Soot comes from incomplete combustion. Also called black carbon, it absorbs light and in turn warms the atmosphere. Although there have been repeated suggestions that reduction of black carbon could be a viable part of decreasing global warming, it has not yet been considered when choosing initiatives to reduce climatic impact. Tami Bond believes that this is because there are four major objections to considering aerosols under hemispheric or global agreements, and that these objections themselves are flawed.

The first objection is that carbon particles will soon be addressed by local regulations, so they should not be considered in global or hemispheric agreements, which focus on pollutants that would not otherwise be addressed by local control. Bond, however, believes that local regulations are unlikely to address some major emitting sectors, and may not address important, high-emitting fractions of the other major emitting sectors.

"Another misconception," says Bond, "is that carbon particles have primarily local effects, and that therefore it is inappropriate to consider them in global regulations." But Bond has shown that climate forcing by carbon particles is not limited to hot spots and that their regional forcing is much greater than their local forcing. "Contour maps showing such 'hot spots' are sometimes used to demonstrate that carbon particles have only local effects," says Bond. "This hot-spot figure is misleading: any source or urban area could be presented in this way, drawing attention to local high concentrations but ignoring most of the forcing. Primary particles do have local impacts, but those are a fraction of their contribution to global climate change."

Many people also believe that aerosols are too complex to address in discussions on climate mitigation. "Aerosol physics is indeed complex," says Bond, "but simple measures, such as those used by the Intergovernmental Panel on Climate Change, already communicate scientists' best estimate of aerosol impact."

Another common objection to considering aerosols under hemispheric or global agreements is that the climatic effects of black carbon are highly uncertain, so they cannot be considered. "While there is substantial uncertainty, aerosols emitted from diesel engines and biofuel for cooking warm climate with a very high probability, and contribute a substantial fraction of the total forcing from these sources," says Bond, based on model simulations that account for a wide range of atmospheric processes and particle properties. "Reducing all aerosols from major sources of black carbon will reduce direct climate warming with a very high probability. This change in climate forcing accounts for at least 25% of the accompanying carbon dioxide forcing with significant probability. Thus, this fraction of radiative forcing should not be ignored."

Because of the uncertainties, Bond believes that more work remains for aerosol scientists: "Even if simple metrics can be accepted, many gaps in our understanding of aerosols remain." However, she says, despite the uncertainties, “some of the major objections to considering aerosols under hemispheric or global agreements are illusory, and vanish when examined closely".