Simulated Variation Characteristics of Oceanic CO2 Uptake, Surface Temperature, and Acidification in Zhejiang Province, China

Since preindustrial times, atmospheric CO2 content increased continuously, leading to global warming through the greenhouse effect. Oceanic carbon sequestration mitigates global warming; on the other hand, oceanic CO2 uptake would reduce seawater pH, which is termed ocean acidification. We perform Earth system model simulations to assess oceanic CO2 uptake, surface temperature, and acidification for Zhejiang offshore, one of the most vulnerable areas to marine disasters. In the last 40 years, atmospheric CO2 concentration increased by 71 ppm, and sea surface temperature (SST) in Zhejiang offshore increased at a rate of 0.16°C/10a. Cumulative oceanic CO2 uptake in Zhejiang offshore is 0.3 Pg C, resulting in an increase of 20% in sea surface hydrogen ion concentration, and the acidification rate becomes faster in the last decade. During 2020–2040, under four RCP scenarios, SST in Zhejiang offshore increases by 0.3–0.5°C, whereas cumulative ocean carbon sequestration is 0.150–0.165 Pg C. Relative to RCP2.6, the decrease of surface pH in Zhejiang offshore is doubled under RCP8.5. Furthermore, simulated results show that the relationship between CO2 scenario and oceanic carbon cycle is nonlinear, which hints that deeper reduction of anthropogenic CO2 emission may be needed if we aim to mitigate ocean acidification in Zhejiang offshore under a higher CO2 concentration scenario. Our study quantifies the variation characteristics of oceanic climate and carbon cycle fields in Zhejiang offshore, and provides new insight into the responses of oceanic carbon cycle and the climate system to oceanic carbon sequestration.


INTRODUCTION
Atmospheric CO 2 concentration has reached 410.07 ± 0.10 ppm (parts per million) by 2019, and increased by 46% since preindustrial time, which is primarily resulting from human activities of fossil fuel burning and land use changes [1]. Observational-based estimates showed that during 1750-2019, total anthropogenic CO 2 emissions were 700 ± 75 Pg C (1 Pg C 10 15 g of carbon) [1]. About 41% of these emissions stayed in the atmosphere, whereas about 24% of these emissions were absorbed by the ocean, which is considered as a main sink of atmospheric CO 2 [1]. Besides, the ocean also plays an important role in regulating climate by key air-sea interaction processes [2][3][4][5][6].
Increased atmospheric CO 2 causes global warming (GW) through the greenhouse effect. Global warming has become one of the most challenging global issues and the core issue of global change [7][8][9][10][11]. Carbon capture and sequestration is a major player in reducing atmospheric CO 2 concentrations and the resultant global warming. However, the absorption of anthropogenic CO 2 by the ocean would also decrease seawater pH, causing ocean acidification and having important effects on the oceanic carbon cycle and ecosystem [12]. Therefore, it is important to investigate the response mechanisms of oceanic carbon cycle and climate system to oceanic CO 2 sequestration. In addition, owing to the inhomogeneous physical and biogeochemical features in different oceanic basins or areas, vulnerability assessment of different oceanic areas under the impacts of climate change has become a forefront topic of international scientific research [13][14][15][16].
Besides GW, ocean acidification is another significant impact induced by greenhouse gas emissions [12,17,18]. With the increased annual CO 2 emissions, ocean acidification has been exacerbated in recent years [19]. The main concern of ocean acidification originates from its potentially adverse effects of CaCO 3 saturation state (Ω) reductions on marine calcifying organisms. Reduced Ω would decrease the calcification rate and increase the CaCO 3 dissolution rate of calcifying organisms, making their skeletons or shells vulnerable [20][21][22][23]. For instance, aragonite is the main constituent of corals' calcareous endoskeleton; therefore, corals surrounded by seawater which is undersaturated with respect to aragonite (Ω A < 1) could encounter adverse effects. There are intriguing evidences that CaCO 3 could also dissolve even in supersaturated seawater [24,25].
Marine organisms in the China seas are ecologically and economically important. China is known as one of the most important countries of ocean aquaculture industry, where more than 70% of the major cities and 50% of the populations are concentrated in the eastern and southern coastal regions. Maintaining sustainable development of resources and environment in offshore and coastal regions is an urgent strategic requirement for the future development of the country [26,27]. Zhejiang is a highly developed province in East China, which takes a leading position in the national marine economic development strategy. Therefore, it is necessary to assess the variation characteristics of oceanic climate, carbon cycle, and acidification in Zhejiang offshore. However, due to the lack of historical observational data in oceanic physical and chemical fields, uncertainties exist in assessing changes in oceanic climate, acidification, and biogeochemical processes in regional areas.
In this study, the variation characteristics of oceanic CO 2 uptake, SST, and ocean acidification in historical times and future trends in Zhejiang offshore are quantified. In order to acquire reasonable assessments, reanalysis data and numerical simulated results are used. The data and model used are introduced in Data and Method section, changes in oceanic carbon cycle and climate system are analyzed in Results section, and the summary and advice for adapting to climate change and ocean acidification are in Conclusion and Discussion section.

