Review of Environment, Energy and Economics - Re3 Short-lived Climate Pollutants: a Closer Look into Near- and Long-term Benefits from their Reduction


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Short-lived Climate Pollutants: a Closer Look into Near- and Long-term Benefits from their Reduction
by Elisa Calliari
Environment - Articles

According to several commenters, the encouraging figures included in the recently released Chinese Intended Nationally Determined Contributions (INDC) should be regarded as the latest battle in the “pollution war” – using the words by Premier Li Keqiang- declared by China in recent years. Protecting climate would be mainly driven by the national interest in reducing the toxic air pollution levels resulting from the rapid coal-driven economic growth and safeguarding public health, food, water and energy supply. It is widely acknowledged that acting on the so-called Short-Lived Climate Pollutants (SLCPs) can both provide important climate and societal benefits in the near-term. In this article, we review the most recent literature on SLCPs mitigation by focusing especially on its implication for achieving the 2°C target, and with the aim of complementing and directing policy debate accordingly with the latest scientific evidence.

Keywords: Short-Lived Climate Pollutants, climate change mitigation, air pollution, co-benefits
JEL Classification: Q53, Q54

Suggested citation: Calliari, Elisa, Short-lived Climate Pollutants: a Closer Look into Near- and Long-term Benefits from their Reduction (September 10, 2015). Review of Environment, Energy and Economics (Re3),

In policy discussion, SLCPs indicate those substances that, despite their short lifespan in the atmosphere, considerably contribute to the radiative forcing that drives climate change. They include methane (CH4), black carbon (BC), tropospheric ozone (O3), and hydro-fluorocarbons (HFCs). In addition to their contribution to global warming, BC and CH4 are also responsible for substantial negative local impacts on human health and agricultural production (UNEP 2011a).

Acting on SLCPs can therefore provide opportunities for win-win outcomes in terms of climate protection and societal benefits. Consistently, it has been campaigned by a number of international initiatives, the most prominent being the Climate and clean air coalition to reduce short-lived climate pollutant (CCAC). The Coalition was launched in 2012 by the governments of Bangladesh, Canada, Ghana, Mexico, Sweden and the United States, along with the United Nations Environment Programme (UNEP) to assess progress in tackling SLCPs and mobilizing resources for sustained action (Climate and Clean Air Coalition 2014). The World Bank (WB), recognizing the development benefits associated with a reduction in climate pollutants, also recently committed to scale up its activities specifically targeting SLCPs mitigation (The World Bank 2013). Along this line, it recently assessed the contribution of potential policies in the transport, industry and energy efficiency sectors in both reducing SLCPs emissions and capturing development gains (The World Bank 2014). According to the study, not only health and food security would be improved but also climate benefits would be immediately materializing. Early action on SLCPs would slow down the rate of near-term warming, avoid or push back dangerous tipping points and give vulnerable societies more time to adapt.

Focusing on SLCPs has proved to be attractive for a number of reasons. Firstly, the benefits stemming from SLCPs reduction materialize near in space and time, as they can be enjoyed locally (Shindell 2013) and within a generational time frame (Shoemaker, et al. 2013). Therefore, incentives for reducing SLCPs appear to be stronger than those associated with carbon dioxide (CO2) mitigation, whose effects will be instead dispersed globally and unfold in longer time scales. Moreover, the technologies and policies needed for abating SLCP are already available: their cost of implementation would be thus quite low compared with what required for a low-carbon transformation of the socio-economic system (Blackstock and Allen 2012). Acting on SLCPs, however, could only be complementary and not substitutable for CO2 mitigation (The World Bank 2014, Shindell, Kuylenstierna, et al. 2012, UNEP and WMO 2011). Stabilizing climate around a certain temperature -i.e. the 2°C relative to pre-industrial level- requires CO2 emissions to be urgently reduced and eventually stopped (Rogelj, et al. 2014). Therefore, the argument employed by some policy literature in supporting action on SLCPs as a mean to “buy time” until political consensus is reached on CO2 mitigation (Victor, Kennel and Ramanathan 2012, Wallack and Ramanathan 2009) would ultimately commit the world to increased warming.

A closer look to SLCPs and their impacts on the climatic and socio-economic systems
Black carbon (BC) is a component of soot, deriving from incomplete combustion of fuels and biomass. It is emitted by a variety of sources, including diesel engines, industry, residential solid fuel (i.e, coal and biomass), and open burning (The World Bank 2014). Dominant sources of BC are differentiated across regions, with combustion of coal and biomass accounting for the lion share in Africa and Asia and diesel engines contributing up to the 70% of emissions in Europe, North and Latin America (Bond, et al. 2013).

