Posts Tagged 'regionalmodeling'

Exposure to extremes in multiple global change drivers: characterizing pH, dissolved oxygen, and temperature variability in a dynamic, upwelling dominated ecosystem

In upwelling systems, fluctuations in seawater pH, dissolved oxygen (DO), and temperature can expose species to extremes that differ greatly from the mean conditions. Understanding the nature of this exposure to extremes, including how exposure to low pH, low DO concentrations, and temperature varies spatiotemporally and in the context of other drivers, is critical for informing global change biology. Here, we use a 4-yr time series of coupled pH, DO, and temperature observations at six nearshore kelp forest sites spanning the coast of California to characterize the variability and covariance among these drivers. We further compare observed properties to those derived from a high-resolution coupled physical-biogeochemical simulation for the central California current system. We find the intensity, duration, and severity of exposure to extreme conditions beyond heuristic, biologically relevant pHT (< 7.7), and DO (< 4.6 mg L−1) values were greatest at sites with strong upwelling. In contrast, sites with relatively weaker upwelling had little exposure to pH or DO conditions below these heuristic values but had higher and more variable temperature. The covariance between pH, DO, and temperature was highest in sites with strong upwelling and weakest in sites with limited upwelling. These relationships among pH, DO, and temperature at the observation locations were mirrored in the model, and model output highlighted geographic differences in exposure regimes across the California marine protected area network. Together, these results provide important insight into the conditions marine ecosystems are exposed to relevant to studies of global change biology.

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A shift in the mechanism of CO2 uptake in the Southern Ocean under high emission-scenario

The Southern Ocean is a major region of ocean carbon uptake, but its future changes remain uncertain under climate warming. Here we show the projected shift in the Southern Ocean CO2 sink using a suite of Earth System Models, revealing changes in the mechanism, position and seasonality of the carbon uptake. Dominant CO2 uptake shifts from the Subtropical to the Antarctic region under the high-emission scenario by the end of the 21st century. The warming-driven sea-ice melt, increased ocean stratification, mixed layer shoaling, and a weaker vertical carbon gradient will together reduce the winter outgassing in the future, which will trigger the switch from mixing-driven outgassing to solubility-driven uptake in the Antarctic region during the winter season. The future Southern Ocean carbon sink will be poleward-shifted, operating in a hybrid mode between biologically-driven summertime and solubility-driven wintertime uptake with further amplification of biological uptake by the increasing Revelle Factor.

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Taphonomy and dissolution rates of the razor clam Ensis magnus shells: current status and projected acidification scenarios


  • Natural variability of seawater (TaΩaragonite and pCO2) revealed an increase of acidification though such change did not suppose abrupt detrimental effects for taphonomic characteristics of shells (length, thickness, organic content or strength).
  • Temperature affected negatively shell strength and thickness, although the large correlation between the environmental variables would disturb the individual characterization of environmental parameters.
  • Dissolution rates of shells subjected to projected laboratory scenarios were significantly greater for cold-acidic environment (more corrosive) as compared to warm-acidic. Mean dissolution time (DT50) for cold-acidic scenario was reduced by half (15 years) as compared to current water chemistry conditions (30 years).
  • More recent shells are being secreted in a progressively less saturated carbonate environment (at an annual rate of change of −0.0127 for Ωaragonite) and accordingly, were more prone to suffer dissolution (and weakening) in projected laboratory scenarios.
  • Marine shells support ecosystem services including refuge for multiple species, substrate to attach and settle of fauna that may change in future environments or may bring changes in the ecological interactions of our coastal areas affecting biodiversity and optimal functioning of the ecosystem services.


The analysis of the natural variability of seawater (TaΩaragonite and pCO2) at Rodas Beach (NW Iberian Peninsula, Spain) revealed an increase of acidification. However, such pH change was not linked to any detrimental effect of the shell taphonomic characteristics of live razor clams harvested during distinct temporal series (length, thickness, organic content or strength). Temperature affected negatively shell strength and thickness, although the large correlation between the environmental variables would limit the individual characterization. Modelled trends in pH (and Ωaragonite) showed a significant decrease in the last 20 years, despite Ω > 1. Therefore, more recent shells are being secreted in a progressively less saturated carbonate environment and, consequently, more prone to suffer dissolution (and weakening) in projected climatic scenarios. When shells of harvested razor clams were exposed to projected climatic scenarios in the laboratory, dissolution rates were significantly greater for cold-acidic scenarios (more corrosive) as compared to warm-acidic. The median dissolution time (DT50) for shells under the cold-acidic scenario was reduced by half (15 years) when compared to the values observed for shells under current water chemistry conditions (30 years).

