The Smithian-Spathian transition of the late Early Triassic was a critical period of environmental and biological upheavals, following the end-Permian mass extinction. Changes in carbonate deposition during this period have been attributed to intensified upwelling along shelf margins, but relevant studies are scarce. Here, we present calcium isotopes of bulk marine carbonate (δ44/40Cacarb) from a Smithian–Spathian boundary (SSB) succession (Guryul Ravine section, Kashmir) on the southern margin of the Neo-Tethys. Our smoothed δ44/40Cacarb curve reveals a ~ 0.2‰ negative shift (from ~ − 1.1‰ to ~ − 1.3‰) across the SSB, concurrent with a ~ +10‰ shift in δ13Ccarb. While increased Ca isotopic fractionation could play a role, we specifically examine potential impacts due to changes in marine Ca fluxes. Using a Ca-cycle mass balance model, we explore scenarios of decreased carbonate burial flux (Fcarb), decreased riverine flux (Friv), and a combination of these processes. The modeling suggest that a pulse decrease in Fcarb by 40% over ~0.06 Myr match the negative shift in δ44/40Cacarb at Guryul Ravine. We infer that this decrease was likely related to intensified upwelling of acidic deep seawater due to invigorated global-oceanic circulation during the SSB cooling event. We suggest that the regionally diverse excursions in δ44/40Cacarb in the Tethyan region could be attributed to spatially varied upwellings in the shelf margin. The upwelling of acidic and anoxic deep seawater may have driven the second-order extinction of ammonoids and conodonts at the beginning of the SSB cooling event.
Continue reading ‘Calcium isotopes reveal shelf acidification on southern Neotethyan margin during the Smithian-Spathian boundary cooling event’Posts Tagged 'globalmodeling'
Calcium isotopes reveal shelf acidification on southern Neotethyan margin during the Smithian-Spathian boundary cooling event
Published 15 May 2023 Science ClosedTags: biogeochemistry, chemistry, globalmodeling, modeling, paleo
CMEMS-LSCE: a global 0.25-degree, monthly reconstruction of the surface ocean carbonate system
Published 12 May 2023 Science ClosedTags: chemistry, globalmodeling, methods, modeling
Observation-based data reconstructions of global surface ocean carbonate system variables play an essential role in monitoring the recent status of ocean carbon uptake and ocean acidification as well as their impacts on marine organisms and ecosystems. So far ongoing efforts are directed towards exploring new approaches to describe the complete marine carbonate system and to better recover its fine-scale features. In this respect, our research activities within the Copernicus Marine Environment Monitoring Service (CMEMS) aim at developing a sustainable production chain of observation-derived global ocean carbonate system datasets at high space-time resolution. As the start of the long-term objective, this study introduces a new global 0.25° monthly reconstruction, namely CMEMS-LSCE, for the period 1985–2021. The CMEMS-LSCE reconstruction derives datasets of six carbonate system variables including surface ocean partial pressure of CO2 (pCO2), total alkalinity (AT), total dissolved inorganic carbon (DIC), surface ocean pH, and saturation states with respect to aragonite (Ωar) and calcite (Ωca). Reconstructing pCO2 relies on an ensemble of neural network models mapping gridded observation-based data provided by the Surface Ocean CO2 ATlas (SOCAT). Surface ocean AT is estimated with a multiple linear regression approach, and the remaining carbonate variables are resolved by CO2 system speciation given the reconstructed pCO2 and AT. 1σ-uncertainty associated with these estimates is also provided. Here, σ stands for either ensemble standard deviation of pCO2 estimates or total uncertainty for each of the five other variables propagated through the processing chain with input data uncertainty. We demonstrate that the 0.25°-resolution pCO2 product outperforms a coarser spatial resolution (1°) thanks to a higher data coverage nearshore and a better description of horizontal and temporal variations in pCO2 across diverse ocean basins, particularly in the coastal-open-ocean continuum. Product qualification with observation-based data confirms reliable reconstructions with root-of-mean–square–deviation from observations less than 8 %, 4 %, and 1 % relative to the global mean of pCO2, AT (DIC), and pH. The global average 1σ-uncertainty is below 5 % and 8 % for pCO2 and Ωar (Ωca), 2 % for AT and DIC, and 0.4 % for pH relative to their global mean values. Both model-observation misfit and model uncertainty indicate that coastal data reproduction still needs further improvement, wherein high temporal and horizontal gradients of carbonate variables and representative uncertainty from data sampling would be taken into account in priority. This study also presents a potential use case of the CMEMS-LSCE carbonate data product in tracking the recent state of ocean acidification.
