In one dramatic stroke, President Obama has removed any doubts that he intends to break sharply from President George W.Bush’s policies on yet another vital issue - this time repudiating Mr.Bush’s passive approach to climate change. At a news conference on January 26, Mr.Obama directed the Environmental Protection Agency to consider immediately California’s application to set its own rules on greenhouse-gas emissions from cars and trucks. Mr.Bush had rejected that application.

Once California receives permission to move ahead - as it surely will - 13 states, and possibly more, are expected to impose similar rules. The result will be to force automakers here and overseas to begin producing cars and trucks that are considerably more fuel efficient than today’s models and on a faster timetable.

The California decision is of great significance not only for that reason but for what it says about Mr.Obama’s commitment to the cause of reversing the rise in greenhouse gases. Mr.Bush began his tenure by breaking a campaign promise to regulate carbon dioxide and by withdrawing the United States from the Kyoto agreement on climate change. Mr.Obama begins his with a clear signal that he will not hesitate to use the regulatory levers provided by the Clean Air Act and other federal statutes to fight global warming.

California has long had the right to set stronger air pollution standards than the rest of the nation, provided it has federal permission. Its earlier requests to set stronger air pollution standards were routinely approved, but in this case the Bush administration said no, dredging up all manner of arguments to support its case. One was that California had not demonstrated“extraordinary and compelling”reasons to limit greenhouse gases; another was that a national regulatory system was preferable to state-bystate laws - even though the administration itself had shown no interest in a national system.

In a companion move, Mr.Obama directed the Transportation Department to finalize the interim nationwide fuelefficiency standards called for in the 2007 energy bill. These standards would eventually require fuel-efficiency increases in the American car and light-truck fleet to roughly 35 miles per gallon (15 kilometers per liter) by 2020 from the current average of 27 m.p.g.(11.5 k.p.l.)

The California standards would require automakers to reach the same 35 m.p.g. target four years ahead of the federal timetable. The California rules cannot by themselves stop the rise in greenhouse gases. In addition to regulatory controls, Mr.Obama must eventually embrace a broader strategy involving major federal investments in clean-energy technologies and, down the road, some effort to put a price on greenhouse-gas emissions in order to unlock private investment. But after eight years of inaction, this is a wonderful start.