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Science, Law & the Environment Emerging Topics in Environmental Law

Fact Check: Are Carbon Emissons Contributing to Acid Rain?

Posted in Clean Water Act, Emerging Policy, Geeky Science Stuff, Ocean Acidification, Water Quality

This morning’s twitter feed brought me the latest from Crosscut on Governor Inslee’s Carbon Emissions Reduction Taskforce. The most important part of the article was the announcement that the next Taskforce meeting (on July 29th) will include a rollout of a draft plan to meet the goals of reducing greenhouse gas emissions in the state to 1990 levels by 2020, to 25 percent below 1990 levels by 2035, and to 50 percent below 1990 levels by 2050. It sounds like that plan could involve cap and trade, and we know that the Governor is interested in exploring a low carbon fuel standard in Washington as well.

The article is worth reading because it provides good context on this issue. However, one paragraph at the end of the article caught my eye, and it deserves correction:

Carbon emissions have been linked to acid rain, which falls into oceans, lakes and rivers. Acid rain is increasing the acidity of the water along Washington’s shores including Puget Sound, which has begun killing baby oysters and harming other shellfish harvested in the Northwest. Washington’s shellfish industry is worth about $270 million annually. Carbon emissions are also linked to global warming, which influences how snow packs melt, which in turn affects how much water is available for farming. As he moves toward announcing his preliminary plan, Inslee has been visiting shellfish sites.

The statement that “carbon emissions have been linked to acid rain” is incorrect. While it is true that carbon dioxide concentrations control the pH of rainwater (this is basic aquatic chemistry) and that an increase in carbon dioxide will decrease the pH of rainwater, the acid that is formed as carbon dioxide equilibrates with rainwater is a weak acid–resulting in rainwater being naturally acidic and having a pH of about 5.5. Because carbonic acid is a relatively weak acid, the ability of carbon dioxide alone to generate true “acid rain” is very limited. Acid rain is caused by industrial emissions of sulfur dioxide and nitrogen oxides (which form much stronger acids when equilibrated in rainwater). In some areas of the United States, sulfur dioxide and nitrogen oxides have, historically, pushed the pH of rainwater down to the low 4 range, but this mostly happens in the eastern part of the United States and is related to fossil fuel combustion (primarily coal). And, with increasing controls on sulfur dioxide and nitrogen oxide emissions, we’ve actually been fairly successful in reducing acid rain in the United States. EPA has a good explanation of the chemistry and sources of acid rain here. In short, I don’t think it is fair to say that “carbon emissions have been linked to acid rain.”

Connecting acid rain to ocean acidification is also dubious at best. Rainwater, even though it is slightly acidic, is rapidly buffered by the presence of carbonates in terrestrial systems, although the sulfuric and nitric acid formed in true acid rain can, at times, overwhelm the buffering capacity of some streams. The cause of ocean acidification is a bit more complex than the concept of acidic rain flowing into the oceans. It is driven by the air-sea exchange of carbon dioxide, and the resulting changes in seawater chemistry that make carbonate formation difficult for shell-building organisms. If you want more background on the chemistry, the Pacific Marine Environmental Laboratory’s website is a good place to start, and I reviewed the chemistry as it relates to shell-building organisms as part of my response to the tiff between Cliff Mass and Craig Welch a while back.

For some, this may appear to be a minor error in reporting. However, in an era where most people receive scientific information through the media, accuracy in reporting science is critical.