4.3: The Ten Solutions

In this section, we’ll introduce the ten solutions, show how they fit into the six solutions clusters, and describe each of them briefly. The following chapters will provide in-depth exploration of each of these solutions.

These ten solutions represent an integrated approach to climate change across a wide range of expertise and sectors. These solutions are described as scalable solutions because they can first be implemented in local or regional living laboratories. Lessons learned can then be scaled up to national and global levels.

Figure 4.3.1 gives a visual overview of the six clusters, ten solutions, and three levers (discussed under Solution #1 below). Table 4.3.1 defines the ten solutions and their relationship to the six solutions clusters.

Figure 4.3.1 The six clusters, three levers, and ten solutions. From Ramanathan et al. 2017.

Table 4.3.1 The ten solutions

Solutions

I. Science Pathways

1. Bend the warming curve immediately by reducing short-lived climate pollutants (SLCPs) and sustainably by replacing current fossil-fueled energy systems with carbon-neutral technologies and by extracting carbon dioxide from the air and sequestering it or repurposing it for commercial uses.

II. Societal Transformation

2. Foster a global culture of climate action through coordinated public communication and education at local to global scales.
3. Deepen the global culture of climate collaboration.

III. Governance

4. Scale up subnational models of governance and collaboration around the world to embolden and energize national and international action.

IV. Markets and Regulations

5. Adopt market-based instruments to create efficient incentives for businesses and individuals to reduce CO₂ emissions.
6. Narrowly target direct regulatory measures—such as rebates and efficiency and renewable energy portfolio standards—at high-emissions sectors not covered by market-based policies.

V. Technology Measures

7. Promote immediate widespread use of mature technologies, such as photovoltaics, wind turbines, battery and hydrogen fuel cell electric light-duty vehicles, and more efficient end-use devices, especially in lighting, air conditioning, appliances, and industrial processes.
8. Aggressively support and promote innovations to accelerate the complete electrification of energy and transportation systems and improve building efficiency.
9. Immediately make maximum use of available technologies combined with regulations to reduce methane emissions by 50% and black carbon emissions by 90%.

VI. Ecosystem Management

10. Regenerate damaged natural ecosystems and restore soil organic carbon to improve natural sinks for carbon (through afforestation, reducing deforestation, and restoration of soil organic carbon). Implement food waste reduction programs and energy recovery systems to maximize utilization of food produced and to recover energy from food that is not consumed.

I. The science pathways cluster

This cluster describes emission pathways that were derived from climate science with the primary goal of keeping the warming below perceived dangerous levels. Until about 2015, the threshold for dangerous warming was generally perceived to be 2°C. However, recent data on the impacts of the 1°C warming that has already taken place (from preindustrial times to 2015)—for example, on extreme weather and on the melting of the Greenland and West Antarctic ice sheets—have led climate scientists and policymakers to conclude that the threshold for dangerous warming should be redefined to 1.5°C. It should be noted, however, that data from past climates suggest that even a warming of 1.5°C, if it is allowed to persist for more than a century, could lead to 6 to 9 meters of sea level rise (Chapter 1 for a discussion of the Eemian interglacial period 130,000 years ago).

Solution #1:

Bend the warming curve immediately by reducing short-lived climate pollutants (SLCPs) and sustainably by replacing current fossil-fueled energy systems with carbon-neutral technologies and by extracting carbon dioxide from the air and sequestering it or repurposing it for commercial uses. Achieve the SLCP reduction targets prescribed in Solution #9 by 2030 to cut projected warming by approximately 50% before 2050. To limit long-term global warming to 1.5°C, achieve carbon neutrality by 2050 and in addition extract as much as 500 billion to 1 trillion tons of carbon dioxide from the air by 2100. Solutions #7 to #9 cover technological solutions, and Solution #10 describes ecosystem solutions to accomplish these targets.

