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The non-steady state oceanic CO2 signal: its importance, magnitude and a novel way to detect it
B. I. McNeil,R. J. Matear
Biogeosciences (BG) & Discussions (BGD) , 2013,
Abstract: The role of the ocean has been pivotal in modulating rising atmospheric CO2 levels since the industrial revolution, sequestering nearly half of all fossil-fuel derived CO2 emissions. Net oceanic uptake of CO2 has roughly doubled between the 1960s (~1 Pg C yr 1) and 2000s (~2 Pg C yr 1), with expectations that it will continue to absorb even more CO2 with rising future atmospheric CO2 levels. However, recent CO2 observational analyses along with numerous model predictions suggest the rate of oceanic CO2 uptake is already slowing, largely as a result of a natural decadal-scale outgassing signal. This recent CO2 outgassing signal represents a significant shift in our understanding of the oceans role in modulating atmospheric CO2. Current tracer-based estimates for the ocean storage of anthropogenic CO2 assume the ocean circulation and biology is in steady state, thereby missing the new and potentially important "non-steady state" CO2 outgassing signal. By combining data-based techniques that assume the ocean is in a steady state, with techniques that constrain the net oceanic CO2 uptake signal, we show how to extract the non-steady state CO2 signal from observations. Over the entire industrial era, the non-steady state CO2 outgassing signal (~13 ± 10 Pg C) is estimated to represent about 9% of the total net CO2 inventory change (~142 Pg C). However, between 1989 and 2007, the non-steady state CO2 outgassing signal (~6.3 Pg C) has likely increased to be ~18% of net oceanic CO2 storage over that period (~36 Pg C). The present uncertainty of our data-based techniques for oceanic CO2 uptake limit our capacity to quantify the non-steady state CO2 signal, however with more data and better certainty estimates across a range of diverse methods, this important and growing CO2 signal could be better constrained in the future.
The non-steady-state oceanic CO2 signal: its importance, magnitude and a novel way to detect it
B. I. McNeil,R. J. Matear
Biogeosciences Discussions , 2012, DOI: 10.5194/bgd-9-13161-2012
Abstract: The ocean's role has been pivotal in modulating rising atmospheric CO2 levels since the industrial revolution, sequestering over a quarter of all fossil-fuel derived CO2 emissions. Net oceanic uptake of CO2 has roughly doubled between the 1960's (~1 Pg C yr 1) and 2000's (~2 Pg C yr 1), with expectations it will continue to absorb even more CO2 with rising future atmospheric CO2 levels. However, recent CO2 observational analyses along with numerous model predictions suggest the rate of oceanic CO2 uptake is already slowing, largely as a result of a natural decadal-scale outgassing signal. This recent and unexpected CO2 outgassing signal represents a paradigm-shift in our understanding of the oceans role in modulating atmospheric CO2. Current tracer-based estimates for the ocean storage of anthropogenic CO2 assume the ocean circulation and biology is in steady state, thereby missing the new and potentially important "non-steady-state" CO2 outgassing signal. By combining data-based techniques that assume the ocean is in steady-state, with techniques that constrain the net oceanic CO2 uptake signal, we show how to extract the non-steady-state CO2 signal from observations. Over the entire industrial era, the non-steady-state CO2 outgassing signal (~13 ± 10 Pg C) is estimated to represent about 9% of the total net CO2 inventory change (~142 Pg C). However between 1989 and 2007, the non-steady-state CO2 outgassing signal (~6.3 Pg C) has likely increased to be ~18% of net oceanic CO2 storage over that period (~36 Pg C), a level which cannot be ignored. The present uncertainty of our data-based techniques for oceanic CO2 uptake limit our capacity to quantify the non-steady-state CO2 signal, however with more data and better certainty estimates across a~range of diverse methods, this important and growing CO2 signal could be better constrained in the future.
