By William James SHARM EL-SHEIKH, Egypt (Reuters) -U.S. President Joe Biden addressed the COP27 climate conference in Egypt on Thursday, saying the global climate crisis posed an existential threat to the planet and promising that the United States was doing its part to combat it. “The climate crisis is about human security, economic security, environmental security, national security, and the very life of the planet,” Biden said, before outlining steps the United States, the world’s second-biggest greenhouse gas emitter, was taking. “I can stand here as president of the United States of Ameri…