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Keywords:

  • CFCs;
  • anthropogenic CO2;
  • North Pacific

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[1] From model simulations of the uptake of CFC-11, CFC-12 and anthropogenic CO2, we investigate the method of using CFC-derived water ages to determine the anthropogenic CO2 accumulation in the ocean. The CFC age method is best suited to water younger than 30 years where CFC water ages are most reliable. For water younger than 30 years, the CFC-12 and CFC-11 age method can estimate the global 1980–1990 accumulation of anthropogenic CO2 to within 10% and 22%, respectively. With the exception of the Southern Ocean south of 50°S and the equatorial upwelling areas, these CFC-derived estimates of the water column inventory of anthropogenic CO2 are within 10% of the simulated values. Our results suggest that the CFC age method provides another way of determining the accumulation of anthropogenic CO2 in the ocean. We applied the CFC age method to observed CFC data to estimate the accumulation of anthropogenic CO2 in the North Pacific (20°N–65°N) between 1980 and 1990. For water younger than 30 years the CFC-derived estimate of accumulation is 1.1 Gt C. In comparison, the estimate from the simulated CFC distribution is 1.3 Gt C. Our model simulation overestimated the accumulation north of 40°N and underestimated the penetration of anthropogenic CO2 into the subtropical gyre. From the observed CFC data we estimate the anthropogenic CO2 uptake by the North Pacific to be 1.1 Gt C/yr for the 1980s.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[2] The ocean plays an important role in the global carbon cycle, absorbing approximately 30% of our present anthropogenic CO2 emissions. However, the oceanic uptake of CO2 is uncertain. Spatial information on the ocean inventory of anthropogenic CO2 provides valuable information to assess model estimates of anthropogenic CO2 uptake and storage. The validation of models is crucial to reducing the uncertainty in the global carbon budget and providing confidence in the future oceanic CO2 uptake predicted by these models.

[3] Methods to estimate the ocean inventory of anthropogenic CO2 were first independently proposed by Brewer [1978] and Chen and Pytkowicz [1979]. The original method was extensively applied but the calculated values had large uncertainties [Shiller, 1981; Broecker et al., 1985]. Gruber et al. [1996] proposed a new technique (hereinafter referred to as the GSS method) for quantifying the ocean inventory of anthropogenic CO2 that addressed some of the shortcomings of the original method. The method was applied in the Atlantic [Gruber, 1998] and the Indian [Sabine et al., 1999a, 1999b] Oceans to provide information to assess ocean carbon models [Caldeira and Duffy, 2000].

[4] The GSS method separates the large natural spatial variability in dissolved inorganic carbon (DIC) concentrations from the anthropogenic CO2 signal to provide estimates of the anthropogenic CO2 concentration (Canthro). This requires subtracting from the observed in situ DIC concentration (C), the DIC value in equilibrium with the preindustrial atmosphere (Ceq), the DIC changes due to the remineralization of organic and inorganic carbon(Cbio) and the DIC value for the carbon disequilibrium between the atmosphere and surface ocean for the time when the atmospheric CO2 was sequestered from the atmosphere (Cdiseq).

  • equation image

Ceq is calculated using the preindustrial atmospheric fCO2 value and the in situ temperature (T), salinity (S) and the preformed alkalinity concentration (Alko). Alko is computed by fitting the surface alkalinity with a linear function of surface temperature, salinity, dissolved phosphate and dissolved oxygen. The DIC change due to the remineralization of organic matter is estimated from the changes in apparent oxygen utilization (AOU) and a constant ratio of C/O2 (rC/O2) for the remineralization. The DIC change due to inorganic carbon remineralization is computed by subtracting the preformed alkalinity concentration (Alk0) from the in situ alkalinity concentration with a correction for the change in alkalinity due to the remineralization of organic matter using a constant ratio nitrate to oxygen (rN/O2). In summary, the Cbio is given by

