Reducing soot emissions as a climate policy measure
Particle emissions are reduced out of concern for human health. Soot particles, however, also have an effect on the global climate and contribute to local warming in the Arctic. Is a different mitigation strategy in order when we take warming effects into account?
Emissions of soot have a warming effect in the atmosphere, and since pre-industrial times have contributed 0.2(±0.15) W/m2 (compared to, e.g., nitrous oxide with 0.16 (±0.02) W/m2). Yet particle emissions are not covered by the Kyoto Protocol or traditional measures for reducing emissions of greenhouse gases. In addition to the direct warming effect, particles also have an indirect effect via long-range transport and deposition of soot on snow of about 0.1 (±0.10) W/m2. Through the NORKLIMA project “Climate Effects of Reducing Black Carbon Emissions”, CICERO has looked at various strategies for how the world can reduce the radiative forcing from emissions of soot, and what it will cost.
Soot is emitted to the atmosphere as a result of incomplete combustion of coal, oil, and biofuels, as well as forest and vegetation fires. Soot particles have a relatively short lifetime in the atmosphere, but have a direct warming effect. Their climate effect varies depending on their location because of different climates, radiative conditions, and deposition in different regions. Much indicates that emissions in Africa, Asia, and Latin America contribute far more strongly to direct warming than emissions in Europe. However, emissions from high latitudes in the Northern Hemisphere are deposited on snow in the Arctic, and this blackening of the snow contributes to local warming in the northern regions. Soot on snow makes the snow darker, which gives a direct radiative forcing. Moreover, snow will melt more quickly so that the dark ground emerges and will absorb the radiation more effectively earlier in the spring. This effect, which is characterized as a feedback in the climate system, is of course particularly strong when the radiative forcing works directly on the snow. Estimates from climate models suggest that the effect of soot deposition on the global mean temperature is 2–4 times greater than that of the radiative forcing from CO2. The contribution through this feedback mechanism will be significantly greater for emissions in Europe compared with other regions because European emissions to a much greater degree are deposited on snow.
Combustion emits not only soot particles, but also organic particles, which have a cooling effect on the climate. For combustion of biomass, warming soot particles and cooling organic particles more or less cancel each other out in terms of their radiative effect. This means that measures directed at reducing emissions from open combustion of biomass are not an effective climate measure when we look at the climate effect of particles. The difference in the climate effect between various regions provides arguments for reducing emissions most in the regions where the climate effect is greatest. However, there are also large regional differences when it comes to what it will cost to reduce emissions. In Europe and North America, many measures to reduce emissions have already been carried out in the context of health. This means that many of the salient measures are more expensive to carry out here than in, for example, China and most developing countries. Asia has also a large population and high absolute emissions of particles.
The warming in the Arctic can be a good argument for nevertheless reducing emissions of particles in Europe even though the costs are high and the potential for large emissions reductions is small. To achieve larger cuts in radiative forcing from soot particles, the emissions in Asia and other developing countries also have to be reduced. In this region, the benefits in the form of health effects will clearly be greatest. Does this mean that we should reduce emissions of soot particles instead of CO2? No. CO2 is the most important greenhouse gas and has, in contrast to soot particles, a long lifetime in the atmosphere. However, measures to reduce emissions of soot can be cost-effective in the short and medium term if we are worried about the warming that is taking place today, for example in the Arctic.