The oceans have absorbed about 40 percent of the manmade carbon dioxide (CO2), and mankind doesn’t seem likely to make fewer demands anytime soon. Just how much CO2 can be forced into the ocean is a question that arose when scientists noticed that despite ever-increasing amounts of carbon dioxide in the atmosphere, the oceans inhaled it at different rates over the years. UCSB’s Tim Devries may have found out why.
The geoscientist and his colleagues, Mark Holzer of the University of New South Wales in Sydney and François Primeau of UC Irvine, took apart data since the 1980s, and some pre-industrial estimates from 1765, to look at the problem from the other end of the telescope. Given the carbon cycle since the 1980s, what was the pattern of change in certain tracer elements, they asked, namely temperature, salinity, two chlorofluorocarbons (CFC 11 and 12), and carbon-14. Their paper, “Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning,” appeared in the February 9 issue of Nature.
They limited their query to the upper 1,000 meters of water, but expanded it to include all the oceans of the world. What they discovered was that the total carbon exchange depended on whether more came up out of the water than went down into it. The deeper parts of the ocean are rich with ancient carbon from limestone, rocks, dead creatures, and so on. With cold temperatures and windy conditions, as prevailed in the 1990s as compared to the 1980s, CO2 emerged to such an extent that it overwhelmed the amount being absorbed into surface waters. The rolling layers of water also took anthropogenic CO2 deeper into the ocean. The reverse has happened through 2014, with warmer conditions and less overturning circulation in the upper ocean.
In some areas, the circulation rate varied by 50 percent from one decade to the next, though the pattern did not hold throughout the world. Some areas of the Northern Hemisphere oceans had fairly equal rates of carbon exchange through the ’80s and ’90s, the researchers found, and upwelling in the tropics in the 2000s belied the strength of the trade winds.
The model does not separate natural variations from anthropogenic forcing, they noted, but shows a trend toward future circulation weakening. “One thing seems clear,” Devries told The Independent. “Changes in ocean circulation can have a big impact on how much CO2 the oceans absorb, and we need to better understand how these changes will affect oceanic CO2 uptake in the future.”