Last summer, more than 1,000 environmentalists in the U.K. staged a weeklong protest in a "Climate Camp" at Heathrow Airport, where about 70 people were arrested. Their immediate purpose was to block a planned expansion of Heathrow, but the protests highlighted a growing complaint in Europe—that the ride to global-warming catastrophe is being fueled not only by coal-fired power plants and SUVs, but also by the ever-rising number of commercial jets. Now governments are starting to listen.