Journal of Oceanography, Vol. 60, pp. 5 to 15, 2004
Temporal Evolution of the North Pacific CO2 Uptake Rate
CHRISTOPHER L. SABINE1*, RICHARD A. F EELY1, YUTAKA W. WATANABE2 and M ARILYN LAMB1
1
2
NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115, U.S.A.
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
(Received 10 September 2003; in revised form 29 November 2003; accepted 30 November 2003)
The recent changes in the North Pacific uptake rate of carbon have been estimated
using a number of different techniques over the past decade. Recently, there has
been a marked increase in the number of estimates being submitted for publication.
Most of these estimates can be grouped into one of five basic techniques: carbon
time-series, non-carbon tracers, carbon tracers, empirical relationships, and inverse
calculations. Examples of each of these techniques as they have been applied in the
North Pacific are given and the estimates summarized. The results are divided into
three categories: integrated water column uptake rate estimates, mixed layer increases, and surface pCO 2 increases. Most of the published values fall under the
water column integrated uptake rate category. All of the estimates varied by region
and depth range of integration, but generally showed consistent patterns of increased
uptake from the tropics to the subtropics. The most disagreement between the methods was in the sub-arctic Pacific. Column integrated uptake rates ranged from 0.25
to 1.3 mol m –2yr–1. The mixed layer uptake estimates were much more consistent,
with values of 1.0–1.3 µ mol kg–1yr–1 based on direct observations and multiple linear regression approaches. Surface pCO 2 changes showed the most obvious regional
variability (0.5–2.5 µ atm yr–1) reflecting the sensitivity of these measurements to
differences in the physical and biological forcing. The different techniques used to
evaluate the changes in North Pacific carbon distributions do not completely agree
on the exact magnitude or spatial and temporal patterns of carbon uptake rate. Additional research is necessary to resolve these issues and better constrain the role of
the North Pacific in the global carbon cycle.
Keywords:
⋅ Carbon cycle,
⋅ anthropogenic
CO 2,
⋅ interannual
variability,
⋅ North Pacific.
ventories, respectively (Sabine et al., 2002). This relatively low inventory compared to the area of the North
Pacific (nearly 25% of global ocean area) primarily results from the shallow ventilation of mode and intermediate waters. The deep waters of the North Pacific (>2000
m) are among the oldest in the global oceans and thus
have not been exposed to anthropogenic CO2 contamination.
Although the inventory estimates constrain the relative importance of the North Pacific’s ability to store anthropogenic CO2 over century time-scales, it does not
directly give the anthropogenic CO2 uptake rate, or how
the ocean sink has varied over time. Anthropogenic CO2
uptake rate can be estimated in two ways, either by examining the net transfer of CO2 into the ocean or by documenting changes in ocean carbon concentrations over
time. In order to qualify as a net sink for anthropogenic
CO2, however, there must be a net uptake of CO2 relative
to the preindustrial fluxes. Given the spatial and temporal variability of the ocean carbon system, this can be very
1. Introduction
The global CO 2 survey, conducted as a part of the
World Ocean Circulation Experiment (WOCE) and the
Joint Global Ocean Flux Study (JGOFS), was completed
in the 1990s. These data have now been synthesized into
a unified data set and used to estimate the total anthropogenic CO2 inventories of the major ocean basins (Johnson
et al., 1998; Sabine et al., 1999, 2002; Feely et al., 2001;
Lamb et al., 2002; Wanninkhof et al., 2003; Lee et al.,
2003).
The North Pacific Ocean is an important sink region
for atmospheric CO2. Subtropical surface waters are estimated to have total anthropogenic CO2 concentrations of
up to 40–50 µmol kg–1 and the total inventory of anthropogenic CO2 in the North Pacific is estimated to be 16.5
Pg C or 37% and 15% of the total Pacific and global in* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
5
2.
Approaches for Estimating CO2 Uptake Rate in
the North Pacific
Every study of the oceanic uptake rate of CO2 has
one or more unique aspects to it. Either they use different
data sets, examine different time periods, integrate over
different depth intervals, or use different methods. This
makes it very difficult to directly compare these different
estimates. However, it can be useful to group the various
estimates by general approach and consider the similarity or differences of these approaches. For this study we
have grouped the techniques into five basic categories:
carbon time-series, non-carbon tracers, carbon tracers,
empirical relationships, and inverse calculations.