Continue reading ‘CMEMS-LSCE: a global 0.25-degree, monthly reconstruction of the surface ocean carbonate system’Response of ocean acidification to atmospheric carbon dioxide removal
Published 9 May 2023 Science ClosedTags: chemistry, globalmodeling, mitigation, modeling
Artificial CO2 removal from the atmosphere (also referred to as negative CO2 emissions) has been proposed as a potential means to counteract anthropogenic climate change. Here we use an Earth system model to examine the response of ocean acidification to idealized atmospheric CO2 removal scenarios. In our simulations, atmospheric CO2 is assumed to increase at a rate of 1% per year to four times its pre-industrial value and then decreases to the pre-industrial level at a rate of 0.5%, 1%, 2% per year, respectively. Our results show that the annual mean state of surface ocean carbonate chemistry fields including hydrogen ion concentration ([H+]), pH and aragonite saturation state respond quickly to removal of atmospheric CO2. However, the change of seasonal cycle in carbonate chemistry lags behind the decline in atmospheric CO2. When CO2 returns to the pre-industrial level, over some parts of the ocean, relative to the pre-industrial state, the seasonal amplitude of carbonate chemistry fields is substantially larger. Simulation results also show that changes in deep ocean carbonate chemistry substantially lag behind atmospheric CO2 change. When CO2 returns to its pre-industrial value, the whole-ocean acidity measured by [H+] is 15%-18% larger than the pre-industrial level, depending on the rate of CO2 decrease. Our study demonstrates that even if atmospheric CO2 can be lowered in the future as a result of net negative CO2 emissions, the recovery of some aspects of ocean acidification would take decades to centuries, which would have important implications for the resilience of marine ecosystems.
Continue reading ‘Response of ocean acidification to atmospheric carbon dioxide removal’Robust estimates of the total alkalinity from satellite oceanographic data in the global ocean
Published 8 May 2023 Science ClosedTags: chemistry, field, globalmodeling, methods, modeling
Total alkalinity (TA) is a key parameter to understand the dynamics of biogeochemical properties in the global ocean and the effects of climate change on ocean acidification, ocean carbon cycle, and carbonate chemistry. To date, global surface ocean distributions of TA were investigated using multiple regional regression approaches which require smoothening techniques due to severe boundary effects in different oceanic regions/basins across latitudes/longitudes. To reduce the uncertainties and produce spatially and temporally consistent TA products, a novel single linear regression (SLR) approach was developed in this study to estimate TA fields in the global surface ocean waters. The SLR formulation was derived using the continuous in-situ measurements of sea surface salinity (SSS) collected from the different oceans. The performance of the SLR was assessed using independent in-situ/satellite derived TA data and the results from three existing algorithms. In general, the SLR-based global surface ocean TA fields from both in-situ and satellite data agreed well with in-situ measured TA data with a mean relative error less than 1%, which is much lower compared to the error with the existing algorithms. Studies were also conducted to examine the spatiotemporal variability and trends in the global surface ocean climatology of SSS and TA fields in the context of current climate change impacts.
