Black carbon or BC is formed through the incomplete combustion of fossil fuels, biofuel, and biomass, and is emitted in both anthropogenic and naturally occurring soot. Black carbon warms the planet by absorbing heat in the atmosphere and by reducing albedo, the ability to reflect sunlight, when deposited on snow and ice. Black carbon stays in the atmosphere for only several days to weeks, whereas CO2 has an atmospheric lifetime of more than 100 years.
Black carbon is a potent climate forcing agent, estimated to be the second largest contributor to global warming after carbon dioxide (CO2). Because black carbon remains in the atmosphere only for a few weeks, reducing black carbon emissions may be the fastest means of slowing climate change in the near-term.
Estimates of black carbon’s climate forcing (combining both direct and indirect forcings) vary from the IPCC’s conservative estimate of + 0.3 watts per square meter (W/m2) + 0.25, to the most recent estimate of 1.0-1.2 W/m2 (see Table 1), which is “as much as 55% of the CO2 forcing and is larger than the forcing due to the other greenhouse gasses (GHGs) such as CH4, CFCs, N2O, or tropospheric ozone.”
In some regions, such as the Himalayas, the impact of black carbon on melting snowpack and glaciers may be equal to that of CO2. Black carbon emissions also significantly contribute to Arctic ice-melt, which is critical because “nothing in climate is more aptly described as a ‘tipping point’ than the 0°C boundary that separates frozen from liquid water—the bright, reflective snow and ice from the dark, heat-absorbing ocean.” Hence, reducing such emissions may be “the most efficient way to mitigate Arctic warming that we know of.” ,
Since 1950, many countries have significantly reduced black carbon emissions especially from fossil fuel sources, primarily to improve public health, and “technology exists for a drastic reduction of fossil fuel related BC” throughout the world.
In its 2007 report, the IPCC estimated for the first time the direct radiative forcing of black carbon from fossil fuel emissions at + 0.2 W/m2, and the radiative forcing of black carbon through its effect on the surface albedo of snow and ice at an additional + 0.1 W/m2. More recent studies and public testimony by many of the same scientists cited in the IPCC’s report estimate that emissions from black carbon are the second largest contributor to global warming after carbon dioxide emissions, and that reducing these emissions may be the fastest strategy for slowing climate change.
Black carbon is formed through the incomplete combustion of fossil fuels, biofuel, and biomass, and is emitted in both anthropogenic and naturally occurring soot. Black carbon warms the planet by absorbing heat in the atmosphere and by reducing albedo, the ability to reflect sunlight, when deposited on snow and ice. Black carbon stays in the atmosphere from several days to weeks, whereas CO2 has an atmospheric lifetime of more than 100 years.
Given black carbon’s relatively short lifespan, reducing black carbon emissions would reduce warming within weeks. Control of black carbon, “particularly from fossil-fuel sources, is very likely to be the fastest method of slowing global warming” in the immediate future, according to Dr. Mark Jacobson of Stanford University, and he believes that major cuts in black carbon emissions could slow the effects of climate change for a decade or two. Reducing black carbon emissions could help keep the climate system from passing the tipping points for abrupt climate changes, including significant sea-level rise from the disintegration of the Greenland and/or Antarctic ice sheets.
“[E]missions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions,” according to Dr. V. Ramanathan and Dr. G. Carmichael. They calculate black carbon’s combined climate forcing at 1.0 – 1.2 W/m2, which “is as much as 55% of the CO2 forcing and is larger than the forcing due to the other [GHGs] such as CH4, CFCs, N2O or tropospheric ozone.” Other scientists estimate the total magnitude of black carbon’s forcing between + 0.2 to 1.1 W/m with varying ranges due to uncertainties.2 (See Table 1.) This compares with the IPCC’s climate forcing estimates of 1.66 W/m2 for CO2 and 0.48 W/m2 for CH4. (See Table 2.) In addition, black carbon forcing is two to three times as effective in raising temperatures in the Northern Hemisphere and the Arctic than equivalent forcing values of CO2.
Jacobson calculates that reducing fossil fuel and biofuel soot particles would eliminate about 40% of the net observed global warming. (See Figure 1.) In addition to black carbon, fossil fuel and biofuel soot contain aerosols and particulate matter that cool the planet by reflecting the sun’s radiation away from the Earth. When the aerosols and particulate matter are accounted for, fossil fuel and biofuel soot are increasing temperatures by about 0.35°C.