  • equation image

[5] In computing Cdiseq, the GSS method assumed that the ocean has been operating in steady state, that the disequilibrium has remained constant within the isopycnal outcrop region and that water is transported along isopycnal surfaces. For isopycnal surfaces that contain an anthropogenic CO2 signal, one utilizes the age of water to estimate Cdiseq; either CFCs or tritium data provide a convenient way of estimating water ages. From the water age and atmospheric history of CO2 one computes Cdiseq by subtracting from the in situ DIC concentration the calculated Ceq value and the organic and inorganic remineralization effect. This removes the anthropogenic signal and one is left with an estimate of Cdiseq,

  • equation image

where τ is the water age and t the sampling date and fCO2(t − τ) denotes the atmospheric CO2 fugacity when the water parcel was last in contact with the atmosphere. Finally, Cdiseq is averaged along isopycnal surfaces to provide the estimated air-sea disequilibrium value.

[6] The DIC changes due to the remineralization of organic and inorganic carbon are large and small errors in either the C/O2 ratio or oxygen saturation of the surface water introduce large uncertainties into the estimated anthropogenic CO2 concentration [Wanninkhof et al., 1999; Sabine et al., 1999a, 1999b]. Furthermore, the assumption that Cdiseq is only a function of density and independent of time is not valid for upwelled deep water that contains no anthropogenic signal. Finally, to apply this method requires hydrographic and in situ DIC measurements which limits the available data for calculating anthropogenic CO2. For this study we explore whether the water mass ages inferred from CFC-11 are sufficient to determine anthropogenic CO2. We extend the recent study by Watanabe et al. [2000] that used CFC ages to estimate anthropogenic CO2 by using model simulations to assess the success and limitations of using this approach to estimate the accumulation of anthropogenic CO2 in the ocean.

[7] From model simulations of the oceanic uptake of CFC-11, CFC-12 and anthropogenic CO2 we demonstrate that CFC-derived ages can estimate the global decadal accumulation of anthropogenic CO2 in the ocean to within 10% error. Guided by the model results we apply the approach to the North Pacific CFC observations to compute the 1980–1990 accumulation of anthropogenic CO2. Finally, we compare the modeled CFC water ages and anthropogenic CO2 accumulation to the values derived from the observations to assess the model performance.

2. Age-Derived Estimate of Anthropogenic CO2

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[8] As pointed out by GSS and used by Watanabe et al. [2000], another method for determining anthropogenic CO2 concentrations can be derived by substituting Cdiseq from equation (3) into equation (1) to give

  • equation image

Where fCO2(t − τ) denotes the atmospheric CO2 fugacity when the water parcel was last at the surface and τ is the water age and t is the year in which one wants to calculate anthropogenic CO2 concentration. The advantage in using equation (4) to calculate anthropogenic CO2 is that one only needs water mass ages and standard hydrographic data (i.e., temperature, salinity, phosphate and oxygen) to determine anthropogenic CO2. However, it is unproven that water mass ages can accurately determine anthropogenic CO2. Also, equation (4) assumes that the disequilibrium term has not changed since preindustrial times and the water mass ages are not changing with time. Once the age of the water is determined one can obtain the anthropogenic CO2 concentration for any year.

3. Model Assessment of the Water Age Method

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[9] To test the ability of equation (4) to estimate anthropogenic CO2 accumulation, we use the CSIRO Ocean Carbon Model [Matear and Hirst, 1999] to simulate the oceanic uptake of CFC-11, CFC-12 and anthropogenic CO2. The CFC-11 and CFC-12 simulation follows the OCMIP 2 CFCs protocol and the comparison of our CFC-11 simulation with the other models in the OCMIP 2 study is given by Dutay et al. [2002]. In the model, the accumulation of anthropogenic CO2 in the ocean is determined from the difference in DIC between a run with the atmosphere CO2 fixed at 280 μatm and a run where the atmospheric CO2 increased according to observations between 1880 and 1990. For both these runs, the biogeochemical model used simulated temperature, salinity, oxygen, phosphate, alkalinity and DIC in the ocean [Matear and Hirst, 1999].

