Connecticut, Maine, and Massachusetts sued the federal government yesterday to force it to regulate carbon dioxide emissions. The lawsuit, which alleges that the emissions fuel global warming and should therefore be governed by the Clean Air Act, marks the first time a state has sued the government to force it to take action on climate change. The three states want to see CO2 classified as a “criteria pollutant” under the act, which would oblige the U.S. EPA to set allowable atmospheric levels, as it currently does for ozone, lead, sulfur dioxide, and other forms of pollution. The states’ argument turns on the definition of a “pollutant,” with the lawyers arguing that although CO2 does not pose a direct threat to human health, the long-term dangers of climate change make the emissions at least as hazardous as, say, smog. So far, the EPA has declined to comment on the suit.