Posts Tagged 'global'

Response of ocean acidification to a gradual increase and decrease of atmospheric CO2

We perform coupled climate–carbon cycle model simulations to examine changes in ocean acidity in response to idealized change of atmospheric CO2. Atmospheric CO2 increases at a rate of 1% per year to four times its pre-industrial level of 280 ppm and then decreases at the same rate to the pre-industrial level. Our simulations show that changes in surface ocean chemistry largely follow changes in atmospheric CO2. However, changes in deep ocean chemistry in general lag behind the change in atmospheric CO2 because of the long time scale associated with the penetration of excess CO2 into the deep ocean. In our simulations with the effect of climate change, when atmospheric CO2 reaches four times its pre-industrial level, global mean aragonite saturation horizon (ASH) shoals from the pre-industrial value of 1288 to 143 m. When atmospheric CO2 returns from the peak value of 1120 ppm to pre-industrial level, ASH is 630 m, which is approximately the value of ASH when atmospheric CO2 first increases to 719 ppm. At pre-industrial CO2 9% deep-sea cold-water corals are surrounded by seawater that is undersaturated with aragonite. When atmospheric CO2 reaches 1120 ppm, 73% cold-water coral locations are surrounded by seawater with aragonite undersaturation, and when atmospheric CO2 returns to the pre-industrial level, 18% cold-water coral locations are surrounded by seawater with aragonite undersaturation. Our analysis indicates the difficulty for some marine ecosystems to recover to their natural chemical habitats even if atmospheric CO2 content can be lowered in the future.
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Global fisheries economics in the face of change in climate

Climate change and changes in biogeochemical conditions of the ocean lead to changes in distribution of marine species and ocean productivity. These changes would affect fisheries, food security, livelihood of fishing communities and eventually the whole economy in different countries. This thesis uses simulation modelling to assess the direct impacts of change in physical and biogeochemical conditions of the ocean on marine fisheries and the socio-economic implications at both global and regional scales. I develop a new global database of fishing cost, and provide an overview of current fishing cost patterns at national, regional, and global scales. The outcomes lay the foundation for the subsequent economic analysis in the thesis, and should also be useful for other future fisheries economic studies. Using these results and other data from the Sea Around Us Project, I estimate the change in landings of over 800 species of fish within the Exclusive Economic Zones (EEZs) under climate change scenarios based on dynamic bioclimate envelope model (DBEM), and an empirical model. About 75% of EEZs are projected to show declines in landings under the Special Report on Emission Scenario (SRES) A2. Most of them are in developing countries, which are socio-economically more vulnerable to climate change. In West Africa, which is one of the most vulnerable regions to climate change, our model projects that there will be a reduction in landings in the 2050s, with some countries experiencing declines of more than 50% under the “business-as-usual” scenario. This substantial decline not only affects the food supply and security in the region, but also has a negative impact on employment opportunities and the downstream economic impact on the whole society. I also analyze how change in climate and ocean acidity under scenarios of anthropogenic CO₂ emission is expected to affect the economics of marine fisheries in the Arctic region. My model only projected a slight decrease in catch potential of marine fish and invertebrates under the impact of ocean acidification in the 2050s. Future studies accounting for the synergistic effects among climate change, ocean acidification and other factors on marine ecosystems are needed.

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Assessing the potential of calcium-based artificial ocean alkalinization to mitigate rising atmospheric CO2 and ocean acidification

Enhancement of ocean alkalinity using Calcium-compounds, e.g. lime has been proposed to mitigate further increase of atmospheric CO2 and ocean acidification due to anthropogenic CO2 emissions. Using a global model, we show that such alkalinization has the potential to preserve pH and the saturation state of carbonate minerals at close to today’s values. Effects of alkalinization persist after termination: atmospheric CO2 and pH do not return to unmitigated levels. Only scenarios in which large amounts of alkalinity (i.e. in a ratio of 2:1 with respect to emitted CO2) are added over large ocean areas can boost oceanic CO2 uptake sufficiently to avoid further ocean acidification on the global scale, thereby elevating some key biogeochemical parameters, e.g. pH significantly above preindustrial levels. Smaller-scale alkalinization could counteract ocean acidification on a sub-regional or even local scale, e.g. in upwelling systems. The decrease of atmospheric CO2 would then be a small side effect.

