Fassbender, Andrea J., Sarah Schlunegger, Keith B Rodgers, and John P Dunne, June 2022: Quantifying the role of seasonality in the marine carbon cycle feedback: An ESM2M case study. Global Biogeochemical Cycles, 36(6), DOI:10.1029/2021GB007018. Abstract
Observations and climate models indicate that changes in the seasonal amplitude of sea surface carbon dioxide partial pressure (A-pCO2) are underway and driven primarily by anthropogenic carbon (Cant) accumulation in the ocean. This occurs because pCO2 is more responsive to seasonal changes in physics (including warming) and biology in an ocean that contains more Cant. A-pCO2 changes have the potential to alter annual ocean carbon uptake and contribute to the overall marine carbon cycle feedback. Using the GFDL ESM2M Large Ensemble and a novel analysis framework, we quantify the influence of Cant accumulation on pCO2 seasonal cycles and sea-air CO2 fluxes. Specifically, we reconstruct alternative evolutions of the contemporary ocean state in which the sensitivity of pCO2 to seasonal thermal and biophysical variation is fixed at preindustrial levels, however the background, mean-state pCO2 fully responds to anthropogenic forcing. We find near-global A-pCO2 increases of >100% by 2100, under RCP8.5 forcing, with rising Cant accounting for ∼100% of thermal and ∼50% of nonthermal pCO2 component amplitude changes. The other ∼50% of nonthermal pCO2 component changes are attributed to modeled changes in ocean physics and biology caused by climate change. Cant-induced A-pCO2 changes cause an 8.1 ± 0.4% (ensemble mean ± 1σ) increase in ocean carbon uptake by 2100. This is because greater wintertime wind speeds enhance the impact of wintertime pCO2 changes, which work to increase the ocean carbon sink. Thus, the seasonal ocean carbon cycle feedback works in opposition to the larger, mean-state feedback that reduces ocean carbon uptake by ∼60%.
Climate change can drive shifts in the seasonality of marine productivity, with consequences for the marine food web. However, these alterations in phytoplankton bloom phenology (initiation and peak timing), and the underlying drivers, are not well understood. Here, using a 30-member Large Ensemble of climate change projections, we show earlier bloom initiation in most ocean regions, yet changes in bloom peak timing vary widely by region. Shifts in both initiation and peak timing are induced by a subtle decoupling between altered phytoplankton growth and zooplankton predation, with increased zooplankton predation (top-down control) playing an important role in altered bloom peak timing over much of the global ocean. Only in limited regions is light limitation a primary control for bloom initiation changes. In the extratropics, climate-change-induced phenological shifts will exceed background natural variability by the end of the twenty-first century, which may impact energy flow in the marine food webs.
Morgan, Eric J., Manfredi Manizza, Ralph F Keeling, Laure Resplandy, Sara E Mikaloff-Fletcher, Cynthia D Nevison, Yuming Jin, Jonathan D Bent, Olivier Aumont, Scott C Doney, John P Dunne, Jasmin G John, Ivan D Lima, Matthew C Long, and Keith B Rodgers, August 2021: An atmospheric constraint on the seasonal air–sea exchange of oxygen and heat in the extratropics. Journal of Geophysical Research: Oceans, 126(8), DOI:10.1029/2021JC017510. Abstract
Typically, the surface of the ocean releases oxygen to the atmosphere during summer and takes it up during winter. This cycle is driven by circulation, biology (photosynthesis and respiration), and the seasonal cycle in water temperature, which changes the solubility of oxygen in surface water. We have used measurements of two atmospheric tracers, one which tracks oxygen and one which tracks heat, to estimate the amount of oxygen taken up or released by a change in ocean heat content. By looking at ocean models and atmospheric observations of the two atmospheric tracers, we find that the oxygen exchange between the ocean and atmosphere in the Southern Hemisphere is more responsive to changes in heat content than in the Northern Hemisphere. These hemispheric metrics are useful tests of how ocean models simulate some biological and physical processes.
Deser, Clara, Flavio Lehner, Keith B Rodgers, T R Ault, Thomas L Delworth, P DiNezio, and Arlene M Fiore, et al., April 2020: Insights from Earth system model initial-condition large ensembles and future prospects. Nature Climate Change, 10(4), DOI:10.1038/s41558-020-0731-2. Abstract
Internal variability in the climate system confounds assessment of human-induced climate change and imposes irreducible limits on the accuracy of climate change projections, especially at regional and decadal scales. A new collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences. These data enhance the assessment of climate change risks, including extreme events, and offer a powerful testbed for new methodologies aimed at separating forced signals from internal variability in the observational record. Opportunities and challenges confronting the design and dissemination of future LEs, including increased spatial resolution and model complexity alongside emerging Earth system applications, are discussed.
