Witness the unveiling of Countdown, a major global campaign to cut greenhouse gas emissions. TED has partnered with scientists, policy makers, organizations, activists and more to create an initiative that everyone in the world can be part of. Check out http://countdown.ted.com to learn how you can get involved — and help turn the tide on climate. [Note: there are two unusual features of this TED Talk. One, it's much longer than our normal, extending a full hour. Two, it's made up of contributions from more than a dozen people, including UN Secretary-General Antonio Guterres, Al Gore, Katharine Hayhoe, Jimmy Kimmel and Yuval Noah Harari, among others. We're putting it out there because the topic deserves this kind of prominence.]