Greta Thunberg explained why she settled on a 'death stare' over a discussion with U.S. President Donald Trump at the UN.

Negotiators split their work into two parts, separating discussions on aid for poor countries affected by climate change from those over a deal on international carbon markets.

Thunberg, who turns 17 in January and who is known for her blunt, straightforward speaking manner, has urged immediate action to address what she describes as the global climate crisis.

"She seems like a very happy young girl looking forward to a bright and wonderful future," Trump wrote in a tweet posted late Monday. "So nice to see!"

Greta Thunberg scolded the leaders of the world, saying: "You have stolen my dreams and my childhood with your empty words."

Heads of nations such as Finland and Germany promised to ban coal within a decade.

Montreal Mayor Valérie Plante spoke at the United Nations at the opening of the UN Climate Summit.

As if to underscore the seriousness of the problem, the U.N.’s World Meteorological Organization released a science report Sunday showing that in the last several years, warming, sea level rise and carbon pollution have all accelerated.

“Even though we ourselves aren’t sick, the planet which we live on is, and we are protesting and fighting for it,” said one student.