Data and Method
The historical SST data used in this article are from ERA-Interim high-precision reanalysis data published by the ECMWF (European Center for Medium-Range Weather Forecasts) [28]. The time span is 1980-2019, and the resolution is 0.125°× 0.125°.
Seawater pH is a measurement to quantify the degree of ocean acidification [29]. The increased hydrogen ion concentration tended to reduce carbonate ion concentration ([CO 2− 3 ]) via the following:

Model and Simulations
The University of Victoria Earth System Climate Model (UVic ESCM) version 2.9 used in this study is an intermediate complexity Earth system model [30]. The UVic model is composed of an energy-moisture balance atmospheric model, a 3-D ocean general circulation model, a thermodynamic/dynamic sea ice model, and land and ocean carbon cycle models [31][32][33][34]. The model's horizontal resolution is 1.8°( latitude)×3.6°(longitude), which is similar to most coupled atmosphere-ocean general circulation models (AOGCMs). The ocean model of UVic is the modular ocean model (MOM) version 2.2 developed by the Geophysical Fluid Dynamics Laboratory (GFDL), which has a vertical resolution of 19 levels [35].
First, the UVic model was spun up for 10,000 model years with a fixed preindustrial CO 2 concentration of 280 ppm to reach a quasiequilibrium state of carbon cycle and the climate system. Then, this preindustrial state was used as an initial condition for the calendar year of 1800, 300-years transient simulations were performed (i.e., from 1800 to 2100) [36,37]. From 1800 to 2019, atmospheric CO 2 concentration data were taken from observation-based estimates [38], and after 2019, CO 2 concentrations were taken from the Representation Concentration Pathway scenarios (RCPs) and their extensions up to 2100. The four scenarios used are RCP2.6, RCP4.5, RCP6.0, and RCP8.5, based on different mitigation policies for greenhouse gases [39,40]. The numbers after "RCP" represent that by 2100, the radiative forcing reaches 2.6, 4.5, 6.0, and 8.5 W m −2 , respectively.

Analysis of Marine Chemistry Fields
In this study, we calculate ocean carbonate chemistry fields, such as pH, based on equations in the Ocean Carbon-Cycle Model Intercomparison Project phase 3 (OCMIP-3, http://ocmip5.ipsl. jussieu.fr/OCMIP/). UVic-simulated ocean temperature, salinity, DIC (dissolved inorganic carbon), ALK (alkalinity), and databased estimates of ocean silicate and phosphate concentrations from the GLODAP (Global Ocean Data Analysis Project) [41] are used. The calculations are shown as follows [42].
Thermodynamic Expressions for DIC and ALK are as follows:

Regions of Interest
This study investigates historical and future variation characteristics of oceanic CO 2 uptake, SST, and ocean acidification in Zhejiang offshore, China. Our regions of interest include the oceanic area of 27.0°-31.0°latitude and 120.0°-124.2°longitude ( Figure 1). Zhejiang offshore locates in North West of the East China Sea, the subtropical humid climate zone [44]. In addition to the atmospheric circulation system, including East Asian monsoon, the climate system in Zhejiang offshore could also be affected by the continental climate system and oceanic currents, such as the Kuroshio and coastal upwelling currents [45]. Zhejiang is a highly developed province in East China, which takes a leading position in national marine economic development strategy. Changes in oceanic carbon cycle and climate system in Zhejiang offshore could have drastic effects on millions of people who depend on marine resources.

RESULTS
Model-simulated historical oceanic CO 2 uptake for the global ocean is consistent with observation-based estimates reported by IPCC AR5 (Intergovernmental Panel on Climate Change Fifth Assessment Report) [46]. For example, UVic-simulated cumulative oceanic CO 2 uptake during preindustrial time to 2011 was 147 Pg C, which compares well with the observational range of 155 ± 30 Pg C reported by IPCC AR5 ( Table 1). Model-simulated averaged oceanic CO 2 uptake during 2002 to 2011 was 2.4 Pg C yr −1 , within the observed value of 2.4 ± 0.7 Pg C yr −1 ( Table 1).
Model-simulated key carbon-related tracers are also compared with observation-based estimates from the GLODAP [46]. As shown in Figure 2, simulated vertical profiles of DIC and ALK for Zhejiang offshore largely agree with observed estimates. In addition, UVic-simulated large-scale distributions of key variables in the oceanic carbon cycle [45][46][47], climate [48][49][50], and historical oceanic uptake of carbon and its isotopes [42] also compare well with observation-based estimates.

SST and Ocean Acidity of Zhejiang Offshore in the Last 40 Years
Zhejiang offshore SST ranges from 19 to 22°C, with higher SST farther offshore at the same latitude. From 1980 to 2019, the hottest  season in Zhejiang offshore sea surface is summer, with SST mainly in the range of 25-28°C, whereas the coldest season is winter, with SST in the 13-17°C range. Across the four seasons, SST spatial gradient is larger in spring and winter, and the smallest in summer.
Observation-based estimates show that during 1980-2019, atmospheric CO 2 concentration increased from 339 to 410 ppm. The increment of atmospheric CO 2 concentration led to rising SST through the enhanced greenhouse effect (Figure 3). During 1980-2019, SST of Zhejiang offshore increased at a rate of 0.16°C/10a, which reached its highest level in 1998 (21.8°C), and the lowest in 1981 (20.5°C) (Figure 3). From the perspective of interdecadal change, SST of Zhejiang offshore mainly increased from 1980s to late 1990s, decreased at the beginning of the 21st century, and increased again in the 2010s. Annual mean SST of Zhejiang offshore reached 21.5°C in 2019.
In addition to rising seawater temperatures, another important impact of CO 2 emissions on marine environment is ocean acidification. With the increasing of atmospheric CO 2 concentration, the ocean, as an important carbon sink, continuously absorbs anthropogenic CO 2 from the atmosphere (Figures 1, 4). For instance, simulated results show that during 1980-2019, cumulative oceanic CO 2 uptake in Zhejiang offshore is 0.3 Pg C. The increase of annual oceanic CO 2 uptake in Zhejiang offshore is largely related to the rise of atmospheric CO 2 concentration. Continuous oceanic CO 2 uptake would lead to ocean acidification, resulting in the rise of seawater hydrogen ion concentration, reducing the calcification rate of marine organisms, making their skeletons or shells vulnerable, and consequently, having adverse impacts on the marine ecosystem.
In the last 40 years, atmospheric CO 2 concentration increased by 21%. Simulated results show that the decrease rate of pH in Zhejiang sea surface is closely related to the increasing rate of atmospheric CO 2 concentration and oceanic CO 2 uptake. With the increase of atmospheric CO 2 , the ocean's continuous absorption of CO 2 leads to the exacerbation of ocean acidification in Zhejiang offshore (Figures 1, 4-6).
Compared with low-and high-latitude sea surface, sea surface of Zhejiang offshore, which is located in the mid-latitude, suffered greater acidification. During 1980-2019, sea surface pH at low and high latitudes decreased by 0.06 units, while that at Zhejiang offshore decreased by 0.08 units, 23% greater than the decrease rate of sea surface pH in the global ocean. As atmospheric CO 2 increases, the acidification rate of seawater also accelerates. For the past 20 years, especially the last 10 years (2010-2019), the reduction rate of Zhejiang offshore sea surface pH significantly accelerated compared to 1980-1999. The pH reduction rate increased from 0.017/10a in 1980-1989 to 0.023/10a in 2010-2019 (accelerated by 35%, Figure 6).
Zhejiang offshore is the habitat of a large number of marine organisms, where fisheries amount to about 220,000 km 2 . The main fishery products include fish, shrimp, crab, shellfish, and other calcified organisms. The total allowable catch in Zhejiang offshore ranks the first in China, and in recent years, the annual production all reached three million tons. By conducting pCO 2 / pH perturbation experiments, Wu and Gao concluded that, the effects of ocean acidification and solar UV changes would also suppress photosynthesis in the China seas [51]. Physiology, morphology, and behavior of some marine organisms (e.g., cnidarians and molluscs) would also be influenced by seawater

