Oxygen in the upper waters of the North Pacific, an area that accounts for about 40 percent of the world’s oceans, decreased as much as 15 percent in a little less than two decades between the early 1980s and late 1990s.
The change could be the result of climate shifts occurring at decade-long intervals — which should eventually shift back — or, it could be caused by global warming, says Steven Emerson, UW professor of oceanography. Emerson reported on the findings this week at the Ocean Sciences Meeting in Honolulu. The meeting, sponsored by the American Geophysical Union and American Society of Limnology and Oceanography, continues through Friday.
It has been just in the last year that research teams — two from Japan and one led by Emerson, all working in different areas of the North Pacific — published three separate papers on findings of oxygen decreases of 10 to 15 percent from the top 100 to 600 meters (300 to 2,000 feet) of the ocean, referred to as the upper thermocline. The Emerson paper is based on measurements made during expeditions funded by the National Science Foundation and the UW in 1980 and 1981, which was then compared with data collected during expeditions in 1991 and 1997.
Slower ocean circulation, an increased growth rate — or productivity — of plants in the surface waters or a combination of the two could result in less oxygen, Emerson says.
Emerson’s presentation comes just days after the publication of a paper in the Feb. 7 issue of Nature presenting evidence that circulation has slowed 25 percent since the mid-1970s in the equatorial and north Pacific Ocean, a significant change in the intensity, according to lead author Michael McPhaden, a scientist with the National Oceanic and Atmospheric Administration in Seattle and UW affiliate faculty member.
With slowing circulation, water would linger longer in the thermocline. Plants and organic matter would have more time to decompose, Emerson says, a process that consumes oxygen when carbon released from decaying tissue is oxidized.
Some global-warming circulation models suggest that global warming would result in a decrease of oxygen in the upper thermocline, Emerson says.
Along with findings by scientists such as McPhaden that circulation is slowing, Emerson’s co-presenter at the Ocean Sciences Meeting used man-made chlorofluorocarbons, or CFCs, as tracers to consider the age of water in places where oxygen depletion was measured. Water was spending more time in the thermocline in the 1990s compared to the 1980s, according to Sabine Mecking, who earned her doctorate at the UW and is now a post-doctoral fellow at Woods Hole Oceanographic Institution.
Emerson says other scientists, such as Dave Karl of the University of Hawaii, have evidence that productivity has increased. An increase in the rate or amount of plant growth near the surface means increasing amounts of degradation in the thermocline, a process taking oxygen.
Documenting additional changes and finding ways of measuring changes in circulation and productivity will eventually help scientists improve models of how oceans react on a decadal scale.
“This work is going to get a lot of attention in the next 10 years,” Emerson says.
Emerson, Mecking and Jeffrey Abell, a UW graduate student, were the co-authors of the most recently published paper on the declines in oxygen that appeared in the journal Global Biogeochemical Cycles last fall.