President-elect Donald J. Trump , who has called human-caused climate change a “hoax” perpetuated by the Chinese, will have the power next year to water down President Obama’s climate change regulations and ignore the United States’ commitments under last year’s Paris climate agreement . If he does, experts in climate change policy say, the world may have no way to avoid the most devastating consequences of global warming, including rising sea levels, extreme droughts and food shortages, and more powerful floods and storms.