Black carbon alone is estimated to have a 20-year Global Warming Potential (GWP) of 4,470, and a 100-year GWP of 1,055-2,240. Fossil fuel soot, as a result of mixing with cooling aerosols and particulate matter, has a lower 20-year GWP of 2,530, and a 100-year GWP of 840-1,280.
Over the course of the century, however, the amount of these cooling aerosols in the atmosphere is expected to decrease, largely as a result of reductions in sulfur dioxide emissions. These reductions will unmask warming by other agents, such as black carbon, which these cooling aerosols currently help to offset. At the same time, under the IPCC A1B scenario, black carbon emissions are expected to double, further compounding their warming effect.
According to the IPCC, “the presence of black carbon over highly reflective surfaces, such as snow and ice, or clouds, may cause a significant positive radiative forcing.” The IPCC also notes that emissions from biomass burning, which usually have a negative forcing, have a positive forcing over snow fields in areas such as the Himalayas.
Black carbon is a significant contributor to Arctic ice-melt, and reducing such emissions may be “the most efficient way to mitigate Arctic warming that we know of,” according to Dr. Charles Zender of the University of California, Irvine. The “climate forcing due to snow/ice albedo change is of the order of 1.0 W/m2 at middle- and high-latitude land areas in the Northern Hemisphere and over the Arctic Ocean.” The “soot effect on snow albedo may be responsible for a quarter of observed global warming.” “Soot deposition increases surface melt on ice masses, and the meltwater spurs multiple radiative and dynamical feedback processes that accelerate ice disintegration,” according to NASA scientists Dr. James Hansen and Dr. Larissa Nazarenko. As a result of this feedback process, “BC on snow warms the planet about three times more than an equal forcing of CO2.”. When black carbon concentrations in the Arctic increase during the winter and spring due to Arctic Haze, surface temperatures increase by 0.5 C
Black carbon emissions from northern Eurasia, North America, and Asia have the greatest absolute impact on Arctic warming. However, black carbon emissions actually occurring within the Arctic have a disproportionately larger impact per particle on Arctic warming than emissions originating elsewhere. As Arctic ice melts and shipping activity increases, emissions originating within the Arctic are expected to rise.
In some regions, such as the Himalayas, the impact of black carbon on melting snowpack and glaciers may be equal to that of CO2,. Ramanathan & G. Carmichael, supra note 1, at 221. Warmer air resulting from the presence of black carbon in South and East Asia over the Himalayas contributes to a warming of approximately 0.6C. An “analysis of temperature trends on the Tibetan side of the Himalayas reveals warming in excess of 1C..”. This large warming trend is the proposed causal factor for the accelerating retreat of Himalayan glaciers,which threatens fresh water supplies and food security in China and India.
By Region: Developed countries were once the primary source of black carbon emissions, but this began to change in the 1950’s with the adoption of pollution control technologies in those countries. Whereas the U.S. emits about 21% of the world’s CO2, it emits 6.1% of the world’s soot. The Unites States and the European Union could further reduce their black carbon emissions by accelerating implementation of black carbon regulations that currently take effect in 2015 or 2020 and by supporting the adoption of pending International Maritime Organization (IMO) regulations. Existing regulations also could be expanded to increase the use of clean diesel and clean coal technologies and to develop second-generation technologies.
Today, the majority of black carbon emissions are from developing countries and this trend is expected to increase. The largest sources of black carbon are Asia, Latin America, and Africa. China and India account for 25-35% of global black carbon emissions. Black carbon emissions from China doubled from 2000 to 2006. Existing and well-tested technologies used by developed countries, such as clean diesel and clean coal, could be transferred to developing countries to reduce their emissions.
Black carbon emissions “peak close to major source regions and give rise to regional hotspots of black carbon—induced atmospheric solar heating.”. Such hotspots include, “the Indo-Gangetic plains in South Asia; eastern China; most of Southeast Asia including Indonesia; regions of Africa between sub-Sahara and South Africa; Mexico and Central America; and most of Brazil and Peru in South America.” . Approximately three billion people live in these hotspots.