Frölicher, Thomas L., L Ramseyer, C C Raible, Keith B Rodgers, and John P Dunne, April 2020: Potential predictability of marine ecosystem drivers. Biogeosciences, 17(7), DOI:10.5194/bg-17-2061-2020. Abstract
Climate variations can have profound impacts on marine ecosystems and the socio-economic systems that may depend upon them. Temperature, pH, oxygen (O2) and net primary production (NPP) are commonly considered to be important marine ecosystem drivers, but the potential predictability of these drivers is largely unknown. Here, we use a comprehensive Earth system model within a perfect modelling framework to show that all four ecosystem drivers are potentially predictable on global scales and at the surface up to 3 years in advance. However, there are distinct regional differences in the potential predictability of these drivers. Maximum potential predictability (> 10 years) is found at the surface for temperature and O2 in the Southern Ocean and for temperature, O2 and pH in the North Atlantic. This is tied to ocean overturning structures with memory or inertia with enhanced predictability in winter. Additionally, these four drivers are highly potentially predictable in the Arctic Ocean at surface. In contrast, minimum predictability is simulated for NPP (< 1 years) in the Southern Ocean. Potential predictability for temperature, O2 and pH increases with depth to more than 10 years below the thermocline, except in the tropical Pacific and Indian Ocean, where predictability is also three to five years in the thermocline. This study indicating multi-year (at surface) and decadal (subsurface) potential predictability for multiple ecosystem drivers is intended as a foundation to foster broader community efforts in developing new predictions of marine ecosystem drivers.
Anthropogenically forced changes in ocean biogeochemistry are underway and critical for the ocean carbon sink and marine habitat. Detecting such changes in ocean biogeochemistry will require quantification of the magnitude of the change (anthropogenic signal) and the natural variability inherent to the climate system (noise). Here we use Large Ensemble (LE) experiments from four Earth system models (ESMs) with multiple emissions scenarios to estimate Time of Emergence (ToE) and partition projection uncertainty for anthropogenic signals in five biogeochemically important upper-ocean variables. We find ToEs are robust across ESMs for sea surface temperature and the invasion of anthropogenic carbon; emergence time scales are 20–30 yr. For the biological carbon pump, and sea surface chlorophyll and salinity, emergence time scales are longer (50+ yr), less robust across the ESMs, and more sensitive to the forcing scenario considered. We find internal variability uncertainty, and model differences in the internal variability uncertainty, can be consequential sources of uncertainty for projecting regional changes in ocean biogeochemistry over the coming decades. In combining structural, scenario, and internal variability uncertainty, this study represents the most comprehensive characterization of biogeochemical emergence time scales and uncertainty to date. Our findings delineate critical spatial and duration requirements for marine observing systems to robustly detect anthropogenic change.
Eddebbar, Yassir A., and Keith B Rodgers, et al., May 2019: El Niño-like Physical and Biogeochemical Ocean Response to Tropical Eruptions. Journal of Climate, 32(9), DOI:10.1175/JCLI-D-18-0458.1. Abstract
The oceanic response to recent tropical eruptions is examined in Large Ensemble (LE) experiments from two fully-coupled global climate models, the Community Earth System Model (CESM) and Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M), each forced by a distinct volcanic forcing dataset. Following the simulated eruptions of Agung, El Chichon, and Pinatubo, the ocean loses heat and gains oxygen and carbon, in general agreement with available observations. In both models, substantial global surface cooling is accompanied by El Niño-like equatorial Pacific surface warming a year after the volcanic forcing peaks. A mechanistic analysis of the CESM and ESM2M responses to Pinatubo identifies remote wind forcing from the western Pacific as a major driver of this El Niño-like response. Following eruption, faster cooling over the Maritime Continent than adjacent oceans suppresses convection and leads to persistent westerly wind anomalies over the western tropical Pacific. These wind anomalies excite equatorial downwelling Kelvin waves and the upwelling of warm subsurface anomalies in the eastern Pacific, promoting the development of El Niño conditions through Bjerknes feedbacks a year after eruption. This El Niño-like response drives further ocean heat loss through enhanced equatorial cloud albedo, and dominates global carbon uptake as upwelling of carbon-rich waters is suppressed in the tropical Pacific. Oxygen uptake occurs primarily at high latitudes, where surface cooling intensifies the ventilation of subtropical thermocline waters. These volcanically-forced ocean responses are large enough to contribute to the observed decadal variability in oceanic heat, carbon and oxygen.
The attribution of anthropogenically forced trends in the climate system requires an understanding of when and how such signals emerge from natural variability. We applied time-of-emergence diagnostics to a large ensemble of an Earth system model, which provides both a conceptual framework for interpreting the detectability of anthropogenic impacts in the ocean carbon cycle and observational sampling strategies required to achieve detection. We found emergence timescales that ranged from less than a decade to more than a century, a consequence of the time lag between the chemical and radiative impacts of rising atmospheric CO2 on the ocean. Processes sensitive to carbonate chemical changes emerge rapidly, such as the impacts of acidification on the calcium carbonate pump (10 years for the globally integrated signal and 9–18 years for regionally integrated signals) and the invasion flux of anthropogenic CO2 into the ocean (14 years globally and 13–26 years regionally). Processes sensitive to the ocean’s physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (23 years globally and 27–85 years regionally).
