Records stretching back to 1960 prove what climate models had predicted:  warmer oceans contain less oxygen. Oceanographer Lothar Stramma of the University of Kiel in Germany and his colleagues report in Science that an analysis of historical records and recent samples show that as the globe has warmed, waters with low oxygen content have expanded in the tropical Atlantic and equatorial Pacific oceans.

"The oxygen concentrations in these oxygen-minimum zones have decreased with time," says oceanographer and study coauthor Gregory C. Johnson of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Wash. "The regions of low oxygen have also expanded vertically by both extending deeper into the ocean and closer to the surface."

Fish and other sea life cannot survive in such waters—and this expansion reduces the area where fish can thrive, says oceanographer Janet Sprintall of the Scripps Institution of Oceanography in La Jolla, Calif., who also coauthored the study. She notes that fisheries may be affected as well.

These low-oxygen zones form naturally as colder waters that have absorbed oxygen in the polar regions sink and flow south. They're different from the so-called dead zones that form at the mouths of rivers whose polluted waters prompt algal blooms. As these currents travel, the water loses oxygen as dead algae, plankton and other organic matter decay. But these low-oxygen waters near the equator are expanding, because the water in the polar regions is not as cold and is not absorbing as much oxygen as it used to be.

In fact, the Atlantic waters with less oxygen than many fish need to survive doubled in depth, "from a thickness of 370 meters [1200 feet] in 1960 to 690 meters [2264 feet] in 2006," the researchers write.

Despite the importance of oxygen levels, relatively little information has been collected across the vast watery reaches of earth and this research is confined to six areas in the Atlantic, Pacific and Indian oceans. The latter showed no change, perhaps because its oxygen-poor waters are already relatively deep or its circulation is weaker, according to the researchers.

But more change—and less oxygen—may be coming. "Some climate models suggest that under global warming scenarios, ocean oxygen content will decrease," Johnson says. The lack of oxygen will also affect other elemental cycles on the planet, such as nitrogen and carbon.

"We cannot say whether this trend will continue or … if it will change in the future," Sprintall adds. "The key is to make sure we keep collecting observations of oxygen." Or at least keep track of where the fish no longer live.