Galician coastline, often characterised by pCO2-rich and cold waters due to upwelling system, would represent the most corrosive scenario for the shells according to the responses monitored in our survey which highlight future compromise for the ecosystem services supplied by these hard skeletons. Future climate scenarios might condition performance of bivalves but also more complex processes related to carbonate structures. Local biodiversity may be lowered which may reduce the possibility that many species find shelter and feeding grounds, diminishing the optimal substrate for other organisms as needed elements for optimal services in the ecosystems.

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Reconstruction of surface seawater pH in the North Pacific

In the recent significant rise in atmospheric CO2, seawater’s continuous acidification is altering the marine environment’s chemical structure at an unprecedented rate. Due to its potential socioeconomic impact, this subject attracted significant research interest. This study used traditional linear regression, nonlinear regression random forest, and the BP neural network algorithm to establish a prediction model for surface seawater pH based on data of North Pacific sea surface temperature (SST), salinity (SSS), chlorophyll-a concentration (Chl-a), and pressure of carbon dioxide on the sea surface (pCO2) from 1993 to 2018. According to existing research, three approaches were found to be highly accurate in reconstructing the surface seawater pH of the North Pacific. The highest-performing models were the linear regression model using SSS, Chl-a, and pCO2, the random forest model using SST and pCO2, and the BP neural network model using SST, SSS, Chl-a, and pCO2. The BP neural network model outperformed the linear regression and random forest model when comparing the root mean square error and fitting coefficient of the three best models. In addition, the best BP neural network model had substantially higher seasonal applicability than the best linear regression and the best random forest model, with good fitting effects in all four seasons—spring, summer, autumn, and winter. The process of CO2 exchange at the sea–air interface was the key factor affecting the pH of the surface seawater, which was found to be negatively correlated with pCO2 and SST, and positively correlated with SSS and Chl-a. Using the best BP neural network model to reconstruct the surface seawater pH over the North Pacific, it was found that the pH exhibited significant temporal and spatiotemporal variation characteristics. The surface seawater pH value was greater in the winter than the summer, and the pH decline rate over the past 26 years averaged 0.0013 yr−1, with a general decreasing tendency from the northwest to the southeast. The highest value was observed in the tropical western Pacific, while the lowest value was observed in the eastern equatorial region with upwelling, which is consistent with the findings of previous studies.

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Ocean acidification as a governance challenge in the Mediterranean Sea: impacts from aquaculture and fisheries

Despite the progress in the international and regional governance efforts at the level of climate change, ocean acidification (OA) remains a global problem with profoundly negative environmental, social, and economical consequences. This requires extensive mitigation and adaptation effective strategies that are hindered by current shortcomings of governance. This multidisciplinary chapter investigates the risks of ocean acidification (OA) for aquaculture and fisheries in the Mediterranean Sea and its sub-basins and the role of regional adaptive governance to tackle the problem. The identified risks are based on the biological sensitivities of the most important aquaculture species and biogenic habitats and their exposure to the current and future predicted (2100) RCP 8.5 conditions. To link OA exposure and biological sensitivity, we produced spatially resolved and depth-related pH and aragonite saturation state exposure maps and overlaid these with the existing aquaculture industry in the coastal waters of the Mediterranean basin to demonstrate potential risk for the aquaculture in the future. We also identified fisheries’ vulnerability through the indirect effects of OA on highly sensitive biogenic habitats that serve as nursery and spawning areas, showing that some of the biogenic habitats are already affected locally under existing OA conditions and will be more severely impacted across the entire Mediterranean basin under 2100 scenarios. This provided a regional vulnerability assessment of OA hotspots, risks and gaps that created the baseline for discussing the importance of adaptive governance and recommendations for future OA mitigation/adaptation strategies. By understanding the risks under future OA scenarios and reinforcing the adaptability of the governance system at the science-policy interface, best informed, “situated” management response capability can be optimised to sustain ecosystem services.