Frequently used terms with respect to CO₂ emission sources are defined here:

• Low-carbon refers to energy sources that emit substantially less CO₂ per unit of energy than conventional fossil fuels. Solar, wind, hydroelectric, and nuclear power fall under this category because fossil fuels are used in the production and transportation of the products used in solar cells, wind turbines, and nuclear plants.
• Zero emissions refers to energy sources or systems that truly have zero associated emissions of CO₂ and other greenhouse gases. This is an ideal that is not realized by any current energy sources, including solar, wind, hydroelectric, and nuclear, but could be approached as associated emissions from manufacturing or transportation systems approach zero.
• Renewables are energy sources that are replenished naturally. Solar, wind, hydroelectric, and geothermal fall under this category.
• Carbon-neutral refers to energy sources or systems that absorb as much CO₂ as they emit. An energy source that is derived from fossil fuels can still be carbon-neutral as a whole if the carbon released is captured and stored indefinitely.

As discussed in Section 4.1, climate studies and computer model projections make it clear that the only solutions pathway that sustainably keeps warming below 2°C is one that combines mitigation of both SLCP and CO₂ emissions. We will refer to these different mechanisms to reduce warming as levers to bend the warming curve. The Bending the Curve report, published in 2015, emphasized mainly the carbon and the SLCP levers because its goal was to keep warming below 2°C. Since the threshold for dangerous warming has been decreased to 1.5°C, we need to pull on a third lever, which we refer to as the atmospheric carbon extraction (ACE) lever. Numerous studies since 2015 have shown that we may have to extract as much 500 billion to 1 trillion tons of CO₂ by 2100 to keep the warming below 1.5°C. We have modified the two-lever strategy of the Bending the Curve report to a three-lever strategy as discussed below and shown in Figure 4.3.1 and Table 4.3.1:

The SLCP lever: take immediate action to cut emissions of short-lived climate pollutants.

Because SLCPs—methane, black carbon, and hydrofluorocarbons (HFCs)—have comparatively short lifetimes in the atmosphere, their mitigation provides a rapid reduction in temperatures relative to the business-as-usual path, helping to buy us time for carbon dioxide mitigation. In particular, we must reduce methane emissions by 50%, reduce black carbon emissions by 90%, and phase out HFCs completely by 2030. Solution #9 specifies the measures needed to achieve these goals.

The carbon lever: drastically reduce emissions of carbon dioxide to near-zero levels well before the end of this century.

This lever, as well as the person pulling on it, is intentionally made larger than the SLCP lever in recognition of the immense challenges of making the planet carbon-neutral. Specifically, we will need to cut CO₂ emissions approximately 40% by 2030 and 80% by 2050, with emissions dropping to as close to zero as possible after that. Solutions #7 and #8 describe the technologies needed to achieve these reductions in emissions.

The “CO₂ + SLCPs” pathway in Figure 4.1.1 represents the combined effects of the carbon and SLCP levers. The SLCP lever should reduce projected warming of the planet by approximately 50% by 2050, compared with business-as-usual projections.

The atmospheric carbon extraction (ACE) lever: remove carbon dioxide from the atmosphere, with removal efforts ramping up significantly over the course of this century.

Because carbon dioxide can remain in the atmosphere for centuries or millennia, keeping warming below dangerous levels for the long run requires this third lever. To give an idea of the enormous magnitude of this effort, it should be noted that to keep warming below 1.5°C throughout this century, as much as 1 trillion tons of CO₂ have to be extracted between 2030 and 2100 (corresponding to a rate of roughly 15 billion tons per year), in addition to pulling on the SLCP and carbon levers. Accordingly, the ACE lever is shown with the person having to bend backward along with the backward bending of the lever.