Downscaling the climate change for oceans around Australia
M. A. Chamberlain,C. Sun,R. J. Matear,M. Feng
Geoscientific Model Development Discussions , 2012, DOI: 10.5194/gmdd-5-425-2012
Abstract: At present, global climate models used to project changes in climate do not resolve mesoscale ocean features such as boundary currents and eddies. These missing features may be important to realistically project the marine impacts of climate change. Here we present a framework for dynamically downscaling coarse climate change projections utilising a global ocean model that resolves these features in the Australian region. The downscaling model used here is ocean-only. The ocean feedback on the air-sea fluxes is explored by restoring to surface temperature and salinity, as well as a calculated feedback to wind stress. These feedback approximations do not replace the need for fully coupled models, but they allow us to assess the sensitivity of the ocean in downscaled climate change simulations. Significant differences are found in sea surface temperature, salinity, stratification and transport between the downscaled projections and those of the climate model. While the magnitude of the climate change differences may vary with the feedback parameterisation used, the patterns of the climate change differences are consistent and develop rapidly indicating they are mostly independent of feedback that ocean differences may have on the air-sea fluxes. Until such a time when it is feasible to regularly run a global climate model with eddy resolution, our framework for ocean climate change downscaling provides an attractive way to explore how climate change may affect the mesoscale ocean environment.
Evaluation of a near-global eddy-resolving ocean model
P. R. Oke,D. A. Griffin,A. Schiller,R. J. Matear
Geoscientific Model Development Discussions , 2012, DOI: 10.5194/gmdd-5-4305-2012
Abstract: Analysis of the variability in an 18-yr run of a near-global, eddy-resolving ocean general circulation model coupled with biogeochemistry is presented. Comparisons between modelled and observed mean sea level (MSL), mixed-layer depth (MLD), sea-level anomaly (SLA), sea-surface temperature (SST), and Chlorophyll a indicate that the model variability is realistic. We find some systematic errors in the modelled MLD, with the model generally deeper than observations, that results in errors in the Chlorophyll a, owing to the strong biophysical coupling. We evaluate several other metrics in the model, including the zonally-averaged seasonal cycle of SST, meridional overturning, volume transports through key Straits and passages, zonal averaged temperature and salinity, and El Nino-related SST indices. We find that the modelled seasonal cycle in SST is 0.5–1.5 °C weaker than observed; volume transports of the Antarctic Circumpolar Current, the East Australian Current, and Indonesian Throughflow are in good agreement with observational estimates; and the correlation between the modelled and observed NINO SST indices exceed 0.91. Most aspects of the model circulation are realistic. We conclude that the model output is suitable for broader analysis to better understand ocean dynamics and ocean variability.
Downscaling the climate change for oceans around Australia
M. A. Chamberlain, C. Sun, R. J. Matear, M. Feng,S. J. Phipps
Geoscientific Model Development (GMD) & Discussions (GMDD) , 2012, DOI: 10.5194/gmd-5-1177-2012
Abstract: At present, global climate models used to project changes in climate poorly resolve mesoscale ocean features such as boundary currents and eddies. These missing features may be important to realistically project the marine impacts of climate change. Here we present a framework for dynamically downscaling coarse climate change projections utilising a near-global ocean model that resolves these features in the Australasian region, with coarser resolution elsewhere. A time-slice projection for a 2060s ocean was obtained by adding climate change anomalies to initial conditions and surface fluxes of a near-global eddy-resolving ocean model. Climate change anomalies are derived from the differences between present and projected climates from a coarse global climate model. These anomalies are added to observed fields, thereby reducing the effect of model bias from the climate model. The downscaling model used here is ocean-only and does not include the effects that changes in the ocean state will have on the atmosphere and air–sea fluxes. We use restoring of the sea surface temperature and salinity to approximate real-ocean feedback on heat flux and to keep the salinity stable. Extra experiments with different feedback parameterisations are run to test the sensitivity of the projection. Consistent spatial differences emerge in sea surface temperature, salinity, stratification and transport between the downscaled projections and those of the climate model. Also, the spatial differences become established rapidly (< 3 yr), indicating the importance of mesoscale resolution. However, the differences in the magnitude of the difference between experiments show that feedback of the ocean onto the air–sea fluxes is still important in determining the state of the ocean in these projections. Until such a time when it is feasible to regularly run a global climate model with eddy resolution, our framework for ocean climate change downscaling provides an attractive way to explore the response of mesoscale ocean features with climate change and their effect on the broader ocean.