2.1 Carbon time-series approach
One of the most direct ways to evaluate the ocean
uptake rate of CO2 is to monitor temporal changes in high
quality mixed layer dissolved inorganic carbon (DIC) or
6
C. L. Sabine et al.
340
320
320
300
300
280
280
260
260
240
240
220
220
200
200
Jan-92
Jan-94
Jan-95
Jan-96
Jan-97
Jan-98
Jan-99
Jan-00
Jan-01
180
Jan-02
Jan-93
Jan-94
Jan-95
Jan-96
Jan-97
Jan-98
Jan-99
Jan-00
Jan-01
Jan-02
Jan-93
Jan-94
Jan-95
Jan-96
Jan-97
Jan-98
Jan-99
Jan-00
Jan-01
Jan-02
Jan-93
AOU (µmol/kg : 27.3σθ)
AOU (µmol/kg : 26.9-27.2σθ)
(a)
2380
(b)
DIC (µmol/kg)
2360
2340
2320
2300
2280
2260
Jan-92
2380
2370
(c)
2360
2350
2340
TA (µeq./kg)
difficult to determine.
There are no measurements to constrain the spatial
patterns of the preindustrial fluxes or carbon distributions
in the water column, but since atmospheric CO2 concentrations have varied by less than 20 ppm during at least
the last 11,000 years prior to the anthropogenic perturbation (Indermühle et al., 1999; Flückiger et al., 2002), most
researchers have assumed that the ocean carbon distributions and net air-sea flux was in steady state (e.g., Prentice
et al., 2001). If this assumption is correct, then observations showing recent increases in the CO 2 flux into the
ocean or increases in the ocean carbon concentrations can
be attributed to anthropogenic CO2 uptake.
Recently there has been a marked increase in the
number of CO 2 uptake rate estimates being submitted for
publication. This manuscript examines some of the recent estimates of CO2 uptake rates in the North Pacific.
Although the techniques all agree that there has been a
substantial increase in carbon over the past several decades the exact magnitude is not completely clear. Some
of the differences may be related to the different techniques, but other potential changes in ecosystem structure, circulation, or carbon cycling may also complicate
the interpretation of carbon data. We discuss the different approaches for estimating CO2 uptake rates that have
been used in the North Pacific, including some of the pros
and cons of the various techniques, so the reader can critically evaluate the range of estimates that are available in
the literature. We do not attempt to summarize the various anthropogenic CO2 inventory estimates. Although the
inventories are clearly related to the uptake rate estimates
and the approaches are similar in some cases, anthropogenic inventories involve a different set of assumptions
and have different limitations than the uptake rate estimates.
2330
2320
2310
2300
2290
2280
Jan-92
䊉 26.9
䊏 27.0
䉱 27.1
䉬 27.2
+ 27.3
Fig. 1. Decadal changes of (a) AOU, (b) DIC and (c) TA on the
26.9σθ (ave. 230 m; solid circles), 27.0σθ (ave. 300 m; solid
squares), 27.1 σθ (ave. 380 m; solid triangles), 27.2σθ (ave.
490 m; solid diamonds) and 27.3σθ (ave. 630 m; plus) surfaces at Station KNOT in the western sub-arctic Pacific
observed during the period from 1992 to 2001. Linear regressions of the AOU, DIC and TA on the respective density surfaces (from Wakita et al., 2003).
CO2 partial pressure (pCO 2) from multiple occupations
at a given location. One of the earliest time-series estimations in the North Pacific was based on data from ocean
Station Papa in the northeastern sub-arctic Pacific. Based
on 5 years of pCO2 observations, Wong and Chan (1991)
infer a small net sink of 0.7 mol C m–2yr –1 at Station P.
However, the data did not indicate a change in the net
sink with time. One of the difficulties of this approach is
isolating the anthropogenic component of the signal from
the steady state fluxes that were present during
preindustrial times. The fact that no change in the net flux
could be observed over the 5 year time series does not
necessarily mean that anthropogenic CO2 was not being
taken up, but that the natural seasonal and interannual
variability in this region made it extremely difficult to
Table 1. Decadal changes of DIC on the 26.9–27.3 potential density surfaces at Station Knot. The errors represent standard error
of the slope of a least squares regression line.
Isopycnal surface
Ave. depth a
(m)
∆DIC/∆T
(µmol kg – 1 yr– 1 )
∆DICan th /∆Tb
(µmol kg – 1 yr– 1 )
∆DICex cess/∆Tc
(µmol kg – 1 yr– 1 )
∆DICo rg /∆Td
(µmol kg – 1 yr– 1 )
26.9
27.0
27.1
27.2
27.3
230 ± 37
293 ± 41
379 ± 43
487 ± 48
626 ± 47
2.3 ± 1.0
2.6 ± 0.9
2.5 ± 0.9
1.3 ± 0.6
1.3 ± 0.6
0.6 ± 0.2
0.5 ± 0.1
0.6 ± 0.1
0.7 ± 0.1
0.6 ± 0.1
1.7 ± 1.0
2.1 ± 0.9
1.9 ± 0.9
0.6 ± 0.6
0.7 ± 0.6
0.8 ± 0.8
1.2 ± 0.8
0.8 ± 0.7
0.6 ± 0.5
0.5 ± 0.6
403
2.0 ± 0.8
0.6 ± 0.1
1.4 ± 0.8
0.8 ± 0.7
Average
a
The average depth and standard deviation of isopycnal surface.
