Thunberg, though, told Congress she didn't have any prepared remarks. Instead, she said she was attaching her testimony -- the Intergovernmental Panel on Climate Change's special report on global warming, which reported a temperature increase of 1.5 degrees Celsius above pre-industrial levels.

"I am submitting this report as my testimony because I don't want you to listen to me, I want you to listen to the scientists," she said. "And I want you to unite behind the science. And then I want you to take real action."

Strong words from the teenager, but this isn't the first time she's spoken up to governments.