Would you like to receive climate news in your inbox? Sign up for Climate Fwd: our e-mail newsletter.
WASHINGTON – Earth's oceans are under pressure from climate change, warns a key new United Nations report threatening everything, from the ability to harvest seafood for the benefit of hundreds of millions living on the coasts.
Rising sea levels threaten to drive ever-increasing tropical cyclones and floods, further threatening coastal areas and worsening a phenomenon that is already in danger, but storms like Hurricane Harvey, which devastated Houston two years ago.
"The oceans send us so many warning signals that we have to control emissions," said Hans-Otto Pörtner, a marine biologist at the Alfred Wegener Institute in Germany and lead author of the report. "Ecosystems are changing, food webs are changing, fish stocks are changing and these riots are affecting humans."
For decades, the oceans have served as a critical buffer against global warming, absorbing about a quarter of carbon dioxide from people from power plants, factories and communities Emit cars and absorb more than 90 percent of the excess heat that stores carbon dioxide and other greenhouse gases on Earth. Without this protection, the country would warm up much faster.
The oceans themselves, however, are reported to become hotter and less oxygen rich. As humans continue to pump greenhouse gases into the atmosphere at an accelerating rate, the risks to human food security and coastal communities will greatly increase as marine ecosystems are already threatened by plastic pollution, unsustainable fishing practices and other man-made threats.
The report, written by more than 100 international experts and based on more than 7,000 studies, is the most comprehensive look at the effects of climate change on oceans, ice sheets, mountain snowpacks, and permafrost.  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 forest fires and droughts. However, the report makes it clear that what happens in these outlying regions will have global repercussions.
For example, the report states that extreme flooding, which was rare in the past, could occur on average once a year or more in many coastal regions in this century, as ice sheets in Greenland and the Antarctic melt and sea levels rise. How fast this happens depends critically on mankind's ability to reduce the emission of greenhouse gases that heat the planet.
Worldwide, mountain glaciers are rapidly decreasing affecting the availability of water to millions of people downstream of meltwater to deliver drinking water, irrigate agricultural land, and generate electricity through dams and hydropower ,
However, some of the report's fiercest warnings refer to the ocean, where major changes are already taking place.
The frequency of sea heat waves that can kill fish, seabirds, coral reefs and sea grasses has doubled since the 1980s. Many fish populations migrate far from their habitual locations to find cooler waters, which disrupts the local fishing industry. Floating sea ice in the Arctic Ocean is falling at rates that "are unlikely to have been unparalleled for at least 1,000 years," the report says.
The report warns that more dramatic changes may be imminent. For example, if fossil fuel emissions continue to soar, the maximum amount of marine fish that can be sustainably caught could fall by as much as a quarter by the end of the century. This would have far-reaching implications for global food security: fish and seafood account for about 17 percent of the world's animal protein, and millions of people around the world rely on fishing to make a living.
Heat waves in the ocean are expected to be 20 to 50 times more frequent in this century, depending on how much greenhouse gas emissions increase.
Changes in the ocean can also disrupt the complex and often fragile ecosystems that underpin the marine environment. The report finds that the upper layers of the open ocean have lost between 0.5 and 3.3 percent of their oxygen since 1970 with rising temperatures. And as the ocean absorbs more carbon dioxide, it becomes more acidic, which could make it harder for corals, oysters, shells, and other organisms to build their hard shells.
The Californian stream, a nutrient-rich pattern of water currents in the Pacific Ocean that supports one of the most lucrative fisheries in the world, is already affected by acidification and a decline in oxygen levels. While scientists are still trying to understand the full implications of these changes, there is a risk that fish may migrate through shifts in the food chain.
"When fish leave, it affects the small fishing fleets we have along the California coast," said Gretchen Hofmann, professor of marine biology at the University of California, Santa Barbara, who was not involved in the report , "So there is a risk of real economic and social problems."
While the report recommends that the nations of the world drastically reduce greenhouse gas emissions to reduce the severity of most of these threats, it also indicates that countries need to adapt. Many changes are now inevitable.
For example, even though nations are rapidly halting their greenhouse gas emissions over the coming decades and limiting global warming to well below a 2-degree Celsius increase from pre-industrial levels – a goal of the Paris Agreement, a pact between nations to combat the Warming – The oceans and frozen landscapes would still look very different from today until the end of the century. Hot water coral reefs would still be ravaged. The global sea level could rise 1 to 2 feet in this century as ice sheets and glaciers melt. The fish populations would still be wandering, producing winners and losers among the fishing nations and potentially leading to increased conflict, the report said.
Coastal cities need to build costly bankwalls to deal with these issues, and many are likely to move. The report says they are not low-lying areas. Fisheries managers need to address unsustainable fishing practices in order to prevent the collapse of fish stocks. Nations could also expand protected areas of the ocean to improve the resilience of marine ecosystems to changing conditions.
However, the report also makes it clear that many of these adaptation measures could lose their effectiveness if greenhouse gas emissions continue to rise. In the report's worst-case emission scenario, in which greenhouse gases accumulate uncontrollably in the atmosphere throughout the century, sea levels could rise inexorably for hundreds of years and possibly reach 17 feet or more by 2300, the report said.
"Our fate is probably somewhere in between," said Michael Oppenheimer, climatologist at Princeton University and lead author of the chapter's Report on the Sea Level. "But if you think about the possibility of sea levels increasing or even increasing indefinitely in the centuries to come, this is a very bad sign of coastal culture."
For more climate and environmental news, see on Twitter at @NYTClimate .