It didnt take long for the death threats to start. Alexandria Villaseor, a 13-year-old environmental activist, had just been featured in an Agence France-Presse article republished by Breitbart News about dozens of students staging a die-in at United Nations headquarters in New York.

Villaseor was protesting that day in mid-March for the same reason that she decided to start a school strike four months earlier: to demand that world leaders quit dragging their feet and take swift action to combat global climate change .

Dont stage it, just die, one Breitbart reader commented on the right-wing publications website. I would be more impressed if they doused theselves with gas and set themselves on fire, wrote another. You dont deserve a future, you pathetic halfwit, said a third.

The repugnant online trolling might send most seventh graders cowering, but Villaseor shrugged it off. Her fight is about ensuring that her generation and future ones are left with a habitable planet. She wasnt about to let a bunch of angry deniers get in the way.

I think if more people really understood the climate science, the extinction rate and just all the terrible statistics about what were doing to our planet, they would be motivated, too, she told HuffPost. If everyone just paid attention to the facts, everyone would be a climate activist.

The last five years were the five hottest on record. The amount of carbon dioxide in the atmosphere has soared past 410 parts per million a concentration that hasnt been seen in 3 million years, when sea levels were up to 66 feet higher, according to a recent study . Human-caused climate change is driving sea-level rise , drought, extreme weather and a biodiversity crisis that scientists have declared Earths sixth mass extinction event. As many as 150 species die off each day.

Monday is Earth Day, the 49th anniversary of the birth of the modern environmental movement and a day of action celebrated by more than a billion people around the globe.