BC is associated with several negative impacts within the climate system. With a very short atmospheric lifetime in the order of days or weeks, BC influences climate in a number of ways. Firstly, and as a direct effect, particles absorb visible light due to their dark colour, thus heating the atmosphere and reducing the sunlight which reaches the surface and is then reflected back to the space (Bond, et al. 2013). However, strategies trying to contrast such warming effect through the removal of BC particles should take into account that the simultaneous removal of co-emitted pollutants like organic carbon, SO2, and NOx, having instead cooling effects, could offset the climate benefits deriving from such an option (Anenberg and Schwartz 2012). Secondly, when BC deposits on snow and ice, it both reduces their reflectivity (albedo) and induce their melting, given the increased absorption of heat (Quinn, et al. 2008). Finally, BC alters the properties and distribution of clouds, in terms of reflectivity, lifetime, stability and precipitations, with a net influence on these interactions which is still difficult to clarify (United States Environmental Protection Agency 2012). Through these mechanisms, BC has been responsible for a range of climate impacts, including increased temperature and melting effects in sensitive regions like the Artic and Himalaya (Institute for Governance & Sustainable Development 2013). Effects on rainfall patterns have also been detected, especially with regards to the timing and amount of monsoon precipitation (UNEP and WMO 2011).

BC can also have considerable negative impacts on health, being a primary component of PM2.5 which is associated with cardiopulmonary diseases, asthma, heart attacks and strokes (The World Bank 2014). Indeed, BC particles are able to penetrate deeply into the alveoli of the lungs, carrying very toxic, often carcinogenic, compounds such as polycyclic aromatic hydrocarbons (PAH) (Armstrong, et al. 2004). It has been estimated that, in 2005, outdoor air pollution caused up to 4.4 million premature deaths, with around additional 1.6 million cases due to indoor burning of solid fuel (UNEP and WMO 2011).

Tropospheric ozone is also associated with considerable negative effects on human health, especially in urban settings where it constitute a major component of smog (Silman 2007). Indeed,  changes in O3 concentration between 1860 and 2000 were responsible for around 375 000 respiratory mortalities annually (Fang, et al. 2013). Its oxidizing power is also at the basis of the damages it causes to agriculture, by reducing photosynthesis and other important physiological functions of vegetation and thus leading to weaker plants, inferior crop quality and decreased yields (Avnery, Mauzerall, et al. 2011b). Global estimates of yield losses driven by exposure to O3 in the year 2000 range from 2.2–5.5% for maize, to 3.9–15% and 8.5–14% for wheat and soybean (Avnery, Mauzerall, et al. 2011a). Finally, beyond being a positive climate forcer, O3 affects climate impacting on evaporation rates, cloud formation, precipitation levels, and wind patterns (Cross and Pierson 2013).

O3 has a very short lifetime in the atmosphere, ranging from a few hours to a couple of days (Blasing 2014). It is not directly emitted, but formed by the chemical reaction of a number of precursor pollutants, including methane, nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO) in the presence of sunlight (Cross and Pierson 2013). Therefore, mitigation strategies are primarily based on reduction of CH4 emissions, with the latter mainly deriving from oil and gas production and distribution, coal mining, agriculture and ruminant livestock, solid waste landfills (Institute for Governance & Sustainable Development 2013). CH4 is itself considered a SLCPs, despite a longer lifetime in the atmosphere, with respect to other short-lived forcers, of 12 years (Blasing 2014).

The last category of SLCPs is constituted by hydroflurocarbons (HFCs). HFCs are a group of chemicals created to replace ozone-depleting substances like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which were phased out in 1989 with the entry into force of the Montreal Protocol (Ozone Secretariat 1987). At environmental concentrations, HFCs pose little threat to human health. As for climate, the ability of HFCs to influence it varies significantly and accordingly to the different lifetimes they have in the atmosphere, ranging from days to hundreds of years. Although the concentration of these gases in the atmosphere is currently small, recent studies project a substantial growth in their use (UNEP 2011a).

Benefits of SLCPs mitigation
The benefits deriving from cutting emissions of SLCPs have been recently comprehensively assessed by UNEP and WMO (2011), with the results being later slightly revised by a subset of the same authors (Shindell, Kuylenstierna, et al. 2012). The report analysed nearly 400 measures improving air quality (through reduction of BC and CH4) through the International Institute for Applied Systems Analysis Greenhouse Gas and Air Pollution Interactions and Synergies (IIASA GAINS) model, and their effects on i) human health, ii) crop yields, and iii) temperature. Among them, 14 measures proved to deliver positive results in all the three domains considered. These measures would be able to avoid 2.4 million premature deaths (range 0.7 and 4.6 million) and the annual loss of 52 million tons (range 30–140 million) of maize, rice, soybean and wheat from 2030 onwards (UNEP and WMO 2011) [Note 1]. The majority of gains, both in terms of mortality reduction and crop production would occur in Asia. Comparable results were obtained by UNEP (2011a). Assessing the impact on health and agriculture of 16 air control measures targeting BC and CH4, the report estimated 2.4 million (range 0.7–4.6 million) premature causalities annually avoided (the same as UNEP and WMO, 2011), and 32 million tons (range 21-57 million) of avoided agricultural losses deriving from reductions in O3.

