JAKARTA, Indonesia (AP) — U.S. Secretary of State John Kerry on Sunday called climate change perhaps the world’s “most fearsome” destructive weapon and mocked those who deny its existence or question its causes, comparing them to people who insist the Earth is flat.

In a speech to Indonesian students, civic leaders and government officials, Kerry tore into climate change skeptics. He accused them of using shoddy science and scientists to delay steps needed to reduce emissions of greenhouse gases at the risk of imperiling the planet.