Uptake Of Anthropogenic Carbon Research Articles

A 1998–1992 comparison of inorganic carbon and its transport across 24.5°N in the Atlantic

Abstract In January and February 1998, when an unprecedented fourth repetition of the zonal hydrographic transect at 24.5°N in the Atlantic was undertaken, carbon measurements were obtained for the second time in less than a decade. The field of total carbon along this section is compared to that provided by 1992 cruise which followed a similar path (albeit in a different season). Consistent with the increase in atmospheric carbon levels, an increase in anthropogenic carbon concentrations of 8±3 μmol kg −1 was found in the surface layers. Using an inverse analysis to determine estimates of absolute velocity, the flux of inorganic carbon across 24.5° is estimated to be −0.74±0.91 and −1.31±0.99 PgC yr −1 southward in 1998 and 1992, respectively. Estimates of total inorganic carbon flux depend strongly upon the estimated mass transport, particularly of the Deep Western Boundary Current. The 1998 estimate reduces the large regional divergence in the meridional carbon transport suggested by previous studies and brings into question the idea that the tropical Atlantic constantly outgasses carbon, while the subpolar Atlantic sequesters it. Uncertainty in the carbon transports themselves, dominated by the uncertainty in the total mass transport estimates, are a hindrance to determining the “true” picture. The flux of anthropogenic carbon (C★ANTH) across the two transects is estimated as northward at 0.20±0.08 and 0.17±0.06 PgC yr −1 for the 1998 and 1992 sections, respectively. The net transport of C★ANTH across 24.5°N is strongly affected by the difference in concentrations between the northward flowing shallow Florida Current and the mass balancing, interior return flow. The net northward transport of C★ANTH is opposite the net flow of total carbon and suggests, as has been found by others, that the pre-industrial southward transport of carbon within the Atlantic was stronger than it is today. Combining these flux results with estimates of atmospheric and riverine inorganic carbon input, it is determined that today's oceanic carbon system differs from the pre-industrial system in that today there is an uptake of anthropogenic carbon to the south that is advected northward and stored within the North Atlantic basin.

On the role of heat fluxes in the uptake of anthropogenic carbon in the North Atlantic

The influence of the overturning circulation on the anthropogenic carbon sink in the North Atlantic is investigated with a simple box model. The net air‐sea flux of anthropogenic carbon in the North Atlantic is the result of two opposing fluxes: The first is the uptake caused by the disequilibrium between the rapidly rising atmospheric pCO2 and the dissolved carbon content in the ocean, depending mainly on the water exchange rate between mixed layer and interior North Atlantic ocean. Superimposed is a second flux, related to the northward transport of heat within the Atlantic basin, that is directed out of the ocean, contrary to conventional wisdom. It is caused by a latitudinal gradient in the ratio of seawater alkalinity to total dissolved inorganic carbon that in turn is related to the cooling and freshening of surface water on its way north. This flux depends strongly on the vertical structure of the upper branch of the overturning circulation and on the distribution of undersaturation and supersaturation of CO2 in Atlantic surface waters. A data‐based estimate of anthropogenic carbon inventory in the North Atlantic is consistent with a dominance of the disequilibrium flux over the heat‐flux‐related outgassing at the present time, but, in our model, does not place a strong constraint on the net anthropogenic air‐sea flux. Stabilization of the atmospheric pCO2 on a higher level will change the relative role of the two opposing fluxes, making the North Atlantic a source of anthropogenic carbon to the atmosphere. We discuss implications for the interpretation of numerical carbon cycle models.

Modelling regional responses by marine pelagic ecosystems to global climate change

Current coupled ocean‐atmosphere model (COAM) projections of future oceanic anthropogenic carbon uptake suggest reduced rates due to surface warming, enhanced stratification, and slowed thermohaline overturning. Such models rely on simple, bulk biogeochemical parameterisations, whereas recent ocean observations indicate that floristic shifts may be induced by climate variability, are widespread, complex, and directly impact biogeochemical cycles. We present a strategy to incorporate ecosystem function in COAM's and to evaluate the results in relation to region‐specific ecosystem dynamics and interannual variability using a template of oceanic biogeographical provinces. Illustrative simulations for nitrogen fixers with an off‐line multi‐species, functional group model suggest significant changes by the end of this century in ecosystem structure, with some of the largest regional impacts caused by shifts in the areal extent of biomes.

