Posts Tagged 'Arctic'

Sea-ice loss amplifies summertime decadal CO2 increase in the western Arctic Ocean

Rapid climate warming and sea-ice loss have induced major changes in the sea surface partial pressure of CO2 (pCO2pCO2). However, the long-term trends in the western Arctic Ocean are unknown. Here we show that in 1994–2017, summer pCO2pCO2 in the Canada Basin increased at twice the rate of atmospheric increase. Warming and ice loss in the basin have strengthened the pCO2pCO2 seasonal amplitude, resulting in the rapid decadal increase. Consequently, the summer air–sea CO2 gradient has reduced rapidly, and may become near zero within two decades. In contrast, there was no significant pCO2pCO2 increase on the Chukchi Shelf, where strong and increasing biological uptake has held pCO2pCO2 low, and thus the CO2 sink has increased and may increase further due to the atmospheric CO2 increase. Our findings elucidate the contrasting physical and biological drivers controlling sea surface pCO2pCO2 variations and trends in response to climate change in the Arctic Ocean.

Continue reading ‘Sea-ice loss amplifies summertime decadal CO2 increase in the western Arctic Ocean’

Emergent constraint on Arctic Ocean acidification in the twenty-first century

The ongoing uptake of anthropogenic carbon by the ocean leads to ocean acidification, a process that results in a reduction in pH and in the saturation state of biogenic calcium carbonate minerals aragonite (Ωarag) and calcite (Ωcalc)1,2. Because of its naturally low Ωarag and Ωcalc (refs. 2,3), the Arctic Ocean is considered the region most susceptible to future acidification and associated ecosystem impacts4,5,6,7. However, the magnitude of projected twenty-first century acidification differs strongly across Earth system models8. Here we identify an emergent multi-model relationship between the simulated present-day density of Arctic Ocean surface waters, used as a proxy for Arctic deep-water formation, and projections of the anthropogenic carbon inventory and coincident acidification. By applying observations of sea surface density, we constrain the end of twenty-first century Arctic Ocean anthropogenic carbon inventory to 9.0 ± 1.6 petagrams of carbon and the basin-averaged Ωarag and Ωcalc to 0.76 ± 0.06 and 1.19 ± 0.09, respectively, under the high-emissions Representative Concentration Pathway 8.5 climate scenario. Our results indicate greater regional anthropogenic carbon storage and ocean acidification than previously projected3,8 and increase the probability that large parts of the mesopelagic Arctic Ocean will be undersaturated with respect to calcite by the end of the century. This increased rate of Arctic Ocean acidification, combined with rapidly changing physical and biogeochemical Arctic conditions9,10,11, is likely to exacerbate the impact of climate change on vulnerable Arctic marine ecosystems.

Continue reading ‘Emergent constraint on Arctic Ocean acidification in the twenty-first century’

Fish embryo vulnerability to combined acidification and warming coincides with low capacity for homeostatic regulation

The vulnerability of fish embryos and larvae to environmental factors is often attributed to a lack of adult-like organ systems (gills) and thus insufficient homeostatic capacity. However, experimental data supporting this hypothesis are scarce. Here, by using Atlantic cod (Gadus morhua) as a model, the relationship between embryo vulnerability (to projected ocean acidification and warming) and homeostatic capacity was explored through parallel analyses of stage-specific mortality and in vitro activity and expression of major ion pumps (ATP-Synthase, Na+/K+-ATPase, H+-ATPase) and co-transporters (NBC1, NKCC1). Immunolocalization of these transporters was used to study ionocyte morphology in newly-hatched larvae. Treatment-related embryo mortality until hatch (+20% due to acidification and warming) occurred primarily during an early period (gastrulation) characterized by extremely low ion transport capacities. Thereafter, embryo mortality decreased in parallel with an exponential increase in activity and expression of all investigated ion transporters. Significant changes in transporter activity and expression in response to acidification (+15% activity) and warming (-30% expression) indicate some potential for short-term acclimatization, although likely associated with energetic trade-offs. Interestingly, whole-larvae enzyme capacities (supported by abundant epidermal ionocytes) reached levels similar to those previously measured in gill tissue of adult cod, suggesting that early-life stages without functional gills are better equipped in terms of ion homeostasis than previously thought. This study implies that the gastrulation period represents a critical transition from inherited (maternal) defenses to active homeostatic regulation, which facilitates enhanced resilience of later stages to environmental factors.

