Follow Population Alert on


 US Mid-Century Strategy for Deep Decarbonization

While CO2 accounts for four-fifths of U.S. greenhouse gas (GHG) emissions, the remainder are highly potent heat-trapping gases, many of which have near-term climate impacts due to their shorter “lifetimes” in the atmosphere.

Figure 6.1 shows the contribution of non-CO2 U.S. GHG emissions and their major sources, including:

  • methane (CH4) (55 percent),
  • nitrous oxide (N2O) (31 percent),
  • hydrofluorocarbons (HFCs) (13 percent), and
  • other fluorinated gases such as PFCs, SF6, and NF3 (1 percent) (EPA 2016a).

Absent significant innovation and policy, non-CO2 GHG emissions are projected to increase rapidly. For example, growing global demand for food would drive broader use of nitrogen fertilizer and increased livestock production, resulting in greater N2O and methane emissions.

A growing global population will also increase demand for energy and refrigerants, leading to greater emissions of methane and HFCs in the coming decades.

Additional challenges to mitigating non-CO2 GHG emissions include the diffuse nature of their sources such as individual cattle or air conditioners and the difficulty of detecting and monitoring leaks.

Increased research, development, and demonstration (RD&D) is needed to address these challenges.

In order to compare the climate impacts of each greenhouse gas, scientists use a “global warming potential” (GWP) factor to convert the warming impacts from a non-CO2 gas into carbon dioxide equivalent (CO2e). A GWP assumes a given time period, since different gases have different lifetimes in the atmosphere.
Non-CO2 greenhouse gases are more potent than CO2 at trapping heat within the atmosphere, and thus have high GWPs.
Using the IPCC AR5 GWP estimates for a 100-year time scale,
  • methane is 28- 36 times more powerful than CO2,
  • nitrous oxide is 265-298 times more powerful, and
  • hydrofluorocarbons have GWPs as high as thousands or tens of thousands (EPA 2016b).

Because methane and HFCs cycle out of the atmosphere more quickly than CO2, their 10 and 20-year GWPs are even higher. This also means that near-term global actions on non-CO2 GHG emissions can effectively reduce the rate of near term warming.



Emissions are down only modestly compared to 2005 emissions levels but are 50 percent lower than a projection of emissions in 2050 without mitigation efforts.
The MCS analysis of non-CO2 mitigation potential does not account for major technological advances that may be achievable with increased RD&D investment. The cost estimates used in the analysis assume only minor technological progress over the next few decades. With continued innovation and well-designed policies, we can achieve even deeper non-CO2 reductions than those displayed in the MCS analysis.

As seen in Figure 6.2, the largest share of residual emissions in 2050 is methane from livestock, landfills, and fossil fuel production. Other major sources in 2050 include HFCs from existing equipment and appliances and N2O emissions from crop production.


Fossil fuels are not only the primary source of CO2 emissions but also a major source of methane emissions. Methane is released across the supply chain as part of fossil fuel production, processing, transmission, storage, and distribution. These emissions are both intentional (venting) and unintentional (leaks). Current estimates attribute nearly one-third of total U.S. methane emissions to oil and natural gas systems.
mining also releases methane trapped in coal seams, accounting for 9 percent of U.S. methane emissions (EPA 2016a).
Decarbonizing the energy sector, including transitioning away from fossil fuels to low-carbon energy, will not only reduce CO2 but will also reduce methane emissions associated with fossil fuel extraction and processing. However, fossil fuels will continue to play a role in the U.S. energy mix for some time. The MCS therefore envisions additional measures to reduce methane from oil and gas, including increasing the stringency of current standards and enhancing investments to improve methane emissions measurement, capture, and repair technology. Some of these methane reductions can be achieved highly cost effectively with the recovery and sale of captured methane
The United States has already taken action to better identify and reduce methane emissions from the energy sector. In 2014, President Obama released a national methane strategy targeting the largest sources of methane, including from oil and gas production and coal mines. The strategy also identifies opportunities to reduce methane from agriculture and landfills, which are discussed in subsequent sections. In 2015, the Obama Administration set a goal of reducing methane emissions from the oil and gas sector 40 to 45 percent below 2012 levels by 2025. Canada and Mexico have since also committed to this goal. In recent years, new information from studies of the U.S. oil and gas industry have indicated that methane emissions are much higher than previously understood. EPA updated the U.S. GHG Inventory with this information, resulting in a large increase in its estimates. In May of 2016, EPA finalized the first-ever standards for methane emissions from new and modified oil and gas facilities, and took the first steps in the process of developing emissions standards for existing sources.
Federal agencies are also coordinating a range of voluntary programs and supporting industry efforts and research initiatives to reduce methane emissions by recognizing leaders, through efforts like the Methane Challenge Program and the Oil and Gas Methane Partnership of the Climate and Clean Air Coalition. Additional federal programs are improving measurement and monitoring of oil and gas sector emissions, such as ARPA-E’s MONITOR program, which has invested $30 million to help reduce the cost of detecting and quantifying natural gas leaks (DOE 2014).
Recent evidence indicates that a small fraction of sources may be responsible for a large portion of total oil and gas methane emissions (Brandt, Heath, and Cooley 2016; Zavala-Araiza et al. 2015). Therefore, developing and deploying monitoring capabilities to identify these sources may be particularly effective for targeting mitigation actions. However, doing so is currently challenging due to the fact that these high-emitting sources are dispersed across the United States and may emit intermittently. Continuous monitoring at a large spatial scale via remote sensing technologies could help identify these sources. Use of new satellite, aircraft, and drone capabilities coupled with on-site continuous monitoring and automated infrared imaging have the potential to greatly improve leak detection, monitoring, and repairs.
In coal mining, commercially available technologies can recover and reduce methane emissions. These mitigation technologies include drainage and recovery systems to remove methane from the coal seam before mining or from the area post-mining, destruction of ventilation air-methane, and end-use application for recovered gas (e.g., electricity generation or use as a process fuel for on-site heating).


