Short-lived Climate Forcers
Short-Lived Climate Pollutants (SLCFs) are warming agents (particulate or gaseous) that have atmospheric lifetimes of days to decades. 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 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.
Modelled impact of various reductions in carbon dioxide, methane and black carbon (BC) on global mean temperature.
Figure courtesy of UNEP Integrated Assessment of Black Carbon and Tropospheric Ozone.
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.