The estimated equilibrium change in DIC based on an atmospheric CO 2 increase of 1.8 ppm yr–1.
c
The difference between ∆DIC/∆T and ∆DIC anth/∆T.
d
The estimated change in DIC associated with changes in biology: RC:O∆AOU/∆T, where RC:O = 0.69 (Anderson and Sarmiento,
1994).
b
isolate the anthropogenic signal (Takahashi et al., 1993).
One complicating factor in evaluating time series changes
in pCO2 is the strong influence of temperature on the values. The DIC of the waters are not affected by changes in
temperature, but are still influenced by changes in biology and mixing. A 10 year record of surface DIC measurements at Station P between 1987 and 1998 also did
not indicate a clear anthropogenic increase (Wong et al.,
2002); probably due to strong seasonal vertical mixing
(Takahashi et al., 1993).
A similar approach has been used to look at the first
3 years of data from the Kyodo western North Pacific
Ocean Time-series (KNOT) program (Tsurushima et al.,
2002). As with the Station P data, it is difficult to identify
a clear increase in surface DIC with time because of the
large seasonal signal (100, 69, and 12 µmol kg–1 at Knot,
Station P, and HOT respectively) and strong interannual
variability in mixing and productivity. Deeper in the water column, however, the signal is more evident. Wakita
et al. (2003) has found that both DIC and apparent oxygen Utilization (AOU) have increased while TA remained
constant during the last decade (Fig. 1). The increase of
DIC (∆DIC/∆T, ave. 2.0 ± 0.8 µmol kg–1yr–1) was three
times higher than expected based on the rate of atmospheric CO 2 increases over this time period (Table 1). The
increases in AOU over time imply that there has been a
net change in either export production/remineralization
or ventilation in this region. By multiplying the change
in AOU by the stoichiometric ratio of carbon to oxygen,
one can estimate the effect of the changing biology on
the DIC. The estimated biological effect was not able to
account for all of the excess DIC (observed–equilibrium)
increase (Table 1). Therefore, Watika et al. (2003) infer
that there must be a net outgassing of non-anthropogenic
CO2 due to weakening of ventilation processes.
While surface carbon changes have been difficult to
detect and interpret in the sub-arctic Pacific, anthropogenic increases in DIC have been clearly observed in the
subtropical North Pacific. Winn et al. (1998) observed a
1.0 ± 2.9 µmol kg–1yr–1 average increase over the first 7
years of the Hawaii Ocean Time-series (HOT) program.
The most recent estimates have been revised slightly upward, to 1.2 ± 0.2 µmol kg–1yr–1, based on 13 years of
observations (Dore et al., 2003). A corresponding increase
in the surface pCO2 values is also noted. Dore et al. cautioned, however, that the effects of climate variability on
regional precipitation and evaporation patterns have a
strong influence on the local carbon uptake signal.
The longest time series of pCO2 measurements in
the Pacific Ocean is from the central and eastern equatorial Pacific. From data collected in the 1980s and 1990s,
Takahashi et al. (2003) observed that during the 1980s
the pCO2 of equatorial surface waters was decreasing at
a rate of about 2.0 µatm yr–1, while sea surface temperature (SST) was increasing; and during the decade of the
1990s the reverse was true, pCO2 was increasing at a rate
of about 1.5 µatm yr–1 while SST was decreasing. These
changes were attributed to changes in circulation and
upwelling processes along the equatorial belt, changes in
biological productivity, and/or some combination of these
processes.
Uptake rates inferred from the carbon time series
approach vary not only as a function of time, but also
spatially. Inoue et al. (1995) measured wintertime
seawater pCO2 along a line from 3°N to 35°N at 137°E in
the western North Pacific between 1981 and 1993 and
found that the growth rate of seawater pCO2 north of 15°N
was about the same as the atmosphere (~1.8 ± 0.6 µatm
Temporal Evolution of the North Pacific CO 2 Uptake Rate
7
60
.25
40
Latitude ( N)
yr–1), but south of 15°N it was significantly lower (0.5 ±
0.7 µatm yr–1). The data showed a south-to-north trend
of increasing uptake rate of CO2 from 11°N to 31°N. This
was the first description of regional variations of the CO2
uptake rate in the subtropical North Pacific.
20
.33
.25
.17
0
.25
-20
8
C. L. Sabine et al.
.33
.42
.50
-40
-60
.58
'70s (1968-1978)
.58
.58 <
60
.33
Latitude ( N)
40
.42
.50
20
0
.33
.17
.25
.33
-20
.42
.50
.58
.67
.75
-40
-60
'80s (1978-1988)
.75<
.75
60
40
Latitude ( N)
2.2 Non-carbon tracers approach
An alternative approach for estimating the rate of
increase of the oceanic CO2 is to monitor the changes in
other (preferably conservative) tracers with similar atmospheric histories. The most common approach relies on
the CFC dating technique (e.g., Watanabe et al., 2000).