3.1. Approach

[10] To compute anthropogenic CO2 from the CFC distributions we first convert the simulated CFCs into pCFCs by dividing the concentrations by the solubility computed from temperature and salinity [Warner and Weiss, 1985]. Then we use the pCFC11 and pCFC12 values to obtain two estimates of the water ages from the atmospheric history of CFC-11 and CFC-12 [Warner et al., 1996]. In the CFC simulation, the CFC saturations at the surface are not assumed to be 100%. Rather the model computes the flux of CFCs into the ocean from the pCFC difference between the atmosphere and the ocean and a wind speed dependent gas exchange coefficient [Dutay et al., 2002]. Therefore, the age of the water at the surface is not always zero because a finite time is required for the upper ocean to equilibrate with the atmosphere.

[11] Once the water ages are determined we use equation (4) and the atmospheric history of CO2 to compute the anthropogenic CO2 concentrations. For both the analysis of the model simulations and the subsequent analysis of the North Pacific observations presented in the next section we choose the reference year to be 1990. The choice is arbitrary but the chosen year reflects the mean year of the CFC observations in the North Pacific.

[12] Equation (4) gives the accumulation of anthropogenic CO2 since preindustrial times (1880 in the model simulation) with the assumption that the disequilibrium between the atmosphere and the ocean has not changed since preindustrial time. From the model simulation, we calculate the Cdiseq for any year t as

  • equation image

where

  • equation image

The calculated map of the change in Cdiseq between 1990 and 1880 (preindustrial time) shows large regions where the value has changed by more than 20 μmol/kg (Figure 1). Typically, regions with recently ventilated old water with no anthropogenic signal show the largest change in Cdiseq. The GSS method, which assumes Cdiseq is independent of time would have this error in the anthropogenic CO2 concentrations. Globally this would lead to an overestimate of the anthropogenic CO2 concentrations because the ocean lags the increase in CO2 in the atmosphere, hence Cdiseq becomes more negative with time.

image

Figure 1. From the anthropogenic CO2 simulation, the surface change in Cdiseq between 1990 and 1880. The contour interval is 5 μmol/kg.

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[13] For the calculation of anthropogenic CO2 accumulation from the water ages one can deal with the changing value of Cdiseq since preindustrial time by reducing the period over which one computes the accumulation. If the period is short enough then we can correctly assume that Cdiseq is constant and our estimate of the accumulation of anthropogenic CO2 between years t2 and t1 is given by

  • equation image
  • equation image

To start with we looked at the accumulation of anthropogenic CO2 between 1980 and 1990 (Figure 2). The simulated accumulation of anthropogenic CO2 generally agrees with the CFC-12 age- derived estimates to within ±1 μmol/kg for water with a CFC-12 age younger than 30 years. For water older than 30 years the disagreement increases dramatically. The CFC age calculation is limited to water younger than 1931 since prior to this year the atmosphere had no CFCs. Therefore with a reference year of 1990 we cannot calculate an anthropogenic change for water older than 59 years (1990–1931). For water between 59 and 30 years old the large discrepancy shown in Figure 2 reveals how mixing bias in the CFC age estimate causes an underestimate of the water ages and an overestimate of its anthropogenic CO2 accumulation. Due to the large errors in the CFC method for water older than 30 years we will restrict the CFC age estimates to water younger than 30 years. In the subsequent estimates of the global and water inventory of anthropogenic CO2 accumulation from CFC ages we will omit any accumulation in water older than 30 years.

image

Figure 2. For all oceans, the zonal averaged difference in anthropogenic CO2 accumulation between the simulated value and the value calculated from CFC-12 ages in μmol/kg. Included are contour lines of the water mass age (years) estimated from CFC-12.