Continue reading ‘Robust estimates of the total alkalinity from satellite oceanographic data in the global ocean’The representation of alkalinity and the carbonate pump from CMIP5 to CMIP6 Earth system models and implications for the carbon cycle (update)
Published 3 May 2023 Science ClosedTags: chemistry, globalmodeling, methods, modeling
Ocean alkalinity is critical to the uptake of atmospheric carbon in surface waters and provides buffering capacity towards the associated acidification. However, unlike dissolved inorganic carbon (DIC), alkalinity is not directly impacted by anthropogenic carbon emissions. Within the context of projections of future ocean carbon uptake and potential ecosystem impacts, especially through Coupled Model Intercomparison Projects (CMIPs), the representation of alkalinity and the main driver of its distribution in the ocean interior, the calcium carbonate cycle, have often been overlooked. Here we track the changes from CMIP5 to CMIP6 with respect to the Earth system model (ESM) representation of alkalinity and the carbonate pump which depletes the surface ocean in alkalinity through biological production of calcium carbonate and releases it at depth through export and dissolution. We report an improvement in the representation of alkalinity in CMIP6 ESMs relative to those in CMIP5, with CMIP6 ESMs simulating lower surface alkalinity concentrations, an increased meridional surface gradient and an enhanced global vertical gradient. This improvement can be explained in part by an increase in calcium carbonate (CaCO3) production for some ESMs, which redistributes alkalinity at the surface and strengthens its vertical gradient in the water column. We were able to constrain a particulate inorganic carbon (PIC) export estimate of 44–55 Tmol yr−1 at 100 m for the ESMs to match the observed vertical gradient of alkalinity. Reviewing the representation of the CaCO3 cycle across CMIP5/6, we find a substantial range of parameterizations. While all biogeochemical models currently represent pelagic calcification, they do so implicitly, and they do not represent benthic calcification. In addition, most models simulate marine calcite but not aragonite. In CMIP6, certain model groups have increased the complexity of simulated CaCO3 production, sinking, dissolution and sedimentation. However, this is insufficient to explain the overall improvement in the alkalinity representation, which is therefore likely a result of marine biogeochemistry model tuning or ad hoc parameterizations. Although modellers aim to balance the global alkalinity budget in ESMs in order to limit drift in ocean carbon uptake under pre-industrial conditions, varying assumptions related to the closure of the budget and/or the alkalinity initialization procedure have the potential to influence projections of future carbon uptake. For instance, in many models, carbonate production, dissolution and burial are independent of the seawater saturation state, and when considered, the range of sensitivities is substantial. As such, the future impact of ocean acidification on the carbonate pump, and in turn ocean carbon uptake, is potentially underestimated in current ESMs and is insufficiently constrained.
Continue reading ‘The representation of alkalinity and the carbonate pump from CMIP5 to CMIP6 Earth system models and implications for the carbon cycle (update)’Climate change and the sea: a major disruption in steady state and the master variables
Published 26 April 2023 Science ClosedTags: Antarctic, Arctic, chemistry, field, globalmodeling, modeling
Since the beginning of the industrial revolution, humans have burned enormous quantities of coal, oil, and natural gas, rivaling nature’s elemental cycles of C, N, and S. The result has been a disruption in a steady state of CO2 and other greenhouse gases in the atmosphere, a warming of the planet, and changes in master variables (temperature, pH, and pε) of the sea affecting critical physical, chemical, and biological reactions. Humans have also produced copious quantities of N and P fertilizers producing widespread coastal hypoxia and low dissolved oxygen conditions, which now threaten even the open ocean. Consequently, our massive alteration of state variables diminishes coral reefs, fisheries, and marine ecosystems, which are the foundation of life on Earth. We point to a myriad of actions and alternatives which will help to stem the tide of climate change and its effects on the sea while, at the same time, creating a more sustainable future for humans and ecosystems alike.
Continue reading ‘Climate change and the sea: a major disruption in steady state and the master variables’Long-term slowdown of ocean carbon uptake by alkalinity dynamics
Published 17 April 2023 Science ClosedTags: Antarctic, biogeochemistry, chemistry, globalmodeling, modeling, North Atlantic, North Pacific, South Pacific
Oceanic absorption of atmospheric carbon dioxide (CO2) is expected to slow down under increasing anthropogenic emissions; however, the driving mechanisms and rates of change remain uncertain, limiting our ability to project long-term changes in climate. Using an Earth system simulation, we show that the uptake of anthropogenic carbon will slow in the next three centuries via reductions in surface alkalinity. Warming and associated changes in precipitation and evaporation intensify density stratification of the upper ocean, inhibiting the transport of alkaline water from the deep. The effect of these changes is amplified threefold by reduced carbonate buffering, making alkalinity a dominant control on CO2 uptake on multi-century timescales. Our simulation reveals a previously unknown alkalinity-climate feedback loop, amplifying multi-century warming under high emission trajectories.
