Francois Primeau; UC Irvine photo by Steve Zylius

Earth’s oceans may be losing their ability to absorb carbon dioxide, the worst of the human-generated greenhouse gases believed to drive global warming, according to a new study co-authored by a UC Irvine scientist.

If the study’s conclusions are right, implications for climate change could be serious. The oceans normally act as a giant sponge, mopping up excess carbon from the atmosphere; reducing that ability could, under some scenarios, hasten the planet’s rise in temperature, which scientists say is already being accelerated by human emissions.

“If we keep burning fossil fuels at the same rate, a bigger fraction will accumulate in the atmosphere,” said UC Irvine physical oceanographer Francois Primeau. “The ocean becomes more acidic, diminishing the ocean’s ability to take up the carbon.”

In a paper published in November in the science journal, “Nature,” Primeau and his co-authors describe the novel method they used, based in part on real-world measurements and in part on computer modeling, to create a kind of three-dimensional profile of ocean absorption since 1765.

Direct measurements of ocean chemistry go back roughly 20 years, allowing the scientists to map out how carbon dioxide has been absorbed, carried into the depths and stored there for decades or longer, then returned to the surface.

The more carbon dioxide held in the upwelling water, the less it can absorb from the atmosphere.

The scientists used these measurements to extrapolate backward in time, revealing how carbon dioxide moved through the oceans.

The scientists also relied on data from bubbles trapped in ice cores and other “proxy” measurements, which reveal the composition of the atmosphere in the deep past — including concentrations of carbon dioxide.

Using the computer model to match these concentrations with ocean circulation patterns revealed how well the oceans were able to absorb carbon dioxide between 1765 and 2008.

“In the 1950s, there was a sharp rise, corresponding to the time when we started burning a lot more fossil fuels,” Primeau said.

The critical finding: that increase flattens out significantly starting in the 1980s, and most dramatically in the 2000s.

“That kind of suggests that the fraction of what we’re emitting that goes into the ocean is decreasing,” Primeau said.

Ocean acidification, which results from excess carbon dioxide being absorbed, also could have grim implications for sea life, perhaps interfering with the ability of coral to form skeletons, or tiny marine creatures to form shells.

There are uncertainties in the modeling — for example, the assumption that ocean circulation patterns in the past were the same as they are today. The assumption is likely safe going back a couple of centuries, Primeau said, which covers the era of climate-altering human emissions.

(UC Irvine photo of Francois Primeau by Steve Zylius.)

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