Continue reading ‘Assessing the potential of calcium-based artificial ocean alkalinization to mitigate rising atmospheric CO2 and ocean acidification’

Geologic history of seawater: A MAGic approach to carbon chemistry and ocean ventilation

We explore the relationship between atmospheric O2 and CO2 evolution and seawater chemistry, with particular focus on the CO2-carbonic acid system and ocean ventilation, over the Phanerozoic Eon using a coupled biogeochemical Earth system model (MAGic). This model describes the biogeochemical cycles involving the major components of seawater (Ca, Mg, Na, K, Cl, SO4, CO2 − HCO3 − CO3), as well as components (O2, Fe, P, organic C, reduced S) central to long-term ecosystem productivity. The MAGic calculations show that the first-order input fluxes from weathering of continental rocks of Ca, Mg, and dissolved inorganic carbon (DIC) to the ocean varied in a cyclical manner over the Phanerozoic. The cyclicity is mainly the result of the impact of changing atmospheric CO2 levels, and hence temperature and runoff, on these fluxes, reflecting the nature of hothouse (greenhouse, high CO2 and warm) versus icehouse (low CO2, cool, and continental glaciation) conditions during the Phanerozoic. Uptake of DIC by seafloor basalt-seawater reactions also varied in a corresponding fashion to the weathering fluxes. The fluxes of Ca, Mg, DIC and other seawater constituents removed in oceanic sinks were also calculated and hence with calculated inputs and outputs of seawater constituents, the changes in seawater chemistry through Phanerozoic time could be obtained. Seawater pH increased irregularly during the Phanerozoic from just above 7 in the Cambrian Period, approaching modern average values in the most recent several millions of years. Calcite saturation state also increased with decreasing age. Both pH and calcite saturation state trends exhibited a cyclic overprint of hothouse and icehouse environmental conditions. Dissolved sulfate changed in a cyclical manner reflecting mainly variations in weathering and accretion rates and redox conditions, whereas dissolved potassium exhibited little variation in concentration.

Using our “standard” model results for the chemistry of seawater and changes in atmospheric CO2 and O2 as the basis for a series of sensitivity experiments, we vary the ventilation rate of the global ocean, and quantify the resulting changes in terms of processes such as net primary production, organic carbon burial and oxidation, pyrite weathering, and sulfate reduction. We use these preliminary results to discuss how changes in ocean ventilation affect atmospheric CO2 and O2, and in turn exert changes in the sulfur, organic carbon, and inorganic carbon systems. We postulate that periods of slow plate accretion rates, associated with lower atmospheric CO2, vigorous deep water formation, cooler, drier climatic conditions and greater poleward temperature gradients are more likely to be associated with a strong thermohaline circulation, and thus “enhanced” global ocean mixing. Conversely, periods of higher accretion rates, higher CO2, higher average global temperatures with more equable poleward gradients, and higher sea levels resulting in extensive continental inundation, would be more likely to be coincident with times of reduced mixing of the global ocean. It is important to recognize that the scale of these changes depends on how major tectonic cycles (controlling chemical weathering, CO2 and temperature) in turn affect nutrient supply, global ocean productivity, and global ocean thermohaline circulation. The key to elucidating these changes lies in an understanding of the relationship between long-term tectonic evolution, which leads to changes in climate, sea level, and the global distribution of continental landmasses and the sedimentary environments they host, and the circulation of the global ocean.

Continue reading ‘Geologic history of seawater: A MAGic approach to carbon chemistry and ocean ventilation’

Chapter 26 – Modeling ocean biogeochemical processes and the resulting tracer distributions

Biogeochemical ocean general circulation models are important tools for quantifying the marine carbon cycle and its feedback to the climate system. These models simulate the inorganic carbon cycle and also the organic carbon cycle through a series of simplified process parameterizations. This chapter presents an overview of the major concepts and methods in marine biogeochemical modeling including the combination of models with observations. Because of the climatic relevance of the carbon cycle, major emphasis is placed on it, but some other related matter cycles are also touched upon. New developments in ocean biogeochemistry during the last decade are described including the marine anthropogenic carbon uptake and ocean acidification. The chapter tries to acquaint scientists from other disciplines with marine biogeochemical modeling and provides key literature resources for further in-depth studies.

