Emission rates of BC in the year 2000 by region, indicating major source categories in each region. Source: Bond et al. Click to enlarge.

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.

...Our best estimate of black carbon forcing ranks it as the second most important individual climate-warming agent after carbon dioxide, with a total climate forcing of +1.1 W m^-2 (+0.17 to +2.1 W m^-2 range). This forcing estimate includes direct effects, cloud effects, and snow and ice effects. The best estimate of forcing is greater than the best estimate of indirect plus direct forcing of methane. The large uncertainty derives principally from the indirect climate-forcing effects associated with the interactions of black carbon with cloud processes.

—Bond et al.

The authors of the study organized their major findings into twelve areas:

1. Black carbon properties.
2. Black carbon emissions and abundance
3. Synthesis of black carbon climate forcing terms
4. Black-carbon direct radiative forcing
5. Black carbon cloud effects
6. Black carbon snow and ice effects
7. Impacts of black-carbon climate forcing
8. Net climate forcing by black-carbon-rich source categories
9. Major factors in forcing uncertainty
10. Climate metrics for black carbon emissions
11. Perspective on mitigation options for black carbone emissions
12. Policy implications

The study, a four-year, 232-page effort, led by the International Global Atmospheric Chemistry (IGAC) Project, is likely to guide research efforts, climate modeling, and policy for years to come, the authors and other scientists familiar with the paper said.

The report’s best estimate of direct climate influence by black carbon is about a factor of two higher than most previous work. This includes the estimates in the 2007 Intergovernmental Panel on Climate Change (IPCC) Assessment, 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 assessment notes that emissions in some regions are probably higher than estimated. This is consistent with other research that also hinted at significant under-estimates in 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.

There are exciting opportunities to cool climate by reducing soot emissions but it is not straightforward. Reducing emissions from diesel engines and domestic wood and coal fires is a no-brainer, as there are tandem health and climate benefits. If we did everything we could to reduce these emissions we could buy ourselves up to half a degree (Celsius) less warming—or a couple of decades of respite.

—co-author Professor Piers Forster from the University of Leeds’s School of Earth and Environment in the United Kingdom

However, 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.

—co-lead author Sarah Doherty of the University of Washington in Seattle

The dark particles absorb incoming and scattered heat from the sun (called solar radiation), they can promote the formation of clouds that can have either cooling or warming impact, and they 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 that provide a cooling effect, counteracting black carbon.

The research team quantified 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.

Based on the scientists’ analyses of these different sources, black carbon emission reductions targeting diesel engines and some types of wood and coal burning in small household burners would have an immediate cooling impact.

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, according to the report. The particles’ impacts can also be felt farther south, inducing changes in rainfall patterns from the Asian Monsoon. Curbing black carbon emissions could therefore have significant impact on reducing regional climate change while having a positive impact on human health by reducing the amount of damage the particles cause to the respiratory and cardiovascular systems.

Policy makers, like the Climate and Clean Air Coalition, are talking about ways to slow global warming by reducing black carbon emissions. This study shows that this is a viable option for some black carbon sources and since black carbon is short-lived, the impacts would be noticed immediately. Mitigating black carbon is good for curbing short-term climate change, but to really solve the long-term climate problem, carbon dioxide emissions must also be reduced.

—co-lead author Tami Bond of the University of Illinois at Urbana-Champaign

Resources

• Bond 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