One hundred days ago Thursday, the oil rig Deepwater Horizon began spewing oil into the Gulf of Mexico. As profoundly as the leak of millions of barrels of oil is injuring the Gulf ecosystem, it is only one of many threats to the Earth's oceans that, many experts say, could change the makeup of the oceans as we know them and wipe out a large portion of marine life.

The waters of the Gulf were already heavily fished, and the Gulf has been home to an oxygen-depleted dead zone generated by agricultural runoff rich in nutrients.

The Gulf and the rest of the world's waters also face the uncertain and potentially devastating effects of climate change. Warming ocean temperatures reduce the water's oxygen content, and rising atmospheric carbon dioxide is altering the basic chemistry of the ocean, making it more acidic. There is no shortage of evidence that both of these effects have begun to wreak havoc on certain important creatures.

Human beings created these problems, largely in the two centuries since the Industrial Revolution, but for some researchers, they bring to mind the ancient past. The Earth has seen several mass extinctions, including five that annihilated more than half the planet's species. Experts now believe Earth is in the midst of a sixth event, the first one caused by humans.

"Today the synergistic effects of human impacts are laying the groundwork for a comparably great Anthropocene mass extinction in the oceans, with unknown ecological and evolutionary consequences," Jeremy Jackson of the Scripps Institution of Oceanography at the University of California, San Diego, wrote in a 2008 article published in the journal Proceedings of the National Academy of Sciences.

Frightening parallel

When it comes to the oceans, research shows a parallel to the Permian-Triassic extinction — also known as the Great Dying — which eradicated 95 percent of marine species when the oceans lost their oxygen about 250 million years ago.

The same phenomenon is taking place in many areas of today's oceans. The entry of fertilizers into rivers and subsequently oceans is eating up the oceans' oxygen — that runoff is the primary source of the Gulf of Mexico's 3,000-square-mile (7,770-square-kilometer) dead zone. Around the world, the number of dead zones, some of which are naturally occurring, increased from 149 in 2003 to more than 200 in 2006, according to a 2008 report by the United Nations Environmental Program.

What's more, the ocean surface is warming, driven by the emission of carbon dioxide and other greenhouse gases. This keeps the deeper waters, which are rich in nutrients but low in oxygen, from mixing with the oxygenated surface. According to a 2007 report from the International Panel on Climate Change (IPCC), global surface temperatures increased by 1.1 degrees Fahrenheit (0.6 degrees Celsius) throughout the 21st century, and, according to the National Oceanic and Atmospheric Administration (NOAA), this decade is the warmest since record-keeping began in 1880.

At the time of the third of the Big Five extinctions, the Permian-Triassic, there was only one massive continent and one massive ocean, conditions that disrupted ocean circulation and inhibited oxygen circulation in an already warm world, according to Lee Kump, a geoscientist at Pennsylvania State University. That set the stage for the ultimate trigger, a series of massive volcanic eruptions in Siberia.

The eruptions pumped massive amounts of carbon dioxide into the atmosphere. This warmed the ocean further, exacerbating its oxygen problem. Meanwhile, more storms on land washed more oxygen-eating nutrients into the ocean. Bacteria began producing hydrogen sulfide, which was ultimately expelled into an atmosphere already toxic with carbon dioxide, according to Kump.

A comparison of carbon dioxide release then versus now is telling, Kump said. Siberian volcanoes emitted tens of thousands of gigatons of carbon dioxide into the air over what was probably thousands of years. Humans currently are producing 9 gigatons per year from fossil fuel reservoirs that contain up to 4,000 gigatons.

The rate of carbon dioxide release matters, Kump said, because life has to have time to adapt.

"It's: Would you rather be squeezed or punched?" Kump said. "The Permian extinction was a squeeze that gradually got tighter and tighter … It may ultimately have been more fatal than the punch we are going to get, but the punch is going to hurt more."

Crumbling at the base

The parallel in ocean chemistry between the past and present isn't limited to oxygen depletion. The Permian ocean became more acidic as the climate changed, just as the modern ocean is doing.

The ocean has absorbed about 30 percent of human-produced carbon dioxide to date, and as a result, its waters have experienced a 30 percent increase in acidity, according to Richard Feely, a senior scientist with NOAA's Pacific Marine Environmental Laboratory. If trends continue, ocean acidity will increase by up to 150 percent by the end of this century, he said.

