Wednesday, January 16, 2013

Bounding the Role of Black Carbon in the Climate System: A Scientific Assessment



Soot, known as black carbon to scientists, is the second largest man-made contributor to global warming, and its influence on climate has been greatly underestimated, according to a new international study. The assessment "Bounding the role of black carbon in the climate system: A scientific assessment" was published online today in the Journal of Geophysical Research-Atmospheres, a journal of the American Geophysical Union.

Diesel engines, forest fires and many other sources throw heat-trapping specks of soot – black carbon – into the atmosphere. In addition to causing respiratory health problems, black carbon also warms the climate. For decades, its full impact on climate has been the source of much debate.

The study for the first time presents a comprehensive and quantitative analysis of the role of black carbon on the climate system. CSD's David Fahey, Ph.D., is a co-lead author and Joshua Schwarz a contributing author of the study, led by the International Global Atmospheric Chemistry (IGAC) Project. The IGAC Project of the International Geosphere-Biosphere Programme coordinates and fosters atmospheric chemistry research worldwide.

The research indicates that black carbon ranks second behind carbon dioxide as the major cause of man-made global warming and that its influence on climate has been underestimated, confirming some earlier studies that also showed a significant role for black carbon in climate warming. The study, a four-year, 232-page effort, provides scientific information relevant to research, climate modeling, and policy decisions regarding black carbon.

"This study confirms and goes beyond other research that suggested black carbon has a strong warming effect on climate, just ahead of methane," says Fahey. In fact, the best estimate of direct climate influence by black carbon in this report is about a factor of two higher than most previous work, including the estimates in the last Intergovernmental Panel on Climate Change (IPCC) Assessment released in 2007, which were based on the best available evidence and analysis at that time. Scientists have spent the years since the last IPCC assessment improving estimates, but the new IGAC assessment notes that emissions in some regions are probably higher than estimated. This is consistent with other research that also hinted at significant under-estimating for some regions' black carbon emissions.

The results indicate that there may be a greater potential to curb warming by reducing black carbon emissions than previously thought.

The international team urges caution because the role of black carbon in climate change is complex. "Black carbon influences climate in many ways, both directly and indirectly, and all of these effects must be considered jointly," says co-lead author and snow measurement expert Sarah Doherty, Ph.D., of the NOAA Joint Institute for the Study of the Atmosphere and Ocean (JISAO) at the University of Washington. The dark particles absorb incoming and scattered heat from the sun); they can promote the formation of clouds that can have either a cooling or warming impact; and black carbon can fall on the surface of snow and ice, promoting warming and increasing melting. In addition, many sources of black carbon also emit other particles, which counteract black carbon providing a cooling effect.

The research team quantified all the complexities of black carbon and the impacts of co-emitted pollutants for different sources, taking into account uncertainties in measurements and calculations. The study suggests mitigation of black carbon emissions for climate benefits must consider all emissions from each source and their complex influences on climate.

In addition, the report finds black carbon is a significant cause of the rapid warming in the Northern Hemisphere at mid to high latitudes, including the northern United States, Canada, northern Europe and northern Asia. Its impacts can also be felt farther south, inducing changes in rainfall patterns from the Asian Monsoon.

Citation: T. C. Bond, S. J. Doherty, D. W. Fahey, P. M. Forster, T. Berntsen, B. J. DeAngelo, M. G. Flanner, S. Ghan, B. Kärcher, D. Koch, S. Kinne, Y. Kondo, P. K. Quinn, M. C. Sarofim, M. G. Schultz, M. Schulz, C. Venkataraman, H. Zhang, S. Zhang, N. Bellouin, S. K. Guttikunda, P. K. Hopke, M. Z. Jacobson, J. W. Kaiser, Z. Klimont, U. Lohmann, J. P. Schwarz, D. Shindell, T. Storelvmo, S. G. Warren, C. S. Zender, Bounding the role of black carbon in the climate system: A scientific assessment J. Geophys. Res., in press, doi:10.1002/jgrd.50171, 2013.


Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that 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. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related; namely, 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. A framework is described and used for quantifying climate forcings, including rapid adjustments. 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. We estimate 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 is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. 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. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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