The mayors of 12 major cities from around the world have taken a landmark step to increase the use of zero-emission public transportation late last month.

Mayors of London, Paris, Los Angeles, Copenhagen, Barcelona, Quito, Vancouver, Mexico City, Milan, Seattle, Auckland, and Cape Town all signed the C40 Fossil-Fuel-Free Streets Declaration, which pledges that they will add only fully electric buses to their cities' public transportation from 2025.

Although electric buses are the largest part of the agreement, the mayors also agreed to other measures to reduce carbon output in their respective cities.

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The agreement's language in general calls for "a future where walking, cycling, and shared transport is how the majority of citizens move around our cities."

The long-term goal, described by C40 Cities, is to ensure substantial areas of each city committed to the agreement are emission-free by 2030 and collectively to help fight the growing effects of climate change.

Additionally, each city's mayor committed to introducing measures to elevate public and shared transport, reduce the number of vehicles operating within the city itself, an increase walking rates for citizens.

Proterra Catalyst electric bus

Proterra Catalyst electric bus

The goals will require each city to work with suppliers to envision ways to meet the goals.

The agreement calls for each mayor to "lead by example," especially when procuring the electric bus fleets.

Large cities, especially in California, are starting to add electric buses to their fleets in substantial numbers. The country's largest public-transit system, in New York City, however, is just starting to plan for tests of a dozen or so battery-electric buses.

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The American cities' mayors each described the C40 agreement as a way to lead the country forward in green initiatives and efforts to battle climate change.

Cities and states across the U.S. have taken it upon themselves to implement their own measures as the United States federal government turns away from global alliances to fight climate change.

Specifically, the U.S. is now the only country on the globe not committed to the Paris Climate Accord after Syria announced its intent to sign the agreement.

Proterra Catalyst electric bus

Proterra Catalyst electric bus

“By reaffirming our commitment to a zero-emission transportation system, Seattle is proud to join our friends around the world in demonstrating the power of cities to lead on climate," Tim Burgess, Mayor of Seattle, said.

Cities will report back on their progress every two years to analyze various implementations and track how far each has progressed towards the 2030 end goal.

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Los Angeles, one of the cities committed to the C40 agreement, already began implementing electric buses into its public transport fleet.

It recently placed an order for up to 100 electric buses built by New Flyer. The city has also run Chinese electric-vehicle maker BYD's buses for some time.

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