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The region of the ocean known as the desert of the sea has expanded dramatically over the past decade, according to a new study. Scientists looking at the color of the ocean from space have found that vast areas that were once green with plankton have been turning blue, as marine life becomes more scarce. If it's linked to global warming, as they suspect, this could be another blow for the world's fisheries.

NPR's Richard Harris reports.

RICHARD HARRIS: Just as plants make up the base of the food web on land, tiny, green phytoplankton in the ocean are a critical food stuff for life in the oceans. And Jeff Polovina, at a National Marine Fisheries Service lab in Hawaii, has been watching by satellite as that greenery in the middle of the ocean is fading away.

Mr. JEFF POLOVINA (Oceanographer, National Marine Fisheries Service): The regions that are showing the lowest amount of plant life, which is sometimes referred to as the biological deserts of the ocean, are growing at roughly 1 to 4 percent per year.

HARRIS: One to 4 percent may not sound like all that much, but these regions are huge to begin with. So this marine desert has grown by two and a half million square miles in the past decade. That's an area the size of Texas every year. Polovina makes the analogy to deserts on land creeping into more productive environments.

Mr. POLOVINA: Well, we have the same thing here. These less productive areas are replacing the slightly more productive areas of the ocean.

HARRIS: And it seems to be tied to global warming. His study in Geophysical Research Letters finds that the areas of low productivity are expanding in lockstep with increasing water temperatures. As surface temperatures warm, that prevents colder water from rising up from the depths. And that colder water carries the nutrients that would feed the algae. This kind of change is predicted by scientists studying climate change, but the sea desert has been spreading 10 times faster than the climate scientists predict. So Polovina is a bit cautious, he says this could be a short term fluctuation, not a permanent change.

Mr. POLOVINA: In the next 10 years, maybe it could switch back. Until we get a much longer time series, we don't know.

HARRIS: Whatever the case, the speed of change in the ocean is disturbing to marine scientists like Paul Falkowski at Rutgers University.

Professor PAUL FALKOWSKI (Institute of Marine and Coastal Sciences and the Department of Geology, Rutgers University): It is amazingly fast if true.

HARRIS: And he says marine scientists using more primitive instruments from ships saw this trend starting long ago back in the early days of the Industrial Revolution.

Prof. FALKOWSKI: We can see that over a hundred years, the ocean has gotten clearer and clearer in the center, so this is a long term trend.

HARRIS: He, too, isn't sure that the trend will keep up at its current speed, but the long term outlook is unsettling. He says the South Pacific is currently the clearest and least productive region of the Earth's oceans.

Prof. FALKOWSKI: If we made the entire world's oceans like that, we'd probably reduce the productivity by at least a factor of two, and that would be extraordinarily large change which I don't want to think about.

HARRIS: Fisheries, which are already under heavy stress from nets and lines, would suffer even more, he said. Reefs would suffer, too. In all, the outlook for oceans is bleak. And we may not be fully aware of the situation as it changes. The satellite that made these critical measurements is nearing the end of its lifespan, and there's no full replacement in the works.

So in a few years, U.S. scientists may not be able to look at the color of the ocean from space to keep tracking this trend.

Richard Harris, NPR News.

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