Estimates of anthropogenic carbon uptake from four three‐dimensional global ocean models

We have compared simulations of anthropogenic CO2 in the four three‐dimensional ocean models that participated in the first phase of the Ocean Carbon‐Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85±0.35 Pg C yr−1 for the 1980–1989 average. Regionally, the Southern Ocean dominates the present‐day air‐sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data‐based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data‐based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the data‐based estimates could be provided regionally.

Climate Model Simulations of Effects of Increased Atmospheric CO2and Loss of Sea Ice on Ocean Salinity and Tracer Uptake

Abstract Recent observations show a decrease in the extent of Northern Hemisphere sea ice; this decrease has been attributed to human activities. Climate model simulations are presented that examine how loss of sea ice affects the ocean salinity and density structure, and rates of uptake of an idealized transient tracer. The latter results are indicative of how loss of sea ice might affect the ocean’s rate of uptake of anthropogenic carbon from the atmosphere. In simulations in which there is no freshwater forcing due to sea ice forming or melting, the salinity minimum associated with Antarctic Intermediate Water is much weaker than in simulations of the present-day ocean. This suggests that this salinity minimum is maintained in part by a steady supply of freshwater from melting of Antarctic sea ice. In addition, in simulations with no freshwater forcing due to sea ice, vertical salinity and density gradients in the Southern and Arctic Oceans are weaker than in simulations of the present-day ocean. This ...

Increase in the uptake rate of oceanic anthropogenic carbon in the North Pacific determined by CFC ages

We propose an approach to estimate the rate of increase of oceanic anthropogenic carbon inventory with CFC11 dating technique. This approach relies on the elapsed time from when the water lost contact with atmosphere as determined by CFC age. Furthermore, the assumption is made that it remains constant over a decadal time scale. Finally, we consider only the increase in anthropogenic carbon from one decade to another and not the entire change from the pre-industrial period to the present. The advantages and disadvantages of our approach are discussed. Using this approach, the spatial distributions of the rate of increase of the anthropogenic carbon inventory and the uptake rate of anthropogenic carbon in the North Pacific were obtained. The western North Pacific subtropical region exhibited a maximum in the rate of increase of the anthropogenic carbon inventory of more than 8 g C m −2 year −1 during 1988–1998, which was equivalent to 34% of the total uptake rate in the entire North Pacific. The net total uptake rate of anthropogenic carbon in the whole North Pacific increased with time and was 0.55±0.09 Pg C year −1 during 1988–1998 indicating that the North Pacific absorbs 24% of the whole oceanic uptake of anthropogenic carbon.

Anthropogenic CO2 invasion into the northeast Pacific based on concurrent δ13CDIC and nutrient profiles from the California Current

The stable isotopic signature of dissolved inorganic carbon (δ13CDIC) in the northeast Pacific Ocean is lower in near‐surface waters by ≈1.1‰ relative to values predicted from global oceanic trends of δ13CDIC versus nutrients. A combination of anthropogenic carbon uptake from the atmosphere and thermodynamic, air‐sea gas exchange processes in different water mass source areas account for the isotopic depletion. Here we evaluate the efficacy of using a concurrent nutrient‐δ13C strategy to separate these two effects, with the goal of improving estimates of anthropogenic carbon uptake over the course of the Industrial Revolution. In depth profiles from the sea surface to 2500 m at four stations across the California Current (42°N), nitrate, rather than phosphate, is best correlated to δ13CDIC providing the best choice for this experiment. On the basis of an assumption of no anthropogenic carbon in North Pacific Deep Waters between 1000–2500 m depth (potential densities, σθ ∼ 27.3–27.7), the “anthropogenic— preanthropogenic” carbon isotope shift (Δδ13Ca−p) in near‐surface waters of the northeast Pacific is inferred to be −0.62 ± 0.17‰, while the thermodynamic air‐sea gas exchange signature is estimated at −0.48 ± 0.17‰. Values of Δδ13Ca−p (similar to the regional patterns of Δ14C and Tritium penetration) approach zero for σθ > 26.8, indicating little penetration of anthropogenic carbon into the North Pacific Intermediate Water or the upper North Pacific Deep Water. Our results suggest an upper North Pacific sink of anthropogenic carbon over the past ∼200 years that is ∼40% greater than that estimated for the interval between ∼1970 and ∼1990 by Quay et al., [1992]. Our estimate of the North Pacific inventory of anthropogenic carbon, added to published estimates from the North Atlantic and Indian Ocean, is smaller than model predictions of the total carbon sink, suggesting that a significant portion of anthropogenic carbon enters the deep sea via the Southern Ocean.