A range of technologies can be used to remove carbon dioxide from the atmosphere, including reforestation and agricultural practices that restore degraded soils and enhance the ability of soil to store carbon. Solution #10 focuses on these measures. In addition, CO₂ can be extracted from the air by a variety of chemical and biological processes. These measures are still under experimentation and are not yet scalable to the hundreds of billions of tons of CO₂ removal that will be required. Atmospheric carbon extraction technologies are discussed in more detail in Chapter 18.

Box 4.3.1 provides perspective on the three levers through the metaphor of a relay race.

Box 4.3.1  Your Goal: Winning the Relay Race

All of this sounds super complicated, so let us offer a metaphor: the three levers can be thought of as three runners in a relay team. Solving the challenge of climate change is like running a relay race, and time is against you. The SLCP runner is the starter who sprints forward quickly to gain some time for your team. The baton represents warming of 1.5°C or less. Assuming SLCP mitigation starts by 2020 and is completed by 2040, the SLCP starter can take the baton (1.5°C or less) to the decade of 2040 to 2050. Around this time, the SLCP runner hands the baton over to the carbon runner. Provided your team achieves carbon neutrality (zero CO₂ emissions) by 2050, the carbon runner can take the baton until 2070 at least, with warming still hovering around 1.5°C. By then, despite the efforts of the first two runners to bend the warming curve, the cumulative emissions of CO₂ (since 1850) will be working hard to bend the curve upward. This is when the baton is passed over to the finishing runner in your team, the ACE runner, who takes it to 2100 and beyond, still keeping the warming under 1.5°C.

It’s important not to confuse the timeline of when each runner begins to bend the curve downward with the time when that runner needs to get into action. For the carbon runner to take the baton around 2040, carbon mitigation efforts must begin immediately (by 2020 at the latest) and achieve carbon neutrality by 2050. The ACE runner has to be ready for action beginning around 2030. Why? We may have to take out as much as 1 trillion tons of CO₂ before 2100. This amount is so large that it cannot be done in a few decades. We have to start taking out about 15 billion tons of CO₂ by 2040 and continue at this rate until the end of the century.

Figure adapted from images in shutterstock.com.

II. The societal transformation cluster

Science can define the necessary pathways to avoid dangerous warming, but the pathways will not be realized if there is not broad understanding of the problem at all levels of society and a willingness to take the measures required. The solutions in this cluster focus on communication, education, and collaboration strategies to develop a culture of consensus and support for climate action.

Solution #2:

Foster a global culture of climate action through coordinated public communication and education at local to global scales. Combine technology and policy solutions with innovative approaches to changing social attitudes and behavior.

Increasing societal awareness of the impacts of climate change and the benefits of climate mitigation is critical to solving the climate problem. Building support for the actions necessary to combat global warming will require societal changes in attitudes toward our fellow human beings and toward nature. Solution #2 focuses on communication and education needed to foster these societal transformations. Efforts will include communications targeted toward key stakeholders, including decisionmakers and investors in low-carbon development, but also broad educational efforts at all levels, from kindergarten through college. While it’s important to make the severity and urgency of the climate problem clear, communications should focus on practical, achievable solutions. The goal of climate communication is to motivate action, not to create a sense that the challenge is too overwhelming to tackle. This book and its companion course are examples of the type of educational outreach recommended as part of this solution.

Communication and educational initiatives should also consider the different needs, responsibilities, and abilities to access information of the world’s top 1 billion, middle 3.5 billion, and bottom 3 billion.

Solution #3:

Deepen the global culture of climate collaboration. Design venues where stakeholders, community, and religious leaders converge around concrete problems with researchers and scholars from all academic disciplines, with the overall goal of initiating collaborative actions to mitigate climate disruption.

For a global culture of support to really take root, we will need to engage in dialogue at all levels: international, national, city, and neighborhood. This dialogue will involve a wide range of stakeholders—decisionmakers; community members; researchers and academics; and business, community, and religious leaders—in collaborative action, developing solutions to specific, concrete problems. An understanding of the local-scale impacts of climate change and development of localized mitigation interventions can help motivate participation by a wide spectrum of citizens.