This approach uses the elapsed time from when the water
lost contact with the atmosphere, as determined by CFC
tracer age, together with the evolution of atmospheric CO2
concentrations to estimate the changes in ocean carbon
uptake rate. The advantage of this technique is that one
does not need to know or correct for the preindustrial
fluxes as with the direct CO 2 measurements. Also, the
measurements are not affected by biology which can complicate the interpretation of the direct carbon measurements. This can be a benefit, but this approach is only
valid if an assumption of steady state biology can be made
for the given time and spatial domains. One advantage of
this technique is that if one assumes a steady state circulation, data from multiple cruises can be compiled to examine the large scale patterns in uptake rate. For example, Watanabe et al. (2000) compiled a dataset of approximately 25,000 samples from cruises run between 1986
and 1998 to determine the vertically integrated changes
in Pacific carbon uptake rate over the last three decades
as presented in Fig. 2. Because this approach is based on
tracer ages throughout the water column, one can estimate full water column integrated uptake rates for differing time periods.
The western North Pacific subtropical region exhibited a maximum in the CFC-calculated rate of increase of
more than 0.7 mol C m–2yr–1 during the 1990s. While the
trends in the CFC tracer age based CO2 uptake rates are
generally consistent with the increasing rate of pCO2
change over the latitude range observed by Inoue et al.
(1995), the CFC based estimates suggest a more complicated pattern of uptake rates than can be inferred from
the published CO2 time series approaches. The CFC based
estimates also show significant uptake rates in the subarctic Pacific where surface carbon measurements did not
show obvious trends.
The Watanabe et al. (2000) estimates showed significant increases in the uptake rates between the 1970s
and the 1990s. A similar approach was used by McNeil et
al. (2003) to calculate a global 20 year average anthropogenic CO2 uptake rate for the 1980s and 90s based on
CFC data collected during the WOCE global survey. The
McNeil et al. estimate for the North Pacific (0.39 ± 0.1
.58
.67
20
0
.50
.33
.42
.25
.42
.50
-20
.58
.67
.83
-40
-60
.75
.92
'90s (1988-1998)
100
120
140
1.0 <
160
0
1.0
180
0.17
200
0.42
220
240
0.67
260
280
300
0.83
∆ vertically integrated carbon (mol C m-2 yr-1)
Fig. 2. Distributions of the rate of increase of the anthropogenic CO2 in the Pacific based on the CFC method (mol C
m–2yr–1) for (a) the 1970s, (b) the 1980s, and (c) the 1990s
(adapted from Watanabe et al., 2000).
mol C m–2yr–1), however, was somewhat smaller than the
average Watanabe et al. uptake rate for this period
(0.51 ± 0.09 mol C m–2yr–1). One potential problem with
this approach is the direct reliance on the accuracy of the
CFC age estimates.
2.3 Carbon tracers approach
The advantage of the carbon time-series approach
described in Subsection 2.1 is the fact that the carbon increase is directly measured, requiring fewer assumptions.
The non-carbon tracer approach (Subsection 2.2), on the
other hand, has the advantage of working with a conservative tracer that is not influenced by biology. A compromise approach still relies primarily on ocean carbon measurements, but uses thermodynamic and stoichiometric
relationships to derive a carbon based quasi-conservative
tracer. Perhaps the best example of this is the ∆C* tracer
first described by Gruber et al. (1996). This tracer has
historically been used to determine the total anthropogenic inventory since preindustrial times, but when applied to a time series data-set, the C* tracer can provide
an improved estimate of the DIC uptake rate over time.
The ∆C* method is based on a premise first proposed
by Chen and Millero (1979) and Brewer (1978) that the
anthropogenic CO2 concentration (Cant) can be isolated
from measured DIC values (Cm) by subtracting the contribution of the biological pumps (∆Cbio), the DIC the
waters would have in equilibrium with a pre-industrial
atmospheric CO2 concentration of 280 ppm (Ceq280), and
a term that corrects for the fact that surface waters are
not always in equilibrium with the atmosphere (∆Cdiseq):
Canth = Cm – ∆Cbio – Ceq280 – ∆Cdiseq = ∆C* – ∆Cdiseq, (1)
where:
Canth= Anthropogenic carbon concentration;
Cm = Measured total carbon concentration;
∆Cbio
= Change in DIC as a result of biological activity (both organic and inorganic);
Ceq280 = DIC of waters in equilibrium with an atmospheric CO2 concentration of 280 µatm;
∆Cdiseq = Air-sea difference in CO2 concentration expressed in µmol kg–1 of DIC.
