The U.N. warns that unless the world acts faster than promised, Earth's temperatures will rise to catastrophic, irreversible levels. The U.S. calls the upcoming climate summit the last chance for the world to avoid disaster. Nick Schifrin discusses the crisis with John Kerry, the president's special envoy on climate, and Frans Timmermans, executive vice president of the European Commission. PBS NewsHour is supported by - https://www.pbs.org/newshour/about/funders