Meltwater from the Antarctic Ice Sheet is projected to cause up to one metre of sea-level rise by 2100 under the highest greenhouse gas concentration trajectory (RCP8.5) considered by the Intergovernmental Panel on Climate Change (IPCC). However, the effects of meltwater from the ice sheets and ice shelves of Antarctica are not included in the widely used CMIP5 climate models, which introduces bias into IPCC climate projections. Here we assess a large ensemble simulation of the CMIP5 model ‘GFDL ESM2M’ that accounts for RCP8.5-projected Antarctic Ice Sheet meltwater. We find that, relative to the standard RCP8.5 scenario, accounting for meltwater delays the exceedance of the maximum global-mean atmospheric warming targets of 1.5 and 2 degrees Celsius by more than a decade, enhances drying of the Southern Hemisphere and reduces drying of the Northern Hemisphere, increases the formation of Antarctic sea ice (consistent with recent observations of increasing Antarctic sea-ice area) and warms the subsurface ocean around the Antarctic coast. Moreover, the meltwater-induced subsurface ocean warming could lead to further ice-sheet and ice-shelf melting through a positive feedback mechanism, highlighting the importance of including meltwater effects in simulations of future climate.
Resplandy, Laure, Ralph F Keeling, C Rödenbeck, B B Stephens, S Khatiwala, and Keith B Rodgers, et al., July 2018: Revision of global carbon fluxes based on a reassessment of oceanic and riverine carbon transport. Nature Geoscience, 11(7), DOI:10.1038/s41561-018-0151-3. Abstract
Measurements of atmospheric CO2 concentration provide a tight constraint on the sum of the land and ocean sinks. This constraint has been combined with estimates of ocean carbon flux and riverine transport of carbon from land to oceans to isolate the land sink. Uncertainties in the ocean and river fluxes therefore translate into uncertainties in the land sink. Here, we introduce a heat-based constraint on the latitudinal distribution of ocean and river carbon fluxes, and reassess the partition between ocean, river and land in the tropics, and in the southern and northern extra-tropics. We show that the ocean overturning circulation and biological pump tightly link the ocean transports of heat and carbon between hemispheres. Using this coupling between heat and carbon, we derive ocean and river carbon fluxes compatible with observational constraints on heat transport. This heat-based constraint requires a 20–100% stronger ocean and river carbon transport from the Northern Hemisphere to the Southern Hemisphere than existing estimates, and supports an upward revision of the global riverine carbon flux from 0.45 to 0.78 PgC yr−1. These systematic biases in existing ocean/river carbon fluxes redistribute up to 40% of the carbon sink between northern, tropical and southern land ecosystems. As a consequence, the magnitude of both the southern land source and the northern land sink may have to be substantially reduced.
Carter, Brendan R., Richard A Feely, S Meckling, J N Cross, A M Macdonald, Samantha A Siedlecki, Lynne D Talley, C L Sabine, F J Millero, J H Swift, A G Dickson, and Keith B Rodgers, February 2017: Two decades of Pacific anthropogenic carbon storage and ocean acidification along Global Ocean Ship-based Hydrographic Investigations Program sections P16 and P02. Global Biogeochemical Cycles, 31(2), DOI:10.1002/2016GB005485. Abstract
A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (Canth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30°N, and P16 is a meridional section crossing the North and South Pacific at ~150°W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to ~1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s–2000s) and recent (2000s–2010s) decades along P02, at rates of 0.53 ± 0.11 and 0.46 ± 0.11 mol C m−2 a−1, respectively. By contrast, Canth storage accelerated from 0.29 ± 0.10 to 0.45 ± 0.11 mol C m−2 a−1 along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60°S and 60°N of 6.1 ± 1.5 PgC decade−1 in the earlier decade and 8.8 ± 2.2 PgC decade−1 in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by −0.08 to −0.14 and aragonite saturation state by −0.57 to −0.82.