The three terms to the right of the first equal sign
make up ∆C*, which can be explicitly calculated for each
sample (i.e. ∆C* = C m – ∆Cbio – Ceq280). The fact that
∆C* is a quasi-conservative tracer helps remedy some of
the mixing concerns arising from the earlier techniques
(Sabine and Feely, 2001). The ∆Cdiseq term is evaluated
over small isopycnal intervals using a water-mass age
tracer such as CFCs (Sabine et al., 2002).
The most difficult part of the ∆C* based total anthropogenic CO2 estimates is to properly determine the
air-sea disequilibrium component (∆Cdiseq). When applied
to time series type data, however, one can use the changes
in ∆C* to directly evaluate the uptake rate without having to determine the disequilibrium component as long
as one can assume that the disequilibrium term has remained constant over the time series. To demonstrate the
utility of this approach, ∆C* was calculated for the 10
year time-series at HOT (Fig. 3). The total ∆C* value is
an estimate of the anthropogenic inventory since
preindustrial times. The changes in ∆C* over time represent the uptake rate of anthropogenic CO2. Evaluation at
the Hawaii Ocean Time-series site shows essentially no
changes in inventory below 800 m over the 10 year record.
The inferred increases at 600, 400, and 200 m, however,
were 0.9, 1.1, and 1.6 µmol kg–1yr–1, respectively. Regional changes in watermass properties in near surface
Fig. 3. Contour map of ∆C* in the upper 1000 m as a function
of time at the Hawaii Ocean Time-series station, ALOHA.
Black dots indicate sample locations and times.
waters, however, complicate the interpretation of the ∆C*
calculations in the shallow waters. This approach also
assumes a constant air-sea disequilibrium and constant
Redfield stoichiometry that may not be valid under all
circumstances.
Similarly, changes in δ13C can be used to infer anthropogenic CO2 uptake by the ocean. This technique
takes advantage of the fact that fossil fuel burning, ocean
uptake, and uptake by the terrestrial biosphere all have
unique isotopic signatures in δ13C. Quay et al. (1992) used
the observed change in depth integrated δ13C inventory
profiles between the 1970s and the 1990s to determine
an oceanic CO2 uptake rate of 2.1 ± 0.8 Pg C yr –1. This
number was refined to 1.5 ± 0.6 Pg C yr –1 by Quay et al.
(2003) with the addition of ~25,000 δ 13C measurements
collected during the WOCE global survey. Although Quay
et al. do not break down their anthropogenic CO2 uptake
rate estimates by region, the observed regional changes
in isotopic composition can provide a qualitative assessment of the relative uptake rates. The δ13C of surface
waters decreased at a rate of –0.24 ± 0.02‰ per decade
at Station ALOHA since 1990 while essentially no change
was observed in the sub-arctic Pacific (Sonnerup et al.,
1999; Quay et al., 2003). Gruber et al. (1999) also estimate a smaller decrease of –0.15 ± 0.06‰ per decade in
the tropical Pacific relative to the subtropical gyre. Quay
et al. note similar features in the 13C inventories used to
estimate the anthropogenic uptake, with the largest δ13C
decreases observed in the North Pacific subtropical gyre
and significantly smaller changes in the sub-polar and
equatorial regions.
The δ13C approach has the advantage over the carbon time series or ∆C* approaches in that generally the
anthropogenic δ13C signal in the ocean is large relative
Temporal Evolution of the North Pacific CO 2 Uptake Rate
9
to the seasonal variability compared to DIC or pCO 2
where the seasonal variations are much larger than the
anthropogenic signal. The disadvantage is that although
there appears to be a strong correlation between δ 13C and
anthropogenic CO 2 on a global scale, this relationship
varies significantly as one moves to smaller spatial scales.
This is the reason why it is difficult to take the Quay et
al. global uptake rates and scale them down to a North
Pacific estimate.
2.4 Empirical relationship approach
Another approach that also uses inorganic carbon
measurements relies on an empirical relationship determined by evaluating a multiple linear regression (MLR)
of inorganic carbon against other hydrographic parameters. This approach has been described in detail by
Brewer et al. (1995), and subsequently others (Wallace,
1995; Slansky et al., 1997; Goyet and Davis, 1997; Sabine
et al., 1999; Feely et al., 2003). In the MLR approach,
DIC is fitted from one cruise as a function of common
hydrographic parameters-salinity (S), potential temperature ( θ ), apparent oxygen utilization (AOU), silicate
(Si(OH)4), and phosphate (PO4):
DIC = aθ + bS + cAOU + dSi(OH)4 + ePO4.
(2)
The coefficients from this fit are then used to predict the
DIC on the other cruises. The difference between the
measured and predicted DIC is inferred to be due to the
excess carbon taken up from the atmosphere. In this procedure, a statistical model is used to calculate the coefficients that describe the naturally occurring variations in
oceanic CO2 as opposed to the inferred relationships from
the carbon tracer techniques.