Continue reading ‘Fish embryo vulnerability to combined acidification and warming coincides with low capacity for homeostatic regulation’

Ocean warming and acidification may drag down the commercial Arctic cod fishery by 2100

The Arctic Ocean is an early warning system for indicators and effects of climate change. We use a novel combination of experimental and time-series data on effects of ocean warming and acidification on the commercially important Northeast Arctic cod (Gadus morhua) to incorporate these physiological processes into the recruitment model of the fish population. By running an ecological-economic optimization model, we investigate how the interaction of ocean warming, acidification and fishing pressure affects the sustainability of the fishery in terms of ecological, economic, social and consumer-related indicators, ranging from present day conditions up to future climate change scenarios. We find that near-term climate change will benefit the fishery, but under likely future warming and acidification this large fishery is at risk of collapse by the end of the century, even with the best adaptation effort in terms of reduced fishing pressure.

Continue reading ‘Ocean warming and acidification may drag down the commercial Arctic cod fishery by 2100’

Effect of terrestrial organic matter on ocean acidification and CO2 flux in an Arctic Shelf Sea

Highlights

• In Hudson Bay, a seasonally ice-covered shelf, remineralization of terrestrial organic carbon significantly reduces aragonite-saturation and increases pCO2.

• The effect of terrestrial carbon is offset by marine export production and dissolution of terrestrial calcium carbonate in surface and deep waters, respectively.

• Effects of terrestrial carbon delivery depend on watershed characteristics and the carbon:nitrogen ratio of organic matter.

Abstract

Recent research has focused on the changing ability of oceans to absorb atmospheric CO2 and the consequences for ocean acidification, with Arctic shelf seas being among the most sensitive regions. Hudson Bay is a large shelf sea in northern Canada whose location at the margin of the cryosphere places it in the vanguard of global climate change. Here, we develop a four-compartment box-model and carbon budget using published and recently collected measurements to estimate carbon inputs, transformations, and losses within Hudson Bay. We estimate the annual effects of terrestrial carbon remineralization on aragonite saturation (ΩAr, a proxy for ocean acidification) and on the partial pressure of CO2 (pCO2, a proxy for air-sea CO2 flux) within each compartment, as well as the effects of marine primary production, marine organic carbon remineralization, and terrestrial calcium carbonate dissolution. We find that the remineralization of terrestrial dissolved organic carbon is the main driver of CO2 accumulation and aragonite under-saturation in coastal surface waters, but this is largely offset by marine primary production. Below the surface mixed layer, marine organic carbon remineralization is the largest contributor to CO2 accumulation and aragonite under-saturation, and is partially offset by terrestrial CaCO3 dissolution. Overall, the annual delivery and processing of carbon reduces ΩAr of water flowing through HB by up to 0.17 units and raises pCO2 by up to 165 µatm. The similarities between Hudson Bay and other Arctic shelf seas suggest these areas are also significantly influenced by terrestrial carbon inputs and transformation.

Continue reading ‘Effect of terrestrial organic matter on ocean acidification and CO2 flux in an Arctic Shelf Sea’

Simulation of factors affecting Emiliania huxleyi blooms in Arctic and sub-Arctic seas by CMIP5 climate models: model validation and selection

The observed warming in the Arctic is more than double the global average, and this enhanced Arctic warming is projected to continue throughout the 21st century. This rapid warming has a wide range of impacts on polar and sub-polar marine ecosystems. One of the examples of such an impact on ecosystems is that of coccolithophores, particularly Emiliania huxleyi, which have expanded their range poleward during recent decades. The coccolithophore E. huxleyi plays an essential role in the global carbon cycle. Therefore, the assessment of future changes in coccolithophore blooms is very important.