Note the specific inclusion of religious leaders in the solution statement. Religion is often overlooked as part of the solution to climate change, but religious leaders and religious communities can play a vital role. Both religions and climate scientists want to protect nature (or creation). Religious spaces can be natural venues to discuss the ethical issues raised by climate change. In addition, in the United States, where climate change has become extremely politicized, religious spaces offer scientists and climate solution seekers like you a nonpolitical forum to discuss the problem and its solutions. Climate change is also an issue where science, policy, and religion converge. While scientists and policymakers talk in terms of intergenerational equity and the protection of nature, major religious traditions often frame these same concepts in terms of a duty to care for our fellow human beings and for creation. An excellent example of the broader framing of climate change impacts in human terms is Pope Francis’s climate change encyclical, Laudato Si’: On Care for Our Common Home, published in 2015. Because of the broad and deep penetration of religious faith across the world, religious settings can also facilitate dialogue between members of the top 1 billion and the bottom 3 billion on our planet.

Solutions #2 and #3 will be discussed in Chapters 5, 6, 7, and 8.

III. The governance cluster

In addition to a broad societal consensus for climate action, implementation of the recommended pathways will require support and coordination at all levels of government, from local neighborhoods to international coalitions.

Solution #4:

Scale up subnational models of governance and collaboration around the world to embolden and energize national and international action. Use the California examples to help other state- and city-level jurisdictions become living laboratories for renewable technologies and for regulatory as well as market-based solutions, and build cross-sector collaborations among urban stakeholders because creating sustainable cities is a key to global change.

With the 2015 Paris Agreement as a framework for international action on climate, this solution focuses on governance models from cities, states, and regions that can be scaled up to national and global levels. Cities cover less than 2% of the Earth’s surface but produce more than 60% of global CO₂ emissions. States, cities, and other subnational jurisdictions have the ability to develop innovative solutions that are responsive to local needs, implement them on a relatively short time scale, and make adjustments as needed. The C40 initiative (https://www.c40.org) and the Under2 Coalition initiated by the governor of California are exceptional examples of subnational activities that can leverage international agreements at a local scale, as we’ll see in Section 4.4. In short, they can act as innovative, nimble living laboratories to test, refine, and promote governance and other solutions, which can then be adapted and expanded to strengthen and enhance national and global efforts. Actions under way in California provide particularly relevant examples of subnational models; we’ll take an initial look at some of these in Section 4.4.

Solution #4 will be discussed further in Chapters 9 and 10.

IV. The markets and regulations cluster

To make mitigation a reality, policymakers need to send clear signals to companies and individuals. Appropriate economic and regulatory measures can encourage investment in existing low-emission technologies and innovation for the future. The next two solutions explore market based instruments and direct regulation.

Solution #5:

Adopt market-based instruments to create efficient incentives for businesses and individuals to reduce CO₂ emissions. These can include cap and trade or carbon pricing and should employ mechanisms to contain costs. Adopt the high-quality emissions inventories, monitoring, and enforcement mechanisms necessary to make these approaches work. In settings where these institutions do not credibly exist, alternative approaches such as direct regulation may be the better approach—although often at higher costs than market-based systems.

Both economic theory and real-world experience indicate that the most economically efficient, lowest-cost way to achieve emissions reduction is through market-based incentives. Market-based mechanisms add a cost to emissions that reflect the long-term environmental damages they cause. Two major categories of market instruments are a direct carbon price, such as a carbon tax or fee on emissions, and a system of cap and trade under which total emissions from large sources are capped and allocated through a system of tradable permits. Cap-and-trade systems for carbon dioxide emissions have been implemented in a variety of markets, including California, the northeastern US, and the European Union. In 2017, China initiated a national cap-and-trade market that began with its power sector and will gradually be expanded to other sectors of the economy.

