15.1: Why Should We Mitigate Short-Lived Climate Pollutants?

Climate benefits of SLCP mitigation

Figure 15.1.1 Projections of future warming along business-as-usual and mitigation pathways. From Ramanathan et al. 2017.

We can see the importance of mitigating SLCPs by looking at Figure 15.1.1. We have already seen this figure in Chapter 4. It shows the temperature record (purple line) from 1950 to about 2010 and possible future temperature pathways up to 2100. As we have previously discussed, the planet has already warmed by 1°C. Most researchers conclude that warming of less than 1.5°C (relative to preindustrial times) will have impacts that, while significant, will mostly be manageable. This zone of presumed relatively “safe” temperatures is colored green. Red represents the zone of “dangerous” warming above 2°C.

The curved lines in the figure represent different possible temperature pathways, based on the choices we make now and in the near future. The highest, gray “business as usual” curve shows the evolution of temperatures if human emissions continue to grow unabated. (Some policymakers now call this the “disaster curve” rather than the more benign-sounding “business as usual curve.”) This pathway leads to temperature increases that will likely exceed 4°C by 2100. As described in previous chapters, the impacts of warming in this range are not merely dangerous; they could be catastrophic for human society and natural ecosystems. This is the warming curve we must bend to ensure a sustainable future for our children and their descendants. The other curved lines show possible mitigation scenarios, as described below.

It’s important to keep in mind that, although these pathways are shown as definite lines, there is actually a range of possible temperature trajectories for each of these pathways. Refer back to Box 1.4.3 in Chapter 1 for more details on this concept. The lines represent the most likely temperature outcome for each pathway, as determined by computer models.

The dotted black “CO₂ only” line shows the expected effects if we make significant efforts to reduce carbon dioxide, but not SLCPs. We can see that this scenario bends the warming curve well below the business-as-usual line, but the benefits happen very slowly and don’t really become apparent until after about 2050. In contrast, the solid black “SLCPs only” mitigation line drops below the business-as-usual curve almost immediately because of the short atmospheric lifetimes of SLCPs. This pathway delays the time to cross the 2°C threshold by 20 to 30 years.

A mitigation strategy that focuses on reducing both CO₂ and SLCPs simultaneously—the solid blue line in the figure—is the only pathway that keeps warming below the 2°C threshold throughout this century. Reducing SLCPs bends the curve immediately and buys us time for the long-term effect of CO₂ reduction to take effect. Pursuing both CO₂ and SLCP mitigation simultaneously is our best—indeed, our only—hope for avoiding dangerous warming of the planet.

Commentary on temperature thresholds: From a scientific perspective there are no sharply defined physical thresholds beyond which negative impacts will start to happen. In fact, damaging effects of the current warming of 1°C are already being experienced in most parts of the world and will get progressively worse as temperatures keep increasing, as documented in the 2018 Special Report from the Intergovernmental Panel on Climate Change (IPCC) on the impacts of 1.5°C warming. Our best understanding of the climate system indicates that sustained warming of 2°C would be unprecedented over at least the past 2 million years and would lead to very severe impacts on human health and threaten food and water security. In practice, such temperature thresholds are used as benchmarks to evaluate the effectiveness of mitigation measures. The yellow area between 1.5°C and 2.0°C represents the transition zone between “safe” temperatures and dangerous climate change. Keep in mind that we generally are referring to warming that is averaged globally, even though in reality warming is not evenly distributed, with some regions, such as the Arctic, warming at least twice as much as the global average.

Health and food security benefits of SLCP mitigation

Mitigation of SLCPs has important co-benefits in addition to helping to bend the warming curve. Black carbon and tropospheric ozone have significant negative impacts on human health, and ozone is a major cause of damage to agricultural crops. The mitigation measures detailed in this chapter can save about 2.4 million lives lost each year and about 50 million tons of crops lost each year to air pollution. The health benefits from reduction of black carbon and ozone are valued at about $5 trillion per year. The health benefits and food benefits alone would justify mitigation of these pollutants even without consideration of the cooling benefits from cutting them.