As for the impacts on climate, the two studies attributed to BC and CH4 mitigation a reduction in projected global mean warming of about 0.5°C (range 0.2–0.7°C) by 2050 (UNEP and WMO 2011, Shindell, Kuylenstierna, et al. 2012) and 0.4°C (range 0.1-0.6°C) between 2010 and 2040 (UNEP 2011a), respectively. Other authors (Blackstock and Allen 2012, The World Bank 2014), summing-up the benefits deriving also from HFCs mitigation, identified reductions in the order of 0.6°C (range 0.3-0.8°C) by 2050.

However, an emerging bunch of literature has argued that this benefits could have been overestimated (Pierrehumbert 2014). One major point has been recently been made by Rogelj, et al. (2014), cautioning about the effects that not considering SLCPs-CO2 coevolution could have on estimates. Indeed, BC and CO2 are strongly intertwined as deriving from common combustion sources. Specifically referring to the results obtained by UNEP and WMO (2011), which attributed around 0.2°C of temperature reduction to BC by 2030, they showed that not accounting for CO2–SLCP linkages would actually overestimate possible mitigation effects of BC-related measures by about 50%. Moreover, when accounting for coevolution of BC and CO2, undertaking additional BC reduction initiatives proves to be unnecessary as BC would be anyhow phased out through CO2 mitigation. The same conclusions can be drawn when considering the long term climate benefits of controlling SLCPs in climate stabilization scenarios. Assuming no CO2 mitigation efforts, the maximum temperature influence in 2100 by CH4, HFCs and BC measures is about 0.7 °C, 0.2 °C, and 0.1 °C respectively, with a combined effect of 1°C : by contrast, this influence drops to 0.4 °C, 0.1 °C and <0.05 °C (and a cumulative effect of 0.5°C) when undertaking CO2 mitigation consistent with a 2°C stabilization scenario. Disregarding linkages between SLCPs and CO2 would lead to overestimating the effect of the former by 100%.

These results show that long term mitigation of CH4 and HFC is unavoidable to reach the 2 °C target; on the contrary, the effect of BC measures appears to be negligible. This fact has two important implications. Firstly, it makes a strong case for explicitly distinguishing the role played by CH4 and BC. Although in policy discussion they are both subsumed under the label of SLCPs, they have very different effects on climate and also require differentiated mitigation strategies. Secondly, it shows that stabilizing temperature mostly depends on mitigating CO2 emissions. Thus, reductions in SLCPs should be undertaken in addition to and not instead of CO2 mitigation (Blackstock and Allen 2012, Shoemaker, et al. 2013). While providing important societal benefits in terms of enhanced food security and health, they would not be substitutable for immediate and aggressive CO2-related measures. Moreover, early action on these pollutants brings small benefits from a climatic point of view compared to action undertaken later (Shindell, Kuylenstierna, et al. 2012) and can only be justified by the willingness of grabbing the societal co-benefits they provide.

SCPs reduction can have substantial positive impacts in terms of health and food security and in contrasting near term warming. However, their very little impact on long term climate change, depending on the accumulation of CO2 in the atmosphere, clearly makes the case for keeping SLCPs actions as additional and complementary to CO2 mitigation. Policy frameworks treating them as equivalent could create incentives for inappropriate substitution between CO2 and SLCPs mitigation and thus lead to adverse consequences.

Policies should instead promote integrated approaches, combining both mitigation and air quality objectives so to grab all the co-benefits stemming from their simultaneous implementation. Differently from ancillary benefits, which are side effects deriving from a single policy, co- benefits arise from coordinated GHG mitigation and air-control interventions (Krupnick, Burtraw and Markandya 2000). As recently illustrated by the IPCC Fifth Assessment Report (AR5) (IPCC 2014), the cost of integrated approaches that achieve energy security, air pollution control and stringent mitigation targets at the same time, show the highest cost-effectiveness and provide substantial benefits especially in terms of health and ecosystems protection. Accounting for co-benefits could also allow for correctly assess the cost of mitigation measures and, with the costs of mitigation being partially offset by co-benefits, provide arguments for enhanced climate action.


[Note 1] In Shindell, Kuylenstierna, et al. (2012) the number of avoided premature losses would instead range between between 0.7 and 4.7 million, while crop benefits between and 30-135 million.


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Elisa Calliari, Fondazione Eni Enrico Mattei, Centro Euro-mediterraneo sui Cambiamenti Climatici and Ca’ Foscari University of Venice