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Changes deep in the ocean or high in the mountains are not always as noticeable as some of the other hallmarks of global warming, such as heat waves on land, or wildfires and droughts. But the report makes clear that what happens in these remote regions will have ripple effects across the globe.

For instance, as ice sheets in Greenland and Antarctica melt and push up ocean levels, the report said, extreme flooding that was once historically rare could start occurring once a year or more, on average, in many coastal regions this century. How quickly this happens depends largely on the ability of humanity to reduce emissions of greenhouse gases that are heating the planet.

Around the world, glaciers in the mountains are receding quickly, affecting the availability of water for millions of people who depend on meltwater downstream to supply drinking water, irrigate agricultural land and produce electricity through dams and hydropower.

But some of the report’s starkest warnings concern the ocean, where major shifts are already underway.

The frequency of marine heat waves — which can kill fish, seabirds, coral reefs and seagrasses — have doubled since the 1980s. Many fish populations are migrating far from their usual locations to find cooler waters, throwing local fishing industries into disarray. Floating sea ice in the Arctic Ocean is declining at rates that are “likely unprecedented for at least 1,000 years,” the report said.

There have even been unwelcome surprises. The report notes that some pathogens are proliferating in warmer waters, including vibrio, a bacteria that can infect oysters and other shellfish, and that already sickens some 80,000 Americans who eat raw or undercooked seafood each year. “That’s a good example of how changes in the ocean can affect even people who live far from the coasts,” said Sherilee Harper, a public health expert at the University of Alberta and an author on the report.