By Source: Approximately 20% of black carbon is emitted from burning biofuels, 40% from fossil fuels, and 40% from open biomass burning, according to Ramanathan.. Similarly, Dr. Tami Bond of the University of Illinois, Urbana Champaign, estimates the sources of black carbon emissions as follows:
42% Open biomass burning (forest and savanna burning)
18% Residential biofuel burned with traditional technologies
14% Diesel engines for transportation
10% Diesel engines for industrial use
10% Industrial processes and power generation, usually from smaller boilers
6.0% Residential coal burned with traditional technologies Black carbon sources vary by region. For example, the majority of soot emissions in South Asia are due to biofuel cooking, whereas in East Asia, coal combustion for residential and industrial uses plays a larger role.
Fossil fuel and biofuel soot have significantly greater amounts of black carbon than climate-cooling aerosols and particulate matter, making reductions of these sources particularly powerful mitigation strategies. For example, emissions from the diesel engines and marine vessels contain higher levels of black carbon compared to other sources. Regulating black carbon emissions from diesel engines and marine vessels therefore presents a significant opportunity to reduce black carbon’s global warming impact.
Biomass burning emits greater amounts of climate-cooling aerosols and particulate matter than black carbon, resulting in short-term cooling. However, over the long-term, biomass burning may cause a net warming when CO2 emissions and deforestation are considered. Reducing biomass emissions would therefore reduce global warming in the long-term and provide co-benefits of reduced air pollution, CO2 emissions, and deforestation. Johannes Lehmann of Cornell University estimates that by switching to slash-and-char from slash-and-burn agriculture, which turns biomass into ash using open fires that release black carbonand GHGs, 12% of anthropogenic carbon emissions caused by land use change could be reduced annually, which is approximately 0.66 Gt CO2-eq. per year, or 2% of all annual global CO2-eq emissions.
Ramanathan notes that “developed nations have reduced their black carbon emissions from fossil fuel sources by a factor of 5 or more since 1950. Thus, the technology exists for a drastic reduction of fossil fuel related black carbon.”
Jacobson believes that “[g]iven proper conditions and incentives, [soot] polluting technologies can be quickly phased out. In some small-scale applications (such as domestic cooking in developing countries), health and convenience will drive such a transition when affordable, reliable alternatives are available. For other sources, such as vehicles or coal boilers, regulatory approaches may be required to nudge either the transition to existing technology or the development of new technology.”
Hansen states that “technology is within reach that could greatly reduce soot, restoring snow albedo to near pristine values, while having multiple other benefits for climate, human health, agricultural productivity, and environmental aesthetics. Already soot emissions from coal are decreasing in many regions with transition from small users to power plants with scrubbers.”
Jacobson suggests converting “[U.S.] vehicles from fossil fuel to electric, plug-in-hybrid, or hydrogen fuel cell vehicles, where the electricity or hydrogen is produced by a renewable energy source, such as wind, solar, geothermal, hydroelectric, wave, or tidal power. Such a conversion would eliminate 160 Gg/yr (24%) of U.S. (or 1.5% of world) fossil-fuel soot and about 26% of U.S. (or 5.5% of world) carbon dioxide.”According to Jacobson’s estimates, this proposal would reduce soot and CO2 emissions by 1.63 GtCO2–eq. per year. He notes, however, “that the elimination of hydrocarbons and nitrogen oxides would also eliminate some cooling particles, reducing the net benefit by at most, half, but improving human health,” a substantial reduction for one policy in one country.
For diesel vehicles in particular there are a several effective technologies available. Diesel oxidation catalysts have been in use for over 30 years, can be used on almost any diesel vehicle, and can eliminate 25-50% of black carbon emissions. Newer, more efficient diesel particulate filters (DPFs), or traps, can eliminate over 90% of black carbon emissions, but these devices require ultra low sulfur diesel fuel (ULSD). To ensure compliance with new particulate rules for new on-road and non-road vehicles in the U.S., the EPA first required a nationwide shift to ULSD, which allowed DPFs to be used in diesel vehicles in order to meet the standards. Because of recent EPA regulations, black carbon emissions from diesel vehicles are expected to decline about 70 percent from 2001 to 2020.” Overall, “BC emissions in the United States are projected to decline by 42 percent from 2001 to 2020.By the time the full fleet is subject to these rules, EPA estimates that over 239,000 tons of particulate matter will be reduced annually. Outside of the US diesel oxidation catalysts are often available and DPFs will become available as ULSD is more widely commercialized.
