Short-lived Climate Pollutants

At-a-glance

  • The main short-lived climate pollutants are black carbon, methane, tropospheric ozone, and fluorinated gases.
  • Currently, fluorinated gases (HFCs, perfluorocarbons (PFCs), SF6, and NF3) account for 3 percent of domestic greenhouse gas emissions in terms of carbon dioxide equivalency (CO2e). The EPA has created several voluntary programs aimed at lowering these emissions.
  • Due to their immense contributions to climate change per molecule emitted, reducing short-lived climate pollutants can be very cost-effective. Actions taken in the immediate future to address them could slow the planet’s warming 0.6 degrees by mid-century.

Introduction

There is growing recognition within the scientific and policy communities that efforts to address climate change should focus not only on substantially reducing carbon dioxide emissions, but also on near-term actions to reduce those climate pollutants that remain in the atmosphere for much shorter periods of time. Short-lived climate pollutants—such as black carbon, methane, troposopherhic ozone, and hydrofluorocarbons—account for 40 to 45 percent of global warming to date. Unlike carbon dioxide, short-lived climate pollutants have a shorter atmospheric lifetime but have a high global warming potential, meaning they can warm the Earth faster compared to carbon dioxide. Targeted efforts to reduce these emissions can slow the pace of global warming by 0.6 degrees Celsius by 2050.

U.S. Greenhouse Gas Emissions by Gas, 2015

Key Short-Lived Climate Pollutants

The most significant short-lived climate pollutants are black carbon, methane, tropospheric ozone, and fluorinated gases due to their atmospheric impacts as depicted in the below figure. They primarily come from fossil fuel production and combustion.

Short-Lived Climate Pollutants

Black Carbon

Black carbon results from incomplete combustion of biomass and fossil fuels. Its major sources are diesel cars and trucks, cook stoves, forest fires, and agricultural open burning. Black carbon has a short atmospheric lifetime, on the order of a few days to weeks. Due to a very brief atmospheric lifetime, black carbon’s climate effects are strongly regional. Black carbon particles give soot its black color and, like any black surface, strongly absorb sunlight. In snow-covered areas, the deposition of black carbon darkens snow and ice, increasing their absorption of sunlight and making them melt more rapidly. Black carbon may be responsible for a significant fraction of recent warming in the rapidly changing Arctic, contributing to the acceleration of sea ice loss, and melting of glaciers, which are a major source of fresh water for millions. Additionally, since black carbon contributes to respiratory and cardiovascular illnesses, reductions in this pollutant would have significant co-benefits for human health, particularly in developing countries. Black carbon’s short lifetime also means that its contribution to climate warming

Methane

Methane has an atmospheric lifetime of about 12 years and a global warming potential of 28 over a hundred-year period. It makes up about 10 percent of greenhouse gas emissions in the United States and roughly 16 percent worldwide. Globally, methane emissions are generated primarily by ruminant livestock (produced by bacteria in the rumen of animals as a result of the fermention process, known as enteric fermentation), oil and gas production and distribution, coal mining, solid waste landfills, cultivation of rice, and biomass burning. Reductions in methane emissions also improve local air quality by reducing voltaile organic compounds, hazardous air pollutants, and ground-level ozone, which harms agriculture and human health.

Tropospheric Ozone

Ozone occurs at both the troposphere (ground level) and in the stratosphere. Tropospheric ozone is created from the chemical reactions of carbon monoxide, nitrogen oxides, and volatile organic compounds, known as precursors. Ground-level ozone, carbon monoxide, and nitrogen dioxide are all known as criteria air pollutants, regulated by the EPA due to their harmful health effects. They are found at the source of any fossil fuel combustion, and reducing fossil fuel use at power plants, industrial facilities, and vehicles can reduce ozone levels.

Fluorinated gases

Fluorinated gases (F-gases) are created from human-related activity. There are four types of F-gases: hydrofluorocarbons, perfluorocarbons, nitrogen fluoride, and sulfur hexafluoride. Usage of hydrofluorocarbons and perfluorocarbons have increased in the last two decades because they are good substitutes to ozone depleting substances that were phased out under the Montreal Protocol.

Hydrofluorocarbons are used in air conditioning, refrigeration, foam blowing, aerosols, and as solvents. Hydrofluorocarbons are the fastest-growing greenhouse gases, increasing globally at a rate of 10 percent to 15 percent per year. The global warming potential of HFCs can be thousands of times greater than carbon dioxide’s and can have a significant impact on climate change. The greatest source of HFCs, and the greatest source of any high global warming potential gas, is leakage from refrigeration and air conditioning equipment. Given their high emission rates and, on average, have a relatively short atmospheric lifetime (compared to carbon dioxide), efforts to reduce hydrofluorocarbon emissions in the near term will significantly reduce projected temperature increases in the coming decades. While hydrofluorocarbons now contribute 2 percent of total global warming emissions, their use is expected to grow dramatically over time (see Figure below).

