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This month Tokyo kicked off its most ambitious effort yet: a mandatory program for 1,400 of the area's factories and office buildings to cut their carbon emissions 25 percent from 2000 levels by the end of 2020. The plan includes a carbon cap-and-trade system, the first ever attempted by a metropolitan area. The mechanism sets limits on emissions and requires those who exceed their quotas to buy pollution rights from those who are under their caps.

Tokyo's strategy is reminiscent of California's. The state's landmark legislation known as AB 32 requires polluters to curb their emissions significantly over the next decade. But while opponents, including large oil refiners are bankrolling a campaign to stall that effort in the Golden State, Tokyo is hitting the gas.

More than half the world's population now resides in cities. Metropolitan Tokyo and its surrounding prefectures have about as many people as the entire state of California. The way such teeming places respond to climate change will largely determine whether global warming can be slowed.

"We recognize our role as big mega-city, simply, to be a leader," said Teruyuki Ohno, Director General for Climate Strategy for Tokyo's Bureau of the Environment. "It is because we are capable of doing it that we have the responsibility to do it."

Some say Tokyo doesn't have a choice. Endowed with few natural resources, Japan has long been a champion in energy efficiency and clean technology. But the nation is falling short on a commitment to cut greenhouse gases, which it made more than a decade ago, when it hosted the United Nations convention on climate change in Kyoto.

Japan's federal legislators so far have failed to agree on a cohesive national strategy. Now some local officials, including Tokyo's maverick governor, Shintaro Ishihara, are taking up the slack. Ishihara, best known in the U.S. as the author of the controversial book "The Japan That Can Say No," is a conservative with a green streak. His administration banned older diesel trucks from the city's roads and led Tokyo's failed effort to win the 2016 Olympics with a pledge to make the city's Games the most environmentally friendly ever.

Masahiro Takeda, manager of sustainability for Mori Building Co., one of Tokyo's largest commercial developers, said demand is rising for buildings that save energy and lower tenants' operating costs. Large-scale recycling, greenery, rainwater reuse and waste heat recapture have become standard features in Mori developments.

At Roppongi Hills, a major shopping and office complex built by Mori in the heart of Tokyo, more than one-quarter of the 21-acre site is covered with trees and shrubs - including a rooftop rice paddy. The plants absorb carbon dioxide and lower roof temperatures, which in turn cuts heating and cooling bills.

Tokyo rooftops are also sprouting solar panels. To spur adoption of photovoltaics, the metropolitan government offers its homeowners a subsidy of 100,000 yen (about $1,070) per kilowatt. That comes on top of the federal subsidy of 70,000 yen (about $750) per kilowatt. The metropolitan government is also offering solar incentives to businesses.

Meanwhile, garbage is getting an afterlife. Tough recycling laws over the years have produced results such as a 99 percent reuse rate for asphalt and concrete and 72 percent for paper. What isn't recovered is incinerated and the residue buried. But with landfill space in short supply, the metropolitan government in 2006 launched an initiative known as Tokyo Super Eco Town to take recycling to the next level.

Built on the site of a former garbage dump in Tokyo Bay, the project is home to Tokyo Waterfront Recycle Power Co. and eight other private companies, including one that treats toxic chemicals known as PCBs, or polychlorinated biphenyls, and another that converts the city's food waste into animal feed.

Such efforts have attracted some big-name attention. Rock star Bono stopped by in 2008 to plant a tree in nearby Umi-no-Mori, a wooded waterfront park that's also rising on the former landfill. 

Tokyo is well ahead of other major cities on a variety of environmental issues. Environmentalists are particularly enthusiastic about the government's willingness to push ahead with its cap-and-trade program in the midst of a sluggish global economy.

"Tokyo is the first metropolitan area in the world to try this," said Tetsunari Iida, executive director of the nonprofit Institute for Sustainable Energy Policies in Tokyo. "They want to show it can work."