Another technology for reducing black carbon emissions from diesel engines is to shift fuels to compressed natural gas. In New Delhi, India, a court-ordered shift to compressed natural gas for all public transport vehicles, including buses, taxis, and rickshaws, resulted in a climate benefit, “largely because of the dramatic reduction of black carbon emissions from the diesel bus engines.” Overall, the fuel switch for the vehicles reduced black carbon emissions enough to produce a 10 percent net reduction in CO2-eq., and perhaps as much as 30 percent. The main gains were from diesel bus engines whose CO2-eq. emissions were reduced 20 percent. According to a study examining these emissions reductions, “there is a significant potential for emissions reductions through the [UNFCCC] Clean Development for such fuel switching projects.”
Technologies are also in development to reduce some of the 133,000 metric tons of particulate matter emitted each year from ships. Ocean vessels use diesel engines, and particulate filters similar to those in use for land vehicles are now being tested on them. As with current particulate filters these too would require the ships to use ULSD, but if comparable emissions reductions are attainable, up to 120,000 metric tons of particulate emissions could be eliminated each year from international shipping. That is, if particulate filters could be shown reduce black carbon emissions 90 percent from ships as they do for land vehicles, 120,000 metric tons of today’s 133,000 metric tons of emissions would be prevented. Other efforts can reduce the amount of black carbon emissions from ships simply by decreasing the amount of fuel the ships use. By traveling at slower speeds or by using shore side electricity when at port instead of running the ship’s diesel engines for electric power, ships can save fuel and reduce emissions.
Reynolds and Kandlikar estimate that the shift to compressed natural gas for public transport in New Delhi ordered by the Supreme Court reduced climate emissions by 10 to 30%.
Ramanathan estimates that “providing alternative energy-efficient and smoke-free cookers and introducing transferring technology for reducing soot emissions from coal combustion in small industries could have major impacts on the radiative forcing due to soot.” Specifically, the impact of replacing biofuel cooking with black carbon-free cookers (solar, bio, and natural gas) in South and East Asia is dramatic: over South Asia, a 70 to 80% reduction in black carbon heating; and in East Asia, a 20 to 40% reduction.”
In Delhi, India, a court-ordered shift to compressed natural gas for public transport is estimated to have reduced climate emissions by 10 to 30%.
Public health benefits of particulate matter reductions have been recognized for years. The WHO estimates that air pollution causes nearly two million premature deaths per year. By reducing black carbon, a primary component of fine particulate matter, the health risks from air pollution will decline. In fact, public health concerns have given rise to leading to many efforts to reduce such emissions, for example, from diesel vehicles and cooking stoves. Since black carbon has a damaging impact on plants, reducing it also benefits agriculture.
Many countries have existing national laws to regulating black carbon emissions, including laws that address particulate emissions. Some examples include:
The International Network for Environmental Compliance & Enforcement recently issued a Climate Compliance Alert on Black Carbon.
Table 1 : Estimates of Black Carbon Climate (Radiative) Forcings by Effect
|Black Carbon Radiative Forcing (W/m2)|
|Source||Direct Forcing||Semi-Direct Effect||Dirty Clouds Effect||Snow/Ice Albedo Effect||Total|
|IPCC (2007)||0.2 + 0.15||-||-||0.1 + 0.1||0.3 + 0.25|
|Jacobson (2001, 2004, and 2006)||0.55||-||0.03||0.06||0.64|
|Hansen (2001, 2002, 2003, 2005, and 2007)||0.2 - 0.6||0.3 + 0.3||0.1 + 0.05||0.2 + 0.1|| 0.8 + 0.4 (2001) |
1.0 + 0.5 (2002)
0.7 + 0.2 (2003)
|Hansen & Nazarenko (2004)||-||-||-|| ~ 0.3 globally |
|Ramanathan (2007)||0.9||-||-||0.1 to 0.3||1.0 to 1.2|
Table 2: Estimated Climate Forcings (W/m2)
|Component||IPCC (2007)||Hansen, et al. (2005)|
| title=Nature Geoscience: Global and regional climate changes due to black carbon