Nature: Representatives from more than 175 nations signed the Paris climate accord at the United Nations in New York City on 22 April. Global leaders celebrated the event timed to coincide with Earth Day with considerable fanfare, but countries must still take more action to implement the December 2015 agreement, which seeks to limit global warming to between 1.5 °C and 2 °C. Nature examines what must happen for the accord to take legal effect and, ultimately, to achieve its goal. What is the significance...