左图：1990年代，洛杉矶市烟尘污染严重；右图：该市今天的空气清新很多。(© AP Images)

洛杉矶的新型电动公交车 (© AP Images)

This is the last in a series on U.S. cities’ environmentalism, followingChicago, Pittsburgh, New York and Portland, Oregon.

American cities have become laboratories for novel policies to reduce climate change, reverse environmental degradation and support alternative energy. Because of the nature of the U.S. federal system, cities’ policies are often adopted at the state or national level, putting cities at the center of the environmental movement.

Cutting the carbon

As a result of federal and state automobile emission standards, American cars and buses emit fewer pollutants than ever. But several cities go further and are experimenting with fuel sources that have zero emissions.

With the largest fleet of natural gas buses in the U.S. — about 2,200 — Los Angeles has become a leader in cutting carbon emissions from public transit systems. City officials estimate that since converting to natural gas, this eco-friendly bus fleet has cut greenhouse gas emissions by 136,000 kilograms daily.

Los Angeles is taking steps beyond natural gas. Last year, the city rolled out the first of five electric buses for testing against the city’s serious public-transit needs. Each bus carries 120 passengers and can run for 275 kilometers before recharging.