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[14] We compare the simulated global accumulation of anthropogenic CO2 in water younger than 30 years (based on CFC-12) with the values derived from the CFC-11 and CFC-12 ages (Figure 3). For the accumulation period of 1960 or more recent to 1990 the difference in the global accumulation in water younger than 30 years between the simulated anthropogenic CO2 accumulation and the CFC method is less than 25%. CFC-12 does better than CFC-11 at estimating accumulation (Table 1) because of its more linear atmosphere history prior to 1990. For accumulation between years earlier than 1960 and 1990 the CFC age estimates diverge from the simulated value. The greater accumulation in the CFC-derived estimates reflects the tendency in Cdiseq to become more negative with time.

image

Figure 3. Global accumulation of anthropogenic CO2 in water younger than 30 years from the anthropogenic CO2 simulation (dashed line) and calculated from the CFC-11 (pluses) and CFC-12 (crosses) simulations. The solid line shows the accumulation in the entire water column from the anthropogenic CO2 simulation.

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Table 1. Global Anthropogenic CO2 Accumulation for the 1980–1990 Period Determined From the CFCs and Anthropogenic CO2 Simulations
Method Used to Calculate AccumulationGt C
  • a

    Water younger than 30 years determined from CFC-12 ages.

  • b

    Water younger than 30 years determined from CFC-11 ages.

Water Older Than 30 Years
Simulated accumulation of anthropogenic CO215.6
 
Water Younger Than 30 Years
Simulated accumulation of anthropogenic CO2a12.4
CFC-12 derived accumulationa13.7
CFC-11 derived accumulationb14.7

[15] The CFC age-derived estimates constantly overestimate the accumulation with the global values falling between the value for water younger than 30 years and the value for the entire water column (Figure 3). The CFC-derived estimates of accumulation in water younger than 30 years approximate the total accumulation of anthropogenic CO2 during the 1980–1990 period to within 15%. The agreement suggests that accumulation estimated from the CFC data for water younger than 30 years provides a proxy for the total decadal accumulation of anthropogenic CO2.

[16] To assess the regional success of the CFC age-derived estimate we compare the water column inventory of anthropogenic CO2 accumulation during the 1980–1990 period from the anthropogenic simulation (Ia) to the values derived from the CFC-12 ages for water younger than 30 years (Icfc). For the comparison, we plot Icfc/Ia for Ia inventory in water younger than 30 years (Figure 4a) and for Ia inventory in the entire water column (Figure 4b). With the exception of the high-latitude Southern Ocean and equatorial upwelling areas, the CFC-12 derived estimates are within ±10% of the simulated accumulation in water younger than 30 years. The CFC-12 age-derived estimates are also generally within ±20% of the total simulated accumulation. The North Pacific is definitely a basin where the CFC-12 estimate is successful at predicting the accumulation in water younger than 30 years (within 10%). The obvious problem region in Figure 4 is the high-latitude Southern Ocean where the CFC-derived method greatly overestimates the accumulation. Fortunately, the accumulation in this region is small so the large departure of the Icfc/Ia ratio from 1 does not reflect a large error in the CFC-derived estimate of the global anthropogenic CO2 accumulation.

image

Figure 4. The ratio of the anthropogenic accumulation for the 1980–1990 period from (a) CFC-12 ages younger than 30 years to the simulated anthropogenic accumulation in water younger 30 years, and (b) CFC-12 ages younger than 30 years to the total simulated anthropogenic accumulation. In the plots the bold contour line denotes a ratio of 1.

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3.2. Limitations

[17] For water younger than 30 years, the CFC-12 and CFC-11 age-derived estimates reproduce the simulated global accumulation of anthropogenic CO2 during the 1980 to 1990 period to within 10% and 22%, respectively. In applying the CFC method, key assumptions are made that introduce errors and biases into these estimates. Using the model simulations we attempt to quantify the potential errors in CFC age-derived estimates of anthropogenic CO2 accumulation and we discuss how to minimize their impact. The underlying assumptions in the method are (1) Cdiseq is constant with time, (2) gas saturation of the CFCs equals the gas saturation of anthropogenic CO2, (3) CFCs can accurately determine water ages, and (4) ocean is in steady state. To minimize the impact of assumption one, the estimated accumulation is restricted to a decadal period. From the model simulation of anthropogenic CO2 uptake, the change in the Cdiseq between 1990 and 1980 is less than ±2 μmol/kg with the exception of the eastern equatorial Pacific and the bottom water formation regions in the North Atlantic and Southern Oceans (Figure 5). This assumption causes the CFC age method to overestimate the anthropogenic CO2 accumulation.

image

Figure 5. Simulated surface change in Cdiseq between 1990 and 1980.