The combined impact of CO2-dependent parameterisations of Redfield and Rain ratios on ocean carbonate saturation
K. F. Kvale,K. J. Meissner,M. d'Orgeville,R. J. Matear
Biogeosciences Discussions , 2011, DOI: 10.5194/bgd-8-6265-2011
Abstract: Future changes to the organic carbon and carbonate pumps are likely to affect ocean ecosystem dynamics and the biogeochemical climate. Here, biological dependencies on the Rain and Redfield ratios on pCO2 are implemented in a coupled Biogeochemistry-Ocean Model, the CSIRO-Mk3L, to establish extreme-case carbonate saturation vulnerability to model parameterisation at year 2500 using IPCC Representative Concentration Pathway 8.5. Surface carbonate saturation is relatively insensitive to the combined effects of variable Rain and Redfield ratios (an anomaly of less than 10 % of the corresponding change in the control configuration by year 2500), but the global zonally-averaged ocean interior anomaly due to these feedbacks is up to 130 % by 2500. A non-linear interaction between organic and carbonate pumps is found in export production, where higher rates of photosynthesis enhance calcification by raising surface alkalinity. This non-linear effect has a negligible influence on surface carbonate saturation but does significantly influence ocean interior carbonate saturation fields (an anomaly of up to 45 % in 2500). The strongest linear and non-linear sensitivity to combined feedbacks occurs in low-latitude remineralisation zones below regions of enhanced biological production, where dissolved inorganic carbon rapidly accumulates.
Enhanced biological carbon consumption in a high CO2 ocean: a revised estimate of the atmospheric uptake efficiency
R. Matear,B. McNeil
Biogeosciences Discussions , 2009,
Abstract: A recent mesocosm study under high CO2 conditions has found phytoplankton carbon consumption is elevated beyond typical Redfield ratios (Riebesell et al., 2007). We investigate the efficacy of this elevated biological carbon consumption to increase global oceanic CO2 uptake from the atmosphere in an ocean general circulation model (OGCM). In the OGCM, elevated biological carbon consumption throughout the ocean increased oceanic CO2 uptake by 46 Pg C during 1800 to 2100 period, which is less than half the value estimated by (Riebesell et al., 2007). Our study's lower ratio of oceanic CO2 uptake from the atmosphere caused by enhanced biological carbon consumption (export production) is due to a more realistic 3-D circulation and the resulting spatial patterns in the re-supply of carbon from the interior ocean to the surface. In our OGCM simulations, despite increased biological carbon export to the ocean interior, some regions like the eastern equatorial Pacific and Southern Ocean actually take up less CO2 from the atmosphere. This is due to the pooling of exported carbon at intermediate depths within these regions (analogous to nutrient trapping) and its subsequent re-supply back to the surface that exceeds the enhanced biological carbon export in the high CO2 world. Thus large-scale increases in biological carbon export can lead to some areas where surface ocean pCO2 increases more rapidly than atmospheric CO2. Furthermore, our results demonstrate that enhancing biological carbon export via other means such as iron fertilization is inefficient in regions like the Southern Ocean because of the rapid vertical re-supply of carbon-rich waters. This vertical resupply of carbon-rich waters in the Southern Ocean dampens the oceanic CO2 uptake efficiency due to enhanced biological carbon consumption to be only 16% and suggests a very low efficacy of biological fertilization in the region.
How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?