While carbon prices and cap and trade could reduce emissions, current fossil fuel subsidies support production and consumption and incentivize CO₂ emissions. Fossil fuel subsidies include tax advantages, low-interest loan guarantees, and access to public natural resources at below-market rates. As estimated by the International Monetary Fund (IMF), global fossil fuel subsidies are as much as US$540 billion annually.

According to the IMF, when fossil fuel impacts on mortality due to air pollution (about 3.5 million premature deaths a year) are included, the total subsidy increases to as much as US$5 trillion annually. In comparison, the International Energy Agency estimates that the cost of changing the entire infrastructure of the world to zero-carbon emissions over a 30-year period would only be about US$1 trillion dollars annually, about one-fifth of the subsidy cost.

A recent study estimated that the net effect of continued tax preferences and other subsidies in the US alone would be to increase domestic oil production by 17 billion barrels (equivalent to 6 billion tons of CO₂ emissions) through 2050, relative to a scenario with no subsidies. Removing these subsidies, as well as providing subsidies for low-emission sources as appropriate, would create strong economic incentives to transition to low-carbon sources of energy.

One criticism of market-based initiatives is that added costs (for example, increases in fuel and energy costs) can be passed on to consumers, with a potentially disproportionate impact on the least affluent. These negative impacts can be reduced if some portion of the revenues from cap-and-trade or carbon pricing mechanisms are used to reduce impacts on disadvantaged communities and others who are adversely affected by higher prices.

Solution #6:

Narrowly target direct regulatory measures—such as rebates and efficiency and renewable energy portfolio standards—at high-emissions sectors not covered by market-based policies. Create powerful incentives that continually reward improvements to bring down emissions while building political coalitions in favor of climate policy. Terminate subsidies that encourage emission intensive activities. Expand subsidies that encourage innovation in low-emission technologies.

Regulatory measures are given lower priority than market-based incentives on our solutions list because they are generally less cost-effective. However, direct regulations provide an alternative instrument for emissions reduction, particularly where economic measures may not be technically or politically feasible. Where regulations are necessary, they should be targeted toward high-emission sectors to maximize their impact and designed to contain the costs of compliance.

Solutions #5 and #6 will be covered detail in Chapters 11 and 12.

V. The technology measures cluster

We have set the stage with broad public support for climate solutions along with governance, market, and regulatory instruments for their implementation; this cluster provides the technological means to make those reductions happen. Both wider use of existing technologies and future innovations will be required. The three solutions in this cluster focus on both carbon dioxide and short-lived climate pollutants. These represent the first two levers discussed above: the carbon lever and the SLCP lever. Solutions #7 and #8 represent two stages of pulling the carbon lever. Solution #7 pulls the carbon lever nearly halfway by 2030, and Solution #8 pulls it the rest of the way by 2050. Solution #9 represents pulling the SLCP lever by 2030.

To keep warming below dangerous levels, both of these levers will be required. Fully implemented, the CO₂ reductions in Solutions #7 and #8 could reduce global warming by as much as 1.5°C by 2100, relative to a business-as-usual scenario. In combination with the SLCP reductions envisaged in Solution #9, this solutions cluster gives us a good chance of keeping warming below 2°C during this century and beyond.

Solution #7:

Promote immediate widespread use of mature technologies, such as photovoltaics, wind turbines, battery and hydrogen fuel cell electric light-duty vehicles, and more efficient end-use devices, especially in lighting, air conditioning, appliances, and industrial processes. These technologies will have even greater impact if they are the target of market-based or direct regulatory solutions such as those described in Solutions #5 and #6 and have the potential to achieve a 30% to 40% reduction in fossil fuel CO₂ emissions by 2030.

Figure 4.3.2 Global CO₂ emissions from fossil fuels and industry, 2014. The darker colors indicate the emissions that are most difficult to eliminate; they account for just over a quarter of the global total. Data from Davis et al. 2018.

