Oceans becoming less effective carbon sinks

Posted by Kate Taylor

A new analysis of the oceans' ability to absorb carbon has concluded that climate change is hampering their ability to act as a carbon sink.

Previous studies have yielded conflicting results. But University of Wisconsin-Madison assistant professor Galen McKinley says she and her team have found a likely source of many of these inconsistencies, along with observational evidence that the ocean is taking up less carbon because of the warming caused by the carbon in the atmosphere.

Previous studies have often been based on information clustered along shipping lanes and other easily accessible areas, and then extrapolated to broader swaths of the ocean.

McKinley and her colleagues combined existing data from 1981 to 2009, methodologies, and locations covering most of the North Atlantic into a single time series for each of three large regions called gyres.

They found a high degree of natural variability that often masked longer-term patterns of change, and which 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.

"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?"

The researchers found that rising temperatures are slowing the carbon absorption across a large portion of the subtropical North Atlantic. Warmer water cannot hold as much carbon dioxide, so the ocean’s carbon capacity is decreasing as it warms.

Her research shows, she says, that the oceans' ability to act as a carbon sink is declining, even though its' carbon content may not be rising faster than that of the air.

"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."