Science in Society Archive

Black Carbon Warms the Planet Second Only to CO2

Eighty percent of black carbon emissions come from fossil fuels and biomass burning associated with deforestation; reducing black carbon emissions may be the quickest, cheapest way to save the climate Dr. Mae-Wan Ho

New research shows that airborne soot, or black carbon (BC) aerosols resulting from incomplete combustion, are warming the earth much more than previously thought [1]. According to Veerabhadran Ramanathan at the Scripps Institution of Oceanography San Diego and Greg Carmichael at the University of Iowa, the warming effect of black carbon is 55 percent that of CO2, the biggest contributor to global warming.

The annual emission of BC (for year 1996) was estimated at about 8 Tg (1012g); of which 20 percent comes from biological fuels (wood, dung and crop residues), 40 percent from fossil fuels (diesel and coal) and 40 percent from open biomass burning (associated with deforestation and crop residue burning). High BC emissions occur in both northern and southern hemispheres, the former from fossil fuels and the latter from open biomass burning. BC is often transported long distances, mixing with other aerosols on the way such as sulphates, nitrates, organics, dust and sea salt, to form transcontinental plumes of brown clouds that extend vertically 3 to 5 km. BC is removed from the atmosphere by rain and snowfall; that and direct deposition limits the atmospheric lifetime of BC to about a week.

Major BC sources coincide with atmospheric solar heating and surface dimming

Until about 1950s, North America and Western Europe were the main sources of soot emissions, but now developing nations in the tropics and East Asia are the major source regions. Field observations and satellite sensors reveal that BC concentrations peak close to major source regions, giving rise to regional hotspots of solar heating in 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.

Whereas CO2 heats the earth surface through the greenhouse effect, BC heats the earth by decreasing its albedo in several ways. (Albedo is the fraction of solar energy not absorbed but reflected from the earth back into space.) First it heats the atmosphere by absorbing solar radiation reflected by the earth’s surface to the atmosphere. This is referred to as ‘top of atmosphere’ or TOA heating. Second, soot inside cloud drops and ice crystals decrease the albedo of clouds by enhancing absorption of solar energy. Third, when airborne black carbon particles, or soot, is deposited over snow and sea ice, it darkens the surfaces and decreases the otherwise high albedo, contributing to the melting of Arctic ice.

Ramanathan and Carmichael estimate that TOC heating (the first pathway), is 0.9 W/ m2 (range 0.4 to 1.2 W/m2), which is 55 percent of the CO2 warming of 1.66 W/m2; greater than that due to other greenhouse gases including methane, and much larger than the 0.2 to 0.4 W/m2 estimated previously by the IPCC.

BC also absorbs solar energy directly, a heating effect estimated at 2.6 W/m2. This direct absorption reduces the solar radiation reaching the earth surface, resulting in a dimming effect estimated at -1.7W/m2.

The calculations are complicated by the mix of aerosols that originate from some sources of BC which co-emit organic carbon compounds (such as benzene, ethane and ethyne from wood burning, all harmful to human health [2]) and sulphate, also harmful to human health [3], that tend to have a cooling effect by direct light scattering and interaction with clouds.

BC melting glaciers and Arctic ice

Models that include only the BC contribution leads to a warming from the surface to about 12 km altitude by as much as 0.6 ˚C over most of the northern hemisphere including the Arctic region, comparable to that due to greenhouse gases [1].

BC atmospheric heating may be an important contributing factor to the melting of Himalayan glaciers. Analysis of temperature trend reveals warming in excess of 1 ˚C since the 1950s [4]. Models suggest that movement of the warm air heated by BC from South and East Asia over the Himalayas contributes warming as much as 0.6 ˚C of the region, which is as large as the warming trend due to greenhouse gases [5]. More than two-thirds of the Himalayan glaciers have retreated, with disastrous consequences for downstream agriculture.

BC contributes substantially to melting of snow through direct soot deposition over snow and sea ice. It darkens the snow and enhances solar absorption significantly. Model simulations showed that the deposition of BC from sources in North America and Europe over the Arctic sea ice may have resulted in an Arctic surface warming trend of as much as 0.5 to 1 ˚C [6]. In addition, the study estimated that BC-induced reduction of snow albedo contributes a major warming of 20 W/m2.

Measurements of BC in ice cores indicate that sources and concentrations of BC in Greenland varied greatly since 1788 as the result of forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a seven-fold increase in BC concentration, with most change occurring in winter. At its maximum between 1906 and 1910, the estimated surface warming effect in early summer from BC in Arctic snow was about 3 W/m2, 8 times the typical pre-industrial average [7]. The direct absorption of sunlight by BC heats the Arctic atmosphere by approximately the same amount as human-injected CO2 in spring and summer, when snow and ice are most vulnerable to melting [8]. Black carbon also warms the Arctic, including in winter, by thickening low-level clouds that trap more of Earth’s emitted heat (TOA heating, see above).

A draft white paper from the US Environment Protection Agency point to diesel and open burning (both agricultural and wildfires) as the major sources of BC that reach the Arctic from the eight Arctic Council nations: the United States, Canada, Iceland, Norway, Sweden, Finland, Denmark, and Russia [9]. These sources also comprise the greatest part of BC emissions in near-Arctic regions (north of approximately forty degrees latitude), including much of the European Union, Ukraine and China north of Beijing. With increased shipping expected in and near the Arctic due to sea ice loss, marine sources of BC will be more important. The draft white paper concludes that “There is sufficient evidence to support the reduction of BC emissions from the identified sources (diesel, burning and marine) as a means to slow the rate of warming in the Arctic over the next few decades.” It recommends practical measures such as retrofits of diesel engines with particulate diesel filters, management of springtime biomass burning; and also pointing to significant mortality and morbidity averted due to air quality benefits from reducing particulate emission.