Eddebbar, Yassir A., Matthew C Long, Laure Resplandy, C Rödenbeck, and Keith B Rodgers, et al., May 2017: Impacts of ENSO on air-sea oxygen exchange: Observations and mechanisms. Global Biogeochemical Cycles, 31(5), DOI:10.1002/2017GB005630. Abstract
Models and observations of atmospheric potential oxygen (APO ≃ O2 + 1.1 * CO2) are used to investigate the influence of El Niño–Southern Oscillation (ENSO) on air-sea O2 exchange. An atmospheric transport inversion of APO data from the Scripps flask network shows significant interannual variability in tropical APO fluxes that is positively correlated with the Niño3.4 index, indicating anomalous ocean outgassing of APO during El Niño. Hindcast simulations of the Community Earth System Model (CESM) and the Institut Pierre-Simon Laplace model show similar APO sensitivity to ENSO, differing from the Geophysical Fluid Dynamics Laboratory model, which shows an opposite APO response. In all models, O2 accounts for most APO flux variations. Detailed analysis in CESM shows that the O2 response is driven primarily by ENSO modulation of the source and rate of equatorial upwelling, which moderates the intensity of O2 uptake due to vertical transport of low-O2 waters. These upwelling changes dominate over counteracting effects of biological productivity and thermally driven O2 exchange. During El Niño, shallower and weaker upwelling leads to anomalous O2 outgassing, whereas deeper and intensified upwelling during La Niña drives enhanced O2 uptake. This response is strongly localized along the central and eastern equatorial Pacific, leading to an equatorial zonal dipole in atmospheric anomalies of APO. This dipole is further intensified by ENSO-related changes in winds, reconciling apparently conflicting APO observations in the tropical Pacific. These findings suggest a substantial and complex response of the oceanic O2 cycle to climate variability that is significantly (>50%) underestimated in magnitude by ocean models.
Ritter, B, P Landschützer, Nicolas Gruber, A R Fay, Y Iida, S Jones, S Nakaoka, G-H Park, P Peylin, C Rödenbeck, and Keith B Rodgers, et al., December 2017: Observation-Based Trends of the Southern Ocean Carbon Sink. Geophysical Research Letters, 44(24), DOI:10.1002/2017GL074837. Abstract
The Southern Ocean (SO) carbon sink has strengthened substantially since the year 2000, following a decade of a weakening trend. However, the surface ocean pCO2 data underlying this trend reversal are sparse, requiring a substantial amount of extrapolation to map the data. Here we use nine different pCO2 mapping products to investigate the SO trends and their sensitivity to the mapping procedure. We find a robust temporal coherence for the entire SO, with eight of the nine products agreeing on the sign of the decadal trends, that is, a weakening CO2 sink trend in the 1990s (on average 0.22 ± 0.24 pg C yr−1 decade−1), and a strengthening sink trend during the 2000s (−0.35 ± 0.23 pg C yr−1 decade−1). Spatially, the multiproduct mean reveals rather uniform trends, but the confidence is limited, given the small number of statistically significant trends from the individual products, particularly during the data-sparse 1990–1999 period.
Toyama, K, Keith B Rodgers, B Blanke, D Iudicone, Masao Ishii, Olivier Aumont, and Jorge L Sarmiento, November 2017: Large Re-emergence of Anthropogenic Carbon Into the Ocean’s Surface Mixed Layer Sustained by the Ocean’s Overturning Circulation. Journal of Climate, 30(21), DOI:10.1175/JCLI-D-16-0725.1. Abstract
We evaluate the output from a widely used ocean carbon cycle model to identify the subduction and obduction (re-emergence) rates of anthropogenic carbon (Cant) for climatological conditions during the World Ocean Circulation Experiment (WOCE) era in 1995 using a new set of Lagrangian diagnostic tools. The principal scientific value of the Lagrangian diagnostics is in providing a new means to connect Cant re-emergence pathways to the relatively rapid renewal timescales of mode waters through the overturning circulation.
Our main finding is that for this model with 2.04 PgC/yr of uptake of Cant via gas exchange, the subduction and obduction rates across the base of the mixed layer (MLbase) are 4.96 PgC/yr and 4.50 PgC/yr, respectively, which are twice as large as the gas exchange at the surface. Given that there is net accumulation of 0.17 PgC/yr in the mixed layer itself, this implies the residual downward Cant transport of 1.40 PgC/yr across the MLbase is associated with diffusion. Importantly, the net patterns for subduction and obduction transports of Cant mirror the large-scale patterns for transport of water volume, thereby illustrating the processes controlling Cant uptake. Although the net transfer across the MLbase by compensating subduction and obduction is relatively smaller than the diffusion, localized pattern of Cant subduction and obduction implies significant regional impacts. The median timescale for re-emergence of obducting particles is short (less than 10 years), indicating that re-emergence should contribute to limiting future carbon uptake through its contribution to perturbing the Revelle factor for surface waters.
Relatively rapid re-emergence of anthropogenic carbon (Cant) in the Equatorial Pacific is of potential importance for its impact on the carbonate buffering capacity of surface seawater, and thereby impeding the ocean's ability to further absorb Cant from the atmosphere. We explore the mechanisms sustaining Cant re-emergence (upwelling) from the thermocline to surface layers by applying water mass transformation diagnostics to a global ocean/sea-ice/biogeochemistry model. We find that the upwelling rate of Cant (0.4 PgC yr-1) from the thermocline to the surface layer is almost twice as large as air-sea Cant fluxes (0.203 PgC yr-1). The upwelling of Cant from the thermocline to the surface layer can be understood as a two-step process: the first being due to diapycnal diffusive transformation fluxes and the second due to surface buoyancy fluxes. We also find that this re-emergence of Cant decreases dramatically during the 1982/1983 and 1997/1998 El Niño events.
