As warming intensifies, scientists warn, the oxygen content of oceans across the planet could be more and more diminished, with serious consequences for the future of fish and other sea life.
The Deepwater Horizon oil spill is overshadowing another catastrophe that’s also unfolding in the Gulf of Mexico this summer: The oxygen dissolved in the Gulf waters is disappearing. In some places, the oxygen is getting so scarce that fish and other animals cannot survive. They can either leave the oxygen-free waters or die. The Louisiana Universities Marine Consortium reported this week that this year’s so-called “dead zone” covers 7,722 square miles.
Unlike the Deepwater Horizon disaster, this summer’s dead zone is not a new phenomenon in the Gulf. It first appeared in the 1970s, and each summer it has returned, growing bigger as the years have passed. Its expansion reflects the rising level of fertilizers that farmers in the U.S. Midwest have spread across their fields. Rain carries much of that fertilizer into the Mississippi River, which then delivers it to the sea. Once the fertilizer reaches the Gulf, it spurs algae to grow, providing a feast for bacteria, which grow so fast they use up all the oxygen in their neighborhood. The same phenomenon is repeating itself along many coastlines around the world. This summer, a 377,000-square-kilometer (145,000-square-mile) dead zone appeared in the Baltic Sea. In 2008, scientists reported that new dead zones have been popping up at an alarming rate for the past 50 years. There are now more than 400 coastal dead zones around the world.
As serious as these dead zones are, however, they may be just a foreshadowing of a much more severe crisis to come. Agricultural runoff can only strip oxygen from the ocean around the mouths of fertilizer-rich rivers. But global warming has the potential to reduce the ocean’s oxygen content across the entire planet. Combined with acidification — another global impact of our carbon emissions — the loss of oxygen could have a major impact on marine life.
Scientists point to two reasons to expect a worldwide drop in ocean oxygen. One is the simple fact that as water gets warmer, it can hold less dissolved oxygen. The other reason is subtler. The entire ocean gets its oxygen from the surface — either from the atmosphere, or from photosynthesizing algae floating at the top of the sea. The oxygen then spreads to the deep ocean as the surface waters slowly sink.
Global warming is expected to reduce the mixing of the ocean by making surface seawater lighter. That’s because in a warmer world we can expect more rainfall and more melting of glaciers, icebergs, and ice sheets. Since freshwater is less dense than salt water, the water at the ocean’s surface will become lighter. The extra heat from the warming atmosphere will also make surface waters expand — and thus make them lighter still. The light surface water will be less likely to sink — and thus the deep ocean will get less oxygen. Instead, more of the oxygen will linger near the surface, where it will be used up by oxygen-breathing organisms.
The prospect that global warming could reduce the ocean’s oxygen has led some scientists to wonder if the predicted decline has already begun. It’s a maddeningly hard thing to determine, however. We can be very confident that humans have driven up the concentration of carbon dioxide in the atmosphere because scientists have recorded a steady increase over the course of decades. The signal of human-produced carbon dioxide is stronger than the noise of nature’s ups and downs.
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