Continue reading ‘Chapter 26 – Modeling ocean biogeochemical processes and the resulting tracer distributions’

Biotic and human vulnerability to projected changes in ocean biogeochemistry over the 21st century

Ongoing greenhouse gas emissions can modify climate processes and induce shifts in ocean temperature, pH, oxygen concentration, and productivity, which in turn could alter biological and social systems. Here, we provide a synoptic global assessment of the simultaneous changes in future ocean biogeochemical variables over marine biota and their broader implications for people. We analyzed modern Earth System Models forced by greenhouse gas concentration pathways until 2100 and showed that the entire world’s ocean surface will be simultaneously impacted by varying intensities of ocean warming, acidification, oxygen depletion, or shortfalls in productivity. In contrast, only a small fraction of the world’s ocean surface, mostly in polar regions, will experience increased oxygenation and productivity, while almost nowhere will there be ocean cooling or pH elevation. We compiled the global distribution of 32 marine habitats and biodiversity hotspots and found that they would all experience simultaneous exposure to changes in multiple biogeochemical variables. This superposition highlights the high risk for synergistic ecosystem responses, the suite of physiological adaptations needed to cope with future climate change, and the potential for reorganization of global biodiversity patterns. If co-occurring biogeochemical changes influence the delivery of ocean goods and services, then they could also have a considerable effect on human welfare. Approximately 470 to 870 million of the poorest people in the world rely heavily on the ocean for food, jobs, and revenues and live in countries that will be most affected by simultaneous changes in ocean biogeochemistry. These results highlight the high risk of degradation of marine ecosystems and associated human hardship expected in a future following current trends in anthropogenic greenhouse gas emissions.

Continue reading ‘Biotic and human vulnerability to projected changes in ocean biogeochemistry over the 21st century’

Carbonate ion concentrations, ocean carbon storage, and atmospheric CO2

Reconstructing past ocean [CO32−] allows the paleodepth of the chemical lysocline to be constrained, an important control on past atmospheric CO2. However, the causal mechanisms responsible for observed spatial and temporal variations in [CO32−] are difficult to quantify because of the complicated carbonate chemistry system. Here spatial and temporal variations in [CO32−] are quantitatively and concisely related to variations in ocean carbon storage due to different processes. The spatial variation in [CO32−] is given by Δ[CO32−] = γCsoft + ΔCdis+ (∂Csat/∂TT − ΔCcarb), where Csoft and Ccarb are the dissolved inorganic carbon (DIC) from remineralization of marine soft tissue and CaCO3, respectively, T is seawater temperature, (∂Csat/∂T) is the temperature-solubility sensitivity of DIC, Cdis is the DIC from air-sea disequilibrium, and γ is a carbonate chemistry coefficient. A similar quantitative function for temporal variation in global mean ocean [CO32−] is derived in terms of atmospheric CO2, CaCO3 precipitation and dissolution, and carbon exchanges of terrestrial or fossil fuel origin. Comparing published [CO32−] reconstructions at the Last Glacial Maximum (LGM) and the late Holocene, the quantitative relationships reveal how the spatial distribution of ocean carbon storage was altered. Relative to the Intermediate North Atlantic, the rest of the ocean saw Csoft + Cdis + (∂Csat/∂T)T − Ccarb increase by an extra 570–970 Pg C during the LGM. Assuming that the Intermediate North Atlantic Csoft + Cdis + (∂Csat/∂T)T − Ccarb did not decrease during the LGM, this 570–970 Pg C increase in the rest of the ocean is enough to explain 40%–70% of the observed glacial decrease in atmospheric CO2.

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Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models (update)