SST Changes and Ocean Acidification of Zhejiang Offshore in the Next 20 Years
The UVic model was used to quantify changes in SST and ocean acidification in Zhejiang offshore from 2020 to 2040, under different greenhouse gas emission scenarios (RCP2.6, RCP4.5, RCP6.0, and RCP8.5). Under four RCP scenarios, atmospheric CO 2 concentration would increase by 28-74 ppm in the next 20 years, resulting in a rise of 0.3-0.5°C in Zhejiang offshore SST (Figures 7A,C).
With the continuing increases in atmospheric CO 2 content, Zhejiang offshore keeps absorbing atmospheric CO 2 , resulting in continuous acidification of seawater. During 2020-2040, under different RCP scenarios, simulated cumulative CO 2 uptakes of Zhejiang offshore are 0.150-0.165 Pg C ( Figure 7B). Under RCP8.5, from 2020 to 2040, Zhejiang sea surface pH decreases by 0.07, corresponding to an increase of 17% in hydrogen ion concentration, while under the RCP2.6 scenario, Zhejiang sea surface pH decreases by 0.03, corresponding to an increase of 7% in hydrogen ion concentration ( Figure 7D). Therefore, in the next 20 years,

The Nonlinear Relationship Between Atmospheric CO 2 Scenario and Oceanic Carbon Cycle
Simulated results under different scenarios show that the relationships among the atmospheric CO 2 scenario used, oceanic climate, and carbon cycle fields in Zhejiang offshore  Frontiers in Physics | www.frontiersin.org August 2021 | Volume 9 | Article 718968 6 are nonlinear (Figure 8). First, the RCP scenario of high CO 2 emissions does not necessarily correspond to high atmospheric CO 2 concentration. For example, during 2020-2040, atmospheric CO 2 concentration of RCP4.5 is higher than that of RCP6.0. Therefore, the RCP scenario of higher CO 2 emissions does not necessarily lead to larger oceanic CO 2 uptake, higher SST, or lower surface pH. For instance, at 2040, simulated oceanic CO 2 uptake in Zhejiang offshore is larger under RCP4.5 than that under RCP6.0 (0.0076 Pg C in RCP4.5 and 0.0074 Pg C in RCP6.0, respectively). In 2040, Zhejiang offshore seawater would experience greater warming and acidification under RCP4.5 than under RCP6.0 ( Figures 8B,C). In addition, the RCP scenario of higher CO 2 emissions not necessarily leads to faster change of oceanic CO 2 uptake, SST, or surface pH. For example, at 2040, Δsurface pH/ΔCO 2 is faster under RCP4.5 than that under RCP6.0 (−0.40 units/100 ppm in RCP4.5 versus −0.37 units/100 ppm in RCP6.0, Figure 9).

