A commonly held belief that global warming will diminish（减少） oxygen concentrations in the ocean looks like it may not be entirely true. According to new research published in Science magazine, just the opposite is likely the case in the northern Pacific Ocean, with its anoxic（缺氧的） zone expected to shrink in coming decades because of climate change. An international team of scientists came to that surprising conclusion after completing a detailed assessment of changes since 1850 in the eastern tropical northern Pacific's oxygen minimum zone (OMZ). An ocean layer beginning typically a few hundred to a thousand meters below the surface, an OMZ is by definition the zone with the lowest oxygen saturation in the water column. OMZs are a consequence of microbial respiration and can be hostile environments for marine life.

Using core samples of the seabed in three locations, the scientists measured the isotopic ratio of nitrogen-15 to nitrogen-14 in the organic matter therein; the ratio can be used to estimate the extent of anoxia in these OMZs. The core depth correlates with age, giving the team a picture of how the oxygen content varied over the time period.

From 1990 to 2010, the nitrogen isotope record indicates that oxygen content steadily decreased in the area, as expected. But before that, and particularly clearly from about 1950 to 1990, oceanic oxygen steadily increased, which, according to co-author Robert Thunell, a marine scientist at the University of South Carolina, runs counter to conventional wisdom（传统观点）.

"The prevailing thinking has been that as the oceans warm due to increasing atmospheric greenhouse gases, the oxygen content of the oceans should decline," Thunell says. "That's due to two very simple processes.

"One, as water becomes warmer, the solubility of oxygen decreases in it, so it can hold less oxygen. And two, as the surface of the ocean warms, its density decreases and the oceans become more stratified. When that happens, the surface waters that do have oxygen don't mix down into the deeper waters of the ocean."