Continue reading ‘Long-term slowdown of ocean carbon uptake by alkalinity dynamics’A biogeochemical model of mineral-based ocean alkalinity enhancement: impacts on the biological pump and ocean carbon uptake
Published 14 April 2023 Science ClosedTags: biogeochemistry, chemistry, globalmodeling, methods, mitigation, modeling
Minimizing anthropogenic climate disruption in the coming century will likely require carbon dioxide removal (CDR) from Earth’s atmosphere in addition to deep and rapid cuts to greenhouse gas emissions. Ocean alkalinity enhancement — the modification of surface ocean chemistry to drive marine uptake of atmospheric CO2 — is seen as a potentially significant component of ocean-based CDR portfolios. However, there has been limited mechanistic exploration of the large-scale CDR potential of mineral-based ocean alkalinity enhancement, potential bottlenecks in alkalinity release, and the biophysical impacts of alkaline mineral feedstocks on marine ecology and the marine biological carbon pump. Here we a series of biogeochemical models to evaluate the gross CDR potential and environmental impacts of ocean alkalinity enhancement using solid mineral feedstocks. We find that natural alkalinity sources — basalt and olivine — lead to very low CDR efficiency while strongly perturbing marine food quality and fecal pellet production by marine zooplankton. Artificial alkalinity sources — the synthetic metal oxides MgO and CaO — are potentially capable of significant CDR with reduced environmental impact, but their deployment at scale faces major challenges associated with substrate limitation and process CO2 emissions during feedstock production. Taken together, our results highlight distinct challenges for ocean alkalinity enhancement as a CDR strategy and indicate that mineral-based ocean alkalinity enhancement should be pursued with caution.
Continue reading ‘A biogeochemical model of mineral-based ocean alkalinity enhancement: impacts on the biological pump and ocean carbon uptake’Optimal parameters for the ocean’s nutrient, carbon, and oxygen cycles compensate for circulation biases but replumb the biological pump
Published 13 April 2023 Science ClosedTags: biogeochemistry, chemistry, globalmodeling, modeling
Accurate predictive modelling of the ocean’s global carbon and oxygen cycles is challenging because of uncertainties in both biogeochemistry and ocean circulation. Advances over the last decade have made parameter optimization feasible, allowing models to better match observed biogeochemical fields. However, does fitting a biogeochemical model to observed tracers using a circulation with known biases robustly capture the inner workings of the biological pump? Here we embed a mechanistic model of the ocean’s coupled nutrient, carbon, and oxygen cycles into two circulations for the current climate. To assess the effects of biases, one circulation (ACCESS-M) is derived from a climate model and the other from data assimilation of observations (OCIM2). We find that parameter optimization compensates for circulation biases at the expense of altering how the biological pump operates. Tracer observations constrain pump strength and regenerated inventories for both circulations, but ACCESS-M export production optimizes to twice that of OCIM2 to compensate for ACCESS-M having lower sequestration efficiencies driven by less efficient particle transfer and shorter residence times. Idealized simulations forcing complete Southern Ocean nutrient utilization show that the response of the optimized system is sensitive to the embedding circulation. In ACCESS-M, Southern Ocean nutrient and DIC trapping is partially short-circuited by unrealistically deep mixed layers. For both circulations, intense Southern Ocean production deoxygenates Southern-Ocean-sourced deep waters, muting the imprint of circulation biases on oxygen. Our findings highlight that the biological pump’s plumbing needs careful assessment to predict the biogeochemical response to environmental changes, even when optimally matching observations.