CONCLUSION AND DISCUSSION
Under the background of greenhouse gas emissions and GW, it is an important issue to analyze the variation characteristics in ocean climate and carbon cycle fields, which determines the capacity of the ocean to capture atmospheric CO 2 [53][54][55]. In this article, changes of oceanic CO 2 uptake, SST, and acidification in Zhejiang offshore in last 40 years are assessed. Future changes in the next 20 years are also simulated by the UVic Earth system model. In addition, we also quantify the differences of oceanic carbon cycle under different RCP  scenarios, which contributes to develop a better understanding of oceanic carbon capture and climate adaptation. Our results show that over the past 40 years, with the increasing atmospheric CO 2 content, SST in Zhejiang offshore increased at a rate of 0.16°C/10a. Meanwhile, the increase of annual oceanic CO 2 uptake in Zhejiang offshore is also closely related to the rise of atmospheric CO 2 concentration, which results in an increase of 20% in sea surface hydrogen ion concentration over the past 40 years, and accelerated acidification over the past 10 years.
Previous projects and studies also reported data-based estimates of changes in oceanic climate and seawater acidity.  [60]. Therefore, Zhejiang offshore is one of the most vulnerable areas to climate change and ocean acidification.
Four CO 2 emission scenarios are used to simulate oceanic CO 2 uptake, SST, and pH in Zhejiang offshore during 2020-2040. By 2040, with the rise of atmospheric CO 2 concentration under four RCP scenarios, SST in Zhejiang offshore increases by 0.3-0.5°C, whereas cumulative oceanic CO 2 uptake is 0.150-0.165 Pg C, leading to a decrease of sea surface pH by 0.03-0.07. Compared to RCP2.6, the decrease of surface pH in Zhejiang offshore is doubled under RCP8.5. In addition, the relationship between the CO 2 scenario used and oceanic carbon cycle is nonlinear, which is important because if we want to mitigate ocean acidification in Zhejiang offshore under a higher CO 2 concentration scenario, more effective anthropogenic CO 2 emission reductions may be needed.
This study has investigated the variation characteristics of oceanic climate and carbon cycle fields in Zhejiang offshore on timescales of decades by using an Earth system model. Some processes or feedbacks that are not considered in this study may also have impacts on the ocean carbon cycle and climate system. For instance, ocean acidification tends to decrease the calcification rate of some ocean calcifying organisms, increasing sea surface alkalinity, promoting oceanic CO 2 sequestration, and mitigating ocean acidification. Moreover, this study has not included the interactive effects between ocean acidification and CaCO 3 in the sediments, which is considered to mitigate the chemistry change in the deep ocean on millennia timescales.
Until now, although Zhejiang has taken steps to develop climate change adaptation and emission reduction policies, more meteorological and climatological measures still need to be adopted to reduce the impacts of extreme climate events on coastal regions, for example, 1) strengthening the monitoring and early warning system for marine disasters in Zhejiang province; 2) paying more attention to the adverse impacts of future climate change on marine fisheries, aquaculture, ecosystems, major infrastructure constructions, and city planning in coastal regions; and 3) investigating innovations to cope with the adverse effects of marine disasters, which is crucial for the sustainable development of the oceans. In addition, further observational and modeling studies would be required to develop a better understanding of the response of oceanic carbon cycle and the climate system to oceanic carbon sequestration, which is vital for more reliable projections of future climate and marine ecosystem changes.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.