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[18] One expects the equilibration of CFCs in the surface ocean with the atmosphere to occur faster than anthropogenic CO2. Therefore, assumption two causes the CFC age method to overestimate the accumulation. The saturation of anthropogenic CO2 is computed by dividing the simulated anthropogenic CO2 concentration by the anthropogenic concentration in equilibrium with the atmosphere (Canthrotheorectical). At the surface for the year 1990, the saturation of CFC-12 is compared with the saturation of anthropogenic CO2 (Figure 6). The largest disagreement occurs in regions where the Cdiseq is changing most rapidly with time. However, for surface ocean of the subtropical gyres, the CFC-12 and anthropogenic CO2 saturations are similar.

image

Figure 6. For 1990, the surface ocean anthropogenic CO2 saturation divided by CFC-12 saturation.

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[19] The third assumption is critical to the application of the CFC age method. Due to mixing bias one expects the CFC water ages to underestimate the true water ages producing an overestimate of the anthropogenic CO2 accumulation. Figure 2 shows that for water older than 30 years the CFC age-derived estimates of anthropogenic CO2 accumulation deviates by up to 3 μmol/kg from the simulated value but for water younger than 30 years the difference is generally less than 1 μmol/kg. For the 1980–1990 period, the accumulation estimated from CFC-12 and CFC-11 ages is greater than the simulated anthropogenic accumulation in water younger than 30 years by 10% and 22%, respectively (Table 1). The overestimate reflects the combined errors of assumptions 1, 2, and 3. In comparison the simulated accumulation in water older than 30 years accounts for another 26% more accumulation (Table 1). If we use the difference in accumulation estimated from the CFC-12 and CFC-11 ages to quantify the error associated with water mass ages we get an uncertainty of 1 Gt C (8%). To further assess the uncertainties in CFC concentrations on the CFC age-derived anthropogenic accumulation we assume the CFC concentrations are 5% less than the simulated value. Then we recompute the water ages and the corresponding anthropogenic CO2 accumulation for the 1980 to 1990 period. The calculation provides an upper estimate of the error because it assumes that errors in the CFC concentrations are depth correlated. Using this approach for errors in CFC concentrations, we obtain a decrease in the 1980 to 1990 accumulation of less than 10%. From these two estimates we assign an uncertainty of 10% and 13% to the accumulation in water younger than 30 years due to errors in CFC-12 and CFC-11 water ages, respectively.

[20] If one assumes the CFC-12 ages reflect the true water ages then one can quantify the minimum error associated with assumptions 1 and 2. They cause an overestimate of the accumulation of 1.2 Gt C (10%). By comparing the accumulation from the CFC age method in water younger than 30 years to the total water column simulated accumulation, we are exploiting the systematic bias in the method for overestimating the accumulation. For our model simulations CFC-12 and CFC-11 accumulation estimates are only 13% and 7% less than the total simulated accumulation for 1980–1990 period. One may expect this agreement would be model dependent.

[21] In the calculation of the water mass ages we use the CFC distributions from only 1 year (one realization) to compute the water ages, which implies the water ages are not changing with time. However, recent ocean observations suggest changes are occurring that may impact the rate of ventilation of the ocean interior [Levitus et al., 2000; Matear et al., 2000; Wong et al., 2001] and alter the water ages with time. To investigate the potential errors associated with changes in ocean ventilation we utilize the climate change simulation of Matear et al. [2000] to simulate the uptake of CFCs with and without climate change. From these two simulations we apply the CFC method to the 1990 distributions and estimate the accumulation of anthropogenic CO2 between 1980 and 1990. The difference in the accumulation between these two simulations is small, less than 0.2 Gt C. If we take the simulated change in anthropogenic accumulation with climate change projected by the model for the decade of the 1990s and 2000s [Matear and Hirst, 1999] the decline is less than 1 Gt C and 3 Gt C, respectively. Prior to the year 2000, the steady assumption appears to be adequate for decadal accumulation but for the future estimates of the accumulation continued collection of CFC data appears necessary to correct for potential changes in water ages with time.