Climate justice benefits of SLCP mitigation

It’s also important to bear in mind that, as we discussed in Chapter 2, the negative impacts of these pollutants are borne disproportionately by the global poor. The poorest 3 billion human beings, representing 40% of the global population, have limited access to energy from fossil fuels and contribute only about 5% of global CO₂ emissions. Poverty forces the poorest 3 billion to rely on eighteenth-century technologies such as inefficient wood-burning stoves for cooking. The poorest 3 billion on the planet are far more exposed to threats such as drought, flooding, heat waves, and sea level rise. Meanwhile, the wealthiest 1 billion represent about 13% of the world’s population but emit about 50% of global CO₂ pollution. Their greater resources provide more opportunities to adapt to the impact of global warming, such as using air conditioning to reduce deaths from heat waves. Mitigation of SLCPs would help to reduce these disproportionate impacts. Moreover, some of the solutions, such as more efficient stoves, would result in improved quality of life for many among the global poor.

The disproportionate responsibilities of the global wealthy and vulnerabilities of the global poor add a moral and ethical component to climate change. As discussed in Chapters 6, 7, and 8 on societal transformation, solutions to these issues should involve not only scientists and engineers, but also religious communities, philosophers, ethicists, climate justice advocates, and others from civil society. Mitigating shortlived climate pollutants brings these issues into particularly sharp focus. Addressing them will require an alliance among science, religion, health care, and public policy.

Political benefits of SLCP mitigation

Fortunately, reducing SLCPs poses fewer political barriers than cutting carbon dioxide. First, governments are more likely to agree to emissions reduction strategies that can deliver local benefits. Second, already available technologies and policies (such as air pollution regulations for black carbon and methane and the Montreal Protocol for HFCs) readily allow for deep cuts in these pollutants. Third, unlike reductions in carbon dioxide emissions, whose main benefits arrive only after decades of mitigation efforts, SLCPs mitigation would satisfy the immediate interests of countries because of rapid and visible improvements in health and food security. Visible early success in fighting climate change through limiting SLCPs would also enhance the credibility of climate change policies and thus accelerate progress on the more challenging task of limiting carbon dioxide. A plan to reduce short-lived climate pollutants would align the self-interests of many polluting nations. It is not surprising that the Climate and Clean Air Coalition, formed in 2012 by the United Nations to focus on SLCP mitigation, already has 61 member nations working together to mitigate SLCPs (Box 15.1.1).

Economic costs and benefits of SLCP mitigation

Figure 15.1.2 Black carbon and methane mitigation measures. Numbers indicate the share of warming reduction from each measure as a percentage of the total warming reduction for all 16 measures in the 2011 UNEP report. Labels indicate whether the measure targets black carbon (BC) or methane (CH4). Colors indicate the cost grouping: blue indicates minimal cost or cost savings; green indicates moderate costs; orange or yellow indicates high costs; gray indicates costs that are difficult to quantify. Data from UNEP and WMO 2011.

To date, there are only limited studies on the costs and benefits of measures to reduce SLCPs. However, studies that account for co-benefits such as human health improvements and the gains from reduced crop loss, as well as the avoidance of damage that would otherwise result from warming, show a clear net societal benefit from SLCP mitigation measures. Furthermore, off-the-shelf technologies exist for mitigating most of these emissions.

For black carbon and methane, a comprehensive cost analysis was provided by the 2011 UNEP study that was highlighted in the Overview. That study, which identified the 16 key measures for black carbon and methane mitigation discussed in Figure 15.2, also classified these measures into four groups: measures with minimal costs or that yielded cost savings; measures with moderate costs; measures with high costs; and measures whose costs are difficult to quantify because they depend in part on improved governance mechanisms in developing countries. Figure 15.1.2 summarizes these results for the top 16 measures recommended in the UNEP report. Notice that over half of these measures can be accomplished with minimal costs or cost savings; however, most of these measures have up-front costs for their initial implementation, with the savings realized over many years and not always by those paying the up-front cost. It’s also important to note that these cost estimates do not include savings due to improvements in human health or avoided crop damage. Even measures that are identified as high-cost could be adopted based on health or food security co-benefits. The European Union, for example, has implemented standards for diesel particle filters based primarily on health benefits from improved air quality.