Long‐term variability of the terrestrial and oceanic carbon sinks and the budgets of the carbon isotopes 13C and 14C

The long‐term variability in the terrestrial and oceanic uptake of anthropogenic carbon is investigated. Ice core and direct observations of atmospheric CO2 and 13C are used for the last 200 years. An inverse method called double deconvolution is applied. It is found that the biosphere turned from a carbon source of about 0.5 Gt C yr−1 into a sink of 1 Gt C yr−1 during the first half of this century. This is in qualitative agreement with earlier reconstructions based on atmospheric CO2 data and implies a terrestrial sink to compensate land use emissions during the last five decades. Oceanic and biospheric carbon uptakes are estimated to be 0.9±1.0 and l.l±1.0 Gt C yr−1 as averaged over the 1970–1990 period. Hence ocean uptake is on the low side of current estimates, but our results may be biased as δ13C observations between 1956 and 1982 are missing. Additional uncertainties in calculated carbon sinks are due to uncertainties in model parameters and in fossil emission estimates. Prior to 1950, uncertainties are primarily related to uncertainties in the ice core δ13C data; a Monte Carlo analysis yields a l‐σ uncertainty in the terrestrial and oceanic uptake of ±0.36 Gt C yr−1 when ice core data are smoothed over a 50 year period. The budget of bomb‐produced radiocarbon is reinvestigated. We could not find model solutions that concomitantly match the bomb budget and the observed atmospheric δ13C and prebomb Δ14C decrease. The closure of the budget would require a relatively low oceanic and biospheric 14C uptake that conflicts with the relatively high isotopic uptake rates required to simulate the observed decrease in δ13C and Δ14C. We conclude that recent estimates of bomb test productions and/or the stratospheric 14C decrease are not compatible with published 13C and 14C observations.

Ocean carbon transport in a box‐diffusion versus a general circulation model

We have compared vertical transport of temperature, anthropogenic CO2, natural radiocarbon (14C), and bomb 14C in a global box‐diffusion model (B‐D) and a three‐dimensional (3‐D) ocean general circulation model from the Geophysical Fluid Dynamics Laboratory. Our main objectives were (1) to test the eddy diffusion parameterization of large‐scale vertical transport in ocean box models and (2) to assess the utility of bomb‐produced and natural 14C observations to validate ocean models used to estimate anthropogenic CO2 uptake. Prom the 3‐D model's distributions and fluxes of natural 14C, bomb 14C, and anthropogenic CO2, we have calculated apparent diffusivities (Kap) vertically over the global ocean that range mostly between 4000 and 8000m2 yr−1. These Kap agree quantitatively with diffusivities found by fitting B‐D models to observed distributions of natural and bomb 14C We then used these sets of Kap in different runs of a global B‐D model. Results from all B‐D models runs matched to within 13% those from the 3‐D model for global uptake of anthropogenic CO2 and bomb‐14C penetration depth. Although Kap from 3‐D simulations for bomb 14C vary with time, those from 3‐D runs for anthropogenic CO2 are essentially constant. Still, we found nearly the same results with the B‐D model when Kap from 3‐D bomb 14C simulations are approximated as time invariant. The best agreement (within 3%) between 3‐D CO2 simulations and B‐D model runs was found when applying Kap derived from bomb 14C in the surface and from natural 14C in the deep. Agreement was worse when using Kap from 3‐D simulations for anthropogenic CO2itself, mostly because in this case deeper Kap could only be extrapolated from higher surface values. We have found it appropriate to study global oceanic uptake of anthropogenic CO2 with B‐D model and to validate anthropogenic carbon uptake models using natural and bomb 14C observations. For bomb 14C in the 3‐D model, convective transport was most important during 1955–1964 while atmospheric levels were rising; afterward, atmospheric levels drop, and advective overturning dominates as for natural 14C. Thus 14C seems less than ideal to validate the convective scheme of general circulation models.

