Global ocean acidification is caused primarily by the ocean's uptake of CO 2 as a consequence of increasing atmospheric CO 2 levels. We present observations of the oceanic decrease in pH at the basin scale (50°S-36°N) for the Atlantic Ocean over two decades . Changes in pH associated with the uptake of anthropogenic CO 2 (ΔpHCant) and with variations caused by biological activity and ocean circulation (ΔpHNat) are evaluated for different water masses. Output from an Institut Pierre Simon Laplace climate model is used to place the results into a longer-term perspective and to elucidate the mechanisms responsible for pH change. The largest decreases in pH (ΔpH) were observed in central, mode, and intermediate waters, T he uptake of anthropogenic CO 2 (Cant) by the ocean has lowered seawater pH since preindustrial times. This largescale and long-term change is referred to as "ocean acidification," a process that has led to changes in seawater carbonate chemistry (1-3) with impacts on the chemical speciation of seawater and biogeochemical cycles. A predominant effect is the decrease of carbonate ions in seawater that impacts calcareous marine organisms (4-6). The uptake of Cant is the main cause for the gradual reduction of seawater pH, but biological, physical, and chemical "natural" changes in the ocean, such as changes in the oxidation of organic matter, impact pH as well (7,8).Several studies have focused on ocean acidification in the last decade. Model-based studies have examined pH changes on a global scale (2, 9), and observation-based studies have focused on time-series stations (10-13) and specific regions such as the North Pacific (7, 14) and North Atlantic (8). However, investigations of basin-wide pH changes throughout the water column from direct measurements are sparse (15), in large part because of a dearth of quality referenced pH measurements.In this study, we present the first (to our knowledge) measurement-based changes in pH along meridional lines in the Atlantic from 50°S-36°N using observations from three cruises: OACES/ CO 2 (1993), CITHER-II (1994), and FICARAM-XV (2013) (Fig. 1A and SI Appendix, Table S1). We investigate the total change in pH and refer to it as acidification. The change in pH is separated into anthropogenic (ΔpHCant) and natural (ΔpHNat) components and is evaluated for the water masses along the section over two decades . Both components are related to Cant uptake and to processes such as the remineralization of organic matter and changes in water mass. The observed pH changes are compared with outputs from an Institut Pierre Simon Laplace (IPSL) climate model to place the observations into context and to elucidate the mechanisms controlling anthropogenic and natural pH changes, which are clearly discerned in climate models. ResultsMore than 4,000 pH measurements referenced to the seawater scale (SWS) at 25°C in the Atlantic Ocean (50°S-36°N) were examined to estimate the changes in oceanic pH between 1993 and 2013. We used pH measurements referenced to the SWS at 25°C thr...

Ocean acidification (OA)or the decrease in seawater pH resulting from ocean uptake of CO2 released by human activitiesstresses ocean ecosystems and is recognized as a Climate and Sustainable Development Goal Indicator that needs to be evaluated and monitored. Monitoring OA-related pH changes requires a high level of precision and accuracy. The two most common ways to quantify seawater pH are to measure it spectrophotometrically or to calculate it from total alkalinity (TA) and dissolved inorganic carbon (DIC). However, despite decades of research, small but important inconsistencies remain between measured and calculated pH. To date, this issue has been circumvented by examining changes only in consistently measured properties. Currently, the oceanographic community is defining new observational strategies for OA and other key aspects of the ocean carbon cycle based on novel sensors and technologies that rely on validation against data records and/or synthesis products. Comparison of measured spectrophotometric pH to calculated pH from TA and DIC measured during the 2000s and 2010s eras reveals that (1) there is an evolution toward a better agreement between measured and calculated pH over time from 0.02 pH units in the 2000s to 0.01 pH units in the 2010s at pH > 7.6; (2) a disagreement greater than 0.01 pH units persists in waters with pH < 7.6, and (3) inconsistencies likely stem from variations in the spectrophotometric pH standard operating procedure (SOP). A reassessment of pH measurement and calculation SOPs and metrology is urgently needed.

The variability in the storage of the oceanic anthropogenic CO 2 (C ant ) on decadal timescales is evaluated within the main water masses of the Subtropical North Atlantic along 24.5°N.Inorganic carbon measurements on five cruises of the A05 section are used to assess the changes in C ant between 1992 and 2011, using four methods (ΔC*, TrOCA, φC T 0 , TTD). We find good agreement between the C ant distribution and storage obtained using chlorofluorocarbons and CO 2 measurements in both the vertical and horizontal scales. 4

a b s t r a c tMethods based on CO 2 and chlorofluorocarbon (CFC) data are used to describe and evaluate the anthropogenic CO 2 (C ant ) concentrations, C ant specific inventories, and C ant storage rates in the Equatorial Atlantic Ocean. The C ant variability in the water masses is evaluated from the comparison of two hydrographic sections along 7.5°N carried out in 1993 and 2010. During both cruises, high C ant concentrations are detected in the upper layers, with values decreasing progressively towards the deep layers. Overall, the C ant concentrations increase from 1993 to 2010, with a large increment in the upper North AtlanticDeep Water layer of about 0.18 ± 0.03 lmol kg À1 y À1 . In 2010, the C ant inventory along the whole section amounts to 58.9 ± 2.2 and 45.1 ± 2.0 mol m À2 using CO 2 and CFC based methods, respectively, with most C ant accumulating in the western basin. Considering the time elapsed between the two cruises, C ant storage rates of 1.01 ± 0.18 and 0.75 ± 0.17 mol m À2 y À1 (CO 2 and CFC based methods, respectively) are obtained. Below $1000 m, these rates follow the pace expected from a progressive increase of C ant at steady state; above $1000 m, C ant increases faster, mainly due to the retreat of the Antarctic Intermediate Waters.