Black Carbon

Black carbon (BC) is an aerosol pollutant emitted as a byproduct of the combustion of organic matter, and is a component of soot or ash. Sources of BC include cooking and heating stoves, industrial plants, coke and brick kilns, diesel combustion, agricultural burning, and oil and gas flaring. Researchers estimate that approximately 20 percent of global BC emissions come from the burning of biofuels, 40 percent from fossil fuels, and 40 percent from biomass burning. There are very high levels of uncertainty surrounding how much BC is emitted globally each year, and it is difficult to quantify current and past emissions of BC from biomass burning. As countries industrialize, they tend to become greater sources of BC from fossil fuels. The largest BC emitters are the major emerging economies, including China, India, and Brazil. From 1950 to 1990, the largest emitters were the United States, Europe, and other industrializing regions, particularly in Asia.

Unlike carbon dioxide (CO2), which stays in the atmosphere for centuries, the atmospheric lifetime of BC is on the scale of days to weeks. This short lifetime means that BC, like a number of other short-lived climate forcers such as methane and some hydrofluorocarbons (HFCs), has disproportionately large short-term climate impacts, and that large-scale reduction of BC emissions could significantly contribute to the reduction of near-term climate change.

BC particles also generally travel shorter distances than globally distributed greenhouse gases (GHGs) because of this short atmospheric lifetime, making BC primarily a localized source of pollution and a regional and local mitigation issue. As a major component of fine particulate matter (PM), BC is a large contributor to local air pollution and is detrimental to human health. Black carbon is a large component of soot from household cookstoves, contributing annually to the death of approximately 2 million people in the developing world.

Climate Impacts of Black Carbon

BC is believed to be a large net contributor to global warming, though there is a wide range of estimates for the magnitude of its radiative forcing (a measure of instantaneous warming of the atmosphere). Some scientists believe that BC’s contribution to global climate change is second in magnitude to CO2, accounting for approximately 18 percent of current radiative forcing; other scientists place BC third, behind methane. However, BC has complex and varying effects on warming throughout the atmospheric layers and at the surface.

Generally, the direction and magnitude of BC’s climate forcing in any given instance depends on where it is located in the atmosphere and the nature of other aerosols with which it is mixed. In order to devise optimal mitigation strategies, BC’s climate impacts are best considered in the context of the impacts of all pollutants with which BC is co-emitted.

By itself, BC absorbs solar radiation and contributes to warming by reducing the Earth’s albedo, both in the atmosphere and when deposited on the ground, particularly on snow and ice. However, other aerosols with which BC is consistently mixed (e.g., sulfates, nitrates, organic carbon, and other ash components) contribute to cooling by reflecting solar radiation and assisting in cloud formation. This mix of aerosols, including BC, can form atmospheric brown clouds (ABCs), which can be continental in size and up to 3 mi. (5 km) in height. ABCs have a negative radiative forcing, which may counteract as much as 50 percent of warming from other GHGs. Black carbon and other aerosols can also contribute to cooling near the surface by absorbing solar radiation, thereby causing dimming. However, new research is showing that BC and ABCs contribute significantly to warming in the lower atmosphere by exacerbating solar heating.

Studies have shown BC to have acute warming impacts in landscapes with significant snow and ice coverage, including the Himalayas and the Arctic. Through atmospheric warming, BC is believed to contribute to an increase of 1.08 degrees F (0.6 degree C) in the Himalayas, causing glacial retreat. Through its ability to decrease albedo of snow and ice, BC is posited to be the greatest contributor to melting of Arctic sea ice. Black carbon not only causes climatic impacts, but may also impact the hydrologic cycle, increasing atmospheric humidity and rainfall in certain areas. For example, studies show that BC emissions in south Asia are impacting seasonal monsoon rains. Black carbon emissions are shown to have wide-ranging health effects, particularly for respiratory and cardiovascular health. Thus, reductions in BC emissions are likely to carry significant social and environmental benefits.

Reducing Black Carbon Emissions

Black carbon is rapidly removed from the atmosphere by precipitation (wet removal) and wind or gravity (direct deposition), resulting in a very short atmospheric lifetime of approximately one week. Thus, reducing BC emissions is a shortterm strategy for addressing climate change and has the potential to head off up to several degrees of warming in certain regions.

Given the broad array of BC sources, there are many possible strategies and policies for reducing BC emissions. Air quality standards for PM10 and PM2.5 (particles that are 10 and 2.5 micrometers in size), which regulate particulate matter, control BC emissions indirectly. Use of low-sulfur fuel, high-efficiency engines, and particulate filters can reduce emissions from diesel combustion, which is used for ships, power generators, and transportation, agricultural, and construction vehicles. Reduction and control of forest fires and agricultural burning can reduce BC emissions from biomass. Technology upgrades and fuel switching from coal to natural gas or renewables can reduce BC emissions from coal burning. Using more efficient stoves and furnaces can reduce emissions from burning solid fuels in developing countries. The Global Alliance for Clean Cookstoves, which seeks to increase usage of more efficient cookstoves in developing country households, acknowledges both the health and climate change implications of inefficient burning of biomass, particularly as they relate to BC emissions.

Internationally, there is a policy gap for regulating BC emissions. There is some discussion under the Convention on Long Range Transboundary Air Pollution that BC be included under the Protocol to Abate Acidification, Eutrophication, and Ground-Level Ozone (Gothenburg Protocol). The Arctic Council, an intergovernmental body that facilitates discussion and decisions among the eight Arctic nations, established a task force to determine the effect of short-lived climate forcers, including BC, on Arctic warming and identify technical needs for mitigation. In 2009, the United States committed $5 million toward international cooperation on reducing BC emissions in and around the Arctic.