“The ocean is taking up less carbon because of the warming caused by the carbon in the atmosphere.”
This is the message from a new analysis published online July 10 in the journal Nature Geoscience, which looked at whether oceans are going to be able to continue absorbing one-third of the atmospheric carbon dioxide that we are pumping into the atmosphere.
University of Wisconsin-Madison assistant professor Galen McKinlen and colleagues identified a likely source for the inconsistences in whether this is possible, clearing up a topic that has been confused by varying results.
Together with researchers at the Lamont-Doherty Earth Observatory at Columbia University and the Universite Pierre et Marie Curie in Paris, as well as UW-Madison, the new analysis combines existing data from a range of years, methodologies, and locations spanning most of the North Atlantic.
This provided the researchers with a greater dataset, one that is normally not available to scientists who have to rely upon existing shipping traffic to collect data, and even then, only along a certain set of prescribed routes which minimizes the clarity of the results.
What they found was a high degree of natural variability which often ends up masking longer-term patterns of change; a natural variability which could go a long way to explaining why previous conclusions often end up disagreeing with one another.
The apparent trends in ocean carbon uptake are highly dependent on exactly when, and where, you look; over a 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,” says McKinlen. “This is a big issue in many branches of climate science – what is natural variability, and what is climate change?”
The researchers found that, even though over the past three decades the rate of atmospheric carbon dioxide has been matched by an increase in carbon dioxide uptake, the warmer waters are slowly affecting how much carbon the ocean is able to extract from the atmosphere.
Because warmer water can simply not hold as much carbon as colder water. Increase the water temperature, and you minimise the amount of carbon it can hold.
In watching for effects of increasing atmospheric carbon on the ocean’s uptake, many people have looked for indications that the carbon content of the ocean is rising faster than that of the atmosphere, McKinley says. However, their new results show that the ocean sink could be weakening even without that visible sign.
“More likely what we’re going to see is that the ocean will keep its equilibration but it doesn’t have to take up as much carbon to do it because it’s getting warmer at the same time,” she says. “We are already seeing this in the North Atlantic subtropical gyre, and this is some of the first evidence for climate damping the ocean’s ability to take up carbon from the atmosphere.”