic: Their Climate Impact and Possible Mitigation Strategies; Atmos. Chem. Phys. 2008, 8, Ramanathan, V.; Li, F.; Ramana, M.V.; Praveen, P.S.; Kim, D.; Corrigan, C.E.; Nguyen, H.; Stone, E.A.; Schauer, J.J.; Carmichael, G.R.; Adhikary, B.; Yoon, S.C. Atmospheric Brown Clouds: Hemispherical and Regional Variations in Long-Range Transport, Absorption, and Radiative Forcing; J. Geophys. Res. 2007, 112, D Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia; United Nations Environment Programme, Nairobi, Kenya, Final Rulemaking to Establish Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards: Regulatory Impact Analysis; EPA-420-R ; U.S. Environmental Protection Agency, Washington, DC, awma.org april 2011 em 13