Students play with an inflatable globe as they march to demand action on climate change, in Rome, Friday, March 15, 2019. Students worldwide are skipping class Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. (Alessandro Di Meo/ANSA via AP)

From the South Pacific to the edge of the Arctic Circle, students mobilized by word of mouth and social media are skipping class to protest what they see as the failures by their governments to take tough action against global warming.

Friday's rallies were one of the biggest international climate change actions yet, involving hundreds of thousands of students in more than 100 countries around the globe.

The coordinated "school strikes" were inspired by 16-year-old Swedish activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year.

Since then, the weekly protests have snowballed from a handful of cities to hundreds, fueled by dramatic headlines about the impact of climate change during the students' lifetime. Scientists have backed the protests, with thousands in Britain, Finland, Germany and the United States signing petitions in support of the students.

Thunberg, who has been nominated for the Nobel Peace Prize, said at a rally in Stockholm that the world faces an "existential crisis, the biggest crisis humanity ever has faced and still it has been ignored for decades."

"And you know who you are, you that have ignored this," she said.

Across the globe, protests big and small urged politicians to act against climate change while also highlighting local environmental problems:

Activist Greta Thunberg, foreground, participates in a climate protest, in central Stockholm Sweden, Friday, March 15, 2019. Students worldwide skipped classes Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. (Pontus Lundahl/TT News Agency via AP)

— In India's capital of New Delhi, schoolchildren protested inaction on climate change and demanded that authorities tackle rising air pollution levels in the country, which often far exceed World Health Organization limits.

— In Paris, teenagers thronged the cobblestoned streets around the domed Pantheon building. Some criticized French President Emmanuel Macron, who sees himself as the guarantor of the landmark 2015 Paris climate accord but is criticized by activists for being too business-friendly and not ambitious enough in efforts to reduce emissions.

Raphael Devautour, high school student said it was his first protest. "We can feel that something is happening," he said. "When the youths start acting, it get things moving. We saw it in 1968."

— In South Africa's capital, Pretoria, one protester held a sign reading "You'll Miss The Rains Down in Africa." Experts say Africa, with its population of more than 1 billion people, is expected to be hardest hit by global warming even though it contributes least to the greenhouse gas emissions that cause it.

— Thousands marched in rainy Warsaw and other Polish cities to demand a ban on burning coal, a major source of carbon dioxide. Some wore face masks as they carried banners that read "Make Love, Not CO2."

Hundreds of schoolchildren take part in a climate protest in Hong Kong, Friday, March 15, 2019. Students in more than 80 countries and territories worldwide plan to skip class Friday in protest over their governments' failure to act against global warming. The coordinated 'school strike' was inspired by 16-year-old activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year. (AP Photo/Kin Cheung)

— Speakers at the U.S. Capitol in Washington stood behind a banner that said "We don't want to die."

— Protests in Madrid and more than 50 other Spanish cities drew thousands. The country is vulnerable to rising sea levels and rapid desertification .

— In Berlin, police said as many as 20,000 protesters gathered in a downtown square, waving signs such as "March now or swim later" before marching through the German capital to Chancellor Angela Merkel's office.

Carla Reemtsma, a 20-year-old student who helped organize the protest in Berlin, said she's part of about 50 WhatsApp groups devoted to discussing climate change.

"A lot happens on social media because you can reach a lot of young people very quickly," she told The Associated Press.

High school students demonstrate outside the Pantheon monument in Paris, Friday, March 15, 2019. Students worldwide are skipping class Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. (AP Photo/Francois Mori)

Azalea Danes, a student at the Bronx High School of Science, wasn't a climate activist until two weeks ago when she read about Thunberg's efforts. Now she is one of the top organizers of the youth climate strike in New York City, where she hopes thousands will rally in three places later Friday.

That shows how these protests are organized from the bottom up, she said.

Volker Quaschning, a professor of engineering at Berlin's University of Applied Sciences, said it was easy for politicians to belittle students.

