The study team notes that seas have warmed substantially over the past 50 years and that climate models predict falling levels of oceanic oxygen in response to global warming.
Rising temperatures can prevent oxygen-rich surface waters from circulating to lower depths, since water becomes less dense as it warms, Stramma explained.
"Then you have less ventilated water for deep and middle layers, which means you have less oxygen supply from the surface," he said. "That is a problem for the larger fish, which need a lot of energy."
Commercial fisheries, particularly in the tropical east Atlantic, could suffer, he added.
"This is an area where there are a lot of tunas," Stramma said. To feed, "bigeye tuna go quite deep to 250 meters [850 feet], so for these guys the area where they live reduces.
"We see these low-oxygen zones rising [in depth]," Stramma added. "This means they may move on to the [continental] shelf and influence areas of intense fishing."
Hypoxic zones also severely reduce overall biological productivity, which reverberates throughout the food chain, he said. (Related: "Porpoises Starving in Europe Due to Ocean Warming" [January 10, 2007].)
Other recent studies have also noted a long-term trend of falling oxygen levels in non-tropical seas. In the sub-Arctic Pacific, for instance, reduced oxygen concentrations have been reported at depths of 330 to 1,310 feet (100 to 400 meters) between 1956 and 2006.
Past Episodes
While the new study suggests human-caused global warming as a possible cause, the team notes that oxygen-deprived seas aren't a new phenomenon.
Paleontological records show that Earth has experienced episodes of much more severe ocean suffocation in its past.
For instance, drastically reduced oxygen levels linked to an era of global warming marked the planet's worst ever extinction event: the end of the Permian period 251 million years ago, when more than 90 percent of all marine species were wiped out.
Today, however, other factors may be amplifying the effects of global warming in spurring the growth of tropical hypoxic zones, said Thomas Wagner, professor of earth systems science at Newcastle University, U.K.
Local human impacts due to land use changes may contribute to the effects highlighted in the new study, he said.
"If you warm up the atmosphere, at the same time you increase the activity of water in the atmosphere," which in turn leads to increased rainfall and river flows into tropical coastal waters, Wagner said.
These flows contain organic runoff such as agricultural fertilizers and sewage, fueling algal blooms that consume the oxygen in seas, he said. (Related: "Gulf of Mexico 'Dead Zone' Is Size of New Jersey" [May 25, 2005].)
"This is a second mechanism which may also have an impact on [oceanic] oxygenation in certain areas," he said.
"Many mechanisms can play together, and it's very difficult to identify which is the leading one," he added. "I think we are still far away from being really confident of how an ocean system will respond [to global warming]."
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