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Comprehensive assessment concludes black carbon second-most important human emission in warming climate

A comprehensive assessment of black-carbon climate forcing accepted for publication in the Journal of Geophysical Research: Atmospheres estimates that black carbon, with a total climate forcing of +1.1 W m-2, is the second-most important human emission in terms of its climate-forcing in the present-day atmosphere. Only carbon dioxide has a greater forcing, the international team of researchers who performed the study found.

Black carbon is a type of carbonaceous material with a unique combination of physical properties that plays a unique and important role in Earth’s climate system.

Schematic overview of the primary black-carbon emission sources and the processes that control the distribution of black carbon in the atmosphere and determine its role in the climate system. Source: Bond et al. Click to enlarge.

The assessment includes all known and relevant processes and is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid; mixed-phase and ice clouds; and deposition on snow and ice. The team calculated the effects with climate models, but when possible, they also evaluated them with both microphysical measurements and field observations.

Predominant black-carbon sources are combustion related: fossil fuels for transportation; solid fuels for industrial and residential uses; and open burning of biomass.

Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr-1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models, and should be increased by a factor of almost three. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m-2 with 90% uncertainty bounds of (+0.08, +1.27) W m-2. Total direct forcing by all black carbon sources, without subtracting the pre-industrial background, is estimated as +0.88 (+0.17, +1.48) W m-2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms.

The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m-2 with 90% uncertainty bounds of +0.17 to +2.1 W m-2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate.

—Bond et al.

Sources that emit black carbon also emit other short-lived species that may either cool or warm climate, the authors note. In their study, they estimated the climate forcings from co-emitted species and included them in the framework they developed for the study.

Diesel engines, which contributed about 20% of global BC emissions in 2000, have the lowest co-emissions of aerosols or aerosol precursors of all the major BC sources, the authors found. In order to enable use of the most advanced exhaust controls, sulfur must be removed from the diesel fuel during refining. Therefore, in regions with fewer controls, primary particulate matter emission factors are higher, but SO2 emissions are also higher.

Total climate forcing for BC-rich source categories continuously emitting at year-2000 rates scaled to match observations in 2005. Bond et al. Click to enlarge.

When the principal effects of co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil-fuel and biofuel) have an industrial-era climate forcing of +0.22 (-0.50 to +1.08) W m-2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all emissions from these sources would reduce net climate forcing (i.e., produce cooling).

When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all black-carbon-rich sources becomes slightly negative (-0.06 W m-2 with 90% uncertainty bounds of -1.45 to +1.29 W m-2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon.

—Bond et al.


  • Bond, T. C. et al. (2013) Bounding the role of black carbon in the climate system: A scientific assessment. Journal of Geophysical Research: Atmospheres doi: 10.1002/jgrd.50171


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