4. Calculated Anthropogenic CO2 Inventory in the North Pacific

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[22] During the past several decades, numerous hydrographic sections have been obtained in the North Pacific with CFC data (Figure 7 and Table 2). We use the temperature, salinity, nutrient and CFC data from these cruises to determine anthropogenic CO2 inventory using equation (4). Following the approach used with the model simulations, we convert the CFC data to pCFC values. Then for each section we computed the water ages using the pCFC concentrations method [Warner et al., 1996]. From the water ages and equation (4) we computed the accumulation of anthropogenic CO2 between 1980 and 1990. The water column inventory of anthropogenic CO2 for the North Pacific is shown in Figure 7. The highest inventory is found in the western subtropical gyre and the lowest inventory in high latitudes of the northwest Pacific. Figure 8 shows the CFC-12 derived accumulation of anthropogenic CO2 along the 165°E and 165°W sections. The accumulation of anthropogenic CO2 derived from CFC-11 is nearly identical to Figure 8 hence it is not shown. One observes slightly deeper penetration of the anthropogenic CO2 in the western section than in the eastern section. The maximum penetration is about 1100 m and it occurs in the midlatitude subtropical gyre. The maximum anthropogenic CO2 accumulation occurs in the subtropical gyre.

image

Figure 7. Inventory of anthropogenic CO2 (mol/m2) for the 1990–1980 period between the surface and 30-year old water from the observed North Pacific CFC-12 data. The locations of the sections used for this compilation are denoted by the symbols and they are summarized in Table 2.

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image

Figure 8. From the observed CFC-12 concentrations, the CFC-derived estimate of anthropogenic CO2 accumulation (μmol/kg) along (a) 165°E and (b) 165°W for the 1980–1990 period.

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Table 2. List of North Pacific Cruises Used to Estimate Anthropogenic CO2 Accumulation in the North Pacific
LineDateReferencea
P1Aug 04–Sep 07, 1985Talley [1987]
P1WAug 30–Sep 21, 1992Wong et al. [1998]
P3Mar 30–Jun 03, 1985*
P10Oct 5–Nov 10, 1993Sabine et al. [2000]
P13Aug 4–Oct 21, 1992Dickson et al. [2000]
P15NSep 6–Oct 10, 1994*
P16NFeb 14–Apr 8, 1994*
P17NMay 15–Jun 26, 1993*
GEMS 92-2Aug 15–Sep 1, 1992*

5. Model-Data Comparison

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[23] To evaluate the performance of the model simulations presented in section 3, we first compare the water ages derived from simulated CFC-12 distribution (Figure 9) to the ages derived from the observed CFC-12 concentrations (Figure 9). Unlike the observations, the model's isolines of water ages are flat and lack a clear difference between the subtropics and the high latitudes. In the observations, the 30-year age contour goes from a maximum depth of 900 m in the subtopics to 400 m in the high latitude (Figure 9) while in the model it is nearly a constant 500 m (Figure 9). The underestimation of CFC-11 penetration in the North Pacific subtropical gyre and the overestimation in the high latitude north of 40°N is a common feature of the models in the Ocean Model Inter-Comparison Project (OCMIP) [Dutay et al., 2002].

image

Figure 9. From the simulated CFC-12 concentrations, the CFC-derived water ages in years along (a) 165°E and (b) 165°W.