Figure 4.3.2 shows the major global sources of fossil fuel and industrial carbon dioxide emissions, grouped by sectors. Many of these emissions can be reduced through expansion of currently available technologies, such as electricity generation by solar photovoltaics and wind turbines. Significant technical advances and decreasing costs have led to a rapid increase in the deployment of renewable electricity over the past decade, mostly from photovoltaic solar panels and wind turbines. However, reducing emissions from some sectors will be more challenging and will require innovative new technologies. These difficult-to-eliminate emissions, which account for just over a quarter of the global total, are indicated by darker colors in Figure 4.3.2 and described under Solution #8.

Nuclear power has the advantage of generating on-demand electricity with no direct carbon dioxide emissions, but it is controversial because of the possibility of nuclear accidents and concerns with storage of radioactive waste. Some countries, such as China, are expanding their nuclear power capacity, while others, like Germany, are phasing it out. In the US, there are currently (as of late 2018) only two new nuclear reactors under construction. The high cost of building new nuclear plants means that at present they are generally not economically competitive with alternatives such as solar or wind. However, new designs such as small modular reactors may provide for lower-cost nuclear power in the future, with less nuclear waste and a far lower risk of catastrophic accidents.

In the transportation sector, cars and light-duty trucks with electric motors powered by lithium ion batteries or hydrogen fuel cells could drastically reduce emissions if low-carbon sources were used for battery charging and hydrogen production. Emissions from homes and commercial buildings could be reduced by use of energy-efficient heating and cooling systems, lighting, and appliances. It’s estimated that full implementation of strategies involving existing technologies has the potential to achieve a 30%–40% reduction in fossil fuel emissions by 2030. We can think of this as pulling the carbon lever about a third of the way toward carbon neutrality. A combination of market and regulatory incentives, as discussed in Solutions #5 and #6, could help accelerate this technological transition.

Solution #8:

Aggressively support and promote innovations to accelerate the complete electrification of energy and transportation systems and improve building efficiency. Support development of lower-cost energy storage for applications in transportation, resilient large-scale and distributed micro-scale grids, and residential uses. Support research and development of a portfolio of new energy storage technologies, including batteries, supercapacitors, compressed air, hydrogen, and thermal storage, as well as advances in heat pumps, efficient lighting, fuel cells, smart buildings, and systems integration. These innovative technologies are essential for meeting the target of 80% reduction in CO₂ emissions by 2050 and transitioning to zero emissions soon after.

Moving away from fossil fuels will require electrification of nearly all end uses, including transportation and heating systems, with the electricity generated almost exclusively by carbon-neutral energy sources. Because wind and solar energy production are inherently variable, increasing penetration of renewables depends on affordable systems to store energy during periods of excess power production and to feed it back into the grid when production falls; energy storage is a crucial area of innovation needed for the transition to low-carbon energy, as discussed in Box 4.3.2.

Power generation systems will also become more widely distributed, ranging in scale from large-scale utility power plants to rooftop solar for individual buildings. This will require the development of “smart” electrical systems that can manage power from sources with variable production and a variety of scales. Microgrids that can function independently of the main power grid when necessary would further increase the ability of the grid to handle variable electric generation and power outages. These ideas will be further discussed in Chapters 13 and 14.

Solution #9:

Immediately make maximum use of available technologies combined with regulations to reduce methane emissions by 50% and black carbon emissions by 90%. Phase out hydrofluorocarbons by 2030 by amending the Montreal Protocol. In addition to the climate and health benefits described under Solution #1, this solution will provide access to clean cooking for the poorest 3 billion people who spend hours each day collecting solid biomass fuels and burning them indoors for cooking.

As discussed in Chapter 1, black carbon, methane, ozone, and hydrofluorocarbons (HFCs) are referred to as short-lived climate pollutants (SLCPs) because their lifetimes in the atmosphere—from a few weeks to a few decades—are relatively short compared with that of CO₂. They are also super pollutants with warming effects tens to thousands of times stronger than CO₂. This combination of short lifetimes and powerful warming ability means that targeting SLCPs for reduction can have a significant and comparatively rapid impact on global temperatures, as we saw in Section 4.1. Solution #9 represents pulling the SLCP lever all the way.