"That's why they need our support," he said. "If we do nothing, then parts of this planet could become uninhabitable by the end of the century."

But some politicians praised the students. Danish Prime Minister Lars Loekke Rasmussen showed up at a protest in Copenhagen and tweeted Friday "we must listen to the youth. Especially when they're right: the climate must be one of our top priorities."

A young student takes part in a global school strike for climate change in Canterbury, south east England, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (Gareth Fuller/PA via AP)

Scientists have warned for decades that current levels of greenhouse gas emissions are unsustainable, so far with little effect.

In 2015, world leaders agreed in Paris to a goal of keeping the Earth's global temperature rise by the end of the century well below 2 degrees Celsius (3.6 degrees Fahrenheit) compared with pre-industrial times.

Yet the world has already warmed by 1 degree C since then and is on track for an increase of 4 degrees Celsius, which experts say would have far-reaching consequences for life on the planet

In Germany, environmental groups and experts have criticized government plans to continue using coal and natural gas for decades to come.

Quaschning, one of more than 23,000 German-speaking scientists to sign a letter of support this week, said Germany should stop using all fossil fuels by 2040. This would give less-advanced nations a bit more time to wean themselves off coal, gas and oil while still meeting the Paris goal globally.

Students attend a protest ralley of the 'Friday For Future Movement' in Berlin, Germany, Friday, March 15, 2019. (AP Photo/Michael Sohn)

"This is going to require radical measures and there isn't the slightest sign of that happening yet," said Quaschning.

In Stockholm, Thunberg predicted that students won't let up their climate protests.

"There are a crisis in front of us that we have to live with, that we will have to live with for all our lives, our children, our grandchildren and all future generations," she said. "We are on strike because we do want a future."

Young demonstrators gather on the steps of the Helsinki Cathedral, prior to the start of a protest march of Finnish youth calling for climate protection, in Helsinki, Finland, Friday, March 15, 2019. (Heikki Saukkomaa/Lehtikuva via AP)

Adults stand in solidarity with Indian students in a climate protest in Hyderabad, India, Friday, March 15, 2019. Students in more than 80 countries and territories worldwide plan to skip class Friday in protest over their governments' failure to act against global warming. The coordinated 'school strike' was inspired by 16-year-old activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year. (AP Photo/Mahesh Kumar A.)

Students take part in a protest against climate change, in Aarhus, Denmark, Friday, March 15, 2019. Students in cities worldwide skipped classes Friday in protest over their governments' failure to act against global warming. (Henning Bagger/Ritzau Scanpix via AP)

Hundreds of schoolchildren take part in a climate protest in Hong Kong, Friday, March 15, 2019. Students in more than 80 countries and territories worldwide plan to skip class Friday in protest over their governments' failure to act against global warming. The coordinated 'school strike' was inspired by 16-year-old activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year. (AP Photo/Kin Cheung)

A boy wearing a mask sits behind a banner during a rally for global climate strike for future in Seoul, South Korea, Friday, March 15, 2019. About 150 students and other protesters attended a rally to protest their governments' failure to take sufficient action against climate change. (AP Photo/Lee Jin-man)

Students take part in a protest against climate change, in Aarhus, Denmark, Friday, March 15, 2019. Students in cities worldwide skipped classes Friday in protest over their governments' failure to act against global warming. (Henning Bagger/Ritzau Scanpix via AP)

Students in Cape Town, South Africa take part in a protest, Friday, March 15, 2019 as part of a global student strike against government inaction on climate change. Students in cities worldwide skipped classes to protest their governments' failure to act against global warming. (AP Photo/Nasief Manie)

High school students demonstrate outside the Pantheon monument in Paris, Friday, March 15, 2019. Students worldwide are skipping class Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. Poster at center right reads "Utopians are not us". (AP Photo/Francois Mori)

