Recent suggestions to reduce the accumulation of anthropogenic carbon dioxide in the atmosphere have included ocean fertilization by artificial upwelling. Our coupled carbon‐climate model simulations ...suggest that artificial upwelling may, under most optimistic assumptions, be able to sequester atmospheric CO2 at a rate of about 0.9 PgC/yr. However, the model predicts that about 80% of the carbon sequestered is stored on land, as a result of reduced respiration at lower air temperatures brought about by upwelling of cold waters. This remote and distributed carbon sequestration would make monitoring and verification particularly challenging. A second caveat predicted by our simulations is that whenever artificial upwelling is stopped, simulated surface temperatures and atmospheric CO2 concentrations rise quickly and for decades to centuries to levels even somewhat higher than experienced in a world that never engaged in artificial upwelling.
Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon‐cycle Model ...Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse‐resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr−1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation‐based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep‐ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep‐ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross‐model means of several biogeochemical metrics show better agreement with observation‐based estimates when restricted to those models that best simulate deep‐ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.
Spin‐up of UK Earth System Model 1 (UKESM1) for CMIP6 Yool, A.; Palmiéri, J.; Jones, C. G. ...
Journal of advances in modeling earth systems,
August 2020, 2020-08-00, 20200801, 2020-08-01, Letnik:
12, Številka:
8
Journal Article
Recenzirano
Odprti dostop
For simulations intended to study the influence of anthropogenic forcing on climate, temporal stability of the Earth's natural heat, freshwater, and biogeochemical budgets is critical. Achieving such ...coupled model equilibration is scientifically and computationally challenging. We describe the protocol used to spin‐up the UK Earth system model (UKESM1) with respect to preindustrial forcing for use in the sixth Coupled Model Intercomparison Project (CMIP6). Due to the high computational cost of UKESM1's atmospheric model, especially when running with interactive full chemistry and aerosols, spin‐up primarily used parallel configurations using only ocean/land components. For the ocean, the resulting spin‐up permitted the carbon and heat contents of the ocean's full volume to approach equilibrium over 5,000 years. On land, a spin‐up of 1,000 years brought UKESM1's dynamic vegetation and soil carbon reservoirs toward near‐equilibrium. The end‐states of these parallel ocean‐ and land‐only phases then initialized a multicentennial period of spin‐up with the full Earth system model, prior to this simulation continuing as the UKESM1 CMIP6 preindustrial control (piControl). The realism of the fully coupled spin‐up was assessed for a range of ocean and land properties, as was the degree of equilibration for key variables. Lessons drawn include the importance of consistent interface physics across ocean‐ and land‐only models and the coupled (parent) model, the extreme simulation duration required to approach equilibration targets, and the occurrence of significant regional land carbon drifts despite global‐scale equilibration. Overall, the UKESM1 spin‐up underscores the expense involved and argues in favor of future development of more efficient spin‐up techniques.
Plain Language Summary
Earth system models (ESMs) are an important tool for understandingz the Earth and for projecting how climate change may affect natural and human systems. For simulations of ESMs to separate anthropogenic influences on climate from the background state, the stability of the unperturbed system is critical. However, achieving this equilibrium is both scientifically and computationally challenging. Here, we describe how this was achieved for one such model, UKESM1, for the sixth Coupled Model Intercomparison Project (CMIP6). Due to the cost of the full model, especially when running with atmospheric chemistry and aerosols, much of UKESM1's spin‐up to equilibrium made use of ocean‐ and land‐only configurations. Millennial‐scale spin‐up phases of these component‐only models were used to initialize a final centennial‐scale phase of the full model to reach preindustrial equilibrium targets. The stability and realism of UKESM1's spun‐up state was then evaluated across a broad range of properties. A number of lessons were drawn from this spin‐up including the extreme simulation duration required to reach equilibrium. A key conclusion is the importance of developing efficient techniques to spin‐up ESMs.