Increased carbon dioxide and ocean acidity played a role in all of the Big Five mass extinctions, but in those cases the change in acidity was tens to hundreds of times slower than what's happening now. When changes happen quickly, "the ocean system itself doesn't have time to adapt," Feely said.

Increasingly acidic waters affect a number of species that are key parts of the ocean's ecosystems.

Acidification interferes with the ability of oysters, marine snails and other creatures to build shells or skeletons from calcium carbonate. In oyster hatcheries on the West Coast of the United States, more-acidic waters prevent oyster larvae from forming shells, and have been shown to dissolve the shells of pteropods — small marine snails that feed salmon and other commercially caught fish — from around the living creatures, Feely said.

Not least among the victims of acidification are corals, whose growth is inhibited in affected waters. But climate change poses another problem for coral reefs: Sunlight and small increases in water temperature cause corals to expel the symbiotic algae that provide them with energy, which causes them to turn white, an effect called bleaching that can be short-lived or fatal. Mass bleaching was first observed in the late 1970s. By 2008, an estimated 19 percent of the world's coral reefs had been lost and 35 percent seriously threatened.

Coral bleaching has an impact on not just the corals. Reefs are key habitats for many marine species.

"Perhaps 25 percent of ocean species spend at least part of their life cycle on coral reefs," said Ken Caldeira with the Carnegie Institution for Science. "When we lose corals, we are likely to lose many of these species."

A recently published paper in the journal Nature documented what may be another domino in the decline of the oceans, this one at the very base of the marine food chain. Over the past century, the authors found, global phytoplankton levels have declined by 1 percent per year, a phenomenon they link to warming ocean surfaces.

Incidental victims

Amid the specter of climate change, the most immediate threat to species and ecosystems is overfishing, according to Susan Lieberman, director of international policy with the Pew Environmental Group.

"We should not take out more than is being produced," Lieberman said. "It sounds very logical, but that is not what is happening."

Sharks are emblematic of this problem. The International Union for Conservation of Nature has listed about a third of all open-ocean shark species as threatened with extinction due to overfishing.

In addition to removing seafood, some fishing practices kill other creatures incidentally, a phenomenon known as bycatch. Sea turtles are among the victims of this problem; a study published in April estimated that millions of sea turtles have been inadvertently caught as part of commercial fishing over the past 20 years.

Bottom trawling — in which a large, heavy net is dragged over the sea floor — is another problem, as it destroys habitat, according to the Pew Environmental report "Protecting Life in the Sea." This report cites studies suggesting 90 percent of the world's large fish have disappeared and that nearly one third of the world supply of commercially caught fish has collapsed.

Not everyone believes that fisheries are in immediate peril though.

"The big picture is, if you're looking in places for which we have good data" – Europe, North America, New Zealand, Australia and the high seas – "fish populations are generally stable and in some cases improving, especially in the U.S.," said Ray Hilborn, a population ecologist at the University of Washington. Hilborn is co-author of a 2009 article published in the journal Science that found reason for hope in certain ecosystems, where management practices have prevented or, more frequently, reduced overfishing.

But given the lack of international oversight on fisheries, "I'm not terribly optimistic about their future," Hilborn said.

Boris Worm, a marine biologist at Dalhousie University in Nova Scotia, Canada, and the lead author of the 2009 fisheries study, was less optimistic about the current health of fisheries. "Even in the best places it's very mixed," he said.

Humanity has solutions

Tony Haymet, director of the Scripps Institution of Oceanography at the University of California, San Diego, describes the oceans, like the atmosphere, as victims of "the tragedy of the commons: everybody owns them, and nobody owns them."

"But on the positive side, there are three things that I think that at least we have the prospect of addressing," he said. These include signs of international movement to address overfishing, the creation of marine reserves, and the prospect that the U.S. Senate might finally ratify the United Nations Convention on the Law of the Sea, which establishes international rules governing the use of the oceans.

Although there are hurdles, aquaculture also has the potential to become a safe, sustainable source of protein, he said.

Others aren't so optimistic. Humans have the technology and the knowledge to stop the ecological havoc we are wreaking, but we lack the wisdom to use it responsibly, Caldiera told LiveScience in an e-mail.

"If current trends continue, the extinctions of the coming decades will be clearly visible to future geologists comparable in scale to the great extinction events in Earth's history," he wrote. "I think it will be an enigmatic extinction. Future geologists will try to figure out why we apparently tried to kill off so many species, but they will find it hard to believe that simple reason is stupidity."

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This article was provided by OurAmazingPlanet, a sister site to LiveScience.