Climate change is beginning to affect the lives of communities around the word. With temperature records being broken, yearly, monthly and daily, severe droughts imperiling food systems and bizarre storms disrupting economies and claiming lives around the world, it’s becoming more and more clear that the changes scientists have long warned of have arrived.

For the oceans, it is a different story. Beneath the sea, the impacts of climate change have been visible for years, and several recent studies bring alarming news of its progress.

There are several ways in which oceans respond to increased CO2 pollution, a warming world, melting glaciers and rising seas. First, as the atmosphere warms, oceans absorb much of that heat. In fact, more than 90 percent of heat trapped by greenhouse gases ends up in the ocean.

The rate at which the ocean is warming has increased dramatically over the last two decades; recent research published in Science Advances finds that the surface-most layer of the ocean is now warming four times faster than in was between 1960 and 1990. It also found that oceans may be storing an even greater proportion of the heat caused by global warming than researchers previously thought.

Warming oceans mean that the habitats of sea organisms move toward the polls, or disappear entirely. Sea creatures migrate with them, or go extinct. Another new study, also published in Science Advances, explored what this meant for areas of the ocean where biodiversity is particularly high. Researchers found that six of the ocean’s most biologically rich areas are seeing a rapid decline in that biodiversity because of warming seas coupled with overfishing. The trend is exacerbated by weakening ocean currents, a phenomenon that is also driven by climate change; as ice sheets melt, sending cool water into warming seas, ocean currents are disrupted, and some may shut down entirely.

Marine hot spots (top) were identified on the basis of the spatially explicit information on the equally weighted distribution of fish (1729), marine mammal (124), and seabird (330) species. Colors represent a dimensionless index of biodiversity ranging from 0 (absence of species) to 1 (maximum species richness). Credit: Advances Sciences

Coral reefs under threat

Another visible and tragic side effect of warmer ocean temperatures is coral bleaching. Scientists reacted with frustration last fall, when an obituary for the Great Barrier Reef published by Outside magazine went viral; the reef wasn’t dead yet, and researchers didn’t want the public to have an excuse to give up on trying to save it.

But new observations reinforce how quickly the reef is dying. Australian researchers reported earlier this month that a heat wave in the area of the Great Barrier Reef is causing a mass bleaching event less than a year after another mass bleaching slammed the vast majority of the reef. Bleaching occurs when an environmental disruption causes the algae that live inside the coral to leave, disrupting a symbiotic relationship. The coral becomes weak and may die. Reefs shrink, and the creatures that depend on them to lose their habitat.

Last year’s bleaching was the worst ever recorded in the Great Barrier Reef, and this month’s heat wave, coming so soon after, will compound the damage. “We are seeing a decrease in the stress tolerance of these corals,” said Dr. Neal Cantin of the Australian Institute of Marine Science. “This is the first time the Great Barrier Reef has not had a few years between bleaching events to recover.

Greenpeace has been documenting the coral bleaching with photos and video, showing miles of ghostly white reef. “Once a coral is dead, it’s gone forever,” said Alix Foster Vander Elst, Campaigner for Greenpeace Australia Pacific. “We have on our doorstep the clearest signal that climate change is happening, and that governments aren’t moving fast enough to stop it. We can still stop the reef’s destruction if we dramatically reduce global emissions.

Oceans severely under threat by 2050

Higher water temperatures aren’t the only climate change-related problem facing our oceans. In addition to absorbing warmth from the atmosphere, oceans absorb the CO2 that causes climate change, leading to ocean acidification, which, in turn, has its own nasty side effects. One new study finds that Arctic waters are acidifying particularly quickly, threatening shellfish, plankton and deep sea coral, and the animals, including bears and seals, that feed on them.

Given the many effects of climate change researchers are observing around the globe, the conclusions of another study published earlier this month in Nature Climate Change are perhaps not surprising. With just 15 more years of business-as-usual CO2 emissions, the study finds, more than half of the world’s ocean environments will be suffering from one source of stress, such as warmer or more acidic water, or reduced oxygen and food as a result. By 2050, that figure rises to 86 percent.

Governments can make a difference in slowing these changes — though, according to the Nature Climate Change study, it may be too late for the Arctic. If countries stick to their commitments under the Paris Agreement, climate change will move more slowly, buying non-Arctic ocean ecosystems another 20 years to adapt; organisms might be able to migrate and escape extinction.

But with emissions continuing to remain flat, but not falling, and the US threatening to pull out of the Paris Agreement, it’s unclear how optimistic we can be about that scenario.

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