We use a large initial condition suite of simulations (30 runs) with an Earth system model to assess the detectability of biogeochemical impacts of ocean acidification (OA) on the marine alkalinity distribution from decadally repeated hydrographic measurements such as those produced by the Global Ship-Based Hydrographic Investigations Program (GO-SHIP). Detection of these impacts is complicated by alkalinity changes from variability and long-term trends in freshwater and organic matter cycling and ocean circulation. In our ensemble simulation, variability in freshwater cycling generates large changes in alkalinity that obscure the changes of interest and prevent the attribution of observed alkalinity redistribution to OA. These complications from freshwater cycling can be mostly avoided through salinity normalization of alkalinity. With the salinity-normalized alkalinity, modeled OA impacts are broadly detectable in the surface of the subtropical gyres by 2030. Discrepancies between this finding and the finding of an earlier analysis suggest that these estimates are strongly sensitive to the patterns of calcium carbonate export simulated by the model. OA impacts are detectable later in the subpolar and equatorial regions due to slower responses of alkalinity to OA in these regions and greater seasonal equatorial alkalinity variability. OA impacts are detectable later at depth despite lower variability due to smaller rates of change and consistent measurement uncertainty.
Cheung, William W., Thomas L Frölicher, R G Asch, M C Jones, M Pinsky, Gabriel Reygondeau, Keith B Rodgers, Ryan R Rykaczewski, Jorge L Sarmiento, Charles A Stock, and James R Watson, May 2016: Building confidence in projections of the responses of living marine resources to climate change. ICES Journal of Marine Science, 73(5), DOI:10.1093/icesjms/fsv250. Abstract
The Fifth Assessment Report of the Intergovernmental Panel on Climate Change highlights that climate change and ocean acidification are challenging the sustainable management of living marine resources (LMRs). Formal and systematic treatment of uncertainty in existing LMR projections, however, is lacking. We synthesize knowledge of how to address different sources of uncertainty by drawing from climate model intercomparison efforts. We suggest an ensemble of available models and projections, informed by observations, as a starting point to quantify uncertainties. Such an ensemble must be paired with analysis of the dominant uncertainties over different spatial scales, time horizons, and metrics. We use two examples: (i) global and regional projections of Sea Surface Temperature and (ii) projection of changes in potential catch of sablefish (Anoplopoma fimbria) in the 21st century, to illustrate this ensemble model approach to explore different types of uncertainties. Further effort should prioritize understanding dominant, undersampled dimensions of uncertainty, as well as the strategic collection of observations to quantify, and ultimately reduce, uncertainties. Our proposed framework will improve our understanding of future changes in LMR and the resulting risk of impacts to ecosystems and the societies under changing ocean conditions.
Frölicher, Thomas L., Keith B Rodgers, Charles A Stock, and William W L Cheung, August 2016: Sources of uncertainties in 21st century projections of potential ocean ecosystem stressors. Global Biogeochemical Cycles, 30(8), DOI:10.1002/2015GB005338. Abstract
Future projections of potential ocean ecosystem stressors, such as acidification, warming, deoxygenation and changes in ocean productivity, are uncertain due to incomplete understanding of fundamental processes, internal climate variability, and divergent carbon emissions scenarios. This complicates climate change impact assessments. We evaluate the relative importance of these uncertainty sources in projections of potential stressors as a function of projection lead-time and spatial scale. Internally generated climate variability is the dominant source of uncertainty in mid-to-low latitudes and in most coastal Large Marine Ecosystems over the next few decades, suggesting irreducible uncertainty inherent in these short projections. Uncertainty in projections of century-scale global sea surface temperature (SST), global thermocline oxygen, and regional surface pH is dominated by scenario uncertainty, highlighting the critical importance of policy decisions on carbon emissions. In contrast, uncertainty in century-scale projections of net primary productivity (NPP), low oxygen waters, and Southern Ocean SST is dominated by model uncertainty, underscoring the importance of overcoming deficiencies in scientific understanding and improved process representation in Earth system models are critical for making more robust projections of these potential stressors. We also show that changes in the combined potential stressors emerge from the noise in 39% (34 – 44%) of the ocean by 2016-2035 relative to the 1986-2005 reference period and in 54% (50 – 60%) of the ocean by 2076-2095 following a high carbon emissions scenario. Projected large changes in surface pH and SST can be reduced substantially and rapidly with aggressive carbon emission mitigation, but only marginally for oxygen. The regional importance of model uncertainty and internal variability underscores the need for expanded and improved multi-model and large initial condition ensemble projections with Earth system models for evaluating regional marine resource impacts.
