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 climate-warming substances with much shorter atmospheric lifetimes. These are called short-lived climate forcers (SLCFs). This two-pronged strategy would accomplish two goals:
- 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 forcers would, on the other hand, slow the near-term rate of climate change. Scientists estimate that SLCFs account for 30 to 40 percent of the human-induced warming to date. Yet as SLCFs remain in the atmosphere for periods of only a few days to a few decades, their warming effect is short-lived, and reducing their emissions would result in more immediate benefits. In addition to limiting climate impacts already underway, including important regional impacts such as glacial melting, SLCF reductions would reduce local air pollution and produce other co-benefits. The U.N. Environment Programme recently estimated that aggressive efforts to reduce SLCFs 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 Forcers
Methane has an atmospheric lifetime of about 12 years. Human-induced 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 improve local air quality by reducing ground-level ozone, which harms agriculture and human health, and is itself an SLCF.
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.
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 freshwater 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. They were developed to replace ozone-depleting substances a few decades ago, HFC-134a, the most widely used of these compounds, has an atmospheric lifetime of 13 years.
As many ozone-depleting substances are also potent greenhouse gases, their phase-out under the Montreal Protocol has contributed indirectly but very significantly to climate mitigation efforts to date. The treaty's net contribution to climate mitigation is estimated to be five to six times larger than the Kyoto Protocol's first commitment period targets.
Many countries now favor working through the Montreal Protocol to phase down HFCs. A proposal by the United States, Mexico and Canada would require an 85 percent reduction in specified HFCs by 2033 for developed countries, and 2043 for developing countries. A proposal by the States of Micronesia and Mauritius calls for a 90 percent reduction by developed countries by 2030, but specifies no schedule for developing countries.
- 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: External Peer Review Draft, March 2011.
International Forums Focused on SLCFs: