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WASHINGTON — Climate change is heating the oceans and altering their chemistry so dramatically that it is threatening seafood supplies, fueling cyclones and floods and posing profound risks to the hundreds of millions of people living along the coasts, according to a sweeping United Nations report issued Wednesday.

The report concludes that the world’s oceans and ice sheets are under such severe stress that the fallout could prove difficult for humans to contain without steep reductions in greenhouse gas emissions. Fish populations are already declining in many regions as warming waters throw marine ecosystems into disarray, according to the report by the Intergovernmental Panel on Climate Change, a group of scientists convened by the United Nations to guide world leaders in policymaking.

“The oceans are sending us so many warning signals that we need to get emissions under control,” said Hans-Otto Pörtner, a marine biologist at the Alfred Wegener Institute in Germany and a lead author of the report. “Ecosystems are changing, food webs are changing, fish stocks are changing, and this turmoil is affecting humans.”

Hotter ocean temperatures, combined with rising sea levels, further imperil coastal regions, the report says, worsening a phenomenon that is already contributing to storms like Hurricane Harvey, which devastated Houston two years ago.