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[24] To investigate how model data misfits in the water age impact the accumulation of anthropogenic CO2, we compare the estimated accumulation of anthropogenic CO2 concentrations from the observed CFCs to the accumulation derived from the simulated CFC distributions. For the 1980–1990 period in the North Pacific, the inventory from the CFC method for water younger than 30 years (Figure 10) is similar to values calculated from the CFC observations (Figure 7) and to the simulated anthropogenic inventory (Figure 11). Both show the highest inventories in the subtropical gyre of the northwest Pacific. In the observations, the inventories decrease rapidly from the peak value of 3.5 to 1.0 mol/m2 as one goes eastward along 30°N, but in the model the decrease is more gradual and smaller (3.5 to 1.5 mol/m2). Consistent with the water age comparison, the model overpredicts the penetration of anthropogenic CO2 north of 40°N and underpredicts the penetration in the western subtropical gyre (Figure 12 versus Figure 8). Recent simulations of anthropogenic CO2 uptake in the North Pacific [Xu et al., 2000] also overestimate anthropogenic inventory north of 50°N and underestimate it in the subtropical gyre. The higher inventories in the model in both the high latitude and eastern Pacific leads to a total anthropogenic carbon inventory (1.3 Gt C) that is greater than the observations (1.1 Gt C) (Table 3). The comparison of the simulated CFC-derived accumulations with those from the observations shows that for all latitude bands the model overestimates the accumulation.

image

Figure 10. Water ages in years calculated from the observed CFC-12 data along (a) 165°E and (b) 165°W.

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image

Figure 11. For the 1980–1990 period, the water column inventory of anthropogenic CO2 (mol/m2) from (a) the anthropogenic CO2 simulation and (b) derived from the simulated CFC-12 for water younger than 30 years.

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image

Figure 12. From the CFC-12 simulation, the CFC-12 derived estimate of anthropogenic CO2 accumulation (μmol/kg) between 1980 and 1990 along (a) 165°E and (b) 165°W. The 30-year CFC-12 age contour is denoted by the bold line.

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Table 3. For the 1980–1990 Period, the Accumulation of Anthropogenic CO2 in the North Pacific and the Corresponding Simulated Oceanic Air-Sea Uptake of Anthropogenic CO2
RegionObserved Accumulation Derived From CFC Ages, Gt CSimulated Accumulation Derived From CFC Ages, Gt CSimulated Accumulation Derived From the Anthropogenic CO2 Simulation, Gt CSimulated Air-Sea Flux of Anthropogenic CO2, Gt C
CFC-12CFC-11CFC-12CFC-11For Water Younger Than 30 YearsEntire Water Column
20°N–30°N0.420.430.530.550.540.610.34
30°N–40°N0.340.350.400.410.400.440.46
40°N–50°N0.190.190.250.260.230.260.43
50°N–65°N0.110.110.130.130.120.130.16
Total1.061.081.311.351.291.441.39

[25] In the model, the CFC-derived accumulation of anthropogenic CO2 in the North Pacific in water younger than 30 years approximates the simulated oceanic uptake of anthropogenic CO2 because in the ocean the net transport of anthropogenic CO2 through 20°N is small (Table 3). If we assume that this is applicable to the observations then the observed inventory gives an estimate of the anthropogenic uptake of CO2 for the North Pacific of 1.1 Gt C for the 1980–1990 period.

[26] Excess storage of anthropogenic CO2 in the north and northeast Pacific in the model suggests that the transport of anthropogenic CO2 from the high latitudes into the western subtropics gyre is not properly simulated. We suggest two potential causes of the deficiencies in the simulated water ages and anthropogenic CO2 inventories. One, the eddy-advection parameterization is too strong in the North Pacific and it is removing the sloping isopycnal in the high latitude North Pacific [Danabasoglu and McWilliams, 1995]. A comparison of our simulation to the IGCR model of Dutay et al. [2002] and Xu et al. [2000] demonstrates that using a different eddy parameterization cannot solve the problem. Two, upwelling north of 40°N is too weak and not supplying enough older water to the upper ocean. The lack of a Bering Strait transport in our model may exacerbate the problem by not providing an avenue to transport light freshwater out of the Pacific. This excess storage in the model north of 40°N suggests the model is underestimating the uptake of anthropogenic CO2 north of 40°N because too much anthropogenic CO2 is being stored in the region rather than being transported south.

