The coming century will likely bring dangerous and dramatic changes to the planet's oceans, with ever strengthening storms, annual bleaching of almost all coral reefs, loss of biodiversity and severe impacts on fisheries and aquaculture unless humans slash greenhouse gas emissions.
These are the findings of a comprehensive review of the effects of warming oceans, issued Monday by International Union for the Conservation of Nature (IUCN) and compiled by more than 80 scientists from 12 countries. The report chronicles how the seas have absorbed the vast majority of the excess heat trapped in the atmosphere by carbon emissions in recent decades, and how that energy is already altering the planet's weather systems and ecology, from deep ocean trenches to alpine glaciers.
"Ocean warming is one of this generation's greatest hidden challenges—and one for which we are completely unprepared," Inger Andersen, IUCN's director general, said in a statement. "The only way to preserve the rich diversity of marine life, and to safeguard the protection and resources the ocean provides us with, is to cut greenhouse gas emissions rapidly and substantially."
In many ways, the oceans have helped mitigate the effects of carbon dioxide emitted into the atmosphere. They've absorbed more than 90 percent of the excess heat since the 1970s. If all the warmth that went into the top 1.25 miles of the ocean between 1955-2010 had instead gone into the air, the earth would have warmed 36 degrees Celsius.
Unfortunately, the report explained, the oceans are beginning to catapult that heat back into the atmosphere, with often dire consequences.
The unusually intense El Niño weather pattern in recent years—which has contributed to strong monsoons, droughts and wildfires across the globe—may be an example of a sudden release of stored energy to the surface in the Pacific Ocean. The seas are likely to warm 1 to 4 degrees Celsius by 2100, and models suggest that will translate into stronger, more frequent El Niño events, more intense hurricanes and a general amping up of the hydrological cycle, which brings moisture into the atmosphere from the ocean and then back down as precipitation.
The report includes a series of chapters detailing what it calls the cascading effects of all this warming on everything from seagrass to marine mammals to carbon management.
In general, we should expect to see "homogenization" of biodiversity, the authors wrote, with weaker species disappearing as stronger ones spread into new territory. Already, many marine creatures have been moving toward the poles at a rate of more than five miles per decade. The North Sea Atlantic Cod, for example, has shifted more than 250 miles north for each degree increase in surface temperature.
Few ecosystems face greater threats than coral reefs, which provide habitat for more than 25 percent of marine fish species despite covering less than 0.1 percent of the sea floor. Warming water is linked to bleaching and disease, which can kill corals or reduce reproduction rates. Models suggest that current rates of emissions will cause annual bleaching of almost all reefs by 2050. The report said that because even today's concentrations of carbon dioxide are too high to support healthy reef systems, enhanced protections and restoration of reefs may be needed to help them survive.
One winner in all of this: jellyfish. The gelatinous creatures have proliferated in recent decades, likely a result of overfishing and warming waters harming other creatures and opening a space for these marine opportunists.
All of these ecological changes will, of course, have profound impacts on people. Both fisheries and aquaculture would be "severely affected" if humans fail to substantially reduce greenhouse gas emissions, the report said. The impacts will not be uniform, with nations in the tropics and subtropics suffering the worst effects.
The study, which the authors said is meant to build on the 2013 Intergovernmental Panel on Climate Change report, covered other issues as well, including ocean acidification and the potential links between warming waters and increased spread of tropical diseases. If that wasn't bad enough, the report also warned that increased ocean temperatures could unleash some 2.5 billion tons of methane hydrate—a potent greenhouse gas currently frozen into the sea floor.
Throughout the paper, the authors argued for immediate actions to preserve and protect marine ecosystems and to repair damage where possible. In essence, the paper is not just scientific, but political too.
"The ocean community will have to 'keep the pressure' on the international climate negotiation process to ensure, first, that the mitigation efforts adopted in Paris in 2015 are effectively implemented and, second, that the global ambition is progressively increased," the authors wrote. "It is thus of critical importance that changes in the ocean are taken into account in climate talks."