Heard about the dead zones that are plaguing our planet? No, we’re not talking about the times when you lose reception, and you’re left blinking at the phone like it is 1998. A dead zone is a low-oxygen in an area and recent reports of their expansion have scientists worried.

Scientists started watching oceanic dead zones in the 1970s, and their observed instances have been steadily increasing. They occur in oceans, but they can also exist in lakes like Lake Erie, Pennsylvania. Dead zones are caused by an increase of chemical nutrients in the waters which is called eutrophication. This process causes algae blooms, which promptly strip all the oxygen from the water and pretty much kills everything.

Many times, algae blooms are caused by people who dump chemical fertilizers, sewage, and urban runoff into the water to upset the natural balance (and cause blooms). Natural causes are coastal upwelling and changes in wind and water circulation patterns. And now scientists are concerned that climate change – and the global warming of our planet’s oceans – and getting into the “mass algae bloom and consequent dead zone” business.

So how is global warming causing more dead zones? Because of rising water temperatures, the warmer water on the top of the ocean acts as a cap. This interferes with the natural circulation that normally allows deeper waters that are already oxygen-depleted to reach the surface. The water that “upwells” from the bottom lacks a good amount of oxygen. Therefore, the surface of the ocean is key to recharging the rest of it with oxygen from the air. But not if it doesn’t get there.

Scientists report that lower levels of oxygen in the Earth's oceans, particularly off the United States' Pacific Northwest coast, could further be a sign of climate change. Along the northern west coast, “the almost complete absence of oxygen has left piles of Dungeness crab carcasses littering the ocean floor, killed off 25-year-old sea stars, crippled colonies of sea anemones and produced mats of potentially noxious bacteria that thrive in such conditions.”

Gregory Johnson is an oceanographer with the National Oceanic and Atmospheric Administration in Seattle. As he told reporters in a recent statement to McClatchy newspapers, "The depletion of oxygen levels in all three (Pacific, Atlantic and Indian) oceans is striking." Other experts conclude that as the Earth warms, we will see more and more low-oxygen areas on the oceans.

Scientists point out that this is all consistent with models of global warming. Steve Bograd is an oceanographer at NOAA's Southwest Fisheries Science Center in Southern California. As he points out, "It's a large disturbance in the ecosystem that could have huge biological changes."

For further reading:
Growing low-oxygen zones in oceans worry scientists

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