Key Points
Earth system components and spin‐up protocol of UKESM1 for CMIP6 outlined
Ocean‐only (5,000 years) and land‐only (1,000 years) phases used prior to fully coupled finalizing of spin‐up (500 year)
Evaluation of spin‐up protocol presented, including cross‐component validation of piControl state and drift
The Arctic Ocean is a region that is particularly vulnerable to the impact of ocean acidification driven by rising atmospheric CO2, with potentially negative consequences for calcifying organisms ...such as coccolithophorids and foraminiferans. In this study, we use an ocean-only general circulation model, with embedded biogeochemistry and a comprehensive description of the ocean carbon cycle, to study the response of pH and saturation states of calcite and aragonite to rising atmospheric pCO2 and changing climate in the Arctic Ocean. Particular attention is paid to the strong regional variability within the Arctic, and, for comparison, simulation results are contrasted with those for the global ocean. Simulations were run to year 2099 using the RCP8.5 (an Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) scenario with the highest concentrations of atmospheric CO2). The separate impacts of the direct increase in atmospheric CO2 and indirect effects via impact of climate change (changing temperature, stratification, primary production and freshwater fluxes) were examined by undertaking two simulations, one with the full system and the other in which atmospheric CO2 was prevented from increasing beyond its preindustrial level (year 1860). Results indicate that the impact of climate change, and spatial heterogeneity thereof, plays a strong role in the declines in pH and carbonate saturation (Ω) seen in the Arctic. The central Arctic, Canadian Arctic Archipelago and Baffin Bay show greatest rates of acidification and Ω decline as a result of melting sea ice. In contrast, areas affected by Atlantic inflow including the Greenland Sea and outer shelves of the Barents, Kara and Laptev seas, had minimal decreases in pH and Ω because diminishing ice cover led to greater vertical mixing and primary production. As a consequence, the projected onset of undersaturation in respect to aragonite is highly variable regionally within the Arctic, occurring during the decade of 2000–2010 in the Siberian shelves and Canadian Arctic Archipelago, but as late as the 2080s in the Barents and Norwegian seas. We conclude that, for future projections of acidification and carbonate saturation state in the Arctic, regional variability is significant and needs to be adequately resolved, with particular emphasis on reliable projections of the rates of retreat of the sea ice, which are a major source of uncertainty.
Ocean fertilization: a potential means of geoengineering? Lampitt, R.S; Achterberg, E.P; Anderson, T.R ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
11/2008, Letnik:
366, Številka:
1882
Journal Article
Recenzirano
Odprti dostop
The oceans sequester carbon from the atmosphere partly as a result of biological productivity. Over much of the ocean surface, this productivity is limited by essential nutrients and we discuss ...whether it is likely that sequestration can be enhanced by supplying limiting nutrients. Various methods of supply have been suggested and we discuss the efficacy of each and the potential side effects that may develop as a result. Our conclusion is that these methods have the potential to enhance sequestration but that the current level of knowledge from the observations and modelling carried out to date does not provide a sound foundation on which to make clear predictions or recommendations. For ocean fertilization to become a viable option to sequester CO2, we need more extensive and targeted fieldwork and better mathematical models of ocean biogeochemical processes. Models are needed both to interpret field observations and to make reliable predictions about the side effects of large-scale fertilization. They would also be an essential tool with which to verify that sequestration has effectively taken place. There is considerable urgency to address climate change mitigation and this demands that new fieldwork plans are developed rapidly. In contrast to previous experiments, these must focus on the specific objective which is to assess the possibilities of CO2 sequestration through fertilization.
The Beaufort Gyre is a key feature of the Arctic Ocean, acting as a reservoir for freshwater in the region. Depending on whether the prevailing atmospheric circulation in the Arctic is anticyclonic ...or cyclonic, either a net accumulation or release of freshwater occurs. The sources of freshwater to the Arctic Ocean are well established and include contributions from the North American and Eurasian Rivers, the Bering Strait Pacific water inflow, sea ice meltwater, and precipitation, but their contribution to the Beaufort Gyre freshwater accumulation varies with changes in the atmospheric circulation. Here we use a Lagrangian backward tracking technique in conjunction with the 1/12‐degree resolution Nucleus for European Modelling of the Ocean model to investigate how sources of freshwater to the Beaufort Gyre have changed in recent decades, focusing on increase in the Pacific water content in the gyre between the late 1980s and early 2000s. Using empirical orthogonal functions we analyze the change in the Arctic oceanic circulation that occurred between the 1980s and 2000s. We highlight a “waiting room” advective pathway that was present in the 1980s and provide evidence that this pathway was caused by a shift in the center of Ekman transport convergence in the Arctic. We discuss the role of these changes as a contributing factor to changes in the stratification, and hence potentially the biology, of the Beaufort Gyre region.
