Britain's new King Charles III is an outspoken climate activist who has traveled worldwide to give environmental speeches and advocate a complete transition from fossil fuels.

Charles — who ascended the throne after his mother Queen Elizabeth II died Thursday — has repeatedly warned that climate change will have "devastating" impacts on the environment. As prince, he regularly attended United Nations climate conferences and issued statements or speeches condemning the "lack of climate action."

"The scale and scope of the threat we face call for a global systems level solution based on radically transforming our current fossil fuel based economy to one that is genuinely renewable and sustainable," Charles remarked during a speech at the U.N.'s COP26 summit in November 2021.

"Any leader who has had to confront such life-threatening challenges knows that the cost of inaction is far greater than the cost of prevention," he continued. "So I could only urge you as the world’s decision makers to find practical ways of overcoming differences, so we can all get down to work together to rescue this precious planet and save the threatened future of our young people."

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The then-prince acknowledged that a green transition would cost trillions of dollars, but said the investment was needed to save the world. He added that a global "military style campaign" was required to achieve such a massive and costly transition.

"We need a vast military style campaign to marshal the strength of the global private sector, with trillions at its disposal far beyond global GDP and, with the greatest respect, beyond even the governments of the world’s leaders," he continued. 

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"It offers the only real prospect of achieving fundamental economic transition."

In April, Charles delivered a virtual address for the Our Ocean Conference. During the speech, he said human activity has led to the "rapid decline" in ocean health.

The king has also spoken out against plastic pollution, in favor of rights for indigenous people and for environmental justice in recent years.

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He has also led several environmental campaigns over the last five decades.

Queen Elizabeth II and Prince Charles

Queen Elizabeth II and Prince Charles are pictured. (Paul Edwards/WPA Pool/Getty Images)

"Global warming, climate change and the devastating loss of biodiversity are the greatest threats humanity has ever faced," Charles said at the 2020 World Economic Forum meeting in Davos, Switzerland.

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"What good is all the extra wealth in the world gained from business as usual if you can do nothing with it, except watch it burn in catastrophic conditions?" he added. "The only limit is our willingness to act and the time to act is now."

Charles will have limited power as king to make substantive policy decisions on behalf of the U.K., but could use his position to continuing advocating for climate action.