The MLR approach is demonstrated here using a series of cruises (GEOSECS, NOAA ENP81, WHPs P16N,
P15N, P2, P1, and NOPP) from Alaska to Hawaii between
30–50°N, and 140–180°W between 1973 and 1999. The
DIC data used were corrected to Certified Reference
Materials (CRMs) where available. The GEOSECS DIC
data were also adjusted by –29 µmol kg–1 to account for
known errors in these measurements (Takahashi et al.,
1982; Takahashi, personal communication). After verifying the consistency of all the data in the deep waters, P16N
was used as the reference station to derive the coefficients
for the fit. Details of the analysis are given in Feely et al.
(2003). Results are presented as changes relative to the
reference station, P16N.
The calculated increase in the mixed layer DIC was
1.3 ± 0.2 µmol C kg–1yr–1 (Fig. 4), very similar to the
observed mixed layer changes at the Hawaii Ocean Timeseries site. The average uptake rate below the mixed layer
down to a depth of 1250 m was 0.79 ± 0.4 µmol kg–1yr–1
(Fig. 5). The total 26 year integrated CO2 uptake rate from
10
C. L. Sabine et al.
Fig. 4. Estimated change in DIC content in the North Pacific
mixed layer between 30–50°N and 140–180°W.
the surface to 1250 m is estimated to be 1.1 ± 0.4 mol C
m–2yr–1.
The empirical relationship approach has the advantage that it uses strong correlations with parameters not
affected by anthropogenic CO2 to correct for modest water mass variations that complicate the interpretation of
results in the carbon time series approach. It also has the
advantage that it actually uses the data to derive the relationships rather than relying on independent estimates that
may have been derived under different oceanographic
conditions. The disadvantage of this approach is that it
assumes the correlations derived from one cruise will
apply to data collected at a different time or in a slightly
different location. It is also susceptible to any errors or
systematic biases between cruises for all of the parameters used in the fit. In other words, if there is a systematic bias between the oxygen values on one cruise relative to another, this will directly translate into an inferred
change in carbon.
2.5 Inverse calculations approach
This paper has focused on uptake rate estimates based
on measurements made in the North Pacific and not on
results from numerical models. Inverse models, however,
combine a numerical transport model with ocean inventory measurements to infer air-sea fluxes and anthropogenic CO2 uptake rates. The inversion approach has been
used for years in the atmospheric community (e.g.,
Gurney et al., 2002), but is relatively new to the ocean
community. Gloor et al. (2003) have taken the DIC measurements from the WOCE/JGOFS global survey and the
∆C* based anthropogenic CO2 inventory estimates together with the Princeton ocean general circulation model
to estimate the regional distribution of air-sea fluxes that
Fig. 5. Comparison of North Pacific DIC binned residuals (µmol
kg–1) from the bottom of the MLD to 1250 m.
best represent the observed distributions of DIC and anthropogenic CO2.
The preindustrial fluxes estimated by Gloor et al.
(2003) showed a large efflux in the tropical Pacific with
the remainder of the North Pacific acting as CO2 sinks
(Fig. 6). Anthropogenic fluxes were into the ocean everywhere, but were the strongest (–0.25 Pg C yr–1) in the
tropical Pacific (13°S to 13°N) relative to the North Pacific regions (–0.1 Pg C yr–1 for 13–36°N and 36–62°N).
The combined mean air-sea fluxes for the 1990s were
estimated to be +0.6, –0.5, and –0.2 Pg C yr–1 for the
tropical, mid-latitude, and northern Pacific regions, respectively (Fig. 6).
This approach has the advantage that it full advantage of the very large synthesized carbon data sets to derive CO2 uptake rate estimates. The results, by definition, will be consistent with the total anthropogenic CO2
distributions that have been determined from these data
sets. This approach also explicitly accounts for advective
and diffusive effects at the resolution of the general circulation model (GCM). The disadvantage of this approach
is that the results are strongly tied to the accuracy of both
the water column data and the model transport fields.
Gloor et al. (2003) found that the results were somewhat
model dependent, so the ability of the model to simulate
real ocean transports is critical.
3. Discussion
To facilitate a comparison of the North Pacific uptake rate estimates, we have combined a number of published results into Table 2. In some cases, the published
_
_
x
_
_
_
x
_
_x
xx
x
x
_
_
xx
_
_
_
_
x
_
_
_x
x
xx
x
xx
x
_
_
_
xx
_
_
_
x
_
_x
_
x
_
_
xx
_
_
_
_x
x
_x
_
xx
xx
xx
xx
xx
x
x
_
_
_
_
xx
x
_
_
_
xx
_
_
_
x
_
_x
x
x
xx
_x
xx
x
xx
x
xx
x
xx
_
_x
x
xx
xx
x
_
x
_
_
_
_
xx
xx
x
xx
x
_
_
_
_
_
_
_
_
_
_x
xx
xx
x
_
_
_
xx
_
_
_
_
xx
_
_
_x
_
_
_x
x
x
x
_
_
_
xx
_
_
x
_
_
_x
_
x
xx
Fig. 6. (a) Pre-Industrial atmosphere-ocean flux estimates (PgC
yr –1) for ocean regions as predicted by the inverse method
(dark gray) and forward simulations with two versions of
the Princeton Ocean Biogeochemistry model (lighter grays).