Students attend a demonstrate during a "Climate strike" protest in Lausanne, Switzerland, Friday, March 15, 2019. Students from several countries worldwide plan to skip class Friday in protest over their governments' failure to act against global warming. (Jean-Christophe Bott/Keystone via AP)

Students gather in front of the monument of the Unknown Soldier to demand action on climate change, in Rome, Friday, March 15, 2019. Students worldwide are skipping class Friday to take to the streets to protest their governments' failure to take sufficient action against global warming.(AP Photo/Andrew Medichini)

Activist Greta Thunberg, foreground, participates in a climate protest, in central Stockholm Sweden, Friday, March 15, 2019. Students worldwide skipped classes Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. (Pontus Lundahl/TT News Agency via AP)

Students attend a protest ralley of the 'Friday For Future Movement' in Magdeburg, eastern Germany, Friday, March 15, 2019. (Klaus-Dietmar Gabbert/dpa via AP)

Students attend a protest ralley of the 'Friday For Future Movement' in Berlin, Germany, Friday, March 15, 2019. The building in the background is the Chancelery (AP Photo/Michael Sohn)

Students protest with a large EU flag on Westminster Bridge, London, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (Kirsty O'Connor/PA via AP)

Thousands of students march from St Stephens Green to Leinster House, Dublin, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (Niall Carson//PA via AP)

Youngsters take part in a student climate protest in Parliament Square in London, Friday, March 15, 2019. Students in more than 80 countries and territories worldwide plan to skip class Friday in protest over their governments' failure to act against global warming. The coordinated 'school strike' was inspired by 16-year-old activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year. (AP Photo/Matt Dunham)

Swedish activist Greta Thunberg, right, speaks during a climate change demonstration, in central Stockholm, Sweden, Friday March 15, 2019. Friday's rallies by students around the world were one of the biggest international actions yet to demand more government action to fight climate change. The coordinated 'school strikes,' were inspired by 16-year-old Swedish activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year. (Henrik Montgomery/TT via AP)

Students shout pro-environment slogans during a rally in Madrid, Spain, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take tough action against global warming. (AP Photo/Bernat Armangue)

A girl holds up a poster as thousands of protesting high school students gather outside the Portuguese parliament in Lisbon while taking part in a global school strike for climate change Friday, March 15, 2019. Students worldwide are skipping class Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. Words on the poster read in Portuguese "Do it for the climate". (AP Photo/Armando Franca)

Youngsters lay down as they take part in a student climate protest at the bottom of Westminster Bridge in London, Friday, March 15, 2019. Students in more than 80 countries and territories worldwide plan to skip class Friday in protest over their governments' failure to act against global warming. The coordinated 'school strike' was inspired by 16-year-old activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year. (AP Photo/Matt Dunham)

A girl holds up a fishing trap with a toy baby seal, in front of the Portuguese parliament in Lisbon as thousands of Portuguese high school students stage a protest while taking part in a global school strike for climate change Friday, March 15, 2019. Students worldwide are skipping class Friday to take to the streets to protest their governments' failure to take sufficient action against global warming. (AP Photo/Armando Franca)

A student holds a banner during a demonstration against climate change in Pamplona, northern Spain, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (AP Photo/Alvaro Barrientos)

Students take part in a global protest for climate change in Cambridge city centre, England, Friday March 15, 2019. Angry students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (Stefan Rousseau/PA via AP)

Students in Cape Town, South Africa take part in a protest, Friday, March 15, 2019 as part of a global student strike against government inaction on climate change. Students in cities worldwide skipped classes to protest their governments' failure to act against global warming. (AP Photo/Nasief Manie)

Students take part in a global school strike for climate change in Canterbury, south east England, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (Gareth Fuller/PA via AP)

A young student takes part in a global school strike for climate change in Canterbury, south east England, Friday March 15, 2019. Students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments' failure to take though action against global warming. (Gareth Fuller/PA via AP)

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