Last week at the C40 Mayors Summit, four cities—Athens, Mexico City, Madrid, and Paris—committed to ban diesel-fueled automobiles in their cities by 2025, a move applauded by many environmental supporters. Diesel-fueled vehicles had previously been championed for emitting less carbon dioxide than gasoline-powered vehicles. The ban comes after local governments became attuned to the local pollution effects of diesel vehicles, in particular nitrogen oxides and particulate matter, which manufacturers have been slow to resolve. Environmental groups are hopeful that this move will inspire other cities to set the same standards, thereby encouraging car manufacturers to design their products accordingly. For example, London is now more seriously considering action on pollution emitted by the transportation sector. For the full story, see http://www.bbc.com/news/science-environment-38170794.