As one of the planet's largest single carbon absorbers, the ocean soaks up roughly one-third of all human carbon emissions, reducing atmospheric carbon dioxide and its associated global changes. But whether the ocean can continue mopping up human-produced carbon at the same rate has been the question for long.

Previous studies on the topic have yielded conflicting results, said University of Wisconsin-Madison assistant professor Galen McKinley.

In a new analysis, McKinley and her colleagues identified a likely source of many of those inconsistencies and provide some of the first observational evidence that climate change is negatively impacting the ocean carbon sink.

“The ocean is taking up less carbon because of the warming caused by the carbon in the atmosphere,” said McKinley, an assistant professor of atmospheric and oceanic sciences and a member of the Center for Climatic Research in the Nelson Institute for Environmental Studies.

McKinley and colleagues at UW-Madison, the Lamont-Doherty Earth Observatory at Columbia University, and the Universite Pierre et Marie Curie in Paris expanded their analysis by combining existing data from a range of years (1981-2009), methodologies, and locations spanning most of the North Atlantic into a single time series for each of three large regions called gyres, defined by distinct physical and biological characteristics.

They found a high degree of natural variability that often masked longer-term patterns of change and could explain why previous conclusions have disagreed. They discovered that apparent trends in ocean carbon uptake are highly dependent on exactly when and where you look – on the 10- to 15-year time scale, even overlapping time intervals sometimes suggested opposite effects.

The study is detailed in Nature Geoscience.