Cities from New York to Shanghai could see regular flooding, as sea levels rise faster than previously thought. Glaciers and ice sheets from the Himalayas to Antarctica are rapidly melting. And the fisheries that feed millions of people are shrinking. These are just some of the impacts that emissions of greenhouse gases have already triggered across the planet’s oceans and frozen regions, according to a new landmark report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC).

More than 100 scientists from 36 countries worked on the report—titled the Special Report on the Ocean and Cryosphere in a Changing Climate. It is the last of three special reports from the IPCC following last October’s urgent report that showed the world may only have until 2030 to keep global warming below 1.5 degrees, and August’s report on climate impacts to the planet’s lands.

Two days after a climate summit failed to deliver game-changing pledges to slash carbon emissions, the United Nations warned yesterday that global warming is devastating oceans and Earth’s frozen spaces in ways that directly threaten a large slice of humanity.

Crumbling ice sheets, rising seas, melting glaciers, ocean dead zones, toxic algae blooms—a raft of impacts on sea and ice are decimating fish stocks, destroying renewable sources of fresh water, and incubating superstorms that will ravage some megacities every year, the landmark assessment approved by the 195-nation IPCC.Some of these impacts are irreversible. The report, a digest of 7,000 peer-reviewed studies, is a sobering reminder that record greenhouse gas emissions, mostly from burning fossil fuels, are driving the planet towards a hothouse climate our species could find intolerable.

But it also raises more clearly than ever before a red flag on the need to confront changes that can no longer be averted. For some island nations and coastal cities, that will almost certainly mean finding new places to call home. “Even if we manage to limit global warming, we will continue to see major changes in the oceans,” said Valerie Masson-Delmotte, a researcher at the Laboratory for Climate and Environmental Sciences and an IPCC co-chair.

“But it will at least buy us some time, both for future impacts and to adapt.” The underlying 900-page scientific report is the fourth such UN tome in less than a year, with others focused on a 1.5-Celsius cap on global warming, the decline of biodiversity, as well as land use and the global food system.

All four conclude that humanity must overhaul how it produces, distributes and consumes almost everything to avoid the worst ravages of global warming and environmental degradation. By absorbing a quarter of manmade CO2 and soaking up more than 90 percent of the heat generated by greenhouse gases, oceans have kept the planet livable—but at a terrible cost, the report finds.