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The ocean could profoundly modulate the ever-increasing atmospheric CO2 by air-sea CO2 exchange process, which is also able to cause significant changes of physical and biogeochemical properties in return. In this study, we assessed the long-term average and spatial-temporal variability of global air-sea CO2 exchange flux (FCO2) since 1980s basing on the results of 18 Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth System Models (ESMs). Our findings indicate that the CMIP6 ESMs simulated global CO2 sink in recent three decades ranges from 1.80 to 2.24 Pg C/a, which is coincidence with the results of cotemporaneous observations. What's more, the CMIP6 ESMs consistently show that the global oceanic CO2 sink has gradually intensified since 1980s as well as the observations. This study confirms the simulated FCO2 could reach agreements with the observations in the aspect of primary climatological characteristics, however, the simulation skills of CIMP6 ESMs in diverse open-sea biomes are unevenness. None of the 18 CMIP6 ESMs could reproduce the observed FCO2 increasement in the central-eastern tropical Pacific and the midlatitude Southern Ocean. Deficiencies of some CMIP6 ESMs in reproducing the atmospheric pressure systems of the Southern Hemisphere and the El Niño-Southern Oscillation (ENSO) mode of the tropical Pacific are probably the major causes. [ABSTRACT FROM AUTHOR]

The sinks/sources of carbon in the Yellow Sea (YS) and East China Sea (ECS), which are important continental shelf seas in China, could exert a great influence on coastal ecosystem dynamics and the regional climate change process. The CO2 exchange process across the seawater-air interface, dissolved and particulate carbon in seawater, and carbon burial in sediments were studied to understand the sinks/sources of carbon in the continental shelf seas of China. The YS and the ECS generally have different patterns of seasonal air-sea CO2 exchange. In the YS, regions west of 124°E can absorb CO2 from the atmosphere during spring and winter, and release CO2 to the atmosphere during summer and autumn. The entire YS is considered as a CO2 source throughout the year with respect to the atmosphere, but there are still uncertainties regarding the exact air-sea CO2 exchange flux. Surface temperature and phytoplankton production were the key controlling factors of the air-sea CO2 exchange flux in the offshore region and nearshore region of the YS, respectively. The ECS can absorb CO2 during spring, summer, and winter and release CO2 to the atmosphere during autumn. The annual average exchange rate in the ECS was −4.2±3.2 mmol m−2 d−1 and it served as an obvious sink for atmospheric CO2 with an air-sea exchange flux of 13.7×106 t. The controlling factors of the air-sea CO2 exchange in the ECS varied significantly seasonally. Storage of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the YS and the ECS were 425×106 t and 1364×106 t, and 28.2×106 t and 54.1×106 t, respectively. Long-term observation showed that the DOC content in the YS had a decreasing trend, indicating that the “practical carbon sink” in the YS was decreasing. The total amount of particulate organic carbon (POC) stored in the YS and ECS was 10.6×106 t, which was comparable to the air-sea CO2 flux in these two continental shelf seas. The amounts of carbon sequestered by phytoplankton in the YS and the ECS were 60.42×106 t and 153.41×106 t, respectively. Artificial breeding of macroalgae could effectively enhance blue carbon sequestration, which could fix 0.36×106-0.45×106 t of carbon annually. Organic carbon (OC) buried in the sediments of the YS was estimated to be 4.75×106 t, and OC of marine origin was 3.03×106 t, accounting for 5.0% of the TOC fixed by phytoplankton primary production. In the ECS, the corresponding depositional flux of OC in the sediment was estimated to be 7.4×106 t yr−1, and the marine-origin OC was 5.5×106 t, accounting for 5.4% of the phytoplankton primary production. Due to the relatively high average depositional flux of OC in the sediment, the YS and ECS have considerable potential to store a vast amount of “blue carbon.” [ABSTRACT FROM AUTHOR]

Comprehensive carbon chemistry data were measured from the mainstream of Kuroshio, off eastern Taiwan, in May 2014. Results indicated that variations of pH@25 °C, POC, ΩCa, DIC, pCO2 and RF were closely related to the characteristics of various water types. Phytoplankton photosynthesis played important roles in DIC variation in Kuroshio SurfaceWater (KSW), whereas the DIC variation in Kuroshio Subsurface Water (KSSW) was probably influenced by the external transport of DIC-enriched water from the South China Sea. Vertical profiles of hydrological parameters and carbonate species indicated that the Kuroshio Current off eastern Taiwan could intrude into the ECS shelf as far as 27.9° E, 125.5° N in spring. What is more, the KSW, KSSW and Kuroshio Intermediate Water (KIW) could convey DIC into the East China Sea (ECS) with flux of 285, 305 and 112 Tg C/half year (1 Tg = 1012 g), respectively. The relevant flux of POC was 0.16, 2.93 and 0.04 Tg C/half year, respectively. Consequently, the intrusion of Kuroshio could probably exert a counteracting influence on the potential of CO2 uptake in the ECS, which needs further study. [ABSTRACT FROM AUTHOR]

