New research challenges current understanding of ocean’s role in carbon uptake

Surprising findings from a pair of NOAA buoys show the importance of frequent, long-term observations

While the global ocean has long been recognized as a crucial carbon sink, quantifying the exchange of carbon dioxide (CO₂) between the ocean and atmosphere has become one of the most daunting challenges in Earth science. Researchers have labored for decades to capture observations that would help them understand with greater precision how different ocean regions absorb or release carbon. This exchange influences local and regional changes in ocean chemistry known as ocean acidification. 

A pair of recent studies focused on the California Current Ecosystem revealed new aspects of these processes, underscoring the value of very frequent observations over long time periods for accurately assessing the changing ocean.

“The amount of information you can get from measurements taken three hours apart, compared with monthly averages, is immense,” said Adrienne Sutton, an oceanographer at NOAA’s Pacific Marine Environmental Laboratory who was a co-author on both papers. “This gives us a much richer understanding of what is happening off our coasts. It will lead to better predictions of ocean carbon chemistry.”

The research challenges long-held assumptions about carbon exchange in both open ocean and nearshore environments, especially in response to events like El Niño, marine heatwaves as well as changing ocean acidification. 

The studies analyzed data from a pair of ocean moorings off the coast of southern California equipped with sophisticated sensors developed by NOAA’s Pacific Marine Environmental Laboratory. The buoys, supported by NOAA’s Global Ocean Monitoring and Observing and Ocean Acidification programs and maintained/deployed by Scripps Institution of Oceanography, provide measurements of ocean chemistry via satellite uplink every three hours, capturing a detailed record of ocean carbon cycling in real time.

The waters around the first mooring (designated CCE1,) located in the open ocean about 150 miles southwest of southern California’s Point Conception, normally absorb  CO₂. The second, (CCE2), positioned nearshore, on the shelf break just offshore of Point Conception, is a CO₂ source according to the buoy data. There, localized upwelling processes bring cold waters with elevated levels of dissolved CO₂ from the depths to the surface where it contributes to ocean acidification and is released into the atmosphere.

Source or sink? It depends…

One study, led by Helena C. Frazão during postdoctoral work at the Scripps Institution of Oceanography and published in the Journal of Geophysical Research Oceans, analyzed the exchange of CO₂ across the air-sea interface from the two sites between 2008 to 2022 to investigate how nearshore and offshore environments each responded to seasonal changes and significant climate shifts like El Niño and marine heatwaves.

What Frazão’s research team learned from the more frequent measurements was a surprise. During the 2015-2016 El Niño marine heatwave known as “the Blob,” the open ocean site (CCE1) was shown to be releasing CO₂ to the atmosphere, not absorbing it. Conversely, the warm waters of The Blob suppressed upwelling of CO₂-rich water at the nearshore site (CCE2), increasing absorption of CO₂ and turning a source into a sink.

The hidden hand of high-frequency variability

The second research team led by Ruiming Song, a graduate student at the University of California, Santa Barbara, evaluated data from the nearshore CCE2 buoy from 2011 to 2020. That study tackled a fundamental question: Does relying on monthly averaged data accurately capture the true air-sea CO₂ flux, or does it miss crucial short-term fluctuations?

Their findings, published in Geophysical Research Letters, suggests that a significant re-evaluation of the California coastal upwelling system’s carbon budget may be due. While scientists analyzing monthly averaged data have concluded this region was a net CO₂ sink, Song’s team found that the high-resolution 3-hourly data demonstrated the opposite: the region is, in fact, a net source of CO₂ to the atmosphere. 

The research team discovered that observations taken every three hours allowed them to detect short-lived events characterized by both high winds and strong upwelling that together enhance outgassing of CO₂ from the ocean.

“The same winds that cause upwelling and bring CO₂ to the surface also promote its release to the atmosphere,” said Song. “These episodic seasonal upwelling events cause this region to be a net source of CO₂ rather than a net sink.”

More frequent observations yield a clearer picture

The contrasting responses of nearshore and open ocean environments suggest that the ocean’s capacity to absorb atmospheric CO₂ might be more variable and complex than previously thought, especially in regions that experience strong upwelling or are impacted by marine heatwaves, Sutton said.

The studies also challenge the traditional reliance on monthly averaged data for calculating CO₂ exchange between air and sea by demonstrating that such methods can significantly misrepresent a region’s role as a CO₂ sink or source due to the inability to capture rapid changes over short time spans. 

“We are now questioning our understanding of ocean carbon uptake in other coastal regions where upwelling is common and the same processes could be at play,” Sutton said. We plan to explore this in future work.”

Modified from story by Theo Stein