Box 4.3.2  Examples of Difficult-to-Eliminate Sectors of Carbon Emissions and Required Innovations

Providing reliable electricity

As more sectors are electrified and as a greater portion of electricity is produced by intermittent renewable energy sources, there will be an increasing need to provide reliable, load-following electric systems that can be ramped up quickly to accommodate any mismatch between energy supply and demand. A key technological approach is improved energy storage. One alternative is to use excess electric power to produce hydrogen, which can then be converted back to electricity by using fuel cells. Hydrogen fuel cell technology is already in use to power vehicles, but the bulk of the hydrogen is produced from natural gas. CO₂ emissions from hydrogen generation can be eliminated if hydrogen is produced by electrolysis (the splitting of water into hydrogen and oxygen) using renewable energy sources.

Aviation, shipping, and long-distance road transportation

Advances in battery technology and hydrogen fuel cells have made short-range battery electric and fuel cell vehicles commercial realities. However, eliminating emissions from long-distance transportation will require new technologies. Improved hydrogen fuel cells may prove suitable for long-distance road transport, but aviation and shipping will require power sources with greater energy density (energy content per unit weight). Biofuels are promising candidates since they are carbon-neutral, but they are energy intensive to produce and can take up agriculturally valuable land.

Cement and steel

Cement and steel production are the two highest-emission industrial processes, generating 4% and 5% of global CO₂ emissions, respectively from the burning of fossil fuels to provide the high temperatures required for production and from materials used in production (such as limestone for cement and coke for steel). Reducing CO₂ emissions from cement and steel production will require the development of new chemical and industrial processes. In the case of cement production, it may also be possible to capture and store CO₂ directly from the kiln’s exhaust gases.

Another advantage of SLCP mitigation is that SLCP emissions can generally be reduced more quickly and easily than CO₂, and reductions in SLCP emissions translate into a more immediate impact on the climate than do reductions in CO₂ (Chapter 1). Fossil fuels have been used intensively since the Industrial Revolution and are deeply embedded in a wide range of human activities. As discussed in Solutions #7 and #8, phasing out CO₂ emissions will require several decades and new technological innovations. SLCPs, on the other hand, are generated by fewer sectors of society and can be addressed with existing technologies. Also, SLCP mitigation is often more easily accepted because many of the co-benefits (to health and agriculture sectors) accrue locally.

The two largest sources of black carbon (up to 95% of the total) are diesel vehicles and domestic cooking and heating, with 3 billion people still relying on eighteenth-century technologies that burn firewood, dung, and coal. Black carbon emissions from diesel vehicles can be reduced by about 98% through adding diesel particulate filters. Replacing inefficient solid-fuel-burning stoves in India, China, sub-Saharan Africa, and many countries in South America with less-polluting models can reduce as much as 80% of their black carbon emissions. Such measures not only reduce the warming effect of black carbon soot, but also provide significant health benefits by reducing particulates that can cause respiratory illnesses. Worldwide, roughly 3 million people die prematurely each year because of indoor smoke from cooking, heating, and lighting with solid fuels.

Another major SLCP, methane, can be addressed through a variety of means, including capture and burning of methane emitted by coal mines, oil wells, gas production and distribution facilities, and landfills. Methane emissions from animal manure and wastewater systems can be controlled through anaerobic digesters. Mitigation of methane would avoid 0.5°C warming by 2050.

Ozone in the troposphere (the lowest layer of the Earth’s atmosphere) is another important short-lived climate pollutant. It is not directly referenced in Solution #9, but decomposition of methane is an important source of ozone. Measures to mitigate methane would result in reduced tropospheric ozone as well. Like black carbon, ozone has negative health impacts and can cause respiratory illnesses; moreover, it is a major source of agricultural crop losses.