Continue reading ‘Optimal parameters for the ocean’s nutrient, carbon, and oxygen cycles compensate for circulation biases but replumb the biological pump’Simulated carbon cycle and Earth system response to atmospheric CO2 removal
Published 12 April 2023 Science ClosedTags: chemistry, globalmodeling, mitigation, modeling
To project possible future climate change, it is important to understand Earth system response to CO2 removal, a potential key method to limit global warming. Previous studies examined some aspects of Earth system response to different scenarios of CO2 removal, but lacked a systematic analysis of the carbon cycle and climate system response in a consistent modeling framework. We expanded previous studies by using an Earth system model to examine the response of land and ocean carbon cycle, as well as a set of climate variables to idealized scenarios of atmospheric CO2 removal with different removal rates. In the scenarios considered, atmospheric CO2 increases at a rate of 1% per year to four times of its preindustrial level, and then decreases at a rate of 0.5%, 1%, and 2% per year to the preindustrial level. Simulation results show that a reduction of atmospheric CO2 induces CO2 release from both the ocean and terrestrial biosphere, and to keep atmospheric CO2 at a lower level requires the removal of anthropogenic CO2 not only from the atmosphere, but from the ocean and land carbon reservoirs as well. The response of many variables of the Earth system, including temperature, ocean heat content, sea level, deep ocean acidity, and permafrost area and carbon, lags the decrease in atmospheric CO2 ranging from a few years to many centuries. A few centuries after atmospheric CO2 returns to the preindustrial level, sea level is still substantially higher than the preindustrial level, and permafrost continues losing CO2 to the atmosphere. Our study demonstrates that to offset previous positive CO2 emissions by atmospheric CO2 removal does not mean to offset climate consequence of positive CO2 emissions. Rapid and deep reduction in CO2 emissions is key to prevent and limit increasing risks from further warming. Our study provides new insights into the carbon cycle and climate system response to CO2 removal, which would help to assess future climate change and the associated impacts.
Continue reading ‘Simulated carbon cycle and Earth system response to atmospheric CO2 removal’Paris Agreement could prevent regional mass extinctions of coral species
Published 12 April 2023 Science ClosedTags: abundance, adaptation, biological response, corals, globalmodeling, Indian, modeling, multiple factors, North Atlantic, otherprocess, regionalmodeling, South Pacific, temperature
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.
Continue reading ‘Paris Agreement could prevent regional mass extinctions of coral species’Comparison of the carbon cycle and climate response to artificial ocean alkalinization and solar radiation modification
Published 11 April 2023 Science ClosedTags: biogeochemistry, chemistry, globalmodeling, mitigation, modeling
Carbon dioxide removal and solar radiation modification (SRM) are two classes of proposed climate intervention methods. A thorough understanding of climate system response to these methods calls for a good understanding of the carbon cycle response. In this study, we used an Earth system model to examine the response of global climate and carbon cycle to artificial ocean alkalinization (AOA), a method of CO2 removal, and reduction in solar irradiance that represents the overall effect of solar radiation modification. In our simulations, AOA is applied uniformly over the global ice-free ocean under the RCP8.5 scenario to bring down atmospheric CO2 to the level of RCP4.5, and SRM is applied uniformly over the globe under the RCP8.5 scenario to bring down global mean surface temperature to the level of RCP4.5. Our simulations show that with the same goal of temperature stabilization, AOA and SRM cause fundamentally different perturbations of the ocean and land carbon cycle. By the end of the 21st century, relative to the simulation of RCP8.5, AOA-induced changes in ocean carbonate chemistry enhances global oceanic CO2 uptake by 983 PgC and increases global mean surface ocean pH by 0.42. Meanwhile, AOA reduces land CO2 uptake by 79 PgC and reduces atmospheric CO2 concentration by 426 × 10−6. By contrast, relative to the simulation of RCP8.5, SRM has a minor effect on the oceanic CO2 uptake and ocean acidification. SRM-induced cooling enhances land CO2 uptake by 140 PgC and reduces atmospheric CO2 concentration by 63 × 10−6. A sudden termination of SRM causes a rate of temperature change that is much larger than that of RCP8.5. A sudden termination of AOA causes a rate of temperature change that is comparable to that of RCP8.5 and a rate of ocean acidification that is much larger than that of RCP8.5.