BC perturbs the monsoons

Rainfall has been decreasing over the past 50 years over many regions of the tropics, particularly Africa, South Asia and northern China. These drying patterns cannot be explained solely by global warming. Models are now investigating the effects of BC and associated atmospheric brown clouds (ABC) formed by BC with other aerosols [1].

Emissions of BC and other aerosol precursors from South Asia have increased significantly since the 1950s. This results in a dimming trend of about 7 percent as detected by surface radiometers in India, with concomitant decrease in the evaporation of the Indian Ocean, where similar dimming has occurred, so less moisture is fed to the monsoons in South Asia. The dimming suppresses greenhouse warming over the North Indian Ocean while the greenhouse warming proceeds unabated over the southern Indian Ocean. As a result, the summer-time sea surface north-south temperature (SST) gradient is decreased, and has been decreasing since the 1950s. The decrease of the SST gradient weakens the monsoonal circulation, and hence the monsoon rains during summertime. At the same time, the atmospheric heating gradient has increased. BC solar heating of the atmosphere over South Asia strengthens the monsoon outflow with stronger rising motions over the subcontinent, accompanied by a bigger influx of moisture into south Asia. This effect increases rainfall and peaks during spring when BC heating is at its most intense.

These effects of BC have been invoked to explain the Sahel drought of the 1970s and 1980s.

BC impacts on health worse than previously thought

A new report released in June 2009 from the non-profit Health Effects Institute (set up by the Environmental Protection Agency in the US) finds that risk of premature death from cardiovascular disease from soot is twice as high as previously thought [10]. It goes up by 24 percent for people living in soot-laden areas instead of 12 percent. The study draws on data gathered from 350 000 people over 18 years, and an additional 150 000 people in more recent years. It included 116 American cities, with the highest levels of soot particles in the eastern suburbs of Los Angeles and the Central Valley of California, Birmingham, Alabama; Atlanta; the Ohio River Valley; and Pittsburgh. The sources of the fine BC particles include diesel engines, automobile tires, coal fired power plants and oil refineries.

The health impacts of BC are worse in developing countries [11], where an estimated 1.8 million people die every year from exposure to BC and other emissions from indoor fires. The health impacts of co-emissions such as organic carbons and sulphate aerosols are also known [2, 3]. The global annual infant and adult premature mortalities due to sulphate aerosol exposure are estimated to be nearly 0.14 and 0.85 million, respectively.

Reducing BC emission can result in immediate climate and health benefits

Because the average life time of BC in the air is or the order of 10 days, there is a real possibility of reducing warming quickly by cutting BC emissions, which is much cheaper than cutting CO2 emissions, Ramanathan told a journalist [12]. And it would deliver substantial benefits as a bonus. Given the concentration of BC emissions in Asia, which is also having dire local air pollution effects, reducing black carbon emissions should be an important part of international development projects.

China and India account for ~25 to 35 percent of global BC emissions, and their expanding economies could make them an even larger source. Technologies to reduce black-carbon emissions already exist: newer combustion techniques and after treatments (scrubbing) often reduce particle emissions by several orders of magnitude. Providing alternative energy-efficient and smoke-free cookers and transferring technology for reducing soot emissions from coal combustion in small industries could have major impacts both on health and on the climate.

Ramanathan and Carmichael [1] showed that simply replacing the biological fuels used currently with BC-free cookers such as solar, biogas and natural gas in South and East Asia would have a dramatic effect. Over South Asia, a 70 to 50 percent reduction in BC heating, and in East Asia, a 20 to 40 percent reduction.

ISIS has been proposing anaerobic digestion of organic farm and human waste to generate methane for both developing and developed countries [13-15] (Biogas Bonanza for Third World Development , SiS 27; How to Beat Climate Change & Post Fossil Fuel Economy, SiS 29; Organic Agriculture and Localized Food & Energy Systems for Mitigating Climate Change, SiS 40). This is all the more relevant now in the context of reducing BC emissions.

Andrew Grieshop and colleagues at University of British Columbia, Vancouver, in Canada point out that eliminating all present-day emissions of black carbon globally over the next 50 years would have an approximate climate mitigation effect equivalent to removing 25 Gt C from the atmosphere over the same period [16]. According to conservative estimates, one tonne of black carbon causes about 600 times the warming of one tonne CO2 over a period of 100 years.

Article first published 28/09/09


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Rory Short Comment left 29th September 2009 23:11:27
This is an extremely interesting article. What it says to me is that dealing with climate change is not a 'one size fits all' matter. The reality is that the earths' climate is the consequence of the inter-play of a multiplicity of factors, some of which we know something about and others of which we know nothing as we have yet to discover them, and we should never ever lose sight of that fact in our endeavours to deal with global warming. This does not mean that we should cease our efforts to deal with climate change but that we need to proceed humbly and with the utmost caution. Geo-engineering schemes, for example, should be avoided at all costs because we simply do not know enough to be able to predict their consequences with any certainity at all.

J.A.M.ROSS Comment left 19th October 2009 00:12:25