Global Warming Causes Oxygen Depletion Zones Across World's Oceans

Dead and low-life zones in the world's oceans are expected to expand as global warming continues to raise aquatic temperatures, according to a new report by researchers from the University of Kiel, Germany, and published in the journal Science.

Researchers noted that areas of the world's oceans that have an oxygen content too low to support much or even any life have been growing in central and eastern equatorial Africa and in the equatorial Pacific. Other studies have noted the expansion of these zones off the coast of California and in the Gulf of Mexico, and the most rapid oxygen declines have been observed in the subarctic Pacific.

While many dead zones have been attributed primarily to fertilizer runoff producing algae blooms that consume all of the ocean's oxygen, the recent study concluded that global warming will worsen this trend. The expansion of these zones, in turn, will lead to increasing extinction of ocean life.

It has long been known that colder water holds more oxygen than warmer water, with typical oxygen concentrations of 300 to 400 micromoles per kilogram in cold surface water. But the researchers found that concentrations in parts of the eastern Pacific and northern Indian oceans were as low as 10 micromoles per kilogram, and there were large regions with concentrations below 150. The concentration of oxygen in these areas had decreased by 0.09 to 0.34 micromoles kilogram per year.

"Most marine species have minimum oxygen thresholds that they require for survival," said Steven J. Bograd of the National Oceanic and Atmospheric Association. "As oxygen decreases, these animals will suffer and/or be compelled to move to other areas. Over time, the optimal area for various species will be compressed."

This minimum ranges between 60 and 120 micromoles per kilogram.

Frank A. Whitney of the Canadian Institute of Ocean Sciences also warned that the biological consequences of oceanic oxygen loss will be severe.

"Many species will lose their deep habitat, meaning competition will become stronger in the remaining favorable habitat, and increased vulnerability to predation will likely occur," he said.