For HFCs, the cost-benefit calculus is just as compelling as that for black carbon and methane. The companies that make refrigerants such as HFCs have learned how to profit from the switch to safer substitutes, without increasing the cost to consumers in any significant way. Indeed, when these companies lose the intellectual property protection for their chemicals, they welcome the transition so they can sell their newer substitutes. Moreover, under the Kigali Amendment to the Montreal Protocol, developed countries contribute to a dedicated funding mechanism that pays the incremental cost for developing countries to switch to safer substitutes. We’ll be discussing the costs and benefits of HFC mitigation in more detail in Sections 15.5 and 15.6.

It is important to consider what weight should be given to such cost-benefit analyses when facing an existential threat from runaway climate change. Former California governor Jerry Brown has compared the climate threat to the threat the US faced in World War II, when it is unlikely a cost-benefit approach was used to determine how the US should produce the needed war material. (Wagner and Weitzman’s book Climate Shock explains why, in their view, a cost-benefit analysis shouldn’t be used to determine how to address the existential threat of climate.) Even if a cost-benefit analysis isn’t the most appropriate metric to evaluate climate solutions, however, the studies we’ve discussed show that the cost-benefit arguments for SLCP mitigation are quite compelling.

Box 15.1.1  Climate and Clean Air Coalition

The Climate and Clean Air Coalition (CCAC) is a global organization of governmental, nongovernmental, and intergovernmental entities that have committed to improving the air quality through actions that reduce short-lived climate pollutants (SLCPs), consistent with the recommendation in Solution #9 of reducing global methane emissions by up to 40% and black carbon emissions by up to 80%. The CCAC was launched in 2012 by the United Nations Environment Programme (UNEP) and six countries—Bangladesh, Canada, Ghana, Mexico, Sweden, and the United States. There are 61 state partners (nations) and 67 nonstate partners (such as international finance institutions, regional development banks, and city networks) at the time of this writing. In 2015, CCAC countries contributed about 40% of global black carbon emissions, and CCAC countries could supply about 50% of total mitigation by 2030.

The CCAC’s activities target the main sectors responsible for SLCP emissions: cooking and heating, industry, transport, agriculture, fossil fuels, waste management, refrigeration, and cooling. The CCAC is currently focused on 11 initiatives. Seven are sector-specific initiatives that include diesel, oil and gas, waste, bricks, HFCs, household energy, and agriculture. The remaining four, which include supporting national action and planning, assessments, finance, and health, cut across sector lines to reduce emissions for all SLCPs.

http://ccacoalition.org/en/content/who-we-are.

Moving toward SLCP mitigation

Given the clear net benefits, why have nations not aggressively promoted SLCP mitigation so far? There are several reasons:

• Perhaps the most important reason is that the combined climate, health, and food security benefits of SLCP mitigation have only been recognized since about 2010. While the scientific study of SLCPs is at least 40 years old, scientists studying the health effects of SLCPs as air pollutants were working separately from the scientists studying SLCPs’ climate impacts. The vital role that SLCP mitigation can play in bending the warming curve has only begun to catch the attention of climate scientists in the last 10 years.
• Cost-benefit studies that show the combined societal benefits of SLCP mitigation—from reduced warming together with health and food security benefits—have only become available in recent years.
• Until around 2010, the attention of climate scientists, activists, and policymakers was focused primarily on CO2 emissions. There was concern that a sudden shift in focus to SLCPs would create the impression that action on CO2 mitigation could be delayed or avoided, presenting a “moral hazard.”
• In all areas of environmental science, there is a time lag between scientific findings and policy response.

However, significant progress in SLCP mitigation is now underway. The Climate and Clean Air Coalition (http://ccacoalition.org/en, described in Box 15.1.1) coordinates policies and practices at the international level. In the US, the State of California has enacted legislation to drastically cut emissions of SLCPs. In addition, the United States Climate Alliance, which includes governors whose states represent 40% of the US population, is aggressively pursuing SLCP mitigation and has developed a detailed road map for reducing emissions.