Galbraith, Eric D., John P Dunne, Anand Gnanadesikan, Richard D Slater, Jorge L Sarmiento, Carolina O Dufour, Gregory F de Souza, Daniele Bianchi, M Claret, Keith B Rodgers, and S Sedigh Marvasti, December 2015: Complex functionality with minimal computation: Promise and pitfalls of reduced-tracer ocean biogeochemistry models. Journal of Advances in Modeling Earth Systems, 7(4), DOI:10.1002/2015MS000463. Abstract
Earth System Models increasingly include ocean biogeochemistry models in order to predict changes in ocean carbon storage, hypoxia and biological productivity under climate change. However, state-of-the-art ocean biogeochemical models include many advected tracers, that significantly increase the computational resources required, forcing a tradeoff with spatial resolution. Here, we compare a state-of-the art model with 30 prognostic tracers (TOPAZ) with two reduced-tracer models, one with 6 tracers (BLING), the other with 3 tracers (miniBLING). The reduced-tracer models employ parameterized, implicit biological functions, that nonetheless capture many of the most important processes resolved by TOPAZ. All three are embedded in the same coupled climate model. Despite the large difference in tracer number, the absence of tracers for living organic matter is shown to have a minimal impact on the transport of nutrient elements, and the three models produce similar mean annual pre-industrial distributions of macronutrients, oxygen and carbon. Significant differences do exist amongst the models, in particular the seasonal cycle of biomass and export production, but it does not appear that these are necessary consequences of the reduced tracer number. With increasing CO2, changes in dissolved oxygen and anthropogenic carbon uptake are very similar across the different models. Thus, while the reduced-tracer models do not explicitly resolve the diversity and internal dynamics of marine ecosystems, we demonstrate that such models are applicable to a broad suite of major biogeochemical concerns, including anthropogenic change. These results are very promising for the further development and application of reduced-tracer biogeochemical models that incorporate ‘sub-ecosystem-scale' parameterizations.
Rodgers, Keith B., J-L Lin, and Thomas L Frölicher, June 2015: Emergence of multiple ocean ecosystem drivers in a large ensemble suite with an earth system model. Biogeosciences, 12(11), DOI:10.5194/bg-12-3301-2015. Abstract
Marine ecosystems are increasingly impacted by human-induced changes. Ocean ecosystem drivers – including warming, acidification, deoxygenation and perturbations to biological productivity – can co-occur in space and time, but detecting their trends is complicated by the presence of noise associated with natural variability in the climate system. Here we use Large Initial-Condition Ensemble Simulations with a comprehensive Earth System Model under a historical/RCP8.5 pathway over 1950–2100 to consider emergence characteristics for the four individual and combined drivers. Using a one-standard deviation (67% confidence) threshold of signal-to-noise to define emergence with a 30 yr trend window, we show that ocean acidification emerges much earlier than other drivers, namely during the 20th century over most of the global ocean. For biological productivity, the anthropogenic signal does not emerge from the noise over most of the global ocean before the end of the 21st century. The early emergence pattern for sea surface temperature in low latitudes is reversed from that of subsurface oxygen inventories, where emergence occurs earlier in the Southern Ocean. For the combined multiple-driver field, 41% of the global ocean exhibits emergence for the 2005–2014 period, and 63% for the 2075–2084 period. The combined multiple-driver field reveals emergence patterns by the end of this century that are relatively high over much of the Southern Ocean, North Pacific, and Atlantic, but relatively low over the tropics and the South Pacific. In regions with pronounced emergence characteristics, marine ecosystems can be expected to be pushed outside of their comfort zone determined by the degree of natural background variability to which they are adapted. The results here thus have implications not only for optimization of the ocean observing system, but also for risk assessment and mitigation strategies.
Majkut, J D., Jorge L Sarmiento, and Keith B Rodgers, April 2014: A Growing Oceanic Carbon Uptake: Results from an inversion study of surface pCO2 data. Global Biogeochemical Cycles, 28(4), DOI:10.1002/2013GB004585. Abstract
Concerted community efforts have been devoted to producing an authoritative climatology of air-sea CO 2 fluxes [Takahashi et al., 2009], but identifying decadal trends in CO 2 fluxes has proven to be more challenging. The available surface pCO 2 estimates are too sparse to separate long-term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo [MCMC] sampling as a novel technique for estimating the historical pCO 2 at the ocean surface. The result is a plausible history of surface pCO 2 based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO 2 data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO 2, with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr − 1 of anthropogenic carbon and an increase in the global annual uptake over the 30-year time period of 0.4 ± 0.1 PgC yr − 1 decade − 1. The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling.
