WASHINGTON, D.C. --- During a tense Senate committee hearing last Thursday, the head of the Environmental Protection Agency defended his refusal to let California set limits on the greenhouse gas emissions of automobiles. EPA Administrator Stephen L. Johnson insisted that climate change posed no "compelling and extraordinary" risk to the state, and described such change as "not unique to" and "not exclusive to California." He called climate change "a global problem requiring a global solution or, at least at a minimum, a national solution," the New York Times reported. But internal EPA documents, cited by members of the Senate Committee on Environment and Public Works, listed climate change effects specific to California, including wildfires and species loss. Fifteen states have signed on to follow California's lead in regulating automobile emissions. The governors of three of them --— Maryland, Pennsylvania and Vermont --— testified before the committee Thursday. They asserted that attacking the problem was essential for their residents and the world at large, the Times reported. The states are suing to overturn Johnson's decision to deny California a waiver from the federal Clean Air Act that would allow the state to regulate emissions. California is the only state allowed by the act to apply for such exemptions. The state needed the waiver to enforce a 2002 state law aimed at cutting carbon emissions from vehicle tailpipes by 30 percent over the next eight years. California has employed previous waivers to require automakers to reduce emissions of smog. Over the years, other states have adopted California's program, and the EPA has adopted many of the changes as national policy. Johnson argued that the newly revised federal standard for vehicle fuel efficiency was a better approach to reducing auto emissions because it was more uniform. During his appearance before the committee, Johnson was confronted with a hostile panel of senators, most of them Democrats. The only Republican present, Senator James M. Inhofe of Oklahoma, questioned whether human activity was an important cause of global warming and disputed the notion that there was agreement among scientists that this was the case, the Times reported. The committee chairwoman, Senator Barbara Boxer of California, has been battling with the EPA over access to its records. She wants to determine whether Johnson overruled his staff in blocking state action on global warming. Johnson said he could not recall whether the briefing papers prepared by his staff had included a recommendation. But according to EPA staff document excerpts released by Boxer last week, the staff concluded that California had "compelling and extraordinary conditions" to justify its own tailpipe law.