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Coral adaptive capacity insufficient to halt global transition of coral reefs into net erosion under climate change

Projecting the effects of climate change on net reef calcium carbonate production is critical to understanding the future impacts on ecosystem function, but prior estimates have not included corals’ natural adaptive capacity to such change. Here we estimate how the ability of symbionts to evolve tolerance to heat stress, or for coral hosts to shuffle to favourable symbionts, and their combination, may influence responses to the combined impacts of ocean warming and acidification under three representative concentration pathway (RCP) emissions scenarios (RCP2.6, RCP4.5 and RCP8.5). We show that symbiont evolution and shuffling, both individually and when combined, favours persistent positive net reef calcium carbonate production. However, our projections of future net calcium carbonate production (NCCP) under climate change vary both spatially and by RCP. For example, 19%–35% of modelled coral reefs are still projected to have net positive NCCP by 2050 if symbionts can evolve increased thermal tolerance, depending on the RCP. Without symbiont adaptive capacity, the number of coral reefs with positive NCCP drops to 9%–13% by 2050. Accounting for both symbiont evolution and shuffling, we project median positive NCPP of coral reefs will still occur under low greenhouse emissions (RCP2.6) in the Indian Ocean, and even under moderate emissions (RCP4.5) in the Pacific Ocean. However, adaptive capacity will be insufficient to halt the transition of coral reefs globally into erosion by 2050 under severe emissions scenarios (RCP8.5).

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Projected effects of climate change on marine ecosystems in Southeast Asian seas

The seas of Southeast Asia are home to some of the world’s most diverse ecosystems and resources that support the livelihoods of millions of people. Climate change will bring temperature changes, acidification and other environmental change, with uncertain consequences for human and natural systems, but there has been little regional-scale climate modelling of the marine ecosystem. We present initial dynamically downscaled projections using a biogeochemical model suitable for coastal and shelf seas. A coupled physical-biogeochemical model with a resolution of 0.1° (approximately 11 km) was used to create projections of future environmental conditions under moderate (RCP4.5) and high (RCP8.5) greenhouse gas scenarios. Changes for different parts of the region are presented, including four sensitive coastal sites of key importance for biodiversity and sustainable development: UNESCO Biosphere Reserves at Cu Lao Cham-Hoi An in Vietnam, Palawan in the Philippines and Taka Bonerate-Kepulauan Selayar in Indonesia, and coastal waters of Sabah, Malaysia, which include several marine parks. The projections show a sea that is warming by 1.1 to 2.9°C through the 21st century, with dissolved oxygen decreasing by 5 to 13 mmol m-3 and changes in many other environmental variables. The changes reach all parts of the water column and many places are projected to experience conditions well outside the range seen at the start of the century. The resulting damage to coral reefs and altered species distribution would have consequences for biodiversity, the livelihoods of small-scale fishers and the food security of coastal communities. Further work using a range of global models and regional models with different biogeochemical components is needed to provide confidence levels, and we suggest some ways forward. Projections of this type serve as a key tool for communities and policymakers as they plan how they will adapt to the challenge of climate change.

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Spatiotemporal heterogeneity in the increase of ocean acidity extremes in the Northeast Pacific

The acidification of the ocean (OA) increases the frequency and intensity of ocean acidity extreme events (OAXs), but this increase is not occurring homogeneously in time and space. Here we use daily output from a hindcast simulation with a high-resolution regional ocean model coupled to a biogeochemical-ecosystem model (ROMS-BEC) to investigate this heterogeneity in the progression of OAX in the upper 250 m of the Northeast Pacific from 1984 to 2019. We focus on the temporal and spatial changes in OAX using a relative threshold approach and using a fixed baseline reflecting the initial conditions. Concretely, conditions are considered extreme when the local hydrogen ion concentration ([H+]) exceeds the 99th percentile of the [H+] distribution of the baseline simulation where atmospheric CO2 was held at its 1979 level. Within the 36 years of our hindcast simulation, the increase in atmospheric CO2 causes a strong increase in OAX throughout the upper 250 m, but most accentuated near the surface. On average across the entire Northeast Pacific, for every additional 10 μatm of CO2 in the atmosphere, OAXs occupy an additional 6.3 % of the upper 250 m depth, last 7.6 days longer, and are 0.18 nmol L−1 (~ −0.006 pH units) more intense. This causes the OAXs to occupy at the end of the simulation a more than 10-times larger volume. The more than 11-fold increase in length, and the strong increase in the number of extreme days per year causes 88 % of the surface area in 2019 to experience near permanent extreme conditions. Finally, the model simulates a more than 6-fold intensification of the OAXs, causing also the intensity of the events with return periods of 10 years or more to increase by more than 80 %. Superimposed on these overall trends are very substantial spatial and temporal differences in these changes. The fraction of the volume identified as extreme across the top 250 m increases in the Central Northeast Pacific up to 160-times, while the deeper layers of the nearshore regions experience “only” a 4-fold increase. Throughout the upper 50 m of the Northeast Pacific, OAXs increase relatively linearly with time, but sudden rapid increases in yearly extreme days and OAX duration are simulated to occur in the thermocline of the Central Northeast Pacific. These differences largely emerge from the large spatial differences in the magnitude and nature of variability in [H+], with the transition between the rather variable thermocline waters of the Offshore Northeast Pacific and the very stable waters of the Central Northeast Pacific causing a very sharp transition in the occurrence of OAX. This transition is caused by the limited offshore reach of offshore propagating eddies that are the dominant driver of OAX in the Northeast Pacific. As the OAXs become more extreme, more of them also become undersaturated with respect to aragonite (ΩA < 1), i.e., become corrosive. In the final year of our hindcast, we find that below 100 m OAXs are characterized by corrosive conditions across a wide stretch of the region offshore of the U.S. and Canadian Coasts. The spatially and temporal heterogeneous increases in OAX, including the abrupt appearance of extremes, likely have negative effects on the ability of marine organisms to adapt to the progression of OA and its associated extremes.

