President Trump announced his intention to withdraw from the landmark Paris climate agreement in 2017 and formally notified the United Nations last year. A mandatory yearlong waiting period ended on Wednesday. Of the nearly 200 nations that signed the agreement, the U.S. is the only one to walk away from its promises to reduce greenhouse gas emissions. NPR science reporter Rebecca Hersher shares fives things to know. For additional info and links, check out the episode page.Email the show at shortwave@npr.org.