I n the 1970s, oceanographers and atmospheric scientists began a long-term research project to monitor changes in the oceanic carbon cycle as part of an effort to better understand it and to measure the changes in the amount of dissolved carbon dioxide traceable to human activity. They expected that a clear signal of our increasing release of this greenhouse gas into the atmosphere would reveal itself in the data, and that it would be the only significant signal they would see. ….
Thirty years of measurements and millions of data points later, the first results came in. And they showed that the oceanic cycle of uptake and release was more complex than suspected.
The ocean breathes. The main places of “inhalation,” where carbon dioxide is sequestered, are where deep, cold water is formed by large-scale cooling and sinking of relatively warm surface water. These areas are near Greenland and Iceland in the Northern Hemisphere, where the Gulf Stream flows into high latitudes, cools, and subsides; and offshore of the Antarctic Peninsula in the Southern Hemisphere, where the southern circumpolar current dives downward.
The primary areas of “exhalation,” where carbon dioxide is vented, are where deep cold currents rise to the surface: regions such as the equatorial Pacific or parts of the Indian Ocean.
And the rate of respiration varies on a time scale of decades, driven by changes in atmospheric pressure and wind patterns.
For example, in the North Atlantic Ocean, carbon dioxide absorption decreased by a factor of 2 between 1995 and 2005, compared to the 1980s and early 1990s. During nearly the same time period, 1997-2004, the equatorial Pacific Ocean gave off more carbon dioxide than during the 1980s and early 1990s.
These changes were eventually traced to naturally occurring oscillations in large-scale wind patterns across each ocean basin driven by complex interactions between surface water temperature and atmospheric jet stream distributions. Known as the North Atlantic Oscillation and the Pacific Decadal Oscillation, they settle into one of two modes for a few to ten years, then flip to the other preferred mode. The resultant wind patterns drive changes in ocean currents that affect the rate of upwelling and downwelling over large areas, and hence the rate of carbon dioxide venting and sequestration. So great were these variances due to natural cycles that it became apparent that the human contribution to oceanic carbon dioxide concentration change, though significant and growing, was relatively small. It would take work to accurately isolate it. During the past 10-15 years, physicists with expertise in ocean dynamics have developed mathematical models to simulate global ocean circulations. They’ve then supplied them with the data gathered over the years of observation and “asked” the models to find the average long-term circulation that best matched the data.
Supplied with that solution, they’ve been able to make a more informed judgment about what causes the variance in oceanic carbon dioxide uptake. For example, the original hypothesis that strong winds at high latitudes during the winter during the late 1980s and early 1990s in the North Atlantic enhanced overturning and the production of deep water proved correct, as did the hypothesis that a change to lighter winds between 1997- 2005 did the opposite.
The modeling also helped scientists gain insight into a lingering puzzle: why the net carbon dioxide uptake in the southern hemisphere’s oceans actually decreased, despite increased winds there, too. In that case, the increased winds, driven in part by the growth of the ozone hole, did increase, overturning the production of new deep water. But, the same overturning also brought old carbon-rich deep water to the surface, and the amount of carbon dioxide vented to the atmosphere more than compensated for the production of new deep water and the carbon dioxide sequestered in it.
Research continues to advance rapidly. Scientists hope that soon not only average decadal oceanic circulations will be analyzable, but that annual changes will be forecastable. It’s complex, difficult work, but worth the effort. Our effects on ocean chemistry, its ability to buffer climate change, and the cascading effects of its growing acidification on its ecology are all issues of the utmost import that require as much knowledge as possible in order to address effectively.
Steve Maleski is a meteorologist at the Fairbanks Museum and Planetarium in St. Johnsbury.