At the end of An Inconvenient Truth, the well-received documentary about global warming that appeared in theaters this past summer, the filmmakers provided a list of practical steps that anyone can take to tackle this looming environmental problem. Perhaps the most memorable suggestion was this one: "Vote for leaders who pledge to solve this crisis. Write to Congress. If they don't listen, run for Congress."

Unfortunately, the federal government is lagging behind other nations in the effort to control climate-warming gases--so now state lawmakers are taking the lead. In August the California legislature approved a bill calling for a 25 percent cut in emissions of carbon dioxide and other greenhouse gases by 2020. Although other states have pledged to make similar cuts, California is the first to mandate the emission limits. Moreover, the proposal won the support of both the Democratic-controlled legislature and Republican Governor Arnold Schwarzenegger.

California's Chamber of Commerce opposed the bill, claiming that it would have no significant effect on global climate because polluting businesses would simply relocate outside the state. But this argument is based on a false assumption. Curbing greenhouse gases need not be costly. Improving the energy efficiency of a factory or office building can slash carbon use and save money at the same time. And thanks to recent technological advances, electricity generated from renewable energy sources--wind turbines, solar-thermal systems, geothermal facilities, and so on--is becoming economically competitive with power produced by burning coal or natural gas. Pacific Gas and Electric, one of the biggest utilities in California, broke with the rest of the industry by supporting the emission limits; the company currently generates 12 percent of its electricity from renewable sources (excluding large hydroelectric plants) and plans to increase that share to 20 percent by 2010.

Perhaps the greatest boon of the new California law is that it will encourage other states to take their own stands against global warming. The governors of seven northeastern states in the Regional Greenhouse Gas Initiative (Connecticut, Delaware, Maine, New Hampshire, New Jersey, New York and Vermont) have already agreed to lower their greenhouse gas emissions by 10 percent by 2019; the group recently issued a set of model rules that must now be approved by the states' legislatures or regulatory agencies. Meanwhile a dozen states have joined a federal lawsuit against the Environmental Protection Agency arguing that the Clean Air Act requires the agency to regulate greenhouse gases. The U.S. Supreme Court is expected to rule on that case next year.

But the federal courts may frustrate the states' efforts rather than furthering them. In 2002 California enacted a law calling for a 30 percent cut in the greenhouse gas emissions from cars and trucks sold in the state; two years later trade groups representing the auto industry filed a lawsuit claiming that the pending regulations are illegal because they effectively mandate an improvement in fuel economy and that only the federal government has the power to set such standards. We fervently hope that the courts reject this argument and recognize the fundamental right of states to protect their citizens from the catastrophic consequences of global warming.