Berlin — Teenage climate activist Greta Thunberg called on world leaders Tuesday to acknowledge the seriousness of global warming and that not enough is being done to curb it. Speaking at a conference in the Austrian capital, Vienna, Thunberg suggested it was wrong to portray the worldwide shift necessary to rein in climate change as "primarily an opportunity to create new green jobs, new businesses, green economic growth," as some political and business leaders have done.

"This is above all an emergency and not just any emergency. This is the biggest crisis humanity has ever faced," she said, adding: "This is not something you can like on Facebook."

On Tuesday, Thunberg met with United Nations Secretary-General Antonio Guterres, Austrian President President Alexander Van der Bellen and former California Governor Arnold Schwarzenegger. The actor-turned-politician posted a picture with Thunberg on his official Twitter account on Monday.

Excited to kick off our @R20_AWS in Vienna. I have to admit I was starstruck when I met @gretathunberg. Tune in tomorrow. pic.twitter.com/b0c5FtmZDq — Arnold (@Schwarzenegger) May 27, 2019

The 16-year-old's activism, which started with her protesting alone in front of the Swedish parliament last August, has inspired regular student protests around the world. Green parties in Europe have credited Thunberg with helping propel the issue of climate change into the limelight, boosting their results in last week's European Parliament elections.

Thunberg has often struck a much darker tone than some of her fellow activists and refused to hold back from criticizing her audiences.

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"Politicians one second say climate change is very important, it is the most important topic and we are going to do everything we can to stop it," she said. "And the next second they want to expand airports, build new coal power plants and motorways. And then they fly off in a private jet to attend a meeting on the other side of the world."

U.N. Secretary-General Antonio Guterres, who has invited world leaders to a climate summit in New York in September, said countries aren't doing enough to meet the Paris climate agreement's goal of keeping global warming well below 2 degrees Celsius (3.6 Fahrenheit) by the end of the century compared with pre-industrial times.

United Nations Secretary-General Antonio Guterres, Swedish climate activist Greta Thunberg, Austrian President Alexander Van der Bellen and former California Gov. Arnold Schwarzenegger, from left, pose before the R20 Austrian world summit at Hofburg palace in Vienna, Austria, Tuesday, May 28, 2019. Ronald Zak / AP

Guterres called for an end to fossil fuel subsidies, saying taxpayers' money shouldn't be used to harm the planet.

"Many people still think that to give fossil fuel subsidies is a way to improve living conditions of people," he said. "Nothing (could be) more wrong. What we are doing is to use taxpayers' money, which means our money, to boost hurricanes, to spread drought, to melt glaciers, to bleach corals, in one word: to destroy the world."

"I believe that as taxpayers we would prefer to see our money back than to see our money used to destroy the world," said Guterres.