How long can the oceans soak up CO2?

For now, the world’s oceans are sucking up so much carbon dioxide that it’s helping to slow the rate of global warming. But that’s expected to change in the future, researchers warned after taking a detailed look at the rate of ocean acidification in the northeast Pacific Ocean.

According to a new study led by oceanographers from MIT and Woods Hole Oceanographic Institution, that rate that mirrors the increase of carbon dioxide emissions pumped into the atmosphere. Most of the carbon going into the ocean is lingering the upper layers and changing the water chemistry. In the past decade, the average pH has dropped by 0.002 units per year, which is decreasing the  availability of aragonite, a mineral needed by many marine species to produce their shells.

“The ocean has been the only true sink for anthropogenic emissions since the industrial revolution,” said Sophie Chu, graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences. “Right now, it stores about a quarter to a third of the anthropogenic emissions from the atmosphere. We’re expecting at some point the storage will slow down. When it does, more carbon dioxide will stay in the atmosphere, which means more warming. So it’s really important that we continue to monitor this.”

Chu and her colleagues have published their results in the Journal of Geophysical Research: Oceans. The findings suggest that the northeast Pacific is  sensitive to changes in ocean chemistry and CO2 in particular. The region sits at the end of the world’s ocean circulation system, where it has collected some of the oldest waters on Earth and accumulated with them a large amount of dissolved inorganic carbon, which is naturally occurring carbon that has been respired by marine organisms over thousands of years.

“This puts the Pacific at this already heightened state of high carbon and low pH,” Chu said, explaining that more CO2 in the mix could affect marine species at the base of the food chain, including tiny sea snails called  pteropods.

“These species are really sensitive to ocean acidification,” she said. “It’s harder for them to get enough carbonate to build their shells, and they end up with weaker shells, and have reduced growth rates.”

The study was originally planned to analyze how ocean acidification affects the pteropods, but Chu realized that the data they collected could also be used to gauge changes in the ocean’s anthropogenic carbon storage. The researchers isolated the anthropogenic carbon buildup in the ocean with a complex climate model.

The total amount of anthropogenic carbon appears to be increasing with each year, but Chu said the rate at which the northeast Pacific has been storing carbon has remained relatively the same since 2001. That means that the region could still have a good amount of “room” to store carbon, at least for the foreseeable future. But already, her team and others are seeing in the acidification trends the ocean’s negative response to the current rate of carbon storage.

“It would take hundreds of thousands of years for the ocean to absorb the majority of CO2 that humans have released into the atmosphere,” Chu says. “But at the rate we’re going, it’s just way faster than anything can keep up with.”

Summit County Citizens Voice, 20 September 2016. Article.

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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book