Continue reading ‘Comparison of the carbon cycle and climate response to artificial ocean alkalinization and solar radiation modification’Global surface ocean acidification indicators from 1750 to 2100
Published 7 April 2023 Science ClosedTags: chemistry, globalmodeling, methods, modeling
Accurately predicting future ocean acidification (OA) conditions is crucial for advancing OA research at regional and global scales, and guiding society’s mitigation and adaptation efforts. This study presents a new model-data fusion product covering 10 global surface OA indicators based on 14 Earth System Models (ESMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6), along with three recent observational ocean carbon data products. The indicators include fugacity of carbon dioxide, pH on total scale, total hydrogen ion content, free hydrogen ion content, carbonate ion content, aragonite saturation state, calcite saturation state, Revelle Factor, total dissolved inorganic carbon content, and total alkalinity content. The evolution of these OA indicators is presented on a global surface ocean 1° × 1° grid as decadal averages every 10 years from preindustrial conditions (1750), through historical conditions (1850–2010), and to five future Shared Socioeconomic Pathways (2020–2100): SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. These OA trajectories represent an improvement over previous OA data products with respect to data quantity, spatial and temporal coverage, diversity of the underlying data and model simulations, and the provided SSPs. The generated data product offers a state-of-the-art research and management tool for the 21st century under the combined stressors of global climate change and ocean acidification. The gridded data product is available in NetCDF at the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information: https://www.ncei.noaa.gov/data/oceans/ncei/ocads/metadata/0259391.html, and global maps of these indicators are available in jpeg at: https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/synthesis/surface-oa-indicators.html.
Continue reading ‘Global surface ocean acidification indicators from 1750 to 2100’Early detection of anthropogenic climate change signals in the ocean interior
Published 10 March 2023 Science ClosedTags: chemistry, globalmodeling, modeling
Robust detection of anthropogenic climate change is crucial to: (i) improve our understanding of Earth system responses to external forcing, (ii) reduce uncertainty in future climate projections, and (iii) develop efficient mitigation and adaptation plans. Here, we use Earth system model projections to establish the detection timescales of anthropogenic signals in the global ocean through analyzing temperature, salinity, oxygen, and pH evolution from surface to 2000 m depths. For most variables, anthropogenic changes emerge earlier in the interior ocean than at the surface, due to the lower background variability at depth. Acidification is detectable earliest, followed by warming and oxygen changes in the subsurface tropical Atlantic. Temperature and salinity changes in the subsurface tropical and subtropical North Atlantic are shown to be early indicators for a slowdown of the Atlantic Meridional Overturning Circulation. Even under mitigated scenarios, inner ocean anthropogenic signals are projected to emerge within the next few decades. This is because they originate from existing surface changes that are now propagating into the interior. In addition to the tropical Atlantic, our study calls for establishment of long-term interior monitoring systems in the Southern Ocean and North Atlantic in order to elucidate how spatially heterogeneous anthropogenic signals propagate into the interior and impact marine ecosystems and biogeochemistry.
Continue reading ‘Early detection of anthropogenic climate change signals in the ocean interior’Alkalinity biases in CMIP6 Earth System Models and implications for simulated CO2 drawdown via artificial alkalinity enhancement
Published 6 March 2023 Science ClosedTags: chemistry, globalmodeling, methods, modeling
The partitioning of CO2 between atmosphere and ocean depends to a large degree not only on the amount of dissolved inorganic carbon (DIC) but also of alkalinity in the surface ocean. That is also why, in the context of negative emission approaches ocean alkalinity enhancement is discussed as one potential approach. Although alkalinity is thus an important variable of the marine carbonate system little knowledge exists how its representation in models compares with measurements. We evaluated the large-scale alkalinity distribution in 14 CMIP6 models against the observational data set GLODAPv2 and showed that most models as well as the multi-model-mean underestimate alkalinity at the surface and in the upper ocean, while overestimating alkalinity in the deeper ocean. The decomposition of the global mean alkalinity biases into contributions from physical processes (preformed alkalinity), remineralization, and carbonate formation and dissolution showed that the bias stemming from the physical redistribution of alkalinity is dominant. However, below the upper few hundred meters the bias from carbonate dissolution can become similarly important as physical biases, while the contribution from remineralization processes is negligible. This highlights the critical need for better understanding and quantification of processes driving calcium carbonate dissolution in microenvironments above the saturation horizons, and implementation of these processes into biogeochemical models.
