– Cities including Seattle, Copenhagen and Tokyo are taking the lead in slashing fossil-fuel emissions, picking up the baton from national governments that have squabbled for more than two decades over how to fight climate change.

Urban areas contain about half the world’s population now and will have two-thirds by 2050, according to the United Nations. That makes them “fundamental” to tackling global warming, said Rachel Kyte, the World Bank’s special envoy on ­climate. The measures they’re adopting range from electrifying buses to making cycle paths and requiring more efficient buildings.

Prosaic as they seem, those individual efforts are making a dent in global greenhouse gas pollution. More than 600 towns, cities and regional authorities that are home to 553 million people have ­committed to reducing their emissions by a total of 1 ­billion tons by 2020, according to ICLEI, a global network of ­cities and towns. That’s nearly as much as Japan produces each year.

“You see cities moving ahead, educating almost, federal governments about what’s possible, what they need to do,” Kyte said in Paris, where talks on global warming involving 195 nations are underway. The world can’t halt temperature increases, she said, “without building and living in cities very differently than we do today.”

The work of cities is crucial because they produce 80 percent of economic output and are responsible for 70 percent of greenhouse gas emissions, according to the C40 network of the world’s top megacities. That means they’re more efficient per dollar of output, but also, city dwellers emit more per person than rural ­residents.

“Cities are centers of adaptation and innovation, and they don’t have to wait for international negotiations or congressional action,” Shelley Poticha, director of urban solutions at the Natural Resources Defense Council in Washington, said in an e-mailed reply to questions. “When cities get it right, which they increasingly are learning to do, they advance solutions that larger and even more complex entities like national governments can adopt.”

The most ambitious cities include Seattle, Oslo, Stockholm and Copenhagen, with all vowing to become carbon-neutral. The Danish capital leads the pack with its pledge to do so by 2025 — a full 15 years before the Swedish capital and 25 years before the other two cities. For comparison, the Danish government has promised a 40 percent reduction in carbon emissions by 2020.

Copenhagen’s measures include district heating programs, replacing street lights with LEDs, and making it easier for cyclists to get around, Morten Kabell, the city’s mayor of environmental affairs, said last week in a panel discussion in Paris.

“As cities are acting, our counterparts at the national level, they just keep on talking,” Kabell said. “We have the obligation to act.”

Copenhagen isn’t alone. London is slashing emissions by 60 percent between 1990 and 2025, compared with a national target to cut greenhouse gases in half. Shenzhen’s goal is to lower emissions per dollar of economic output by 21 percent over the five years ending this year. That outstrips China’s goal for a 17 percent cut. And Tokyo’s goal of a 25 percent reduction over the two decades through 2020 compares favorably with Japan’s target of a 3.8 percent reduction between 2005 and 2020. Tokyo has introduced its own cap-and-trade ­program to curb emissions from large office buildings and ­factories.

In the U.S., New York Mayor Bill de Blasio has pledged to cut pollution 80 percent by 2050 on top of a 19 percent reduction in the past decade. Portland and San Francisco also seek 80 percent reductions by 2050. Austin, Texas, plans a 90 percent cut.

Reductions of that scale aren’t easy, even over a few decades. For George Ferguson, mayor of Bristol in southwest England, one of the keys is to work with businesses, which he said have made a “major contribution” to his city’s efforts. Accountancy firm KPMG has advised on funding for environmental efforts, First Group has helped clean up the city’s buses, and Skanska, a Swedish company, has made buildings and infrastructure cleaner.

Cities don’t get it all their own way, Salt Lake City Mayor Ralph Becker said in an interview in Paris. Residents offer opposition when asked to accept cycle lanes that disrupt traffic or water restrictions that affect what plants they can grow, he said.

“The pushback comes when we make changes that disrupt people’s everyday lives,” he said. “There will always be people who will say, ‘We like the idea of adjusting to the impacts on the climate, but don’t change my world.’ ”