BC is a significant component of particulate matter (PM) pollution, which has been linked to adverse health and environmental impacts through decades of scientific research. More recent work also suggests that BC plays an important role in climate change, although there is more uncertainty about its effects on climate. Reducing current emissions of BC may help slow the near-term rate of climate change, particularly in regions such as the Arctic.

In February, the US and coalition partners Bangladesh, Canada, Ghana, Mexico, Sweden and the UN Environment Programme announced the Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants, a new global initiative to seize the opportunity of realizing concrete benefits on climate, health, food and energy resulting from reducing short-lived climate pollutants such as BC. (Earlier post.)

Despite the rapidly expanding body of scientific literature on BC, there is a need for a more comprehensive evaluation of both the magnitude of particular global and regional climate effects due to BC and the impact of emissions mixtures from different source categories.

—Report to Congress on Black Carbon

Effects of BC on climate, as compared to GHGs. Sunlight that penetrates to the Earth’s surface reflects off bright surfaces, especially snow and ice. Clean clouds and non-light-absorbing (transparent) particles scatter or reflect sunlight, reducing the amount of solar energy that is absorbed by the surface. BC suspended in the atmosphere absorbs some incoming solar radiation, heating the atmosphere. Clouds containing BC inclusions in drops and BC interstitially between drops can absorb some incoming solar radiation, reducing the quantity that is reflected. Clouds warmed by the absorbed energy have shorter atmospheric lifetimes and may be less likely to precipitate compared to clean clouds. BC deposited on snow and/or ice absorbs some of the sunlight that would ordinarily be reflected by clean snow/ice, and increases the rate of melting. Most solar radiation is absorbed by the Earth’s surface and warms it. Part of the absorbed energy is converted into infrared radiation that is emitted into the atmosphere and back into space. Most of this infrared radiation passes through the atmosphere, but some is absorbed by GHG molecules like CO2, methane, ozone and others. These gases re-emit the absorbed radiation, with half returning to the Earth’s surface. This GHG effect warms the Earth’s surface and the lower atmosphere. Source: EPA. Click to enlarge.

As requested by Congress, EPA consulted with other federal agencies on key elements of this report, including inventories; health and climate science; and mitigation options. The report also draws from recent BC assessments, including work under the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO); the Convention on Long Range Transboundary Air Pollution (CLRTAP); and the Arctic Council.

Other key messages of the EPA report are:

1. Black carbon is the most strongly light-absorbing component of particulate matter (PM), and is formed by the incomplete combustion of fossil fuels, biofuels, and biomass.

   BC can be defined specifically as a solid form of mostly pure carbon that absorbs solar radiation (light) at all wavelengths.

2. BC is emitted directly into the atmosphere in the form of fine particles (PM_2.5). The United States contributes about 8% of the global emissions of BC. Within the United States, BC is estimated to account for approximately 12% of all direct PM_2.5 emissions in 2005. Many countries have significantly higher PM_2.5 emissions than the United States, and countries with a different portfolio of emissions sources might have a significantly higher percentage of BC.

3. BC contributes to the adverse impacts on human health, ecosystems, and visibility associated with PM_2.5.

   While the scientific community has focused increasingly on trying to identify the health impacts of specific PM_2.5 constituents, such as BC, EPA has determined that there is insufficient information at present to differentiate the health effects of the various constituents of PM_2.5. Thus, EPA assumes that many constituents are associated with adverse health impacts.

4. BC influences climate through multiple mechanisms. In the direct effect, BC absorbs both incoming and outgoing radiation of all wavelengths, which contributes to warming of the atmosphere and dimming at the surface. In the snow/ice albedo effect, BC deposited on snow and ice darkens the surface and decreases reflectivity, thereby increasing absorption and accelerating melting. In other effects, BC also alters the properties of clouds, affecting cloud reflectivity and lifetime (“indirect effects”), stability (“semi-direct effect”) and precipitation.

5. The direct and snow/ice albedo effects of BC are widely understood to lead to climate warming. However, the globally averaged net climate effect of BC also includes the effects associated with cloud interactions, which are not well quantified and may cause either warming or cooling.

   Therefore, though most estimates indicate that BC has a net warming influence, a net cooling effect cannot be ruled out. It is also important to note that the net radiative effect of all aerosols combined (including sulfates, nitrates, BC and OC) is widely understood to be negative (cooling) on a global average basis.

   The sign and magnitude of the net climate forcing from BC emissions are not fully known at present, largely due to remaining uncertainties regarding the effects of BC on clouds. There is inconsistency among reported and many studies do not provide quantitative estimates of cloud impacts. In the absence of a full quantitative assessment, the current scientific basis for understanding BC climate effects is incomplete. Based on a limited number of the recent UNEP/WMO assessment estimated that global average net BC forcing is likely to be positive and in the range of 0.0 to +1.0 W m^-2, with a best estimate of +0.6 W m^-2; however, further work is needed to refine these estimates.

   —Report to Congress

6. Sensitive regions such as the Arctic and the Himalayas are particularly vulnerable to the warming and melting effects of BC.

7. BC contributes to surface dimming, the formation of Atmospheric Brown Clouds (ABCs), and changes in the pattern and intensity of precipitation.

8. BC is emitted with other particles and gases, many of which exert a cooling influence on climate. Therefore, estimates of the net effect of BC emissions sources on climate should include the offsetting effects of these co-emitted pollutants. This is particularly important for evaluating mitigation options. Some combustion sources emit more BC than others relative to the amount of co-pollutants; reductions from these sources have the greatest likelihood of providing climate benefits.

9. BC’s short atmospheric lifetime (days to weeks), combined with its strong warming potential, means that targeted strategies to reduce BC emissions can be expected to provide climate benefits within the next several decades.

10. The different climate attributes of BC and long-lived GHGs make it difficult to interpret comparisons of their relative climate impacts based on common metrics.

11. Based on recent emissions inventories (2000 for global and 2005 for the United States), the majority of global BC emissions come from Asia, Latin America, and Africa. The United States currently accounts for approximately 8% of the global total, and this fraction is declining. Emissions patterns and trends across regions, countries and sources vary significantly.

12. Control technologies are available to reduce BC emissions from a number of source categories.

13. BC mitigation strategies, which lead to reductions in fine particles, can provide substantial public health and environmental benefits.

14. Mitigating BC can also make a difference in the short term for climate, at least in sensitive regions.

15. Selecting optimal BC mitigation measures requires taking into account the full suite of impacts and attempting to maximize co-benefits and minimize unintended consequences across all objectives (health, climate, and environment).

16. Considering the location and timing of emissions and accounting for co-emissions will improve the likelihood that mitigation strategies will be properly guided by the balance of climate and public health objectives.

17. Achieving further BC reductions, both domestically and globally, will require adding a specific focus on reducing direct PM_2.5 emissions to overarching fine particle control programs.

18. The most promising mitigation options identified in this report for reducing BC (and related “soot”) emissions are consistent with control opportunities emphasized in other recent assessments.

19. A variety of other options may also be suitable and cost-effective for reducing BC emissions, but these can only be identified with a tailored assessment that accounts for individual countries’ resources and needs.

Resources

• Report to Congress on Black Carbon