Plancherel, Y, Keith B Rodgers, Robert M Key, A R Jacobson, and Jorge L Sarmiento, July 2013: Role of regression model selection and station distribution on the estimation of oceanic anthropogenic carbon change by eMLR. Biogeosciences, 10(7), DOI:10.5194/bg-10-4801-2013. Abstract
Differencing predictions of linear regression models generated from hydrographic data collected at different times (the eMLR method) was proposed as a means of quantifying the dominant patterns of change in oceanic anthropogenic carbon in the context of sparse data sets subject to natural variability. The ability of eMLR to recover the anthropogenic carbon signal in the North Atlantic was tested using a global circulation and biogeochemistry model. Basin-scale applications of eMLR on horizontal layers can estimate the change in anthropogenic carbon inventory with an accuracy typically better than 10%. Regression model selection influences the distribution of the recovered anthropogenic carbon change signal. The systematic use of statistically optimum regression formulae does not produce the best estimates of anthropogenic carbon change if the distribution of the station locations emphasizes hydrographic features differently in time. Additional factors, such as a balanced station distribution and vertical continuity of the regression formulae should be considered to guide model selection. Accurate results are obtained when multiple formulae are used throughout the water column. Different formulae can yield results of similar quality. The fact that good results are obtained in the hydrographically complex North Atlantic suggests that eMLR can produce accurate estimates in other basins.
The distribution of radiocarbon (14C) in the ocean and atmosphere has fluctuated on timescales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute 14C within the Earth system on inter-annual to centennial timescales. The model, CM2Mc, is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory's CM2M model, uses no flux adjustments, and incorporates a simple prognostic ocean biogeochemistry model including 14C. The atmospheric 14C and radiative boundary conditions are held constant, so that the oceanic distribution of 14C is only a function of internal climate variability. The simulation displays previously-described relationships between tropical sea surface 14C and the model-equivalents of the El Niño Southern Oscillation and Indonesian Throughflow. Sea surface 14C variability also arises from fluctuations in the circulations of the subarctic Pacific and Southern Ocean, including North Pacific decadal variability, and episodic ventilation events in the Weddell Sea that are reminiscent of the Weddell Polynya of 1974–1976. Interannual variability in the air-sea balance of 14C is dominated by exchange within the belt of intense Southern Westerly winds, rather than at the convective locations where the surface 14C is most variable. Despite significant interannual variability, the simulated impact on air-sea exchange is an order of magnitude smaller than the recorded atmospheric 14C variability of the past millennium. This result partly reflects the importance of variability in the production rate of 14C in determining atmospheric 14C, but may also reflect an underestimate of natural climate variability, particularly in the Southern Westerly winds.
Gorgues, T, Olivier Aumont, and Keith B Rodgers, August 2010: A mechanistic account of increasing seasonal variations in the rate of ocean uptake of anthropogenic carbon. Biogeosciences, 7(8), DOI:10.5194/bg-7-2581-2010. Abstract
A three-dimensional circulation model that includes a representation of anthropogenic carbon as a passive tracer is forced with climatological buoyancy and momentum fluxes. This simulation is then used to compute offline the anthropogenic ΔpCO2 (defined as the difference between the atmospheric CO2 and its seawater partial pressure) trends over three decades between the years 1970 and 2000. It is shown that the mean increasing trends in ΔpCO2 reflects an increase of the seasonal amplitude of ΔpCO2. In particular, the ocean uptake of anthropogenic CO2 is decreasing (negative trends in ΔpCO2) in boreal (austral) summer in the Northern (Southern) Hemisphere in the subtropical gyres between 20° N (S) and 40° N (S). In our simulation, the increased amplitude of the seasonal trends of the ΔpCO2 is mainly explained by the seasonal sea surface temperature (SST) acting on the anthropogenic increase of the dissolved inorganic carbon (DIC). It is also shown that the seasonality of the anthropogenic DIC has very little effect on the decadal trends. Finally, an observing system for pCO2 that is biased towards summer measurements may be underestimating uptake of anthropogenic CO2 by about 0.6 PgC yr−1 globally over the period of the WOCE survey in the mid-1990s according to our simulations. This bias associated with summer measurements should be expected to grow larger in time and underscores the need for surface CO2 measurements that resolve the seasonal cycle throughout much of the extratropical oceans.
Sarmiento, Jorge L., M Gloor, Nicolas Gruber, C Beaulieu, A R Jacobson, Sara E Mikaloff-Fletcher, Stephen W Pacala, and Keith B Rodgers, August 2010: Trends and regional distributions of land and ocean carbon sinks. Biogeosciences, 7(8), DOI:10.5194/bg-7-2351-2010. Abstract
We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 Pg C yr−1 between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) Pg C yr−1 to a new relatively constant mean of −1.15 Pg C yr−1 between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.