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Paris Agreement could prevent regional mass extinctions of coral species

Coral reef ecosystems are expected to undergo significant declines over the coming decades as oceans become warmer and more acidic. We investigate the environmental tolerances of over 650 Scleractinian coral species based on the conditions found within their present-day ranges and in areas where they are currently absent but could potentially reach via larval dispersal. These “environmental envelopes” and connectivity constraints are then used to develop global forecasts for potential coral species richness under two emission scenarios, representing the Paris Agreement target (“SSP1-2.6”) and high levels of emissions (“SSP5-8.5”). Although we do not directly predict coral mortality or adaptation, the projected changes to environmental suitability suggest considerable potential declines in coral species richness for the majority of the world’s tropical coral reefs, with a net loss in average local richness of 73% (Paris Agreement) to 91% (High Emissions) by 2080-2090 and particularly large declines across sites in the Great Barrier Reef, Coral Sea, Western Indian Ocean and Caribbean. However, at the regional scale, we find that environmental suitability for the majority of coral species can be largely maintained under the Paris Agreement target, with 0-30% potential net species lost in most regions (increasing to 50% for the Great Barrier Reef) as opposed to 80-90% losses in most areas under High Emissions. Projections for sub-tropical areas suggest that range expansion will give rise to coral reefs with low species richness (typically 10-20 coral species per region) and will not meaningfully offset declines in the tropics. This work represents the first global projection of coral species richness under oceanic warming and acidification. Our results highlight the critical importance of mitigating climate change to avoid potentially massive extinctions of coral species.

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Projected increase in carbon dioxide drawdown and acidification in large estuaries under climate change

Most estuaries are substantial sources of carbon dioxide (CO2) to the atmosphere. The estimated estuarine CO2 degassing is about 17% of the total oceanic uptake, but the effect of rising atmospheric CO2 on estuarine carbon balance remains unclear. Here we use 3D hydrodynamic-biogeochemical models of a large eutrophic estuary and a box model of two generic, but contrasting estuaries to generalize how climate change affects estuarine carbonate chemistry and CO2 fluxes. We found that small estuaries with short flushing times remain a CO2 source to the atmosphere, but large estuaries with long flushing times may become a greater carbon sink and acidify. In particular, climate downscaling projections for Chesapeake Bay in the mid-21st century showed a near-doubling of CO2 uptake, a pH decline of 0.1–0.3, and >90% expansion of the acidic volume. Our findings suggest that large eutrophic estuaries will become carbon sinks and suffer from accelerated acidification in a changing climate.

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Predicting coral reef carbonate chemistry through statistical modeling: constraining nearshore residence time around Guam

To accurately predict the impacts of ocean acidification on shallow-water ecosystems, we must account for the biogeochemical impact of local benthic communities, as well as the connectivity between offshore and onshore water masses. Estimation of residence time can help quantify this connectivity and determine the degree to which the benthos can influence the chemistry of the overlying water column. We present estimates of nearshore residence time for Guam and utilize these estimates to model the effects of benthic ecosystem metabolism on the coral reef carbonate system. Control volume and particle tracking approaches were used to estimate nearshore residence time. These estimates were paired with observed patterns in the reef carbonate system around Guam using water samples collected by NOAA’s National Coral Reef Monitoring Program. Model performance results suggest that when considering the effects of benthic metabolism on the carbonate system, it is paramount to represent the contact time of the water volume with the benthos. Even coarse estimates of residence time significantly increase model skill. We observed the highest predictive skill in models including control volume derived estimates of residence time, but only when those estimates were included as an interaction with benthic composition. This work shows that not only is residence time critically important to better predict biogeochemical variability in coral reef environments, but that even coarse hydrodynamic models can provide useful residence time estimates at management relevant, whole-ecosystem scales.

