"Don't let anyone dim your light," the former first lady wrote on Twitter Thursday to Thunberg. "Like the girls I've met in Vietnam and all over the world, you have so much to offer us all."

Without mentioning Trump, the former first lady added, "Ignore the doubters and know that millions of people are cheering you on."

Trump on Thursday morning tweeted that it was "so ridiculous" for Thunberg to be recognized as Person of the Year.

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

Following the President's tweet, Thunberg updated her Twitter bio to reflect Trump's comments.

Democratic presidential frontrunner Joe Biden, who served as Barack Obama's vice president, chastised Trump for his comments, writing on Twitter, "What kind of president bullies a teenager?"