Reduced ocean oxygen has led to mass extinction in the past

...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.... Canadians for Action on Climate Change