Black Carbon: A Small Particle’s Big Effect on Climate Change

To most individuals, climate change represents a strictly long-term environmental issue caused by a growing abundance of greenhouse gasses in our atmosphere. This view is re-enforced by the few global agreements and national policies that exist. For example, the Kyoto Protocol only seeks to reduce human emission of greenhouse gasses. However, a recent study by Bond et al. makes it clear that any new international agreement must also consider other climate forcing agents. The authors’ expansive report sheds light on a particular climate agent that carries with it nearly immediate, local climate and public health effects: black carbon (BC). Once considered negligible, the acknowledged magnitude of the impact BC has on climate warming has grown with the breadth of the climate science and the sensitivity of global climate models. The model used in this study shows that BC is the second-most important agent of climate warming, after CO₂.

Black carbon is an atmospheric aerosol primarily emitted during the incomplete combustion of biomass, biofuels, and fossil fuels. This particulate “exists as an aggregate of small spheres” and “strongly absorbs visible light”. It is primarily emitted by diesel engines, industrial activity, residential solid fuel burning, and the open burning of biomass. An estimated 7500 Gg per year of BC is currently released into the atmosphere, compared to a preindustrial average (including human activity) of 1400 Gg per year. What makes BC so difficult to study is the heterogeneous nature of its distribution in the troposphere and stratosphere as well as the limited amount of time it spends there (only about three weeks before it is deposited on earth’s surface).

The Bond et al. study is unique in that it quantifies both the short-term effects of BC, which it terms “rapid adjustments”, and the longer-term effects using the more traditional measure of radiative forcing (RF). For purposes of this article, the summation of these two terms is known as “climate forcing”, the estimated total effect BC is having on the climate. In order to understand this aggregate effect, it is useful to break the total effect into three parts:

• The first is direct radiative forcing, which at its most basic level can be described as both the absorption and scattering of sunlight. The BC absorbs sunlight, causing the particles to warm, which in turn warm the air around them. Simultaneously, the particulates both reflect the radiation entering the atmosphere as well as trap the reradiated energy. Overall this is estimated to be net-warming (RF: +0.88 W m^-2)

• The second is cloud effects. This influence represents the most uncertainty in the model because of the limited understanding of the microphysical effects particulates have on cloud systems, the inability to tease out the effects of co-emitted climate forcing agents, the inability to separate rapid adjustments from radiative forcings, and the indirect nature of the effect. The authors’ best estimate of the forcing effect is positive (FR: +0.23 W m^-2) with a change of sign within the confidence interval (i.e. the true effect being net cooling is a possibility).

• The final effect is related to snow and ice. The deposition of BC in arctic and other ice-covered regions can significantly lower snow and ice albedo, or reflectivity. This decreases the amount of energy reradiated into space as well as contributes to the positive feedback mechanism associated with arctic melt. In fact, BC has a much stronger effect on this feedback loop than more homogenous atmospheric forcing agents like CO₂. Although the climate forcing effects are only modestly net-warming (CF: +0.13 W m^-2), increasing global mean temperature will exacerbate the effect in the future.

Overall, this study estimates a climate forcing value of +1.11 W m^-2. This represents a significant increase over past estimates, including that cited by the Intergovernmental Panel on Climate Change. This compares with the radiative forcing effect of +1.56 W m^-2 for CO₂ and +0.86 W m^-2 for CH₄. Holding all else equal, this finding indicates that policy-makers need to exhibit a much higher level of concern over the emission of BC from an environmental perspective. Luckily, there is another side to this coin: public health. Studies have long linked particulate matter to human health problems. In fact, the evidence was so overwhelming that a federal court ordered the US Environmental Protection Agency to tighten its standard for soot emissions (of which BC is a component). New soot standards related to public health will have a positive spillover into the realm of climate policy; however, the evidence presented in this report indicates that clack carbon must also be considered in any comprehensive climate policy.

Feature Photo: cc/(Βethan)