Sulfur dioxide emissions from the burning of coal and oil react with water and oxygen in the air to form sulfate aerosols; acidic compounds that fall to the Earth in the form of acid rain.
Global warming and acid rain are two environmental problems the world will be forced to reckon with in the 21st century. Unfortunately, efforts to mitigate acid rain may actually increase regional warming, according to a university professor.
"It is ironic, in a sense, that in working to solve one environmental problem you exacerbate another problem", said Michael Schlesinger, a professor of atmospheric sciences at the University of Illinois, Urbana-Champaign.
Sulfur dioxide emissions from the burning of coal and oil react with water and oxygen in the air to form sulfate aerosols — acidic compounds that fall to the Earth in the form of acid rain, wreaking havoc on the world’s forests and streams.
Sulfate aerosols also reflect sunlight back into space. "This acts as a negative radiative forcing which partially compensates for the positive radiative forcing due to greenhouse gases", said Schlesinger.
Take away the sulfur dioxide — a gas that doesn’t stray too far from its source of emission — and all of a sudden something that used to mitigate the effects of carbon dioxide is lost, resulting in regional warming.
"In recent studies, we found that decreasing the sulfur dioxide emissions led to significant regional warming in North America, Europe and Asia", said Schlesinger.
Schlesinger and his colleagues based their study on four scenarios for emissions of greenhouse gases that are being produced by the Intergovernmental Panel on Climate Change for the panel’s Third Assessment Report scheduled for completion in 2001.
In these scenarios, sulfur dioxide emissions have been de-coupled from carbon dioxide emissions. Even though the burning of coal and oil produces both carbon dioxide and sulfur dioxide emissions, technologies exist that allow for both low-sulfur fuels and "scrubbers" placed in smoke stacks that clean emissions of sulfur dioxide.
Therefore, in each of the scenarios that Schlesinger and his colleagues examined, sulfur dioxide emissions either leveled off early in the next century or decreased while carbon dioxide emissions continued to rise.
"Thus it appears that mitigation of the acid-rain problem by future reductions in sulfur dioxide emissions exacerbates the greenhouse-warming problem by enhancing the warming in and near the regions where the sulfur dioxide emissions are reduced", he said.
Nevertheless, Schlesinger said that the problem of acid rain and the breathing difficulty he has experienced in Beijing, China, are good enough of a reason to want to reduce emissions of sulfur dioxide.
Schlesinger presented the group’s findings June 2 in Bonn, Germany, at a joint meeting of the IPCC and the Subsidiary Body for Scientific and Technical Advice of the United Nations Framework Convention on Climate Change.