New paper finds a large increase of the CO2 sink in N Atlantic from 2002-2009

Settled science update: A paper published today in the Journal of Geophysical Research finds from an "unprecedented number of high-quality" observations collected from a cruise ship that there was "a large increase of the CO₂ sink in the western tropical North Atlantic from 2002 to 2009." The finding contradicts previous studies that found a decrease or no change in oceanic CO2 uptake.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C08029, 10 PP., 2012
doi:10.1029/2011JC007803

A large increase of the CO₂ sink in the western tropical North Atlantic from 2002 to 2009

Key Points

• Surface fCO2SW [fugacity] shows significantly lower increase trend than the atmosphere
• The slower increase in fCO2SW causes an increase in oceanic CO2 uptake
• Changes in winter SSTs, correlated to ENSO, and in MLD contribute to this trend

Geun-Ha Park

Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida, USA

Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USA

East Sea Research Institute, Korea Institute of Ocean Science and Technology, Uljin, South Korea

Rik Wanninkhof

Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USA

Determining a robust trend of surface ocean carbon dioxide (CO₂) for a period shorter than a decade is challenging due to large seasonal variability and a sparsity of data. Here, we estimate the multiannual trend of surface CO₂ in the region of 19°N–20°N, 65°W–68°W for the period of 2002–2009. We used an unprecedented number of high-quality underway data of the fugacity of CO₂ in surface seawater (fCO_2SW) collected from 137 cruises using an automated system onboard the cruise ship Explorer of the Seas. The growth rate of fCO_2SW was estimated by two de-seasonalization approaches that showed similar and significantly lower values than the atmospheric increases, leading to a large increase in the CO₂ sink. The seasonal difference in the trends was significant, with fCO_2SW values in winter showing no increase, while summer fCO_2SW values lagged only slightly with the atmosphere. We attribute the lack of an increase in winter fCO_2SW values to sea surface temperature changes, which are closely correlated with the El Niño-Southern Oscillation cycle, and to changes in the mixed layer depth. The slower increase of fCO_2SW is also related to decreases in salinity. The 8-year averaged annual net sea-air CO₂ flux was −0.06 ± 0.18 mol m⁻² yr⁻¹ compared to a climatology that shows a flux out of the ocean of +0.11 mol m⁻² yr⁻¹. The increasing flux differs from previous, mostly longer-term results for regional studies and time series stations in the North Atlantic, which suggests a decrease or no change in oceanic CO₂uptake.