Agricultural production contributes over 40 percent of U.S. non-CO2 greenhouse gas emissions in the form of N2O and methane. Agriculture, in particular the use of nitrogen-rich fertilizers to increase crop yields, is the source of three-fourths of annual U.S. N2O emissions. Agricultural methane emissions are largely driven by livestock manure and enteric fermentation (EPA 2016a).
As discussed previously, global demand for food is projected to lead to greater global agricultural-related methane and N2O emissions. In spite of this growth, the MCS analysis points to potential actions to reduce N2O emissions significantly from 2005 levels by 2050. Still, without additional technological innovation, agricultural methane emissions will likely remain a significant GHG source in 2050.
Many technologies and practices are currently available that can reduce methane and N2O emissions associated with agricultural operations. Farmers, ranchers, and land managers across the United States are already using many of these techniques. Through the Department of Agriculture and programs such as the Environmental Quality Incentives Program (EQIP), the United States has promoted increased education, dissemination of online tools, and technical assistance to help farmers manage livestock herds, improve manure management, modify animal diets, and adopt alternative techniques to fertilizer applications. These actions reduce emissions while also maintaining yields and decreasing costs. In 2015, the USDA announced its Building Blocks for Climate Smart Agriculture and Forestry, previously discussed in Chapter 5. Through the Building Blocks, USDA is working closely with farmers, ranchers, and rural communities to implement voluntary, incentive-based practices that improve environmental conditions while also preparing communities for the impacts of climate change. To address N2O, USDA promotes efficient nitrogen stewardship to reduce over-application and nitrogen runoff into waterways through the principles of right timing, right fertilizer type, right placement, and right quantity. Adopting these techniques will enable farmers to maintain yield while decreasing expenses on fertilizer. Another Building Block supports livestock partnerships that use cost-share support and technical assistance to encourage broader deployment of anaerobic digesters, lagoon covers, composting, and solids separators to reduce methane emissions from cattle, dairy, and swine operations.
Building on these actions, greater emissions reductions can be achieved through broader uptake of these existing techniques, greater incentives to promote climate-smart practices, and technological innovation. Investments in animal genetics and breeding could improve the health and value of livestock while reducing feed demand and decreasing livestock-related emissions. Safe food additives like certain types of algae have the potential to significantly reduce methane production in livestock (Kinley and Fredeen 2014). Small-scale anaerobic digesters can capture methane from waste and supply renewable energy for electricity and on-farm equipment. Slow-release fertilizers and other precision agriculture techniques can reduce the amount of nitrogen that is applied to a field. USDA estimates that we can reduce non-CO2 emissions from agriculture by 25 percent or more from current levels by 2050 by successfully expanding existing mitigation options, making new technologies standard practice, and expanding outreach and technical assistance efforts.22 Over time, many of these mitigation solutions can lead to economic gains for farmers and ranchers, including lower fertilizer costs, and increasing health and productivity of livestock.
Addressing methane and N2O from agricultural production will continue to be challenging. Risk aversion, highly competitive agricultural markets, and growing impacts from climate change can make new practices unattractive for farmers and ranchers. Achieving widespread adoption of these practices could require putting in place economic incentives to help overcome potential concerns about lower yields, lower profits, and any costs associated with new technologies and practices. Improved approaches for capturing these emissions reductions in the U.S. GHG Inventory are also needed. By no means are the opportunities laid out in this report exhaustive. The MCS envisions additional focus on policies, incentives, and innovative technologies to scale up non-CO2 mitigation from agriculture.