For the application of the models to assess the potential of ocean alkalinity enhancement to increase ocean carbon uptake and counteract ocean acidification, a back-of-the-envelope calculation was conducted with each model’s global mean surface alkalinity and DIC as input parameters. We find that the degree of compensation of DIC and alkalinity biases at the surface is more important for the marine CO2 uptake capacity than the alkalinity biases themselves. The global mean surface alkalinity bias relative to GLODAPv2 in the different models ranges from -85 mmol kg-1 (-3.6 %) to +50 mmol kg-1 (+2.1 %) (mean: -25 mmol kg-1 or -1.1 %), while for DIC the relative bias ranges from -55 mmol kg-1 (-2.6 %) to 53 mmol kg-1 (+2.5 %) (mean: -13 mmol kg-1 or -0.6 %). Because of this partial compensation, all but two of the CMIP6 models evaluated here overestimate the Revelle factor at the surface and thus overestimate the CO2-draw-down after alkalinity addition by up to 13 % and pH increase by up to 7.2 %. This overestimate has to be taken into account when reporting on efficiencies of ocean alkalinity enhancement experiments using CMIP6 models.
Continue reading ‘Alkalinity biases in CMIP6 Earth System Models and implications for simulated CO2 drawdown via artificial alkalinity enhancement’Long-term slowdown of ocean carbon uptake by alkalinity dynamics
Published 9 February 2023 Science ClosedTags: chemistry, globalmodeling, modeling, multiple factors, temperature
Abstract
Oceanic absorption of atmospheric carbon dioxide (CO2) is expected to slow down under increasing anthropogenic emissions; however, the driving mechanisms and rates of change remain uncertain, limiting our ability to project long-term changes in climate. Using an Earth system simulation, we show that the uptake of anthropogenic carbon will slow in the next three centuries via reductions in surface alkalinity. Warming and associated changes in precipitation and evaporation intensify density stratification of the upper ocean, inhibiting the transport of alkaline water from the deep. The effect of these changes is amplified three-fold by reduced carbonate buffering, making alkalinity a dominant control on CO2 uptake on multi-century timescales. Our simulation reveals a previously unknown alkalinity-climate feedback loop, amplifying multi-century warming under high emission trajectories.
Key Points
- Oceanic uptake of carbon could slow in upcoming centuries through previously unidentified alkalinity-climate feedback
- Reduced upwelling and carbonate buffer enhance the influence of alkalinity on the increase in surface ocean carbon dioxide
- Reductions in surface alkalinity will reduce the rate of carbon uptake on multi-century timescales
Recent trends and variability in the oceanic storage of dissolved inorganic carbon
Published 20 January 2023 Science ClosedTags: chemistry, globalmodeling, modeling
Several methods have been developed to quantify the oceanic accumulation of anthropogenic carbon dioxide (CO2) in response to rising atmospheric CO2. Yet, we still lack a corresponding estimate of the changes in the total oceanic dissolved inorganic carbon (DIC). In addition to the increase in anthropogenic CO2, changes in DIC also include alterations of natural CO2. Once integrated globally, changes in DIC reflect the net oceanic sink for atmospheric CO2, complementary to estimates of the air-sea CO2 exchange based on surface measurements. Here, we extend the MOBO-DIC machine learning approach by Keppler et al. (2020a) to estimate global monthly fields of DIC at 1° resolution over the top 1500 m from 2004 through 2019. We find that over these 16 years and extrapolated to cover the whole global ocean down to 4000 m, the oceanic DIC pool increased close to linearly at an average rate of 3.2+/-0.7 Pg C yr^-1. This trend is statistically indistinguishable from current estimates of the oceanic uptake of anthropogenic CO2 over the same period. Thus, our study implies no detectable net loss or gain of natural CO2 by the ocean, albeit the large uncertainties could be masking it. Our reconstructions suggest substantial internal redistributions of natural oceanic CO2, with a shift from the mid-latitudes to the tropics and from the surface to below 200 m. Such redistributions correspond with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. The interannual variability of DIC is strongest in the tropical Western Pacific, consistent with the El Nino Southern Oscillation.
