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Are we causing a mass extinction in our oceans?

Research shows that many areas of today's oceans have conditions that parallel those of 250 million years ago, when 95 percent of marine species quickly died out.

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Oil from the Deepwater Horizon oil spill is seen near an unprotected island in the Gulf of Mexico near Timbalier Bay, off the coast of Louisiana, Wednesday.

Gerald Herbert/AP

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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.

IN PICTURES: The Gulf oil spill's impact on nature

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.

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