Ocean ecosystems are increasingly stressed by human-induced changes of their physical, chemical and biological environment. Among these changes, warming, acidification, deoxygenation and changes in primary productivity by marine phytoplankton can be considered as four of the major stressors of open ocean ecosystems. Due to rising atmospheric CO2 in the coming decades, these changes will be amplified. Here, we use the most recent simulations performed in the framework of the Coupled Model Intercomparison Project 5 to assess how these stressors may evolve over the course of the 21st century. The 10 Earth system models used here project similar trends in ocean warming, acidification, deoxygenation and reduced primary productivity for each of the IPCC’s representative concentration pathways (RCPs) over the 21st century. For the “business-as-usual” scenario RCP8.5, the model-mean changes in the 2090s (compared to the 1990s) for sea surface temperature, sea surface pH, global O2 content and integrated primary productivity amount to +2.73 (±0.72) °C, −0.33 (±0.003) pH unit, −3.45 (±0.44)% and −8.6 (±7.9)%, respectively. For the high mitigation scenario RCP2.6, corresponding changes are +0.71 (±0.45) °C, −0.07 (±0.001) pH unit, −1.81 (±0.31)% and −2.0 (±4.1)%, respectively, illustrating the effectiveness of extreme mitigation strategies. Although these stressors operate globally, they display distinct regional patterns and thus do not change coincidentally. Large decreases in O2 and in pH are simulated in global ocean intermediate and mode waters, whereas large reductions in primary production are simulated in the tropics and in the North Atlantic. Although temperature and pH projections are robust across models, the same does not hold for projections of subsurface O2 concentrations in the tropics and global and regional changes in net primary productivity. These high uncertainties in projections of primary productivity and subsurface oxygen prompt us to continue inter-model comparisons to understand these model differences, while calling for caution when using the CMIP5 models to force regional impact models.

Continue reading ‘Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models (update)’

Climate change and ocean acidification impacts on lower trophic levels and the export of organic carbon to the deep ocean (update)

Most future projections forecast significant and ongoing climate change during the 21st century, but with the severity of impacts dependent on efforts to restrain or reorganise human activity to limit carbon dioxide (CO2) emissions. A major sink for atmospheric CO2, and a key source of biological resources, the World Ocean is widely anticipated to undergo profound physical and – via ocean acidification – chemical changes as direct and indirect results of these emissions. Given strong biophysical coupling, the marine biota is also expected to experience strong changes in response to this anthropogenic forcing. Here we examine the large-scale response of ocean biogeochemistry to climate and acidification impacts during the 21st century for Representative Concentration Pathways (RCPs) 2.6 and 8.5 using an intermediate complexity global ecosystem model, MEDUSA-2.0. The primary impact of future change lies in stratification-led declines in the availability of key nutrients in surface waters, which in turn leads to a global decrease (1990s vs. 2090s) in ocean productivity (−6.3%). This impact has knock-on consequences for the abundance of the low trophic level biogeochemical actors modelled by MEDUSA-2.0 (−5.8%), and these would be expected to similarly impact higher trophic level elements such as fisheries. Related impacts are found in the flux of organic material to seafloor communities (−40.7% at 1000 m), and in the volume of ocean suboxic zones (+12.5%). A sensitivity analysis removing an acidification feedback on calcification finds that change in this process significantly impacts benthic communities, suggesting that a~better understanding of the OA-sensitivity of calcifying organisms, and their role in ballasting sinking organic carbon, may significantly improve forecasting of these ecosystems. For all processes, there is geographical variability in change – for instance, productivity declines −21% in the Atlantic and increases +59% in the Arctic – and changes are much more pronounced under RCP 8.5 than the RCP 2.6 scenario.

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Global warming amplified by reduced sulphur fluxes as a result of ocean acidification

Climate change and decreasing seawater pH (ocean acidification)1 have widely been considered as uncoupled consequences of the anthropogenic CO2 perturbation2, 3. Recently, experiments in seawater enclosures (mesocosms) showed that concentrations of dimethylsulphide (DMS), a biogenic sulphur compound, were markedly lower in a low-pH environment4. Marine DMS emissions are the largest natural source of atmospheric sulphur5 and changes in their strength have the potential to alter the Earth’s radiation budget6. Here we establish observational-based relationships between pH changes and DMS concentrations to estimate changes in future DMS emissions with Earth system model7 climate simulations. Global DMS emissions decrease by about 18(±3)% in 2100 compared with pre-industrial times as a result of the combined effects of ocean acidification and climate change. The reduced DMS emissions induce a significant additional radiative forcing, of which 83% is attributed to the impact of ocean acidification, tantamount to an equilibrium temperature response between 0.23 and 0.48 K. Our results indicate that ocean acidification has the potential to exacerbate anthropogenic warming through a mechanism that is not considered at present in projections of future climate change.

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Ocean acidification in the IPCC AR5 WG II

OUP book