United States President Donald Trump mocked teenage climate activist Greta Thunberg on Thursday for being chosen as Time magazine's Person of the Year.

"So ridiculous. Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!" he wrote on Twitter.

Earlier this year, following her speech at the United Nations, Trump said that Thunberg "seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"