Abstract
The ocean is currently a significant net sink for anthropogenically remobilised CO2, taking up around 24% of global emissions. Numerical models predict a diversity of responses of the ocean carbon sink to increased atmospheric concentrations in a warmer world. Here, we tested the hypothesis that increased atmospheric forcing is causing a change in the ocean carbon sink using a high frequency observational dataset derived from underway pCO2 (carbon dioxide partial pressure) instruments on ships of opportunity (SOO) and a fixed-point mooring between 2002 and 2016. We calculated an average carbon flux of 0.013 Pg yr−1 into the ocean at the Porcupine Abyssal Plain (PAP) site, consistent with past estimates. In spite of the increase in atmospheric pCO2, monthly average seawater pCO2 did not show a statistically significant increasing trend, but a higher annual variability, likely due to the decreasing buffer capacity of the system. The increasing ΔpCO2 led to an increasing trend in the estimated CO2 flux into the ocean of 0.19 ± 0.03 mmol m−2 day−1 per year across the entire 15 year time series, making the study area a stronger carbon sink. Seawater pCO2 variability is mostly influenced by temperature, alkalinity and dissolved inorganic carbon (DIC) changes, with 77% of the annual seawater pCO2 changes explained by these terms. DIC is in turn influenced by gas exchange and biological production. In an average year, the DIC drawdown by biological production, as determined from nitrate uptake, was higher than the DIC increase due to atmospheric CO2 dissolution into the surface ocean. This effect was enhanced in years with high nutrient input or shallow mixed layers. Using the rate of change of DIC and nitrate, we observed Redfieldian carbon consumption during the spring bloom at a C:N ratio of 6.2 ± 1.6. A comparison between SOO and PAP sustained observatory data revealed a strong agreement for pCO2 and DIC. This work demonstrates that the study area has continued to absorb atmospheric CO2 in recent years with this sink enhancing over time. Furthermore, the change in pCO2 per unit nitrate became larger as surface buffer capacity changed.
Highlights
The cumulate emissions of carbon by human activities since the industrial revolution have caused atmospheric CO2 concentrations to approach 410 ppm in 2018 (Le Quéré et al, 2018a), a number not seen on Earth for the past 3 million years (Seki et al, 2010)
The annual cycles features (Fig. 2) are typical of a mid-latitude northern-hemisphere oceanic area. These were obtained by averaging available data over the restricted Porcupine Abyssal Plain (PAP)-SO footprint
Further decomposition by latitude reveals that the summer peak is mostly driven by measurements in the southern half of the study area, which give an annual cycle closer to a typical subtropical one, while the measurements in the northern half give an annual cycle closer to a typical subpolar record, resembling a sinusoid
Summary
The cumulate emissions of carbon by human activities since the industrial revolution have caused atmospheric CO2 concentrations to approach 410 ppm in 2018 (Le Quéré et al, 2018a), a number not seen on Earth for the past 3 million years (Seki et al, 2010). The atmospheric concentration would be even higher and climate change would proceed much faster had the ocean not taken up approximately a quarter of the cumulative anthropogenic carbon emitted between 1870 and 2016 (Le Quéré et al, 2018b). The North Atlantic is the largest per surface area, taking up a total of 0.7 ± 0.1 Pg C annually (Gruber et al, 2002). The ocean’s ability to take up carbon from the atmosphere is determined by the Revelle factor, the ratio of change in carbon dioxide to the change in total DIC (Broecker et al, 1979; Egleston et al, 2010). Low Revelle factors correspond to high buffer capacity in regions such as the North Atlantic, and reflect an enhanced ability to absorb
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