O. Duteil, W. Koeve, A. Oschlies, O. Aumont, D. Bianchi, L. Bopp, E. Galbraith, R. Matear, J. K. Moore, J. L. Sarmiento,J. Segschneider
Biogeosciences (BG) & Discussions (BGD) , 2012,
Abstract: Phosphate distributions simulated by seven state-of-the-art biogeochemical ocean circulation models are evaluated against observations of global ocean nutrient distributions. The biogeochemical models exhibit different structural complexities, ranging from simple nutrient-restoring to multi-nutrient NPZD type models. We evaluate the simulations using the observed volume distribution of phosphate. The errors in these simulated volume class distributions are significantly larger when preformed phosphate (or regenerated phosphate) rather than total phosphate is considered. Our analysis reveals that models can achieve similarly good fits to observed total phosphate distributions for a~very different partitioning into preformed and regenerated nutrient components. This has implications for the strength and potential climate sensitivity of the simulated biological carbon pump. We suggest complementing the use of total nutrient distributions for assessing model skill by an evaluation of the respective preformed and regenerated nutrient components.
Projected climate change impact on oceanic acidification
Ben I McNeil, Richard J Matear
Carbon Balance and Management , 2006, DOI: 10.1186/1750-0680-1-2
Abstract: Our results show that the direct decrease in pH due to ocean warming is approximately equal to but opposite in magnitude to the indirect increase in pH associated with ocean warming (ie reduced DIC concentration of the upper ocean caused by lower solubility of CO2).As climate change feedbacks on pH approximately cancel, future oceanic acidification will closely follow future atmospheric CO2 concentrations. This suggests the only way to slowdown or mitigate the potential biological consequences of future ocean acidification is to significantly reduce fossil-fuel emissions of CO2 to the atmosphere.Rising atmospheric CO2 concentrations via fossil fuel emissions will lead to an increase in oceanic CO2 via thermodynamic equilibration. Carbon chemistry in seawater undergoes the following equilibrium reactions as CO2 enters the ocean.CO2 +H2O ？ H2CO3 ？ + H+ ？ + 2H+ (1)The pH of seawater is defined by the amount of H+ ions available: pH = -log10[H+]. Increasing CO2 concentrations in the surface ocean via anthropogenic CO2 uptake will have implications for oceanic pH. As shown in equation (1), when CO2 dissolves in water it forms a weak acid (H2CO3), dissociates to bicarbonate generating hydrogen ions (H+), which makes the ocean less basic (pH decreases). Using an ocean-only model forced with atmospheric CO2 projections (IS92a), Caldeira and Wickett [4]predicted a pH drop of 0.4 units by the year 2100 and a further decline of 0.7 by the year 2300.Future acidification (lowering of pH) may adversely impact marine biota, but our present understanding of the potential biological response is limited [1]. It is recognised however that a decrease in pH will alter the acid-base balance with the cells of marine organisms [1]. Marine organisms regulate intercellular pH by the metabolic interconversion of acids and bases, the passive chemical buffering of intra- and extra-cellular fluids, and the active ion transport (e.g. proton transport by extra-cellular respiratory proteins such as
Global warming projection of the change in dissolved oxygen concentrations in low oxygen regions of the oceans
Matear,Richard;
Gayana (Concepción) , 2006, DOI: 10.4067/S0717-65382006000300010
Abstract: global warming projections using a range of climate models included in the ipcc 4th assessment report (ar4) suggest the oceans will warm, the stratification of the upper ocean will increase and the ventilation of the ocean interior will change. these physical changes will impact dissolved oxygen levels in the ocean. using a global warming projection from the csiro (australian commonwealth scientific and research administration) climate model linked to a simple ocean biogeochemical model i investigated how dissolved oxygen levels in the ocean interior change under global warming. the climate simulations project the low oxygen regions like the eastern equatorial pacific will expand. by the end of the century it is projected that the volume of hypoxic water (<10 mmol/kg) in the thermocline of the eastern equatorial pacific ocean will double.
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