Forty-five times as much carbon resides in Earth’s global ocean as in its atmosphere. But the ocean is not an unchanging cache of carbon. It constantly exchanges carbon dioxide with the atmosphere. (See the article by Heather Graven, Physics Today, November 2016, page 48.) Recent research has shown that CO2 uptake by the ocean slowed down in the 1990s but puzzlingly recovered in the 2000s. Now Tim DeVries at the University of California, Santa Barbara, Mark Holzer at University of New South Wales in Australia, and François Primeau at the University of California, Irvine, have shown the cause to be changes in the overturning circulation—the transport of surface waters downward and deep waters upward—in the upper 1000 meters of the global ocean.
The net flow of CO2 at the air–sea boundary depends on the relative concentrations of the greenhouse gas in the ocean and atmosphere. Attention has rightly focused on rising fossil-fuel CO2 in the atmosphere, but the ocean is no static bystander. DeVries and his colleagues developed a model that uses observational data from the 1980s, 1990s, and 2000s to trace out for each decade the upper ocean overturning circulation. As illustrated in the figure, a vigorous overturning in the 1990s enhanced the transport of surface waters to the deep, thus promoting uptake of CO2. But the overturning also brought carbon-rich deep waters to the surface, which could then outgas. Compared with the 1980s, the balance tipped toward a weakening of the net carbon uptake (brown arrow in the figure). In the 2000s the circulation weakened. That blunted the effects of rising fossil-fuel CO2 levels, but not enough to offset the reduction in deep-ocean carbon escaping into the air. Studies over longer time periods will need to sort out to what extent the most recent trend of the 2000s is due to natural variability or global climate change. (T. DeVries, M. Holzer, F. Primeau, Nature 542, 215, 2017.)
