The ocean's "deserts," where it is difficult for marine organisms to survive, are expanding faster than predicted and have been linked to warming ocean waters, a new study shows. These barren areas are found in roughly 20 percent of the world's oceans and are within what are called subtropical gyres, or the permanent swirling expanses of water in the middle of the ocean on either side of the equator. But between 1998 and 2007, these expanses of saltwater with low surface plant life in the Pacific and Atlantic oceans grew by 15 percent, or 2.5 million square miles (6.6 million square kilometers), according to the new study, detailed in a recent issue of the journal Geophysical Research Letters. The expansion is occurring at the same time that sea surface temperatures are warming about 1 percent or 0.02 to 0.04 degrees Celsius a year. The warming imposes tougher barriers between different layers of the ocean waters, preventing deep ocean nutrients from rising to the surface and feeding plant life. "The fact that we are seeing an expansion of the ocean’s least productive areas as the subtropical gyres warm is consistent with our understanding of the impact of global warming," said study co-author Jeffrey J. Polovina, an oceanographer with the U.S. National Marine Fisheries Service in Honolulu. "But with a nine-year time series, it is difficult to rule out decadal variation." Polovina and his colleagues used data from NASA's SeaStar satellite, which maps ocean biological productivity (or the amount of chlorophyll produced by phytoplankton, the microscopic plants that form the base of the ocean food chain) around the world. These maps showed areas of low productivity in the Pacific Ocean expanding outwards from the center toward Hawaii. In the Atlantic Ocean, these low-productivity areas are expanding even faster eastward from the Caribbean toward Africa. These areas now cover roughly 20 million square miles (51 million square kilometers) in the two major oceans.

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Images: Under the Pacific