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Effects of climate change on the Kenyan coral reef eco-system

The coral reef ecosystem is a natural habitat for many marine organisms that has high economic and tourist significance. Nonetheless, this ecosystem has very low tolerance to the effects of changes brought about by increasing sea surface temperatures and ocean acidification. This study sought to investigate the combined effect of rising sea surface temperatures and ocean acidification on the Kenyan coral reef ecosystem. This was achieved by determining the spatial-temporal variability of ocean acidification over the Kenyan coastline; and simulating the combined effect of sea surface temperature increases and ocean acidification on the coral reef ecosystem.

Historical (2000-2021) data on sea surface temperature (SSTs) was obtained from the National Oceanic and Atmospheric Administration (NOAA) and data on dissolved total carbon dioxide (TCO2) and pH from Global Ocean Data Analysis Project (GLODAP). Future (2022-2081) sea surface temperature and dissolved carbon dioxide data was downloaded from Coupled Model Intercomparison Project (CMIP6) experiment for two Shared Socioeconomic Pathways (SSPs) namely SSP2-4.5 and SSP5-8.5. Statistical, graphical and model simulations analyses were applied in the study to investigate the combined effect of increasing SST and ocean acidification on coral reef ecosystem over the Kenyan coastline.

Results indicate that mean sea surface temperature and dissolved carbon dioxide along the Kenyan coastline varied with seasons and had increased between the years 2000-2021. Trend tests of SSTs and TCO2 revealed a significant upward trend at 5% level of significance. Rising SSTs led to bleaching in coral reefs along this coastline whereas TCO2 led to reduced amount of carbonate ion concentration and reduced pH in the sea surface waters which affected the rates of calcification and survival of the coral reefs. The results of the Combined Mortality and Bleaching Output model simulation revealed that bleaching and ocean acidification had negatively affected the coral reef cover resulting in a decline of more than 30% of cover between 2000 and 2021. The results of the simulation also projected that the coral reef cover will continue to decline in the long-term by 52% under SSP2-4.5 and 63% under SSP5-8.5 if the trends in SSTs and TCO2 are maintained.

This study recommends collaborative implementation of climate change policies and practices by national and regional governments, communities and policy makers; enhanced efforts by coastal county governments in Kenya and research organisations to expound on scientific knowledge base while simultaneously implementing sustainable targeted solutions to ensure that the socio-economic benefits of the coral reef ecosystem are sustained.

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Hydrological and biogeochemical controls on estuarine carbonate chemistry along a climate gradient

Increasing global atmospheric CO2 concentrations drive a net flux of CO2 into the oceans, mitigating the impacts of anthropogenic greenhouse gas emissions on the climate. This results in a reduction in pH and carbonate saturation state, a.k.a. ocean acidification, of marine waters. The acidified ocean water may advect into estuaries, leading to estuarine acidification. Many estuaries are highly sensitive to this acidification due to low buffer capacity. Because estuaries provide many important ecosystem services, alterations in their carbonate systems may have significant consequences on ecosystems and the economy. Despite the current understanding that estuaries may play a disproportionately important role in global air-sea CO2 flux, little is known about carbonate systems in subtropical estuaries. Further comprehension of estuarine carbonate systems is vital for quantification of the global carbon cycle. Specifically, subtropical estuaries in the northwestern Gulf of Mexico (nwGOM) exhibit a general long-term decrease in pH and total alkalinity (TA), with lower latitudes experiencing more extreme acidification than higher latitudes.

In Chapter II, sediment cores and slurries from the semiarid Mission-Aransas Estuary of the nwGOM were incubated and surface waters were analyzed for contributions of biogeochemical processes to TA change. Changes in total TA as well as calcium and sulfate ion concentration were examined following known reaction stoichiometry. Ratio of TA: ion changes suggested that carbonate dissolution co-occurred with oxidation of reduced sulfur species, and the latter consumed TA during drought periods in Mission-Aransas Estuary. This biogeochemical (sulfide oxidation) TA consumption has been poorly studied yet may affect TA budget in other semiarid estuaries worldwide.

In Chapter III, river alkalinity total load and concentration were calculated using the United States Geological Survey’s Fortran Load Estimator Program (LOADEST) and long-term trends in alkalinity and discharge of six major nwGOM rivers were determined. Stepwise multiple linear regression methods were used to generate models for predicting estuarine TA based on river alkalinity, year, and net evaporation (evaporation-precipitation). Some rivers were found to have long-term (multidecadal) declines in freshwater discharge, area-weighted alkalinity yield, of alkalinity flow-weighted concentration, with most declines occurring in the southern end of the study region. Freshwater flow-weighted alkalinity concentration (annual alkalinity load for an area divided by discharge) appeared in many of the predictive models for estuarine TA and may play a major role in regulating estuarine TA of the nwGOM. Methods for linking freshwater and estuarine carbonate dynamics are lacking in the scientific literature; this study provides a potentially useful approach for predicting estuarine carbonate chemistry based on freshwater quality and input.

