London -- Thousands of scientists came out in support of children's school strikes on Friday as young people across the world took to the streets again to demand action on climate change. A letter published in the magazine "Science," titled "Concerns of young protesters are justified," had been co-signed by more than 3,000 scientists across the globe, and that number continued to grow.

"The current measures for protecting the climate and biosphere are deeply inadequate," the letter said.

"We see it as our social, ethical, and scholarly responsibility to state in no uncertain terms: Only if humanity acts quickly and resolutely can we limit global warming, halt the ongoing mass extinction of animal and plant species, and preserve the natural basis for the food supply and well-being of present and future generations. This is what the young people want to achieve. They deserve our respect and full support," it continued.

”We declare: Their concerns are justified and supported by the best available science. The current measures for protecting the climate and biosphere are deeply inadequate.”

World leading climate scientists support #FridaysForFuture #schoolstrike4climate https://t.co/1QEQKIHVr8 — Greta Thunberg (@GretaThunberg) April 11, 2019

Young protesters were demonstrating as part of a climate school strike movement which has seen thousands of students skip classes on Fridays for weeks to demand climate action.

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In London, marchers blocked traffic in the central Oxford Circus shopping area. Some carried signs calling for a "Green New Deal," inspired by Alexandria Ocasio Cortez's proposed legislation in the United States. Politicians in Great Britain have also been pushing a Green New Deal, and they told CBS News recently that they hoped the initiative would gain momentum thanks to the the attention garnered by Ocasio Cortez's high-profile campaign.

Greta Thunberg, the 16-year-old girl who started the student climate strike movement, was out in her home of Helsinki, Finland on Friday. Other young people tweeted photos of their protests from India and Uganda.

The climate strikes have gained steam as another protest group, Extinction Rebellion, carries out increasing civil disobedience actions in the U.K., including having a number of almost-naked protesters disrupt a Brexit discussion in the Houses of Parliament last week. That group is also demanding more action from the government to address climate change. They've vowed to "shut down" London with a massive demonstration next week.

"Many social, technological, and nature-based solutions already exist," the letter published on Friday in "Science" said. "The young protesters rightfully demand that these solutions be used to achieve a sustainable society. Without bold and focused action, their future is in critical danger. There is no time to wait until they are in power."