The term “climate change” is often used interchangeably with “global warming.”  The most recent findings on climate change provide clear evidence that ‘human activities, especially the combustion of fossil fuels, are influencing the climate in ways that threaten the well-being and continued development of human society. Global warming is the increase in the average temperature of earth’s near-surface air and oceans since the mid-20th century and its projected continuation. According to the 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC), global surface temperature increased 0.74 ±  0.18 °C (1.33 ± 0.32 °F) during the 20th century1.A result of increasing concentrations of atmospheric aerosols that block sunlight from reaching the surface, which is also called global dimming, which partially countered the effects of warming induced by greenhouse gases.

Whenever a carbon-based fuel is burned CO2 is emitted, butother pollutants that are the unintended by-products of non-ideal combustion are also released. These pollutants include carbon monoxide (CO), oxides of sulfur and nitrogen (SOX and NOX), volatile organic compounds (VOCs), and particulate matter (PM or aerosols). Until recently these pollutants were regarded as having primarily local health and environmental impacts. Some of these species can also impact the global climate system by changing the Earth’s radiative balance. Black Carbon is one of the unique pollutants that have a large direct negative impact on indoor and outdoor air quality like temperature, cloudiness, precipitation, mountain; snow packs glaciers, sea ice, and human health. The lack of access to fossil fuels forces more than three billion from rural populations to burn biomass fuels such as dung, firewood and crops and which contributes to the black carbon emission. Black carbon, the main component of soot, is a product of the incomplete combustion of fossil fuels (primarily coal and diesel fuel), the burning of solid biomass in cook stoves and heating stoves and the open burning of biomass. The black carbon in the global atmosphere emitted from the burning of wood and cow dung in household cooking and through the use of coal to heat homes. Countries in Europe and elsewhere that rely heavily on diesel fuel for transportation also contribute large amounts.

Black carbon or soot is a part of a larger type of emissions, both natural and manmade, called aerosols. These aerosols are very small particles that are suspended in our atmosphere. They can originate from natural sources, such as salt spray formed during ocean wave processes, dust formed from the grinding and erosion of land surfaces, forest fires, and volcanic eruptions. These aerosols play complex role in our atmosphere; they can have either a cooling or warming effect, depending on a variety of other factors. For example, a large volcanic eruption can inject millions of tons of small light coloured particles high into the atmosphere where they reflect sunlight back into space causing a cooling effect. This effect can be measured by a temperature drop on our planet’s surface. Darker particles, however, directly absorb energy as any dark surface will do and therefore have a warming effect. Black carbon heats the region in two ways. Firstly the dark particles absorb heat from the sun and collect at mountain latitudes and secondly, this soot coats the snow, turning it grey and reducing the snow’s albedo – (ability to reflect sunlight), thus accelerating the melting of snow.

Although black carbon has long been thought to be a contributor to global climate change, its contribution traditionally was estimated to be minor compared to the contribution of the main greenhouse gases (GHGs). Black carbon is often transported over long distances, mixing with other aerosols along the way. The aerosol mix can form transcontinental plumes of atmospheric brown clouds, with vertical extents of 3 to 5 km. Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions.

According to the IPCC  (Intergovernmental Panel on Climate Change), black carbon is the third largest contributor to the positive radiative forcing that causes climate change. Brazil, Indonesia, and Central Africa are the area where it is happening the most. Black carbon has also been identified as a driver of important regional climate impacts as black carbon is washed out of the atmosphere by rain, and only has a lifetime of a few weeks. If we stop emitting it, its contribution to climate change would cease quickly. Asia is the single-largest source of global black carbon emissions from contained combustion (combustion in engines, stoves and kilns), more than half of all types of emissions. A major contributor to global black carbon emissions in Asia is from open combustion (forest fires, land clearing through fire, and burning of agricultural wastes). Open combustion also produces emissions of climate-cooling aerosols and most of the studies estimated overall climate neutral or cooling effect. According to NASA Science news, since 1975, global surface temperatures have increased by about 0.9 degrees Fahrenheit, a trend that has taken global temperatures to their highest level in the past millennium.

Black carbon deposition on snow and ice in Arctic and Antarctic areas, causes the surfaces to absorb more of the sun’s heat, and may be responsible for as much warming in the Arctic as combination of anthropogenic forcing combined. The black carbon settling in the Arctic is a major cause of warming there, possibly responsible for 1°C of the 2.5°C of warming already observed in Arctic. Black carbon is also a major threat to the Himalayan glaciers, which in turn are the source of water for rivers of critical human importance, such as the Ganges. The dark soot particles affect the area by absorbing sunlight and heating the air around them. When black carbon gets trapped in the air flanking the Himalayas, it creates a warm layer that then rises into the mountains and accelerates glacial melting. The effect of this regional phenomenon may even be greater than that of global warming from greenhouse gases.

Although black carbon lasts only up to a few weeks in the atmosphere, its warming potential is about 700 times greater than CO2 during the 100 years following emission. Methane, with its warming effects around 25 times greater than CO2, has an atmospheric lifetime of only 10–12 years. Because of the short residence life of black carbon in the atmosphere, focusing on reducing black carbon emissions can result in immediate climate benefits and also help reduce health risks associated with incomplete combustion in many developing countries. The distribution of black carbon emissions is somewhat important because of its relatively short atmospheric lifetime. Emissions near Arctic areas or other areas with year-round snow cover will have a stronger albedo impact than emissions in the tropics, as more black carbon will fall onto the ice.

Reductions in black carbon emissions produce almost immediate benefits in terms of reduced radiative forcing, due to the short average residence time of black carbon in the atmosphere, which ranges from days to weeks, compared to that of greenhouse gases, which last years to centuries. Such reductions also produce large co-benefits for public health. A little attention has also been given to a low-risk, cost-effective and high-reward option, reducing emissions of light-absorbing carbon particles and of the gases that form the ozone. Limiting the presence of black carbon in the atmosphere is an easier, cheaper, and more politically feasible proposition than the most popular proposals for slowing climate change and it would have a rapid effect.

Some commercial products already exist for mitigating the black carbon emission and the technologies which could substantially reduce black carbon emission are in the form of commercially available products. In the case of households, these include cleaner fuels like biogas, natural gas, Liquid Petroleum gas (LPG) or kerosene, or direct solar power and improved cook stoves that burn fuel more efficiently and result in fewer emission of black carbon and other climate-relevant pollutants. Also, there is a requirement to install better filters on diesel engines than it does to curb activities that are critically linked to greenhouse gas emissions.

Control on black carbon can produce rapid regional and global climate benefits. The use of cleaner burning fuels or filters would provide immediate cooling benefits and prevent black carbon emissions as soot only remains in the atmosphere for a few weeks. q

Dr. Uzma Nadeem
unadeem@devalt.org

References
1. IPCC (2007-05-04).

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