By 2025, there will be no more diesel vehicles in Paris, Madrid, Mexico City, or Athens, Greece.

The four cities' mayors, who are attending the C40 Mayors Summit this week in Mexico City, have announced plans to combat air pollution by banning diesel vehicles.

"Mayors have already stood up to say that the climate change is one of the greatest challenges we face," Mayor Anne Hidalgo of Paris said in a statement. "Today, we also stand up to say we no longer tolerate air pollution and the health problems and deaths it causes — particularly for our most vulnerable citizens. Big problems like air pollution require bold action, and we call on car and bus manufacturers to join us."

Mayors and local leaders representing 86 cities — including Mayor Rahm Emanuel of Chicago, Mayor Eduardo Paes of Rio de Janeiro, and Deputy Mayor Ai Xuefeng of Shenzhen — have convened at the C40 summit to discuss plans to address climate change.

Cities have increasingly been leading the charge on climate action, since more than half of the world's population lives in cities, a share that is expected to rise to 70% by 2050. That means cities also account for a large portion of greenhouse-gas emissions, putting them in a powerful position to fight climate change.

At the summit, mayors discussed a new report outlining actions cities could take that would account for 40% of the emissions reductions necessary to keep the global temperature from rising by more than 1.5 degrees Celsius. To do that, the report found, they would need to ensure that emissions peak by 2020 and then drop dramatically.

Removing cars is one way to achieve that.

"Our goal is to ultimately remove all cars from the center of Athens in the years to come," the mayor of Athens, Giorgos Kaminis, said in a statement.

Other cities have also initiated efforts to eliminate cars from their centers — the Norwegian capital, Oslo, plans to implement a car ban by 2019, and Brussels and Copenhagen, Denmark, are home to Europe's two biggest car-free zones.

Those efforts, Mayor Miguel Ángel Mancera of Mexico City said, "ease congestion in our roadways and our lungs."