Currently, there are a large number of climate models that give projections for various oceanographic, meteorological, and biochemical variables in the Arctic. However, individual climate models can have large biases when compared to historical observations. The main goal of this research was to select an ensemble of climate models that most accurately reproduces the state of environmental variables that influence the coccolithophore E. huxleyi bloom over the historical period when compared to reanalysis data. We developed a novel approach for model selection to include a diverse set of measures of model skill including the spatial pattern of some variables, which had not previously been included in a model selection procedure. We applied this method to each of the Arctic and sub-Arctic seas in which E. huxleyi blooms have been observed. Once we have selected an optimal combination of climate models that most skilfully reproduce the factors which affect E. huxleyi, the projections of the future conditions in the Arctic from these models can be used to predict how E. huxleyi blooms will change in the future.

Here, we present the validation of 34 CMIP5 (fifth phase of the Coupled Model Intercomparison Project) atmosphere–ocean general circulation models (GCMs) over the historical period 1979–2005. Furthermore, we propose a procedure of ranking and selecting these models based on the model’s skill in reproducing 10 important oceanographic, meteorological, and biochemical variables in the Arctic and sub-Arctic seas. These factors include the concentration of nutrients (NO3, PO4, and SI), dissolved CO2 partial pressure (pCO2), pH, sea surface temperature (SST), salinity averaged over the top 30 m (SS30 m), 10 m wind speed (WS), ocean surface current speed (OCS), and surface downwelling shortwave radiation (SDSR). The validation of the GCMs’ outputs against reanalysis data includes analysis of the interannual variability, seasonal cycle, spatial biases, and temporal trends of the simulated variables. In total, 60 combinations of models were selected for 10 variables over six study regions using the selection procedure we present here. The results show that there is neither a combination of models nor one model that has high skill in reproducing the regional climatic-relevant features of all combinations of the considered variables in target seas. Thereby, an individual subset of models was selected according to our model selection procedure for each combination of variable and Arctic or sub-Arctic sea. Following our selection procedure, the number of selected models in the individual subsets varied from 3 to 11.

The paper presents a comparison of the selected model subsets and the full-model ensemble of all available CMIP5 models to reanalysis data. The selected subsets of models generally show a better performance than the full-model ensemble. Therefore, we conclude that within the task addressed in this study it is preferable to employ the model subsets determined through application of our procedure than the full-model ensemble.

Continue reading ‘Simulation of factors affecting Emiliania huxleyi blooms in Arctic and sub-Arctic seas by CMIP5 climate models: model validation and selection’

Higher sensitivity towards light stress and ocean acidification in an Arctic sea‐ice associated diatom compared to a pelagic diatom

Thalassiosira hyalina and Nitzschia frigida are important members of Arctic pelagic and sympagic (sea‐ice associated) diatom communities. We investigated the effects of light stress (shift from 20 to 380 µmol photons m‐2 s‐1, resembling upwelling or ice break‐up) under contemporary and future pCO2 (400 vs. 1000 µatm).

The responses in growth, elemental composition, pigmentation and photophysiology were followed over 120 h and are discussed together with underlying gene expression patterns.

Stress response and subsequent re‐acclimation were efficiently facilitated by T. hyalina, which showed only moderate changes in photophysiology and elemental composition, and thrived under high‐light after 120 h. In N. frigida, photochemical damage and oxidative stress appeared to outweigh cellular defenses, causing dysfunctional photophysiology and reduced growth. pCO2 alone did not specifically influence gene expression, but amplified the transcriptomic reactions to light stress, indicating that pCO2 affects metabolic equilibria rather than sensitive genes.

Large differences in acclimation capacities towards high‐light and high pCO2 between T. hyalina and N. frigida indicate species‐specific mechanisms in coping with the two stressors, which may reflect their respective ecological niches. This could potentially alter the balance between sympagic vs. pelagic primary production in a future Arctic.