(b) Same as in (a) except that anthropogenic CO2 fluxes for
the year 1990 are shown. (c) Same as (a) except that the
total contemporary fluxes (sum of pre-industrial and anthropogenic perturbation) are compared with the air-sea CO2
fluxes estimated using the pCO 2 climatology of Takahashi
et al. (2002) (square dependence of gas exchange coefficient on wind speed and cubic dependence), and the mean
estimates from TransCom3 inversions (adapted from Gloor
et al., 2003).
values had to be converted to common units requiring
assumptions about the area or integration depth. The results are divided into three categories: integrated water
column uptake rate estimates, mixed layer increases, and
surface pCO2 increases. Most of the published values fall
Temporal Evolution of the North Pacific CO 2 Uptake Rate
11
12
C. L. Sabine et al.
*Uptake not integrated all the way to the surface.
Table 2. Summary of published CO2 uptake rates in Pacific Ocean.
under the water column integrated uptake rate category.
These values varied by region and depth range of integration with values ranging from 0.25 to 1.3 mol m–2yr –1.
The mixed layer uptake rate estimates were much more
consistent, with values of 1.0–1.3 µmol kg–1yr–1 based
on direct observations and MLR approaches. It should be
noted, however, that all of the estimates presented here
were based primarily on subtropical measurements. Although not quantified, mixed layer changes were not easily distinguished from the large seasonal variability in
the sub-arctic Pacific based on observations at Station P
and the KNOT station (Wong et al., 2002; Tsurushima et
al., 2002). Surface pCO2 changes also showed regional
variability that is likely due to differences in the physical
and biological forcing (Inoue et al., 1995; Feely et al.,
2003; Takahashi et al., 2003).
In general, the direct carbon observation approaches
indicated that carbon uptake rates increased from the tropics to the subtropics in the North Pacific. This trend is
seen both in the water column data as well as the mixed
layer DIC and surface pCO2 measurements. Comparable
decreases are seen in the δ 13C results with latitude. The
estimates with the most spatial information are the CFC
age based values. The maps presented in Fig. 2 are generally consistent with the carbon observations, but suggest more structure with both zonal and meridional gradients. While these patterns cannot be easily resolved with
the existing carbon time series observations, they are consistent with the total anthropogenic CO2 inventory distributions inferred from the ∆C* approach (Sabine et al.,
2002; McNeil et al., 2003).
The CFC age based estimates overall suggest a
smaller sink than the carbon observation approaches.
These differences may be related to the difficulty in removing the biological and preindustrial signals from the
carbon measurements. It is also possible that the CFC
estimates are too low because of the assumptions that CFC
ages represent true watermass ages, that there is a constant CO 2 equilibration relative to CFCs, and that the
watermass age structure and biological pumps have remained constant over decadal time scales. The CFCs are
also of limited use in the future since atmospheric concentrations are decreasing. This represents a significant
departure from the atmospheric history of CO2 and reduces the utility of this tracer for age dating. Also, several recent studies have suggested that oceanic conditions
may be changing due to greenhouse warming effects and/
or climate variability, calling into question the assumptions of steady state circulation (Ono et al., 2001;
Watanabe et al., 2001; Emerson et al., 2001). Changes in
circulation and/or biology actually affect all of the current approaches for estimating anthropogenic CO2 uptake
in the ocean.
The largest discrepancies between the various ap-
proaches are observed in the sub-arctic Pacific. If one
were to only look at surface values in the sub-arctic Pacific, one might conclude that there is no anthropogenic
CO2 uptake in this region. Results from the MLR work
presented here as well as the water column studies of
Wakita et al. (2003), however, show that anthropogenic
increases can be observed below the seasonal mixed layer.
The CFC age approach also shows uptake in the sub-arctic region, although the rate of change appears to decrease
north of the Kuroshio Extension.
The ocean inversion results potentially represent the
most sophisticated approach for estimating uptake rates
because they combine the state-of-the-art three dimensional flow field estimates with the extensive full water
column carbon measurements. In addition to the inversion results, Fig. 6 also summarizes results from a range
of prognostic models and total fluxes from Takahashi et
al. (2002) and the TransCom-3 atmospheric inversions
(Gurney et al., 2002). Comparison with the prognostic
models shows that the data did alter the inversion results
away from the initial model estimates. When normalized
to a unit area (Table 2), the inversion estimates give the
highest uptake rates in the 36°N to 62°N box. Although
this may appear to contradict the previous statements
about the observations in the sub-arctic Pacific, this result is somewhat misleading because the southern boundary of the inversion box also includes areas south of the
Kuroshio Extension where very high uptake rates were
estimated from the CFC age approach. Comparisons between the present day fluxes from the inversion and the
observationally based pCO2 fluxes of Takahashi et al.