Plain Language Summary
The Beaufort Gyre, a clockwise ice and water circulation in the Arctic Ocean, is an important feature of the Arctic because it stores a large volume of fresh—relative to the rest of the ocean—water. Depending on the atmospheric circulation driving it, the Beaufort Gyre can either accumulate or release this freshwater. The sources of relatively freshwater to the Beaufort Gyre are Arctic rivers, the Bering Strait, and melting sea ice. By tracking virtual particles in a high‐resolution ocean model, we investigate how these sources have changed in recent decades, and identify a change in the pathways bringing them to the Beaufort Gyre. This change in ocean circulation was found to correlate with a change in the mixed layer depth in the model.
Key Points
Lagrangian particle tracking technique used to investigate advective pathways associated with different sources of Beaufort Gyre water
Change in advective pathways associated with Pacific inflow to the gyre occurred between 1980s and 2000s
This change in pathways correlates with a shoaling of the modeled mixed layer depth in the Beaufort Gyre region
Artificial ocean iron fertilization (OIF) enhances phytoplankton productivity and is being explored as a means of sequestering anthropogenic carbon within the deep ocean. To be considered successful, ...carbon should be exported from the surface ocean and isolated from the atmosphere for an extended period (e.g., the Intergovernmental Panel on Climate Change's standard 100 year time horizon). This study assesses the impact of deep circulation on carbon sequestered by OIF in the Southern Ocean, a high‐nutrient low‐chlorophyll region known to be iron stressed. A Lagrangian particle‐tracking approach is employed to analyze water mass trajectories over a 100 year simulation. By the end of the experiment, for a sequestration depth of 1000 m, 66% of the carbon had been reexposed to the atmosphere, taking an average of 37.8 years. Upwelling occurs predominately within the Antarctic Circumpolar Current due to Ekman suction and topography. These results emphasize that successful OIF is dependent on the physical circulation, as well as the biogeochemistry.
Key Points
Carbon export to 1000 m does not guarantee sequestration in the Southern Ocean
OIF requires consideration of the circulation as well as biogeochemistry
Sequestered carbon is dispersed widely out of the Southern Ocean by circulation
Axonal degeneration contributes to clinical disability in the acquired demyelinating disease multiple sclerosis. Axonal degeneration occurs during acute attacks, associated with inflammation, and ...during the chronic progressive phase of the disease in which inflammation is not prominent. To explore the importance of interactions between oligodendrocytes and axons in the CNS, we analysed the brains of rodents and humans with a null mutation in the gene encoding the major CNS myelin protein, proteolipid protein (PLP1, previously PLP). Histological analyses of the CNS of Plp1 null mice and of autopsy material from patients with null PLP1 mutations were performed to evaluate axonal and myelin integrity. In vivo proton magnetic resonance spectroscopy (MRS) of PLP1 null patients was conducted to measure levels of N-acetyl aspartate (NAA), a marker of axonal integrity. Length-dependent axonal degeneration without demyelination was identified in the CNS of Plp1 null mice. Proton MRS of PLP1-deficient patients showed reduced NAA levels, consistent with axonal loss. Analysis of patients' brain tissue also demonstrated a length-dependent pattern of axonal loss without significant demyelination. Therefore, axonal degeneration occurs in humans as well as mice lacking the major myelin protein PLP1. This degeneration is length-dependent, similar to that found in the PNS of patients with the inherited demyelinating neuropathy, CMT1A, but is not associated with significant demyelination. Disruption of PLP1-mediated axonal--glial interactions thus probably causes this axonal degeneration. A similar mechanism may be responsible for axonal degeneration and clinical disability that occur in patients with multiple sclerosis.
