The Arctic Ocean has experienced rapid warming and sea ice loss in recent decades, becoming the first open-ocean basin to experience widespread aragonite undersaturation [saturation state of aragonite (Ω _arag ) < 1]. However, its trend toward long-term ocean acidification and the underlying mechanisms remain undocumented. Here, we report rapid acidification there, with rates three to four times higher than in other ocean basins, and attribute it to changing sea ice coverage on a decadal time scale. Sea ice melt exposes seawater to the atmosphere and promotes rapid uptake of atmospheric carbon dioxide, lowering its alkalinity and buffer capacity and thus leading to sharp declines in pH and Ω _arag . We predict a further decrease in pH, particularly at higher latitudes where sea ice retreat is active, whereas Arctic warming may counteract decreases in Ω _arag in the future.

Previous work has not led to a clear understanding of the causes of spatial pattern in global surface ocean dissolved inorganic carbon (DIC), which generally increases polewards. Here, we revisit this question by investigating the drivers of observed latitudinal gradients in surface salinity-normalized DIC (nDIC) using the Global Ocean Data Analysis Project version 2 (GLODAPv2) database. We used the database to test three different hypotheses for the driver producing the observed increase in surface nDIC from low to high latitudes. These are (1) sea surface temperature, through its effect on the CO2 system equilibrium constants, (2) salinity-related total alkalinity (TA), and (3) high-latitude upwelling of DIC- and TA-rich deep waters. We find that temperature and upwelling are the two major drivers. TA effects generally oppose the observed gradient, except where higher values are introduced in upwelled waters. Temperature-driven effects explain the majority of the surface nDIC latitudinal gradient (182 of the 223 µmol kg−1 increase from the tropics to the high-latitude Southern Ocean). Upwelling, which has not previously been considered as a major driver, additionally drives a substantial latitudinal gradient. Its immediate impact, prior to any induced air–sea CO2 exchange, is to raise Southern Ocean nDIC by 220 µmol kg−1 above the average low-latitude value. However, this immediate effect is transitory. The long-term impact of upwelling (brought about by increasing TA), which would persist even if gas exchange were to return the surface ocean to the same CO2 as without upwelling, is to increase nDIC by 74 µmol kg−1 above the low-latitude average.

Carbon dioxide partial pressure (pCO₂) in surface water was continuously measured every 3 h from July 2012 to June 2013 using an autonomous pCO₂ system (MAPCO₂) deployed on a moored buoy on the East China Sea shelf (31°N, 124.5°E). Sea surface pCO₂ and pH had the largest variations in summer, ranging from 215 to 470 μatm, and 7.941 to 8.263 (averagely 8.084 ± 0.080), respectively. They varied little in winter, ranging from 328 to 395 μatm, and 8.003 to 8.074 (averagely 8.052 ± 0.010), respectively. The seasonal average sea surface pCO₂ was respectively 335 ± 70 μatm, 422 ± 43 μatm, 362 ± 11 μatm, and 311 ± 59 μatm in summer, autumn, winter, and spring, and was overall undersaturated with respect to atmosphere on a yearly basis. Although the average sea surface pCO₂ in summer was below the atmospheric level, the net CO₂ flux has suggested a CO₂ source status due to the influence of typhoon. Our observation thus demonstrated the significant, even dominant impact of episodic typhoon events on surface ocean CO₂ chemistry and air–sea CO₂ gas exchange, which would be impossible to capture by shipboard observation. The high wind stress and curl associated with the northward movement of typhoon induced complex sea surface water movement, vertical mixing, and subsequent biological drawdown, which differed in pre‐, onset, and post‐typhoon stages. Based on our estimates, the degassing fluxes during typhoon reached as high as 82 mmol m⁻² CO₂ and 242 mmol m⁻² CO₂ in summer and autumn, respectively, accounting for twice as large as the summer CO₂ sink during non‐typhoon period, and 28% of the total CO₂ source in autumn.

The Arctic Ocean has turned from a perennial ice‐covered ocean into a seasonally ice‐free ocean in recent decades. Such a shift in the air‐ice‐sea interface has resulted in substantial changes in the Arctic carbon cycle and related biogeochemical processes. To quantitatively evaluate how the oceanic CO₂sink responds to rapid sea ice loss and to provide a mechanistic explanation, here we examined the air‐sea CO₂flux and the regional CO₂sink in the western Arctic Ocean from 1994 to 2019 by two complementary approaches: observation‐based estimation and a data‐driven box model evaluation. ThepCO₂observations and model results showed that summer CO₂uptake significantly increased by about 1.4 ± 0.6 Tg C decade⁻¹in the Chukchi Sea, primarily due to a longer ice‐free period, a larger open area, and an increased primary production. However, no statistically significant increase in CO₂sink was found in the Canada Basin and the Beaufort Sea based on both observations and modeled results. The reduced sea ice coverage in summer in the Canada Basin and the enhanced wind speed in the Beaufort Sea potentially promoted CO₂uptake, which was, however, counteracted by a rapidly decreased air‐seapCO₂gradient therein. Therefore, the current and future Arctic Ocean CO₂uptake trends cannot be sufficiently reflected by the air‐seapCO₂gradient alone because of the sea ice variations and other environmental factors.

The distributions of dissolved O₂and CO₂have not previously been systematically compared across the global surface ocean, despite their significance for life and climate. Here we analyze carbon dioxide and oxygen concentrations relative to saturation (equilibrium with the atmosphere) in surface waters, using two large datasets (ship-collected and float-collected data). When applied to a high-quality global ship-collected dataset, CO₂and O₂concentrations relative to saturation exhibit large seasonal and geographic variations. However, linear fits of CO₂and O₂deviations from saturation (ΔCO₂against ΔO₂) yield y-intercepts close to zero, which suggests a requirement for data validity. We utilize this finding to investigate the accuracy of carbonate system data from biogeochemical-Argo floats. We find significant discrepancies in ΔCO₂-ΔO₂y-intercepts compared to the global reference, implying overestimations of float-based CO₂release in the Southern Ocean. We conclude that this technique can be applied to data from autonomous platforms for quality assessment.