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 (CO2) emissions, but also on near-term actions to reduce those climate pollutants that remain in the atmosphere for much shorter periods of time. With atmospheric lifetimes on the order of a few days to a few decades, the primary short-lived climate pollutants (SLCPs) are methane, black carbon and certain hydrofluorocarbons. SCLPs are responsible for 30-40 percent of global warming to date. Actions to reduce their emissions could reduce by half the amount of warming that would occur over the next few decades.
In the past few years, considerable attention has shifted to these compounds. New policies (described below) have advanced both within the United States and by the international community aimed at reducing emissions of these pollutants.
Reductions Required in both CO2 and SLCPs
To effectively slow the rate and magnitude of climate change, a strategy that significantly reduces both carbon dioxide and SLCPs is critical:
- Reducing CO2 emissions limits the ultimate amount of warming. Because CO2 represents by far the largest source of climate-warming emissions, and because it stays in the atmosphere for hundreds of years, large reductions in CO2 emissions are required to meet any long-term climate stabilization goal, such as the 2°C goal set by the international community.
- Reducing emissions of short-lived climate pollutants would, on the other hand, effectively slow the near-term rate of climate change. Because SLCPs remain in the atmosphere for a relatively short period of time (compared to CO2) reducing their emissions would result in more immediate benefits. In addition to limiting climate change impacts already underway, including important regional impacts such as glacial melting, SLCP reductions would reduce local air pollution and produce other co-benefits. The U.N. Environment Programme recently estimated that aggressive efforts to reduce SLCPs would avoid 2.4 million premature deaths by 2030 and reduce warming between now and 2040 by a half a degree.
Key Short-Lived Climate Pollutants
Methane has an atmospheric lifetime of about 12 years and a global warming potential of 25 times that of carbon dioxide. It makes up approximately 9 percent of GHG emissions in the United States and roughly 14 percent worldwide. Methane emissions result primarily from oil and gas production and distribution, coal mining, solid waste landfills, cultivation of rice and ruminant livestock, and biomass burning. Reductions in methane emissions also improve local air quality by reducing ground-level ozone, which harms agriculture and human health, and is itself a SLCP.
Black carbon (BC) 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.
Because of a very brief atmospheric lifetime measured in weeks, black carbon's climate effects are strongly regional. BC 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. BC may be responsible for a significant fraction of recent warming in the rapidly changing Arctic, contributing to the acceleration of sea ice loss. BC also is contributing to the melting of Himalayan glaciers, a major source of fresh water for millions of people in Asia, and may be driving some of the recent reduction in snowpack in the U.S. Pacific Northwest.
Black carbon's short lifetime also means that its contribution to climate warming would dissipate quickly if emissions were reduced. Additionally, since BC contributes to respiratory and cardiovascular illnesses, reductions in BC emissions would have significant co-benefits for human health, particularly in developing countries.
Hydrofluorocarbons (HFCs) are a family of industrially produced chemicals widely used in refrigeration and air conditioning, foam blowing, and other applications. They were developed to replace ozone-depleting substances (primarily chlorofluorcarbons and hydrochlorofluorocarbons – CFCs and HCFCs) a few decades ago. While HFCs now contribute around 1 percent of total global warming emissions, their use is expected to grow dramatically over time. HFC-134a, the most widely used of these compounds, has an atmospheric lifetime of 13 years and a global warming potential of 1300.
Because ozone-depleting substances (CFCs and HCFCs) are also potent greenhouse gases, their phase-out under the Montreal Protocol has contributed significantly to climate mitigation efforts to date. The treaty's net contribution to climate mitigation (taking into account the growth of HFCs as replacements) is estimated to be five to six times larger than the Kyoto Protocol's first commitment period targets.
Policy Actions in the United States to Limit SLCPs
President Obama included specific actions aimed at reducing emissions of SLCPs in his Climate Action Plan, announced in June 2013. The plan called for the Environmental Protection Agency to use its regulatory authority under the Clean Air Act to limit the use of HFCs where more climate-friendly alternatives are available. In August 2014, under its Significant New Alternatives Program, EPA proposed to limit the use of certain HFCs in mobile air conditioning, specified types of foams and aerosol applications.
In September 2014, the Obama administration announced a series of voluntary commitments from chemical firms, manufacturers and retailers to move rapidly away from HFC-134a and similar compounds and to shift to more environmentally friendly replacements. The announcement also called for the federal government to shift its purchasing to products that use alternatives to HFCs.
The Climate Action Plan also called for additional domestic actions to curb methane emissions, and created an interagency task force to develop a comprehensive strategy for identifying and reducing methane emissions. As a first step in implementing this strategy in April 2014, EPA issued for public comment five white papers on emission sources and reduction technologies for reducing methane in the oil and gas sector. On January 14, 2015, the Administration announced a series of actions aimed at reducing methane emissions in the United States by 40-45 percent from 2012 levels by 2025. The announcement calls for EPA to directly regulate methane emissions from new or modified oil and gas production sources and natural gas processing and transmission sources. It also calls for the agency to issue guidelines for states to reduce VOC emissions from existing oil and gas systems in nonattainment areas and within the Ozone Transport Region. The Bureau of Land Management is to take action to reduce emissions from existing oil and gas wells leased on public lands.