Landfills are the third largest source of methane emissions in the United States, contributing 11 percent of non-CO2 emissions. As seen in Figure 6.2, landfill methane remains a significant share of non-CO2 emissions in 2050 under the MCS Benchmark scenario. When organic materials, such as food waste, decompose in the absence of oxygen, methane is produced. Methane emissions are similarly generated from municipal and industrial wastewater treatment activities, although centralized aerobic wastewater treatment facilities limit the amount of methane released. Municipal wastewater is also a source of N2O emissions—human sewage emits N2O during both the nitrification and denitrification of urea, ammonia, and proteins.
In July 2016, EPA finalized stringent standards to reduce methane emissions from new and existing landfills that will result in reductions of 8 million metric tons annually in 2025. The standard requires the installation of gas collection systems that capture methane to either flare or put to productive use, such as powering on-site equipment. EPA also supports smaller landfills in implementing landfill gas capture through voluntary programs like the Landfill Methane Outreach Program. The efficiency of biogas collection systems is currently around 85 percent (EPA 2008). This may be improved with technological advances and as new landfills are designed with gas collection in mind. Some of the remaining fugitive emissions from landfills could be reduced by installing and maintaining bio-based systems such as bio covers or bio filters that oxidize methane emissions.
While these measures can reduce methane once created, other actions can help prevent methane production entirely. For example, food waste reduction and diversion programs cut the amount of organic waste decomposing in landfills. Approximately 133 billion pounds of food end up in landfills because it is either deemed cosmetically unfit or will not stay fresh long enough to be shipped far distances, making it the single greatest contributor to municipal landfills (USDA 2015). In addition to exacerbating methane emissions, food waste contributes to excess fossil fuel and water use, while also putting increasing pressures on cropland as global food demand grows. In September 2015, USDA and EPA, along with many private sector and food bank partners, announced a national target to reduce food waste 50 percent by 2030, including through encouraging farmers to donate more of their imperfect produce to the hungry (USDA 2015). Multiple states, including Massachusetts, Vermont, and Connecticut, have implemented regulations to reduce food waste from commercial sources (Perry 2014). Scaling up these waste diversion programs would help significantly reduce landfill emissions in the future.
Finally, methane emissions in wastewater treatment could be significantly reduced by 2050 through currently available mitigation options, such as anaerobic biomass digesters and centralized wastewater treatment facilities. Improved operational practices, such as controlling dissolved oxygen levels during treatment or limiting operating system upsets, can also help reduce N2O emissions from wastewater treatment.


Fluorinated gases are man-made and used in a range of applications. They are highly potent greenhouse gases, trapping hundreds to thousands of times more heat than carbon dioxide. The vast majority of fluorinated gases emitted are hydrofluorocarbons (HFCs). A substitute for ozone-depleting substances, HFCs are primarily used for refrigeration and air conditioning.
Absent regulation, emissions of HFCs in the United States and globally were expected to double between 2015 and 2030, due both to the phase-out of ozone-depleting substances under the Montreal Protocol on Ozone-Depleting Substances and the overall growth of air conditioning and refrigeration around the world (EPA 2012, Velders et al. 2009). Fortunately, HFC emissions reductions are achievable by preventing or reducing leaks and transitioning to the use of low-GWP alternatives. The Obama Administration has reduced HFCs through both international diplomacy and domestic actions.
Over the past several years, the Obama Administration announced a series of executive actions to address HFCs. Under the Significant New Alternatives Policy (SNAP), EPA lists acceptable alternatives used in aerosols, foam-blowing, refrigeration, and other sectors. In 2015 and 2016, EPA finalized two regulations to prohibit the use of certain HFCs and HFC-containing blends across a variety of end-uses where safer and more climate-friendly alternatives are available. In September 2016, EPA also finalized a regulation that would strengthen existing refrigerant management requirements and extend safe handling, reuse, and disposal requirements to HFCs.
Along with these regulatory measures, the White House announced a series of private-sector commitments to cut HFC usage. The combination of private-sector commitments and executive actions in the United States is estimated to reduce domestic reliance on HFCs and contribute to a reduction in cumulative global consumption by more than 1 billion MtCO2e through 2025.
Significant progress has also been made this year on the international front. In October 2016, the United States worked with nearly 200 other countries to adopt an amendment under the Montreal Protocol to phase down the production and consumption of HFCs. Under the Kigali Amendment to the Montreal Protocol, the United States and other countries listed under Article 2 of the Montreal Protocol committed to phase down production and consumption of HFCs by 85 percent by 2036, while the rest of the world committed to 80 to 85 percent reductions by 2047. The United States is also working with partners in the Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants (CCAC), launched in 2012, to promote climate-friendly alternatives and standards for HFCs.
Achieving HFC reductions beyond those shown in Figure 6.2 will depend on addressing the existing stock of refrigerators and air conditioners, which already contain HFCs and have potential to leak into the atmosphere over the coming decades. For example, EPA can help to reduce or eliminate the leaking of HFCs from various types of refrigerant-containing equipment through targeted partnership programs such as its GreenChill program, which partners with food retailers to, among other things, lower refrigerant charge sizes and eliminate leaks. EPA can also scale up partnership programs such as the Responsible Appliance Disposal Program to prevent emissions through the proper disposal of appliances by ensuring recovery and reclamation or destruction of refrigerants and foam.
Additional RD&D support to ensure new alternatives to HFCs continue to enter the market may also be important, including both new molecules and new uses for existing alternatives, though private sector players are already leading the way on this front.