Continue reading ‘Higher sensitivity towards light stress and ocean acidification in an Arctic sea‐ice associated diatom compared to a pelagic diatom’

Freshening of the western Arctic negates anthropogenic carbon uptake potential

As human activities increase the atmospheric concentration of carbon dioxide (CO2), the oceans are known to absorb a significant portion. The Arctic Ocean has long been considered to have enormous potential to sequester anthropogenic CO2, and mitigate emissions. The frigid waters make CO2 more soluble, and as sea ice melts, greater surface area is exposed to absorb CO2. However, sparse data have made quantifying the amount of anthropogenic CO2 in the Arctic difficult, stimulating much debate over the basin’s contribution to CO2 sequestration from the atmosphere. Using three separate cruises in 1994, 2005, and 2015 in the Canada and Makarov basins, we analyze the decadal variability in anthropogenic CO2 uptake in the central western Arctic. Here we show, from direct carbon system measurements spanning two decades, that despite increased atmospheric CO2, total dissolved inorganic carbon has actually decreased, with minimal anthropogenic CO2 uptake. The reduction in dissolved CO2 results from a dilution of total alkalinity by increased freshwater supply, particularly river water. Changes in the freshwater budget of the western Arctic override its uptake potential, resulting in a weak sink, or possibly source of CO2.

Continue reading ‘Freshening of the western Arctic negates anthropogenic carbon uptake potential’

The recent state and variability of the carbonate system of the Canadian Arctic in the context of ocean acidification

Ocean acidification driven by the uptake of anthropogenic CO2 by the surface oceans constitutes a potential threat to the health of marine ecosystems around the globe. The Arctic Ocean is particularly vulnerable to acidification due to its relatively low buffering capacity and, thus, is an ideal region to study the progression and effects of acidification before they become globally widespread. The appearance of undersaturated surface waters with respect to the carbonate mineral aragonite (ΩA < 1), an important threshold beyond which the calcification and growth of some marine organisms might be hindered, has recently been documented in the Canada Basin and adjacent Canadian Arctic Archipelago. Nonetheless, few of these observations were made in the last five years and the spatial coverage in the latter region is poor. Additionally, the strong variability inherent to this dynamic shelf environment renders the temporal imprint of ocean acidification on carbonate system parameters (pH, pCO2, DIC, Ω) virtually indistinguishable on decadal timescales. We use a dataset of carbonate system parameters measured in Canadian Arctic Archipelago (CAA) and its adjacent basins to describe the recent state of these parameters across the Canadian Arctic and investigate the amplitude and sources of the system's variability. Our findings reveal that, in addition to the surface of the Canada Basin, the entire water column of the Queen Maud Gulf was undersaturated with respect to aragonite in 2015 and 2016. We also estimate that approximately a third of the interannual variability in surface DIC in the CAA results from fluctuations in biological activity.

Continue reading ‘The recent state and variability of the carbonate system of the Canadian Arctic in the context of ocean acidification’

Origin and accumulation of an anthropogenic CO2 and 13C Suess effect in the Arctic Ocean

We determined the impact of anthropogenic CO2 (Cant) accumulation on the δ13C of dissolved inorganic carbon (DIC) in the Arctic Ocean (i.e., the 13C Suess effect) based on δ13C measurements during a GEOTRACES cruise in 2015. The δ13C decrease was estimated from the amount of Cant change derived by the transit time distribution (TTD) approach and the ratio of the anthropogenic δ13C/DIC change (RC). A significant Cant increase (up to 45 μmol kg−1) and δ13C decrease (up to −0.9‰) extends to ~2000 m in the Canada and Makarov Basin. We find distinctly different RC values for the intermediate water (300–2000 m) and upper halocline water (<200 m) of −0.020 and −0.012‰ (μmol kg−1)−1, respectively, which identifies two sources of Cant accumulation from North Atlantic and North Pacific. Furthermore, estimated RC for intermediate waters is the same as the RC observed in the Greenland Sea and the rate of anthropogenic DIC increase estimated for intermediate waters at 0.9 μmol kg−1 yr−1 is identical to the estimated rate in the Iceland Sea. These observations indicate that the high rate of Cant accumulation and δ13C decrease in the Arctic Ocean is primarily a result of the input of Cant, via ventilation of intermediate waters, from the Nordic Sea rather than local anthropogenic CO2 uptake within the Arctic Basin. We determine the preindustrial δ13C (δ13CPI) distributions and find distinct δ13CPI signatures of the intermediate and upper halocline waters that reflect the difference in δ13CPI–PO4 relationship of Atlantic and Pacific source water.

Continue reading ‘Origin and accumulation of an anthropogenic CO2 and 13C Suess effect in the Arctic Ocean’


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