In Chapter IV, CO2 flux of the Trinity-San Jacinto Estuary (Galveston Bay) was calculated and compared to results from discrete samples for carbonate parameters. Inferences about spatial and temporal patterns in CO2 flux as well as ecosystem metabolism were made based on results. The Trinity-San Jacinto Estuary was found to be a net sink for atmospheric CO2, but with high seasonal and spatial variability. Specifically, large freshwater inflows in spring stimulated photosynthesis in the estuary, which increased the sink behavior. Seasons with less freshwater inflow resulted in higher heterotrophy and CO2 emission in some regions of the estuary.

This research increases knowledge and research capacity in the nwGOM region on estuarine acidification and carbonate chemistry. Causes of acidification in major estuaries within the region were addressed along a latitudinal climatic gradient. This will aid with better management of fresh and estuarine water resources in the nwGOM. The results of this research will also clarify the role of semiarid, subtropical estuaries in the global carbon cycle and expand our range of knowledge on carbonate system analyses of estuaries.

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Observations of seawater carbonate chemistry in the Southern California Current

The ocean has taken up roughly a quarter of the total anthropogenic carbon emissions (Gruber et al., 2019). This addition causes changes in carbonate system equilibrium, decreasing ocean pH, which impacts marine organisms, ecosystems, and humans reliant on marine resources (Doney et al., 2020). The study of the changing carbonate chemistry and its impact on the ocean requires the refinement of measurement techniques, observational programs, models and the sharing of data. Chapter 1 focuses on measurement techniques by assessing the stability of tris pH buffer in artificial seawater stored in bags. These bagged reference materials can be used by both benchtop and autonomous instruments to aid in quality control of measurements of carbonate chemistry. Chapter 2 focuses on continued observation, with the oldest inorganic carbon time series in the Pacific. This time series in the Southern California Current helps confirm the rate of anthropogenic ocean acidification observed in other regions of the ocean. Chapter 3 focuses on models by using seasonal cycles determined in Chapter 2 to build a mixed layer carbon budget at the location of the time series. Chapter 4 focuses on the sharing of data by summarizing and publishing previously unavailable observations of carbonate chemistry in the Southern California Current going back as far as 1983.

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Understanding the seasonality, trends, and controlling factors of Indian Ocean acidification over distinctive bio-provinces

The Indian Ocean (IO) is witnessing acidification as a direct consequence of the continuous rising of atmospheric CO2 concentration and indirectly due to rapid ocean warming, which disrupts the pH of the surface waters. This study investigates the pH seasonality and trends over various bio-provinces of the IO and regionally assesses the contribution of each of its controlling factors. Simulations from a global and a regional ocean model coupled with biogeochemical modules were validated with pH measurements over the basin and used to discern the regional response of pH seasonality (1990–2010) and trend (1961–2010) to changes in Sea Surface Temperature (SST), Dissolved Inorganic Carbon (DIC), Total Alkalinity (ALK), and Salinity (S). DIC and SST are significant contributors to the seasonal variability of pH in almost all bio-provinces. Total acidification in the IO basin was 0.0675 units from 1961 to 2010, with 69.3% contribution from DIC followed by 13.8% contribution from SST. For most of the bio-provinces, DIC remains a dominant contributor to changing trends in pH except for the Northern Bay of Bengal and Around India (NBoB-AI) region, wherein the pH trend is dominated by ALK (55.6%) and SST (16.8%). Interdependence of SST and S over ALK is significant in modifying the carbonate chemistry and biogeochemical dynamics of NBoB-AI and a part of tropical, subtropical IO bio-provinces. A strong correlation between SST and pH trends infers an increasing risk of acidification in the bio-provinces with rising SST and points out the need for sustained monitoring of IO pH in such hotspots.