HFCs are primarily used as refrigerants in air-conditioning systems, refrigerators, and auto cooling systems. Substitutes with far lower warming potential are already available. Left unchecked, HFC emissions alone would warm the planet by 0.1°C by 2050 and 0.5°C to 1.0°C by 2100.

Solution #9 will be covered in more detail in Chapter 15.

VI. The ecosystem management cluster

The previous five clusters focus on mitigating our emissions of climate damaging pollutants. However, most projections indicate that for long-term temperature stability we will also need to remove CO₂ from the atmosphere. This cluster focuses on reducing emissions from managed ecosystems, particularly agricultural lands and rangelands, and managing ecosystems to enhance their ability to absorb CO₂ from the atmosphere. This represents a portion of the third and last of our three levers, the atmospheric carbon extraction (ACE) lever. It should be noted, Solution #10 by itself cannot meet more than a third of the carbon extraction requirements of 500 billion to 1 trillion tons of CO₂ extraction by 2100. We will most likely have to resort to direct capture of carbon dioxide from the air, using some of it for commercial and residential needs and sequestering the remaining carbon. However, thus far only pilot projects exist for direct capture, and there are yet no clear pathways to scale these up to the level of carbon capture required. These technologies are discussed in Chapter 18.

Solution #10:

Regenerate damaged natural ecosystems and restore soil organic carbon to improve natural sinks for carbon (through afforestation, reducing deforestation, and restoration of soil organic carbon). Implement food waste reduction programs and energy recovery systems to maximize utilization of food produced and recover energy from food that is not consumed. Global deployment of these measures has the potential to reduce as much as 25% of the current annual emissions of about 40 billion tons of CO₂. In addition, Solution #10 will help meet the recently approved sustainable development goals of the United Nations by creating wealth for the poorest 3 billion.

After fossil fuels, the second largest anthropogenic source of CO₂ is deforestation. Burning or clearing trees for agriculture and croplands is estimated to release about 2 billion tons of CO₂ into the atmosphere annually. Reducing deforestation would reduce these emissions; reforestation (restoration of forest cover in deforested areas) and afforestation (the planting of trees in areas that did not previously have forest cover) would actually remove CO₂ from the atmosphere. Creating payment mechanisms for the environmental services provided by forest ecosystems can be an effective mechanism to promote reduced deforestation, while providing an income source for forest-dependent communities around the world.

Restoration of degraded ecosystems, including wetlands and mangrove swamps, and soil management and restoration can provide another mechanism for CO₂ reduction. Soils contain significant quantities of organic carbon in the form of plant matter, microbes, and other organisms. Intensive agriculture tends to disturb the soil, promoting CO₂ release. Encouraging alternative agricultural and grazing practices, including reducing tillage of agricultural fields and promoting greater biodiversity, can promote CO₂ absorption and storage in the form of organic carbon.

One caveat: the capacity of forests and agricultural soils to store carbon is not unlimited. For example, a 2018 study by the US National Academies of Sciences, Engineering and Medicine estimated that the capacity of agricultural soils to store carbon gradually drops to zero over two to four decades as the soils approach carbon saturation.

Reducing food waste is another key element of Solution #10 and one of the most significant actions we can take in addressing climate change. Globally, about one-third of food production is wasted; in the US, this figure rises to 40%. When food is wasted, the energy and associated emissions that went into its production, transportation, and storage are wasted as well. Further, food waste in landfills is a major source of methane emission.

It’s estimated that combined, these measures for reduced deforestation, afforestation, reforestation, soil carbon restoration, ecosystem restoration, and reduced food waste could reduce greenhouse emissions by about the equivalent of 10 billion tons of CO₂ annually, about 25% of our current CO₂ emissions. This solution will be explored further in Chapter 16.