Pulse response functions are cost-efficient tools to model the link between carbon emissions, atmospheric CO 2 and global warming

The development of models to simulate the redistribution of anthropogenic carbon and the climate response to greenhouse gas forcing requires a substantial effort. The behavior of such models can be characterized by pulse response functions which allows one to build simple substitute models. We have used mixed-layer pulse response functions to describe the surface-to-deep mixing of tracers in the ocean and biospheric decay response functions to describe carbon turnover in the land biota. We build a simple carbon cycle-climate model using response functions. For the Princeton-GFDL ocean model, we find that the agreement between the complete model and its pulse substitute model is better than 4% for the cumulative uptake of anthropogenic carbon applying the IPCC stabilization scenarios S450 and S750. We have simulated the transient temperature response to an increasing CO 2 concentration (1% yr −1) prescribing climate sensitivities of 2.1 and 4.6 K for a CO 2 doubling in the substitute model. We find a global surface temperature warming of 1.6 and 2.4 K after 80 years in good agreement with the corresponding results (1.5 to 2.7 K) of 9 atmosphere-ocean general circulation models (Houghton et al., 1996). The pulse model is efficient; a 1000 year simulation of the pulse substitute model requires 25 seconds of CPU time on a workstation.

An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake

An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake

An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake

Establishing the link between atmospheric CO 2 concentration and anthropogenic carbon emissions requires the development of complex carbon cycle models of the primary sinks, the ocean and terrestrial biosphere. Once such models have been developed, the potential exists to use pulse response functions to characterize their behaviour. However, the application of response functions based on a pulse increase in atmospheric CO 2 to characterize oceanic uptake, the conventional technique, does not yield a very accurate result due to nonlinearities in the aquatic carbon chemistry. Here, we propose the use of an ocean mixed-layer pulse response function that characterizes the surface to deep ocean mixing in combination with a separate equation describing air-sea exchange. The use of a mixed-layer pulse response function avoids the problem arising from the nonlinearities of the carbon chemistry and gives therefore more accurate results. The response function is also valid for tracers other than carbon. We found that tracer uptake of the HILDA and Box-Diffusion model can be represented exactly by the new method. For the Princeton 3-D model, we find that the agreement between the complete model and its pulse substitute is better than 4% for the cumulative uptake of anthropogenic carbon for the period 1765–2300 applying the IPCC stabilization scenarios S450 and S750 and better than 2% for the simulated inventory and surface concentration of bomb-produced radiocarbon. By contrast, the use of atmospheric response functions gives deviations up to 73% for the cumulative CO 2 uptake as calculated with the Princeton 3-D model. We introduce the use of a decay response function for calculating the potential carbon storage on land as a substitute for terrestrial biosphere models that describe the overturning of assimilated carbon. This, in combination with an equation describing the net primary productivity permits us to exactly characterize simple biosphere models. As the time scales of biospheric overturning are one key aspect to determine the amount of anthropogenic carbon which might be sequestered by the biosphere, we suggest that decay response functions should be used as a simple and standardized measure to compare different models and to improve understanding of their behaviour. We provide analytical formulations for mixed-layer and terrestrial biosphere decay pulse response functions which permit us to easily build a substitute for the ‘Bern’ carbon cycle model (HILDA). Furthermore, mixed-layer response functions for the Box-Diffusion, a 2-D model, and the Princeton 3-D model are given. DOI: 10.1034/j.1600-0889.1996.t01-2-00006.x