Greta Thunberg, other activists, deliver messages at UN youth climate summit

The first UN youth climate summit kicked off in New York City on Saturday with a panel of speakers that includes 16-year-old Swedish environmental activist Greta Thunberg.

She delivered a short statement, explaining that she wanted to leave more time for other speakers, and that she would be addressing Monday's UN summit on climate change.

"Yesterday, millions of people across the globe marched and demanded real climate action, especially especially young people. We showed that we are united and that we, young people, are unstoppable," Thunberg said.

 After listening to Thunberg and other youth climate activists, UN Secretary-General Antonio Guterres credited young people with changing him from a pessimist to an optimist in the fight against climate change.

Thunberg was on Capitol Hill on Wednesday, wrapping up a six-day visit to Washington, appearing with other young activists before the House foreign affairs subcommittee on climate change.

The teen activist urged politicians to "just tell them the truth," when asked how children can get more involved in learning about climate change and doing something about it.

On Friday, Thunberg joined a rally in New York, where it's estimated at least 100,000 people marched, many of them teens who followed the call for a climate strike. Thunberg started the climate strike movement last year, calling for weekly demonstrations.

Young people marched in cities around the world on Friday to demand that politicians heading to the UN summit on climate change take immediate action to combat climate change.

In an interview published by The Associated Press on Saturday, Thunberg was asked for her impression about the political situation in the United States around climate change.

"It's a bit worse than in other countries," she said. "The arguments for continuing to not do anything and the empty words and promises and lies are the same. Some countries are more extreme than others but it's not much different."

She said nothing may come from the upcoming summit, but added that "giving up cannot be an option."

The UN has called climate change the "defining issue of our time." Scientists have warned that global warming will subject Earth to rising seas, as well as more heat waves, droughts, storms and flooding.