Continue reading ‘Recent trends and variability in the oceanic storage of dissolved inorganic carbon’Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-CO2 greenhouse gases
Published 18 January 2023 Science ClosedTags: chemistry, globalmodeling, modeling
Future ocean acidification mainly depends on the continuous ocean uptake of CO2 from the atmosphere. The trajectory of future atmospheric CO2 is prescribed in traditional climate projections with Earth System Models, leading to a small model spread and apparently low uncertainties for projected acidification, but a large spread in global warming. However, climate policies such as the Paris Agreement define climate targets in terms of global warming levels and as traditional simulations do not converge to a given warming level, they cannot be used to assess uncertainties in projected acidification for these warming levels. Here, we perform climate simulations that converge to given temperature levels using the Adaptive Emission Reduction Algorithm (AERA) with the Earth System Model Bern3D-LPX at different setups with different transient climate response to cumulative carbon emissions (TCRE) and choices between reductions in CO2 and non-CO2 forcing agents. With these simulations, we demonstrate that uncertainties in surface ocean acidification are an order of magnitude larger than the usually reported inter-model uncertainties from simulations with prescribed atmospheric CO2. Uncertainties in acidification at a given stabilized temperature are dominated by TCRE and the choice of emission reductions of non-CO2 greenhouse gases. High TCRE and relatively low reductions of non-CO2 greenhouse gases, for example, necessitate relatively strong reductions in CO2 emissions and lead to relatively little ocean acidification at a given temperature level. The results suggest that choices between reducing emissions of CO2 versus non-CO2 agents should consider the economic costs and ecosystem damage of ocean acidification.
Continue reading ‘Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-CO2 greenhouse gases’Limits and CO2 equilibration of near-coast alkalinity enhancement
Published 18 January 2023 Science ClosedTags: chemistry, globalmodeling, mitigation, modeling
Ocean alkalinity enhancement (OAE) has recently gained attention as a potential method for carbon dioxide removal (CDR) at gigatonne (Gt) scale, with near-coast OAE operations being economically favorable due to proximity to mineral and energy sources. In this paper we study critical questions which determine the scale and viability of OAE. Which coastal locations are able to sustain a large flux of alkalinity at minimal pH and ΩArag (aragonite saturation) changes? What is the interference distance between adjacent OAE projects? How much CO2 is absorbed per unit of alkalinity added? How quickly does the induced CO2 deficiency equilibrate with the atmosphere? Choosing relatively conservative constraints on ΔpH or ΔOmega, we examine the limits of OAE using the ECCO LLC270 (0.3∘) global circulation model. We find that the sustainable OAE rate varies over 1–2 orders of magnitude between different coasts and exhibits complex patterns and non-local dependencies which vary from region to region. In general, OAE in areas of strong coastal currents enables the largest fluxes and depending on the direction of these currents, neighboring OAE sites can exhibit dependencies as far as 400 km or more. At these steady state fluxes most regional stretches of coastline are able to accommodate on the order of 10s to 100s of megatonnes of negative emissions within 300 km of the coast. We conclude that near-coastal OAE has the potential to scale globally to several Gt CO2 yr−1 of drawdown with conservative pH constraints, if the effort is spread over the majority of available coastlines. Depending on the location, we find a diverse set of equilibration kinetics, determined by the interplay of gas exchange and surface residence time. Most locations reach an uptake efficiency plateau of 0.6–0.8 mol CO2 per mol of alkalinity after 3–4 years, after which there is only slow additional CO2 uptake. Regions of significant downwelling (e.g., around Iceland) should be avoided by OAE deployments, as in such locations up to half of the CDR potential of OAE can be lost to bottom waters. The most ideal locations, reaching a molar uptake ratio of around 0.8, include North Madagascar, California, Brazil, Peru and locations close to the Southern Ocean such as Tasmania, Kerguelen and Patagonia, where the gas exchange appears to occur faster than the surface residence time. However, some locations (e.g., Hawaii) take significantly longer to equilibrate (up to 8–10 years) but can still eventually achieve high uptake ratios.
Continue reading ‘Limits and CO2 equilibration of near-coast alkalinity enhancement’
