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Idealog—in the ideas business

Ocean’s ability to absorb CO2 at risk from climate change

We’re lucky to be living in a country surrounded by so much beautiful ocean. But as well as its aesthetic function, the ocean plays a critical role in CO2 absorption, sucking up as much as one-third of all human-made carbon emissions. But how exactly is climate change affecting its ability to fulfil this critical function? A new analysis by the University of Wisconsin-Madison finds some conflicting results. 

As one of the planet's largest single carbon absorbers, the ocean takes 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 is still up in the air. Previous studies on the topic have yielded conflicting results, says University of Wisconsin-Madison assistant professor Galen McKinley. 

In a new analysis published online July 10 in Nature Geoscience, McKinley and her colleagues identify 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," says 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. 

The analysis differs from previous studies in its scope across both time and space. One of the biggest challenges in asking how climate is affecting the ocean is simply a lack of data, McKinley says, with available information clustered along shipping lanes and other areas where scientists can take advantage of existing boat traffic. With a dearth of other sampling sites, many studies have simply extrapolated trends from limited areas to broader swaths of the ocean. 

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. 

"Because the ocean is so variable, we need at least 25 years' worth of data to really see the effect of carbon accumulation in the atmosphere," she says. "This is a big issue in many branches of climate science -- what is natural variability, and what is climate change?" 

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