SEATTLE — The U.S. Environmental Protection Agency said Thursday it will consider ways the states can address rising acidity levels in oceans, which pose a serious threat to shellfish and other marine life.

The agency’s decision was announced in a legal settlement with the Center for Biological Diversity. The environmental group sued the EPA last year for not requiring Washington state to list its coastal waters as impaired by rising acidity under the Clean Water Act.

“It’s one of the most important threats to water quality right now,” said Miyoko Sakashita, a senior attorney at the group’s San Francisco office. “It’s affecting waters around the world, and it’s particularly stark in the waters off the West Coast.”

Oceans are becoming more acidic as they absorb excess carbon dioxide from the atmosphere — a problem Sakashita referred to as “global warming’s evil twin.”

The changing chemistry of the waters affects many types of sea life, but especially anything that grows a shell or hard covering. Some scientists believe it is likely to blame for die-offs in Northwest oyster stocks over the past several years.

“Protection of the nation’s water quality, including the health of our ocean waters, is among EPA’s highest priorities,” the agency said in a statement. “EPA is interested in learning more about how to protect our ocean and coastal waters from acidification.”

Previously, states have taken steps to address rising acidity levels in lakes and streams under the Clean Water Act, but this is the first time the EPA has agreed to consider ocean acidity. The Center for Biological Diversity is petitioning each coastal state to address the issue, Sakashita said.

In the settlement agreement, the EPA said it would take public comment on the increasing acidity of oceans, on ways states can determine if their coastal waters are affected, and on how states can limit pollutants that cause the problem.

Such measures could include regional cap-and-trade systems to limit carbon-dioxide emissions from the burning of fossil fuels or requiring industrial plants to reduce their emissions as a condition of any discharge permits granted under the Clean Water Act, Sakashita said.

She compared it to the way the states have used the Clean Water Act in the past to regulate mercury emissions and acid rain. The problem is global, she said, but any steps toward reducing emissions help.

“It would be complementary to any other types of climate solutions we have out there,” she said.

The American Petroleum Institute, which represents hundreds of oil and gas companies, sought unsuccessfully to intervene in the lawsuit. It claimed that its members have refineries in Washington state whose permits under the Clean Water Act to discharge wastewater off Washington’s coast could be affected by the lawsuit’s outcome.

“API is now reviewing the settlement and looks forward to seeing EPA’s notice for comments on the ocean listings issue,” spokesman Bill Bush said in an e-mail.