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Restoration and coral adaptation delay, but do not prevent, climate-driven reef framework erosion of an inshore site in the Florida Keys

For reef framework to persist, calcium carbonate production by corals and other calcifiers needs to outpace loss due to physical, chemical, and biological erosion. This balance is both delicate and dynamic and is currently threatened by the effects of ocean warming and acidification. Although the protection and recovery of ecosystem functions are at the center of most restoration and conservation programs, decision makers are limited by the lack of predictive tools to forecast habitat persistence under different emission scenarios. To address this, we developed a modelling approach, based on carbonate budgets, that ties species-specific responses to site-specific global change using the latest generation of climate models projections (CMIP6). We applied this model to Cheeca Rocks, an outlier in the Florida Keys in terms of high coral cover, and explored the outcomes of restoration targets scheduled in the coming 20 years at this site by the Mission: Iconic Reefs restoration initiative. Additionally, we examined the potential effects of coral thermal adaptation by increasing the bleaching threshold by 0.25, 0.5, 1 and 2˚C. Regardless of coral adaptative capacity or restoration, net carbonate production at Cheeca Rocks declines heavily once the threshold for the onset of annual severe bleaching is reached. The switch from net accretion to net erosion, however, is significantly delayed by mitigation and adaptation. The maintenance of framework accretion until 2100 and beyond is possible under a decreased emission scenario coupled with thermal adaptation above 0.5˚C. Although restoration initiatives increase reef accretion estimates, Cheeca Rocks will only be able to keep pace with future sea-level rise in a world where anthropogenic CO2 emissions are reduced. Present results, however, attest to the potential of restoration interventions combined with increases in coral thermal tolerance to delay the onset of mass bleaching mortalities, possibly in time for a low-carbon economy to be implemented and complementary mitigation measures to become effective.

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Effect of plankton composition shifts in the North Atlantic on atmospheric pCO2


Marine carbon cycle processes are important for taking up atmospheric CO2 thereby reducing climate change. Net primary and export production are important pathways of carbon from the surface to the deep ocean where it is stored for millennia. Climate change can interact with marine ecosystems via changes in the ocean stratification and ocean circulation. In this study we use results from the Community Earth System Model version 2 (CESM2) to assess the effect of a changing climate on biological production and phytoplankton composition in the high latitude North Atlantic Ocean. We find a shift in phytoplankton type dominance from diatoms to small phytoplankton which reduces net primary and export productivity. Using a conceptual carbon-cycle model forced with CESM2 results, we give a rough estimate of a positive phytoplankton composition-atmospheric CO2 feedback of approximately 60 GtCO2/°C warming in the North Atlantic which lowers the 1.5° and 2.0°C warming safe carbon budgets.

Key Points

  • Biological production decreases significantly in the high latitude North Atlantic in Community Earth System Model version 2 under the SSP5-8.5 scenario
  • Phytolankton type dominance shifts from diatoms to small phytoplankton
  • A positive feedback loop is diagnosed where changes in the physical system decrease biological production, reducing oceanic uptake of CO2
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A space-time mosaic of seawater carbonate chemistry conditions in the north-shore Moorea coral reef system

The interplay between ocean circulation and coral metabolism creates highly variable biogeochemical conditions in space and time across tropical coral reefs. Yet, relatively little is known quantitatively about the spatiotemporal structure of these variations. To address this gap, we use the Coupled Ocean Atmosphere Wave and Sediment Transport (COAWST) model, to which we added the Biogeochemical Elemental Cycling (BEC) model computing the biogeochemical processes in the water column, and a coral polyp physiology module that interactively simulates coral photosynthesis, respiration and calcification. The coupled model, configured for the north-shore of Moorea Island, successfully simulates the observed (i) circulation across the wave regimes, (ii) magnitude of the metabolic rates, and (iii) large gradients in biogeochemical conditions across the reef. Owing to the interaction between coral net community production (NCP) and coral calcification, the model simulates distinct day versus night gradients, especially for pH and the saturation state of seawater with respect to aragonite (Ωα). The strength of the gradients depends non-linearly on the wave regime and the resulting residence time of water over the reef with the low wave regime creating conditions that are considered as “extremely marginal” for corals. With the average water parcel passing more than twice over the reef, recirculation contributes further to the accumulation of these metabolic signals. We find diverging temporal and spatial relationships between total alkalinity (TA) and dissolved inorganic carbon (DIC) (≈ 0.16 for the temporal vs. ≈ 1.8 for the spatial relationship), indicating the importance of scale of analysis for this metric. Distinct biogeochemical niches emerge from the simulated variability, i.e., regions where the mean and variance of the conditions are considerably different from each other. Such biogeochemical niches might cause large differences in the exposure of individual corals to the stresses associated with e.g., ocean acidification. At the same time, corals living in the different biogeochemical niches might have adapted to the differing conditions, making the reef, perhaps, more resilient to change. Thus, a better understanding of the mosaic of conditions in a coral reef might be useful to assess the health of a coral reef and to develop improved management strategies.

