A teenage climate activist addressed world leaders at a United Nations conference in Poland this week, accusing them of “behaving like children” on climate change.

Greta Thunberg, a 15-year-old from Sweden, warned attendees of the conference in Katowice, Poland, that global warming is “an existential threat,” and spoke of the necessity of youth activism in environmental issues.

“For 25 years, countless people have come to the UN climate conferences begging our world leaders to stop emissions and clearly that has not worked as emissions are continuing to rise,” she said, according to The Guardian.

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“So I will not beg the world leaders to care for our future. I will instead let them know change is coming, whether they like it or not,” she said.

Thunberg started a protest that has inspired more than 20,000 students globally to walk out of school to prompt lawmakers to take action on the climate.

“Since our leaders are behaving like children, we will have to take the responsibility they should have taken long ago,” Thunberg said at the conference. “We have to understand what the older generation has dealt to us, what mess they have created that we have to clean up and live with. We have to make our voices heard.”

Thunberg met with U.N. Secretary-General Antonio Guterres, according to The Guardian.

The teen activist originally spent two weeks protesting outside Swedish Parliament instead of going to school, and now does so every Friday, vowing to stop when the country reduces its carbon emissions by 15 percent annually.

“Why should we be studying for a future that soon may be no more? This is more important than school, I think,” she told the conference.

Representatives of nearly 200 nations are attending the conference in Poland for two weeks of talks on climate change and hammering out concrete action to implement the Paris accord, which President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE pulled out of last year.

The conference comes weeks after the U.N. released a dire report on the impending consequences of global warming.