evidence: Our oceans support an abundance of life with an average pH level of 8.1, making seawater slightly basic. But experts estimate that over the course of the 21st century, the pH of ocean water could dip down to 7.8. That may sound like a small change, but the last time the ocean pH was this low was some 14 to 17 million years ago, when the Earth was a very different place. Scientists predict the change will have serious ramifications for ocean ecologies, food security, and economies, big and small, that depend on marine industries. [...] How do we know that the decrease in ocean pH isn’t part of a natural cycle? Because research has shown that the rise in fossil fuels and atmospheric CO2 correlates with the decline of ocean pH. And while correlation doesn't always equal causation, Suatoni explains, "the consistency between what scientists theorize and what we’ve observed—as well as the lack of good alternative explanations—gives us high confidence that carbon dioxide pollution is causing ocean acidification."