Humanity has only two years left “to save the world” by making dramatic changes in the way it spews heat-trapping emissions and it has even less time to act to get the finances behind such a massive shift, the head of the United Nations climate agency said. With governments of the world facing a 2025 deadline for new and stronger plans to curb carbon pollution, nearly half of the world's populations voting in elections this year, and crucial global finance meetings later this month in Washington, United Nations executive climate secretary Simon Stiell said Wednesday he knows his warning may sound melodramatic.