Reducing emissions of carbon dioxide to combat climate change would also reduce acid rain. There are at present no economically competitive "end-of-pipe" or "scrubbing" technologies for removing carbon dioxide (CO2) from industrial smokestacks. CO2 emissions can only be cut by reducing the use of fossil fuel, whether by lowering energy use per unit of economic production or substituting low- and non-carbon fuel for high-carbon fuels. On average, reducing one tonne (1,000 kg) of carbon simultaneously reduces about 20 kg of sulphur dioxide and eight kg of nitrogen oxides from stationary sources such as factories, and about 0.5 and 9 kg respectively from mobile sources such as cars. From the point of view of policies to cut GHG emissions, this secondary benefit of reducing acid rain may be considerable; it may even, at least initially, exceed the primary benefit of reduced warming by a factor of 10 or 20. If these figures are accurate, they lend additional support to economic arguments for carbon taxes and other policies for reducing GHG emissions.
Reducing emissions of sulphur dioxide to combat acid rain would probably affect the rate of global warming. Rather than reducing fossil fuel use, which cuts emissions of all fuel-related gases, policies to control SO2 emissions call for installing flue gas desulphurisation and other end-of-pipe technologies and for substituting low-sulphur fuels for high-sulphur fuels. While leaving GHGs untouched, these technologies will reduce the sulphur dioxide and other sulphuric particles floating in the atmosphere as "aerosols". By scattering and reflecting the sun's light away from the earth (causing bright red sunsets), these particles have a cooling effect on the lower atmosphere. In the highly industrialised Northern Hemisphere, sulphur particles in the atmosphere may be offsetting at least half of the current man-made warming. In the less-industrialised southern hemisphere, the offsetting effect may be almost negligible. The potency of sulphur cooling has recently been demonstrated with the volcanic eruption of Mount Pinatubo in June 1991. This massive release of sulphur particles into the stratosphere seems to have cooled the earth by 0.5 C on average between May 1991 and May 1992.
A warmer atmosphere would influence the occurrence and severity of acid rain. Many of the chemical reactions that lead to the formation of acid rain are affected by temperature. Higher temperatures usually accelerate reactions, suggesting that global warming is likely to increase the formation of acidic materials. The likely net effect on acid rain is unclear, however, since global warming will also cause changes in cloud, wind, and precipitation patterns.
Because of these linkages, there are benefits to designing acid rain and climate change policies simultaneously. Cost benefit analyses of policies to reduce GHG emissions normally suggest that reductions should take place as long as the marginal costs of reduction are smaller than the marginal benefits of avoided global warming. Because reducing GHG emissions also reduces acid rain, however, the marginal benefits of this secondary reduction should also be considered in the analysis. Similarly, a cost benefit analysis of reducing sulphur emissions should consider the marginal benefits of avoided acid rain, the marginal costs of making the reductions, plus the additional costs of increased global warming due to the reduction in atmospheric aerosols.
For further Reading:
D.M. Newberry, Acid Rain, Economic Policy 11(October), 297-346, 1992.
D.W. Pearce, The Secondary Benefits of Greenhouse Gas Control, Centre for Social and Economic Research on the Global Environment (CSERGE), University of East Anglia and University College London, Working Paper GEC 92-12, 1992.
IPCC, Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment, Cambridge: Cambridge University Press, 1992.
J.B. Smith and D.A. Tirpak, ed., The Potential Effects of Global Climate Change on the United States. Appendix F: Air Quality, US Environmental Protection Agency, Washington, DC, 1989.