Posts Tagged 'modeling'

Amplification of global warming through pH-dependence of DMS-production simulated with a fully coupled Earth system model

We estimate the additional transient surface warming ΔTs caused by a potential reduction of marine dimethyl sulfide (DMS) production due to ocean-acidification under the high emission scenario RCP8.5 until the year 2200. Since we use a fully coupled Earth system model, our results include a range of feedbacks, such as the response of marine DMS-production to the additional changes in temperature and sea-ice cover. Our results are broadly consistent with the findings of a previous study that employed an off-line model set-up. Assuming a medium (strong) sensitivity of DMS-production to pH, we find an additional transient global warming of 0.30 K (0.47 K) towards the end of the 22nd century when DMS-emission are reduced by 7.3 Tg S yr−1 or 31 % (11.5 Tg S yr−1 or 48 %). The main mechanism behind the additional warming is a reduction of cloud albedo, but a change in short-wave radiative fluxes under clear-sky conditions due to reduced sulfate aerosol load also contributes significantly. We find an approximately linear relationship between reduction of DMS-emissions and changes in top of the atmosphere radiative fluxes as well as changes in surface temperature for the range of DMS-emissions considered here. For example, global average Ts changes by −0.041 K per 1 Tg S yr−1 change in sea-air DMS-fluxes. The additional warming in our model has a pronounced asymmetry between northern and southern high latitudes. It is largest over the Antarctic continent, where the additional temperature increase of 0.56 K (0.89 K) is almost twice the global average. We find that feedbacks are small on the global scale due to opposing regional contributions. The most pronounced feedback is found for the Southern Ocean, where we estimate that the additional climate change enhances sea-air DMS-fluxes by about 9 % (15 %), which counteracts the reduction due to ocean acidification.

Continue reading ‘Amplification of global warming through pH-dependence of DMS-production simulated with a fully coupled Earth system model’

Eutrophication-induced acidification of coastal waters in the northern Gulf of Mexico: Insights into origin and processes from a coupled physical-biogeochemical model

Nutrient inputs from the Mississippi/Atchafalaya River system into the northern Gulf of Mexico promote high phytoplankton production and lead to high respiration rates. Respiration coupled with water column stratification results in seasonal summer hypoxia in bottom waters on the shelf. In addition to consuming oxygen, respiration produces carbon dioxide (CO2), thus lowering the pH and acidifying bottom waters. Here we present a high-resolution biogeochemical model simulating this eutrophication-driven acidification and investigate the dominant underlying processes. The model shows the recurring development of an extended area of acidified bottom waters in summer on the northern Gulf of Mexico shelf that coincides with hypoxic waters. Not reported before, acidified waters are confined to a thin bottom boundary layer where the production of CO2 by benthic metabolic processes is dominant. Despite a reduced saturation state, acidified waters remain supersaturated with respect to aragonite.

Continue reading ‘Eutrophication-induced acidification of coastal waters in the northern Gulf of Mexico: Insights into origin and processes from a coupled physical-biogeochemical model’

Light and temperature control physiological responses of Halimeda to ocean acidification

The oceans have absorbed excess carbon dioxide (CO2) resulting from anthropogenic activities such as the burning of fossil fuels and deforestation. As a result, seawater chemistry has shifted causing an increase in bicarbonate ions (HCO32-) and hydrogen ions (H+) and leading to a reduction in carbonate (CO32-) concentration. This shift in seawater chemistry leads to a decrease in aragonite saturation state and pH. Eventually, the ocean will accumulate most of the extra CO2 produced over many years resulting in extreme acidified conditions where aragonite saturation levels will not support the chemical process of calcification that is vital to marine calcifiers. This thesis investigates the combined effects of elevated pCO2 with temperature and light on the calcification and photosynthesis of the green calcareous algae Halimeda. Halimeda, is a major contributor to sediment production for coral reef accretion and island reef formation. Based on carbonate data from biologists and geologists it is estimated that vertical accretion of CaCO3 by Halimeda ranges between 0.18 to 5.9 m in 1000 years. The role that light plays in the coupling between photosynthesis and calcification in Halimeda macroloba was investigated experimentally through a combination of two pCO2 levels (360 and 1200 uatm) and three irradiances (80, 150, and 595 μmol quanta m-2 s-1). A decrease in calcification at low light intensity and elevated pCO2 suggests that light is a limiting factor for the physiology of H. macroloba. The effects of elevated pCO2 and temperature on the photosynthesis and calcification of Halimeda incrassata were tested through two experiments using two pCO2 levels (390 and 900 uatm) and four temperatures (26, 29, 30 and 34 °C). Elevated temperature can mitigate the effects Ocean Acidification (OA) in H. incrassata. An estimate of current carbonate production by H. incrassata in Key Biscayne Florida Lagoon was obtained from biomass, CaCO3 content and turnover rate. Calcification rates from laboratory experiments were used to estimate future (200 years from now) seasonal carbonate production rates, which were then compared against current summer carbonate production. Future summer carbonate production rates were not affected by elevated pCO2 in relationship to current summer carbonate production. Elevated temperatures ~2 °C above summer maximum average could promote calcification of H. incrassata under ocean acidification conditions and, therefore, overall carbonate production of the reef. Results throughout the thesis revealed that the tolerance of the green calcareous algae Halimeda to OA could change depending on light and temperature conditions. In a more acidic future ocean, growth rates and sediment production of Halimeda will be affected under low light and temperature and will be enhanced under high light and and moderate elevated temperatures.

