For years scientists have urged national leaders to tackle climate change, based on the  assumption that prevention efforts would require the coordinated actions of entire nations to be effective. But as anyone who has watched the past 15 years of international climate negotiations can attest, most countries are still reluctant to take meaningful steps to lower their production of greenhouse gases, much less address issues such as how to help developing countries protect themselves from the extreme effects of climate change. Frustrated by the ongoing diplomatic stalemate, a number of urban leaders have decided to take matters into their own hands, adopting solutions that already exist or inventing new ones for limiting greenhouse gas emissions and preparing for the effects of ongoing global warming.