 

Projected Growth in HFCs and Climate Forcing from Emissions

Costs

The costs associated with reducing short-lived climate pollutants are similar to the costs of reducing other greenhouse gases. Costs can vary depending where they come from and the available technologies to reduce those emissions. Typically, they will be lower for reductions achieved through efficiency improvements or using substitutes that emit less carbon—these options may even generate a net savings. For example, achieving energy efficiency in buildings and appliances costs less than reducing the carbon intensity of power plants, where capital-intensive technologies that need to be installed may take more time.

In addition, emissions from a fixed, identifiable source present more cost-effective emissions reduction options than other diffuse emissions sources. For example it is easier to address, fugitive emissions from a single building than emissions from mobile, fossil-fuel burning cars and trucks. Actions taken to reduce greenhouse gas emissions elsewhere can also help reduce high global warming gases.

Policy Actions on High GWP Gas and SCLP Mitigation

International Efforts

Climate and Clean Air Coaliation (CCAC)

The Climate and Clean Air Coalition (CCAC) is a cooperative public-private initiative that promotes national and international action but establishes no legal obligations or authorities. Formed in 2012, the coalition includes the United Nations Environment Program (UNEP) and other governments around the world, united to address short-lived climate pollutants and to deliver action and benefits on: “climate, public health, energy efficiency, and food security.”

Montreal Protocol

The Montreal Protocol is an international treaty aimed at eliminating ozone depleting substances, which are also potent greenhouse gases. The treaty’s net contribution to climate mitigation is about five to six times larger than the Kyoto Protocol’s first commitment period targets.

At the 2016 Meeting of the Parties to the Montreal Protocol in Kigali, Rwanda, countries around the world agreed to a legally binding commitment to reduce HFC emissions that could prevent up to 0.5 degrees Celsius of warming by 2100. This landmark agreement includes targets and timetables to replace HFCs with cleaner alternatives, provisions that restrict countries who ratified the protocol from trading with countries who have not, and a commitment by richer countries to finance poorer countries’ transition to the new standards.

U.S. Efforts

New Source Performance Standards

Under the Obama Administration, EPA issued New Source Performance Standards (NSPS) to address methane emissions from new, modified and reconstructed sources in the oil and gas industry. It set emission limits for methane, covered additional sources located at oil wells and processing plants than the previous 2012 NSPS rule, and required owners and operators to check and repair leaks.

Voluntary Efforts

The U.S. EPA has established several voluntary programs aimed at reducing high-GWP emissions. One is the SF6 Emission Reduction Partnership for Electric Power Systems, where utilities partner with EPA and voluntarily commit to reducing sulfur hexafluoride emissions. Since its creation in 1999, these utilities have decreased the ratio of sulfur hexafluoride emissions relative to the total amount of sulfur hexafluoride in their equipment. In September 2014, the Obama administration announced a series of voluntary commitments from chemical firms, manufacturers and retailers, and the federal government to move rapidly away from HFC-134a and similar compounds and to shift to more environmentally friendly replacements.

U.S. EPA SNAP

The EPA also maintains a regulatory program called the Significant New Alternatives Policy Program (SNAP). Under this program, the EPA may evaluate and control substitutes to ozone depleting substances to ensure that they are more environmentally benign than the substances they seek to replace. In August 2014, under the SNAP Program, EPA proposed to limit the use of certain HFCs in mobile air conditioning, certain types of foams, and aerosol applications. In August 2017, the U.S. Court of Appeals for the District of Columbia Circuit ruled EPA lacked the authority to require manufacturers of HFCs to replace HFCs based on climate change. As of January 2018, the Court of Appeals has denied rehearing the case. In May 2016, EPA finalized standards for new and existing sources in the oil and gas sector which would reduce methane, volatile organic compounds (VOCs) and toxic air emissions. These standards applied to hydraulically fractured oil wells and to new, reconstructed, and modified processes and equipment. Overall, the standards were anticipated to reduce methane emissions by 40–45 percent by 2025.

State Level Action

A 2016, California law established the most stringent state restrictions on short-lived climate pollutants. It sets goals to cut methane and hydrofluorocarbon gases by 40 percent and black carbon by 50 percent below 2013 levels by 2030.

In addition, California, Colorado, Wyoming, Ohio, and Pennsylvania have adopted or in the process of adopting regulations to control methane from oil and gas operations.