Continue reading ‘Light and temperature control physiological responses of Halimeda to ocean acidification’

Proton pumping accompanies calcification in foraminifera

Ongoing ocean acidification is widely reported to reduce the ability of calcifying marine organisms to produce their shells and skeletons. Whereas increased dissolution due to acidification is a largely inorganic process, strong organismal control over biomineralization influences calcification and hence complicates predicting the response of marine calcifyers. Here we show that calcification is driven by rapid transformation of bicarbonate into carbonate inside the cytoplasm, achieved by active outward proton pumping. Moreover, this proton flux is maintained over a wide range of pCO2 levels. We furthermore show that a V-type H+ ATPase is responsible for the proton flux and thereby calcification. External transformation of bicarbonate into CO2 due to the proton pumping implies that biomineralization does not rely on availability of carbonate ions, but total dissolved CO2 may not reduce calcification, thereby potentially maintaining the current global marine carbonate production.

Continue reading ‘Proton pumping accompanies calcification in foraminifera’

CO2 seawater acidification by CCS-simulated leakage: Kinetic modelling of Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediment using pH-static leaching tests

A modified pH-dependent leaching test with continuous pH control that employed CO2 to acidify a seawater-sediment mixture is used to address Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediments under the influence of acidification processes. Long-term (480 h) leaching experiments at pH values of 7.0, 6.5 and 6.0 are performed. The different evolutionary patterns of the redox potential and Fe release at pH = 6 with respect to the other pH values shows the need to assess the influence of the initial Fe content in seawater upon elemental release. Hence, assays at pH = 6.0 are conducted using natural seawater with Fe concentrations between 9.02 and 153 μg/L. A set of in-series reactions for trace elements, Fe and other ions associated with Fe is proposed to model a Fe/multi-ion-dependent mechanism for trace metal release. The maximum concentration of each contaminant that can be released from the sediment and the kinetic parameters of the proposed model are completed for the studied pH values, for good consistency between the experimental and simulated mobilisation of each studied element.

Continue reading ‘CO2 seawater acidification by CCS-simulated leakage: Kinetic modelling of Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediment using pH-static leaching tests’

Risks of ocean acidification in the California Current food web and fisheries: ecosystem model projections

The benefits and ecosystem services that humans derive from the oceans are threatened by numerous global change stressors, one of which is ocean acidification. Here, we describe the effects of ocean acidification on an upwelling system that already experiences inherently low pH conditions, the California Current. We used an end-to-end ecosystem model (Atlantis), forced by downscaled global climate models and informed by a meta-analysis of the pH sensitivities of local taxa, to investigate the direct and indirect effects of future pH on biomass and fisheries revenues. Our model projects a 0.2-unit drop in pH during the summer upwelling season from 2013 to 2063, which results in wide-ranging magnitudes of effects across guilds and functional groups. The most dramatic direct effects of future pH may be expected on epibenthic invertebrates (crabs, shrimps, benthic grazers, benthic detritivores, bivalves), and strong indirect effects expected on some demersal fish, sharks, and epibenthic invertebrates (Dungeness crab) because they consume species known to be sensitive to changing pH. The model’s pelagic community, including marine mammals and seabirds, was much less influenced by future pH. Some functional groups were less affected to changing pH in the model than might be expected from experimental studies in the empirical literature due to high population productivity (e.g., copepods, pteropods). Model results suggest strong effects of reduced pH on nearshore state-managed invertebrate fisheries, but modest effects on the groundfish fishery because individual groundfish species exhibited diverse responses to changing pH. Our results provide a set of projections that generally support and build upon previous findings and set the stage for hypotheses to guide future modeling and experimental analysis on the effects of OA on marine ecosystems and fisheries.

Continue reading ‘Risks of ocean acidification in the California Current food web and fisheries: ecosystem model projections’

Spatial competition dynamics between reef corals under ocean acidification

Climate change, including ocean acidification (OA), represents a major threat to coral-reef ecosystems. Although previous experiments have shown that OA can negatively affect the fitness of reef corals, these have not included the long-term effects of competition for space on coral growth rates. Our multispecies year-long study subjected reef-building corals from the Gulf of Aqaba (Red Sea) to competitive interactions under present-day ocean pH (pH 8.1) and predicted end-of-century ocean pH (pH 7.6). Results showed coral growth is significantly impeded by OA under intraspecific competition for five out of six study species. Reduced growth from OA, however, is negligible when growth is already suppressed in the presence of interspecific competition. Using a spatial competition model, our analysis indicates shifts in the competitive hierarchy and a decrease in overall coral cover under lowered pH. Collectively, our case study demonstrates how modified competitive performance under increasing OA will in all likelihood change the composition, structure and functionality of reef coral communities.

Continue reading ‘Spatial competition dynamics between reef corals under ocean acidification’

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Ocean acidification in the IPCC AR5 WG II

OUP book