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Simulated impact of ocean alkalinity enhancement on atmospheric CO2 removal in the Bering Sea


Ocean alkalinity enhancement (OAE) has the potential to mitigate ocean acidification (OA) and induce atmospheric carbon dioxide (CO2) removal (CDR). We evaluate the CDR and OA mitigation impacts of a sustained point-source OAE of 1.67 × 1010 mol total alkalinity (TA) yr−1 (equivalent to 667,950 metric tons NaOH yr−1) in Unimak Pass, Alaska. We find the alkalinity elevation initially mitigates OA by decreasing pCO2 and increasing aragonite saturation state and pH. Then, enhanced air-to-sea CO2 exchange follows with an approximate e-folding time scale of 5 weeks. Meaningful modeled OA mitigation with reductions of >10 μatm pCO2 (or just under 0.02 pH units) extends 100–100,000 km2 around the TA addition site. The CDR efficiency (i.e., the experimental seawater dissolved inorganic carbon (DIC) increase divided by the maximum DIC increase expected from the added TA) after the first 3 years is 0.96 ± 0.01, reflecting essentially complete air-sea CO2 adjustment to the additional TA. This high efficiency is potentially a unique feature of the Bering Sea related to the shallow depths and mixed layer depths. The ratio of DIC increase to the TA added is also high (≥0.85) due to the high dissolved carbon content of seawater in the Bering Sea. The air-sea gas exchange adjustment requires 3.6 months to become (>95%) complete, so the signal in dissolved carbon concentrations will likely be undetectable amid natural variability after dilution by ocean mixing. We therefore argue that modeling, on a range of scales, will need to play a major role in assessing the impacts of OAE interventions.

Key Points

  • We used regional ocean model to simulate single point-source ocean alkalinity enhancement in the Bering Sea
  • The steady state carbon dioxide removal efficiency was near one in years 3+ of the simulation
  • The meaningful modeled ocean acidification mitigation is confined to the region near the alkalinity addition

Plain Language Summary

The Intergovernmental Panel on Climate Change suggests that carbon dioxide (CO2) removal (CDR) approaches will be required to stabilize the global temperature increase at 1.5–2°C. In this study, we simulated the climate mitigation impacts of adding alkalinity (equivalent to 667,950 metric ton NaOH yr−1) in Unimak Pass on the southern boundary of the Bering Sea. We found that adding alkalinity can accelerate the ocean CO2 uptake and storage and mitigate ocean acidification near the alkalinity addition. It takes about 3.6 months for the Ocean alkalinity enhancement impacted area to take up the extra CO2. The naturally cold and carbon rich water in the Bering Sea and the tendency of Bering Sea surface waters to linger near the ocean surface without mixing into the subsurface ocean both lead to high CDR efficiencies (>96%) from alkalinity additions in the Bering Sea. However, even with high efficiency, it would take >8,000 alkalinity additions of the kind we simulated to be operating by the year 2100 to meet the target to stabilize global temperatures within the targeted range.

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Feedbacks of CaCO3 dissolution effect on ocean carbon sink and seawater acidification: a model study

The oceanic absorption of atmospheric CO2 acidifies seawater, which accelerates CaCO3 dissolution of calcifying organisms, a process termed dissolution effect. Promoted CaCO3 dissolution increases seawater ALK (alkalinity), enhancing ocean carbon sink and mitigating ocean acidification. We incorporate different parameterizations of the link between CaCO3 dissolution and ocean acidification into an Earth System Model, to quantify the feedback of the dissolution effect on the global carbon cycle. Under SRES A2 CO2 emission scenario and its extension with emissions of 5,000 PgC in ~400 years, in the absence of the dissolution effect, accumulated ocean CO2 uptake between year 1800 and 3500 is 2,041 PgC. The consideration of the dissolution effect increases ocean carbon sink by 195–858 PgC (10–42%), and mitigates the decrease in surface pH by 0.04–0.17 (a decrease of 10–48% in [H+] (hydrogen ion concentration)), depending on the prescribed parameterization scheme. In the epipelagic zone, relative to the Arc-Atlantic Ocean, the Pacific-Indian Ocean experiences greater acidification, leading to greater dissolution effects and the resultant stronger feedbacks on ocean carbon sink and acidification in the Pacific-Indian Ocean. Noteworthy, the feedback of dissolution effect on ocean carbon sink can be comparable with or stronger than the feedback from CO2-induced radiative warming. Our study highlights the potentially critical role played by CaCO3 dissolution effect in the ocean carbon sink, global carbon cycle and climate system.

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