SEATTLE -- States with coastal water that is becoming more acidic because of carbon dioxide should list them as impaired under the

the U.S. Environmental Agency said.

The federal agency's memo Monday to states recognizes carbon dioxide as not only an air pollutant but a water pollutant, and notes the serious impacts that ocean acidification can have on aquatic life.

refers to the decrease in the alkalinity of oceans, which is caused by the absorption of excess carbon dioxide from the atmosphere. As water becomes more acidic, scientists have raised concern about dissolving coral reefs and potential effects on fish and other sea life.

"Ocean acidification is one of the biggest threats to our marine environment," said Miyoko Sakashita, a senior attorney at Center for Biological Diversity. This EPA action "really gave the green light to using the Clean Water Act to address ocean acidification," she said.

The EPA's memo stems from a legal settlement with the Center for Biological Diversity, which sued the EPA last year for not requiring Washington state to list its coastal waters as impaired by rising acidity.

The memo said in 2012, states should begin to list bodies of water that suffer from ocean acidification as impaired, but it also acknowledged there's currently not enough information in many states to support listings for that reason.

Currently, about 40,000 bodies of water are listed nationwide as impaired.

Sandy Howard, a spokeswoman with the

, said Tuesday the state is working with federal agencies to find more accurate and reliable methods of measuring pH, which shows how alkaline or acidic something is.

She said the listing program, however, is not the correct tool to fix the problem of greenhouse gas emissions.

The program focuses on local water quality fixes, she said, while the issue of greenhouse gas emissions is a global one.