6. Discussion and Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[27] Using model simulations of CFC-11, CFC-12 and anthropogenic CO2 uptake we compare the CFC-derived water age estimates of anthropogenic CO2 accumulation with the simulated accumulation. The limitation in using CFC-derived ages is that for water older than 1960 (30 years) the ages are underestimated because of mixing bias which leads to a large overestimate of anthropogenic CO2 accumulation. Restricting our CFC estimates of accumulation to water younger than 30 years shows that the CFC-12 and CFC-11 method overestimates the global 1980–1990 simulated anthropogenic CO2 accumulation in water younger than 30 years by 10% and 22%, respectively. The three key assumptions of the CFC age method are (1) Cdiseq is constant with time, (2) saturation of CFCs equals the saturation of anthropogenic CO2 and (3) pCFCs can be used to determine water age. All three assumptions cause the method to overestimate the accumulation. This enables the CFC-derived estimates from water younger than 30 years to approximate the total global accumulation. For our model simulations, the CFC-12- and CFC-11-derived estimates from water younger than 30 years are within 13% and 7%, respectively, of the simulated global accumulation for the 1980–1990 period. The spatial map of the water column inventory of anthropogenic CO2 accumulation for the 1980–1990 period for water younger than 30 years from the CFC method also agrees with simulated accumulation to within 10% except in the high-latitude Southern Ocean and equatorial upwelling areas. The large overestimate of the accumulation in these regions reflects the errors in the three key assumptions of the CFC method. In these regions, the ventilation of old water with low anthropogenic and CFC concentrations causes the CFC method to overestimate the anthropogenic accumulation.

[28] Our modeling work supports the use of water ages for estimating anthropogenic CO2 accumulation. The utilization of an age tracer to estimate anthropogenic CO2 concentrations allows one to address two of the limitations of the GSS method. One, we do not need to remove the large but uncertain biological signal from the in situ DIC measurements prior to computing anthropogenic CO2 concentrations. Second, we can minimize the assumption that the air-sea carbon disequilibrium is constant by reducing the period over which we compute the change in anthropogenic CO2 concentrations. The approach of converting CFC water ages to anthropogenic CO2 accumulation does not require a linear relationship between CFC and anthropogenic CO2 because one uses the observed atmospheric history to transform CFC to anthropogenic CO2. In our model simulations there is no linear relationship between the inventory of pCFC and anthropogenic CO2, but we can still recover the anthropogenic CO2 concentrations from the CFC concentrations using the atmospheric history of the two gases. By incorporating other time tracers like natural 14C or CFC-113 one could extend the calculation of anthropogenic CO2 to older water and improve the method. The use of age tracers provide another method for estimating anthropogenic CO2 that can be easily implemented and be used to assess the other methods for estimating anthropogenic CO2, like the multiple linear regression method [McNeil et al., 2001].

[29] To demonstrate the CFC method we applied it to North Pacific data to assess our model simulations. For the North Pacific, the estimated accumulation of anthropogenic CO2 between 1980–1990 in water younger than 30 years is 1.3 Gt C from the simulated CFCs compared to 1.1 Gt C from the observed CFCs. The model had excess storage of anthropogenic CO2 in the north and northeast Pacific, which suggests that the transport of anthropogenic CO2 from the high latitudes into the western subtropical gyre is not properly simulated. The model simulations suggest that the CFC-derived estimate of anthropogenic CO2 accumulation is approximately equal to uptake during the same period (1.4 Gt C). Using this result and the observed CFC data we estimate that the anthropogenic CO2 uptake for the North Pacific (north of 20°N) was approximately 1.1 Gt C for the 1980s.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

[30] We thank B. McNeil and B. Tilbrook for the helpful comments to the manuscript. R.J.M. acknowledges the support of Environment Australia Climate Change Research Program. C.S.W. thanks Fisheries and Oceans Canada for salary support and the Canadian Panel for Energy Research and Development (PERD) for financial support under PERD grant 52539 on Oceanic Uptake of CO2.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Age-Derived Estimate of Anthropogenic CO2
  5. 3. Model Assessment of the Water Age Method
  6. 4. Calculated Anthropogenic CO2 Inventory in the North Pacific
  7. 5. Model-Data Comparison
  8. 6. Discussion and Conclusion
  9. Acknowledgments
  10. References
  11. Supporting Information

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