Rodgers, Keith B., Robert M Key, Anand Gnanadesikan, Jorge L Sarmiento, John P Dunne, and A R Jacobson, et al., September 2009: Using altimetry to help explain patchy changes in hydrographic carbon measurements. Journal of Geophysical Research, C09013, DOI:10.1029/2008JC005183. Abstract
Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O2) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O2 column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O2 column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictability's CO2/Repeat Hydrography program).
Correction: 10.1029/2009JC005835
Rodgers, Keith B., Jorge L Sarmiento, Olivier Aumont, Cyril Crevoisier, C de Boyer Montégut, and N Metzl, June 2008: A wintertime uptake window for anthropogenic CO2 in the North Pacific. Global Biogeochemical Cycles, 22, GB2020, DOI:10.1029/2006GB002920. Abstract
An ocean model has been forced with NCEP reanalysis fluxes over 1948–2003 to evaluate the pathways and timescales associated with the uptake of anthropogenic CO2 over the North Pacific. The model reveals that there are two principal regions of uptake, the first in the region bounded by 35–45°N and 140–180°E, and the second along a band between 10–20°N and between 120°W and 180°E. For both of these regions, the dominant timescale of variability in uptake is seasonal, with maximum uptake occurring during winter and uptake being close to zero or slightly negative during summer when integrated over the basin. A decadal trend toward increased uptake of anthropogenic CO2 consists largely of modulations of the uptake maximum in winter. For detection of anthropogenic changes, this implies that in situ measurements will need to resolve the seasonal cycle in order to capture decadal trends in ΔpCO2. As uptake of anthropogenic CO2 occurs preferentially during winter, observationally based estimates which do not resolve the full seasonal cycle may result in underestimates of the rate of uptake of anthropogenic CO2. There is also a sizable circulation-driven decadal trend in the seasonal cycle of sea surface ΔpCO2 for the North Pacific, with maximum changes found near the boundary separating the subtropical and subpolar gyres in western and central regions of the basin. These changes are due to a trend in the large-scale circulation of the gyres, which itself is driven by a trend in the wind stress over the basin scale. This trend in the three-dimensional circulation is more important than the local trend in mixed layer depth (MLD) in contributing to the decadal trend in ΔpCO2.
Gorgues, T, C Menkes, Olivier Aumont, Y Dandonneau, G Madec, and Keith B Rodgers, March 2007: Indonesian throughflow control of the eastern equatorial Pacific biogeochemistry. Geophysical Research Letters, 34, L05609, DOI:10.1029/2006GL028210. Abstract
Two model simulations were performed to address the influence of the Indonesian throughflow (ITF) on the biogeochemical state of the equatorial Pacific. A simulation where the ITF is open is compared with an experiment where it is closed, and it is first shown that the impacts on the physical circulation are consistent with what has been found in previous modelling studies. In terms of biochemistry, closing the ITF results in increased iron concentration at the origin of the Equatorial Undercurrent (EUC). But the 11Sv of water otherwise transferred to the Indian Ocean remain in the equatorial Pacific, which result in a 30 m deepening of the thermocline/ferricline in the eastern Pacific. This deepening decreases the iron concentration of the equatorial wind driven upwelled water and cancels the iron increase advected by the EUC. The iron decrease of the equatorial upwelled water leads to decrease primary production by 15% along the equator.
Naegler, T, Philippe Ciais, Keith B Rodgers, and I Levin, June 2006: Excess radiocarbon constraints on air-sea gas exchange and the uptake of CO2 by the oceans. Geophysical Research Letters, 33, L11802, DOI:10.1029/2005GL025408. Abstract
We re-assess the constraints that estimates of the global ocean excess radiocarbon inventory (IE ) place on air-sea gas exchange. We find that the gas exchange scaling parameter a q cannot be constrained by IE alone. Non-negligible biases in different global wind speed data sets require a careful adaptation of a q to the wind field chosen. Furthermore, a q depends on the spatial and temporal resolution of the wind fields. We develop a new wind speed- and inventory-normalized gas exchange parameter a qN which takes into account these biases and which is easily adaptable to any new estimate of IE. Our study yields an average estimate of a q of 0.32 ± 0.05 for monthly mean winds, lower than the previous estimate (0.39) from Wanninkhof (1992). We calculate a global annual average piston velocity for CO2 of 16.7 ± 2.9 cm/hr and a gross CO2 flux between atmosphere and ocean of 73 ± 10 PgC/yr, significantly lower than results from previous studies.
Orr, James C., V J Fabry, Olivier Aumont, Laurent Bopp, Scott C Doney, Richard A Feely, Anand Gnanadesikan, Nicolas Gruber, A Ishida, Fortunat Joos, Robert M Key, Keith Lindsay, E Maier-Reimer, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, Keith B Rodgers, C L Sabine, Jorge L Sarmiento, R Schlitzer, Richard D Slater, I J Totterdell, M-F Weirig, Y Yamanaka, and Andrew Yool, September 2005: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059), DOI:10.1038/nature04095. Abstract
Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms such as corals and some plankton will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.