Surface partial pressure of CO 2 ( p CO 2 ) and pertinent parameters (i.e., pH, total alkalinity, dissolved oxygen, chlorophyll a) were investigated in the southern Yellow Sea (SYS) and the East China Sea (ECS) basing on two surveys conducted in June and August of 2013. The results suggested carbonate chemistry dynamics and related controlling factors were provided with significant temporal and spatial variations in different subregions of these two continental shelf seas. The western of SYS (SYSW) was CO 2 -undersaturated both in June and August, with the average F CO 2 −1.88 mmol m −2 d −1 and −3.72 mmol m −2 d −1 , respectively. The phytoplankton initiated CO 2 -absorption and the suspended sediment induced CO 2 -emission jointly controlled the air–sea CO 2 exchange there. The center of SYS (SYSC) also behaved as an obvious CO 2 sink (−1.57 mmol m −2 d −1 and −3.99 mmol m −2 d −1 in June and August, respectively), probably due to elevated TA/DIC ratio and the subsequent effects of spring bloom. As for the Yangtze River estuary (YRE), it changed from an obvious CO 2 sink (−1.28 mmol m −2 d −1 ) in June into a very weak CO 2 source (0.04 mmol m −2 d −1 ) in August. This change was probably associated with the rising of seawater temperature and monthly variation of Yangtze River discharge. The inner shelf of ECS (ECSS) experienced obvious air–sea CO 2 flux changes during from June (−8.88 mmol m −2 d −1 ) to August (−0.36 mmol m −2 d −1 ) as well. Biological DIC consumption in the upper layer and DIC regenerated from respiration in the subsurface jointly controlled this p CO 2 variation. As a whole, the SYS and ECS acted as an obvious CO 2 sink during summer and could absorb atmospheric CO 2 with the average air–sea flux ( F CO 2 ) −2.68 mmol m −2 d −1 . The summary of air–sea CO 2 flux in the ECS and SYS during recent two decades indicated the ECS served as quite a stable CO 2 sink, whereas the SYS experienced obvious change. Discharge of Yangtze River and anthropogenic nutrients loading could profoundly affect the variations of p CO 2 and F CO 2 in future. [ABSTRACT FROM AUTHOR]

The partial pressure of CO2 (pCO2) and air-sea CO2 exchange flux (FCO2) in the southern Yellow Sea (SYS, 120–125°E, 31.5–37°N) were investigated basing on the field surveys conducted in April of 2011, and June, July, October of 2012. With significant spatial variations, surface pCO2 ranged from 243 to 574μatm, 206 to 620μatm, 102 to 655μatm and 328 to 557μatm in April, June, July and October, respectively. Nearshore area of Shandong Peninsula and Jiangsu Shallow (depth<50m) were pCO2-supersaturated (pCO2=400–600μatm), as the result of intensive water mixing which brought the bottom CO2-rich water to the surface layer. Conversely, offshore area of SYS center (depth>50m) was pCO2-undersaturated (pCO2<390μatm) in April, June and October, but supersaturated in July. Phytoplankton production sustained by abundant nutrient and suitable hydrodynamic conditions was of great importance for this undersaturated pCO2. Moreover, extreme low pCO2 (pCO2<300μatm) was observed in the Changjiang plume (32.5–33.5°N, 123–125°E) in July, which was also related with the biological uptake of CO2. Average air-sea CO2 exchange flux of the SYS in April, June, July and October was −3.16±0.40mmolm−2 d−1,−4.56±0.34mmolm−2 d−1, −0.36±0.51mmolm−2 d−1, and 6.67±0.57mmolm−2 d−1, respectively. As a whole, the SYS behaved as a weak CO2 sink during April to October, with an average flux for about −0.35mmolm−2 d−1. As for the controlling factors for pCO2 variation, temperature played the dominant role in October, whereas the non-temperature factors, such as vertical mixing, Changjiang plume and biological activity, were considered as the primary controlling factors in June and July. Spatially, the control of temperature on pCO2 was predominant in the offshore SYS; the non-temperature factors were predominant in the shallow nearshore area, especially in coast of Shandong Peninsula and the Jiangsu Shallow. [ABSTRACT FROM AUTHOR]