The Arctic Ocean is rapidly changing. With warming waters, receding sea ice, and changing circulation patterns, it has been hypothesized that previously closed ecological pathways between the Pacific ...and Atlantic Oceans will be opened as we move toward a seasonally ice‐free Arctic. The discovery of the Pacific diatom Neodenticula seminae in the Atlantic suggests that a tipping point may have already been reached and this “opening up” of the Arctic could already be underway. Here, we investigate how circulation connectivity between the Pacific and Atlantic Oceans has changed in recent decades, using a state‐of‐the‐art high‐resolution ocean model and a Lagrangian particle‐tracking method. We identify four main trans‐Arctic pathways and a fifth route that is sporadically available with a shorter connectivity timescale. We discuss potential explanations for the existence of this “shortcut” advective pathway, linking it to a shift in atmospheric and oceanic circulation regimes. Advective timescales associated with each route are quantified, and seasonal and interannual trends in the main four pathways are discussed, including an increase in Fram Strait outflow relative to the Canadian Archipelago. In conclusion, we note that while tipping points for ecological connectivity are species dependent, even the most direct routes require multiannual connectivity timescales.
Plain Language Summary
With a warming Arctic Ocean, it has been suggested that the ocean currents that connect the Pacific to the Atlantic may change. This could have potential biological consequences, including bringing Pacific species of plankton to the Atlantic. We investigate how the pathways bringing Pacific water to the Atlantic have changed, identify a pathway that takes less time that other routes to bring waters from Pacific to the Atlantic (but that is only occasionally available), and note that even the shortest timescales are over 2 years.
Key Points
The shortest Pacific to Atlantic connectivity timescale is >2 years, so tipping point for a species requiring single‐summer transit is highly improbable
Advective pathway associated with shortest connectivity timescales is via Barrow Canyon and Canadian Archipelago, but is only sporadically available
A sporadically available pathway was linked to the anomalously cyclonic Arctic
Sea‐ice‐free summers are projected to become a prominent feature of the Arctic environment in the coming decades. From a shipping perspective, this means larger areas of open water in the summer, ...thinner and less compact ice all year round, and longer operating seasons. Therefore, the possibility for easier navigation along trans‐Arctic shipping routes arises. The Northern Sea Route (NSR) is one trans‐Arctic route, and it offers a potential 10 day shortcut between Western Europe and the Far East. More ships transiting the NSR means an increased risk of an accident, and associated oil spill, occurring. Previous research suggests that current infrastructure is insufficient for increased shipping. Therefore, should an oil spill occur, the window for a successful clean‐up will be short. In the event of a failed recovery, the long‐term fate of the unrecovered pollutants must be considered, at least until the next melt season when it could become accessible again. Here we investigate the role of oceanic advection in determining the long‐term fate of Arctic pollutants using a high‐resolution ocean model along with Lagrangian particle‐tracking to simulate the spread of pollutants. The resulting “advective footprints” of pollutants are proposed as an informative metric for analyzing such experiments. We characterize the circulation along different parts of the NSR, defining three main regions in the Eurasian Arctic, and relate the distinctive circulation pathways of each to the long‐term fate of spilled oil. We conclude that a detailed understanding of ocean circulation is critical for determining the long‐term fate of Arctic pollutants.
Plain Language Summary
The Earth's climate is changing and the Arctic Ocean is projected to experience ice free summers within decades. This would enable more commercial shipping, which in turn makes an Arctic shipping accident more likely. This could lead to oil (or other pollutants) being spilled into the ocean. Because of the harsh Arctic environment, an oil spill may not be successfully recovered, so we need to consider where it will go in the following months and years. We released virtual “particles” into a computer model of the ocean and tracked their progress for 2 years. In this time, particles traveled, on average, 1,223 km. This demonstrates that pan‐Arctic modeling is needed in the event of an unrecovered pollutant spill. Unrecovered oil from one season may be accessible the next spring. By analyzing the spread of our particles, we found that on average 676,917 km2 would need to be searched to find it, but that this is highly dependent on where the spill occurs. Finally, we noted that in some places, particularly the Barents Sea, there was a risk that spilled pollutants could become entrained into deep water, rendering them irrecoverable.
Key Points
Pan‐Arctic modeling of advective pathways is necessary to understand the long‐term (2 years) fate of pollutants spilled in the Arctic Ocean
Long‐term fate of pollutants dependent on where the spill occurs: the Northern Sea Route can be split into three main regimes
Subduction into deep water could render pollutants irrecoverable. This is a risk especially for spills occurring in the Barents Sea
PDF