International Initiatives to Limit SLCPs
Climate and Clean Air Coalition: In February 2012, Secretary of State Hillary Clinton announced the creation of the Climate and Clean Air Coalition. Directed specifically at reducing global emissions of SCLPs, this coalition has grown from six governments at the tine of its announcement to 36 state and regional governmental partners, and numerous international and non-governmental organizations. Since its inception, it has developed initiatives across a wide range of sectors including diesel engines, cook stoves, HFCs, oil and gas, and waste.
Montreal Protocol Action on HFCs
A proposal by the United States, Mexico and Canada would require an 85 percent reduction in specified HFCs by 2035 for developed countries, and 2045 for developing countries. While efforts in past years to achieve agreement on this or related proposals have been unsuccessful, the United States and others continue to make the case at annual meetings of the Protocol and in other multilateral and bilateral discussions for using this treaty as a mechanism for achieving worldwide reductions in HFCs. This issue will once again be discussed at the 2015 Meeting of the Parties scheduled for October in Dubai.
High-Level Bilateral and Multilateral Discussions:
President Obama and President Xi of China in a statement issued in June 2013 agreed to work together using the Montreal Protocol to phase-down HFCs.
In St. Petersburg, Russia at the G-20 meeting in September 2013, the member nations included in their final declaration a statement supporting the use of the Montreal Protocol to phasedown HFCs.
In their joint statement on Sept. 30, 2014, President Obama and Prime Minister Modi of India recognized the need to use the institutions and expertise of the Montreal Protocol to reduce emissions of HFCs.
On May 9, 2013, Rep. Scott Peters introduced the Super Pollutant Emissions Reduction Act, which would create a task force aimed at developing strategies and actions to reduce SLCPs.
On September 18, 2014, Senators Christopher Murphy, Susan Collins, and nine others introduced a similar bill, the Super Pollutants Act of 2014.
- Bachmann, John and Seidel, Stephen, Domestic Policies to Reduce the Near-Term Risk of Climate Change. Center for Climate and Energy Solutions, 2013.
This paper sets out a series of cost-effective steps that the Obama Administration can implement under existing authorities that would deliver substantial near-term reductions in the rate of climate change.
- Fast Action to Reduce the Risks of Climate Change: U.S. Options to Limit Short-Lived Climate Pollutants, Feb. 2012
- Bodansky, Daniel, Multilateral Climate Efforts Beyond the UNFCCC, Center for Climate and Energy Solutions, Nov. 2011.
This report looks at a number of multilateral entities that could play a role in addressing certain of the SLCFs including: the Montreal Protocol as a possible venue for HFCs and the Convention on Long-Range Transboundary Air Pollution as a possible venue for BC, methane and other ozone-precursors.
- Bachmann, John, Black Carbon: A Science/Policy Primer, Center for Climate and Energy Solutions (formerly the Pew Center on Global Climate Change), 2009.
This paper summarizes current knowledge on the effects of soot components—black carbon and organic particles—on climate, and identifies sources and technologies to mitigate their impacts. It also presents perspectives on the potential role of soot mitigation approaches in developing more comprehensive climate strategies.
- What is Black Carbon?, Center for Climate and Energy Solutions (formerly the Pew Center on Global Climate Change), April 2010.
This factsheet provides an overview of black carbon as a major contributor to global climate change. It describes why reducing black carbon is a win-win scenario for both climate and health reasons.
- Read Eileen Claussen's statement on the Climate and Clean Energy Coalition to Reduce Short-Lived Climate Pollution
- UNEP and WMO, Integrated Assessment of Black Carbon and Tropospheric Ozone: Summary for Decision Makers, 2011
- UNEP, Near-term Climate Protection and Clean Air Benefits: Actions for Controlling Short-Lived Climate Forcers, 2011.
- Montzka, S.A., Dlugokency,E.J., Butler, J.H., Non-CO2 Greenhouse Gases and Climate Change. Nature, 476, 43-50. August 2011.
- Shindell, D. et. al., Simultaneously Mitigating Near-Term Climate Change and Improving Human Health and Food Security, Science, 335, 183-188 Jan 13, 2012.
- UNEP, HFCs: A Critical Link in Protecting Climate and the Ozone Layer, Nov. 2011.
- Velders, G.J.M., Fahey, D.W., Daniel, J.S., McFarland, M, Andersen, S.O., The Large Contribution of Projected HFC Emissions to Future Climate Forcing. Proc. Nat. Acad. Sci. 106, 10949-10954. 2009
- US EPA, Report to Congress on Black Carbon: March 2012.
International Forums Focused on SLCPs: