PRESIDENT OBAMA’s announcement on Tuesday that he will use his executive authority to regulate carbon-dioxide emissions from existing power plants, and to finalize rules for plants that have yet to be built, is reason to celebrate — but also an occasion for mourning. The announcement reflects Congress’s refusal to pass serious laws to curb greenhouse-gas emissions. Ultimately, if America is to fully contribute to an effective global response to the perils of a warming planet, Congress must stop dithering.

The dangers of climate change are rising as the burning of fossil fuels and deforestation worldwide boost atmospheric concentrations of carbon dioxide. In March that concentration passed 400 parts per million — a concentration not seen for several million years, according to the best available evidence. It is on a pace to climb far higher in the coming decades, unless emissions are curbed. The United States is only one piece of that global puzzle, but it is an essential one.

For many years, advocates of serious action on climate change have rightly emphasized the importance of putting a price on carbon so that energy prices reflect the damage to society caused by carbon dioxide emissions. Whether through a cap-and-trade system, carbon tax, or an equivalent measure, a carbon price would boost the development of low-carbon technologies while deterring the use of carbon-intensive fuels.

Not long ago, both sides of the aisle agreed on this principle, which has the support of mainstream economists. The administration of President George Bush pioneered a cap-and-trade system in the early 1990s, to deal with acid rain, and as presidential candidates in 2008, Barack Obama and John McCain both promoted cap-and-trade systems that would have slashed greenhouse-gas emissions over the next 40 years.