(2002) are reasonably good for the North Pacific. Although these first inversion estimates only determined
average fluxes for relatively large regions, as the carbon
data sets grow and models improve the inverse calculations will also improve.
4. Conclusions
The different techniques used to evaluate the North
Pacific carbon uptake rates do not completely agree on
the exact magnitude but generally agree in their overall
trends. The one exception to this is in the sub-arctic Pacific. The divergence in results in this region is likely
related to the large seasonal and interannual variability
in both the physical and biological controls. This variability may invalidate some of the assumptions associated with attempts to relate these changes to an anthropogenic CO2 uptake rate in the discussed techniques.
Additional research is necessary to resolve these issues
and better constrain the role of the North Pacific in the
global carbon cycle. The tremendous area, diversity of
habitats, and corrosiveness of the Pacific waters with respect to carbonate minerals provide the potential for significant changes in carbon cycling in this ocean as a re-
Temporal Evolution of the North Pacific CO 2 Uptake Rate
13
sult of future climate change. Some of these changes may
lead to changes in the role of the Pacific as a sink for
anthropogenic CO2. The global CO2 survey data and estimates provided here make an important baseline for assessing future changes in the Pacific carbon cycle.
Acknowledgements
This manuscript summarizes the work of many investigators that have studied the North Pacific carbon
cycle. We wish to acknowledge all of those that contributed to the Pacific Ocean data set complied for this project,
including those responsible for the carbon measurements,
the CFC measurements, and the Chief Scientists. This
research was funded by NOAA/DOE grant GC99-220 and
NSF grant OCE-0136897. We also thank PICES working
group 13 members and the JGOFS scientists for fostering an environment of international collaboration. This
publication is PMEL contribution number 2618.
References
Anderson, L. A. and J. L. Sarmiento (1994): Redfield ratios of
remineralization determined by nutrient data analysis. Global Biogeochem. Cycles, 8, 65–80.
Brewer, P. G. (1978): Direct observation of the oceanic CO2
increase. Geophys. Res. Lett., 5, 997–1000.
Brewer, P. G., D. M. Glover, C. M. Goyet and D. K. Schafer
(1995): The pH of the North Atlantic Ocean: Improvements
to the global model for sound absorption in sea water. J.
Geophys. Res., 100(C5), 8761–8776.
Chen, C.-T. A. and F. J. Millero (1979): Gradual increase of
oceanic CO2. Nature, 277, 205–206.
Dore, J. E., R. Lukas, D. W. Sadler and D. M. Karl (2003):
Climate-driven changes to the atmospheric CO2 sink in the
subtropical North Pacific Ocean. Nature, 424, 754–757.
Emerson, S., S. Mecking and J. Abell (2001): The biological
pump in the subtropical North Pacific Ocean: Nutrient
sources, redfield ratios, and recent changes. Global
Biogeochem. Cycles, 15(3), 535–554.
Feely, R. A., C. L. Sabine, T. Takahashi and R. Wanninkhof
(2001): Uptake and storage of carbon dioxide in the oceans:
The global CO 2 survey. Oceanography, 14(4), 18–32.
Feely, R. A., Y. Nojiri, A. Dickson, C. L. Sabine, M. F. Lamb
and T. Ono (2003): CO2 in the North Pacific Ocean, PICES
Working Group 13 Final Report. PICES Scientific Report
No. 24, 2003, 49 pp.
Flückiger, J., E. Monnin, B. Stauffer, J. Schwander, T. F.
Stocker, J. Chappellaz, D. Raynaud and J. M. Barnola
(2002): High resolution Holocene N2O ice core record and
its relationship with CH 4 and CO2. Global Biogeochem.
Cycles, 16, doi:10.1029/2001GB001417.
Gloor, M., N. Gruber, J. Sarmiento, C. L. Sabine, R. A. Feely
and C. Rodenbeck (2003): A first estimate of present and
preindustrial air-sea CO 2 flux patterns based on ocean interior carbon measurements and models. Geophys. Res. Lett.,
30, 1010, doi:10.1029/2002GL015594.
Goyet, C. and D. Davis (1997): Estimation of total CO 2 concentration throughout the water column. Deep-Sea Res I,
44(5), 859–877.
14
C. L. Sabine et al.
Gruber, N., J. L. Sarmiento and T. F. Stocker (1996): An improved method for detecting anthropogenic CO 2 in the
oceans. Global Biogeochem. Cycles, 10, 809–837.
Gruber, N., C. D. Keeling, R. B. Bacastow, P. R. Guenther, T. J.
Lueker, M. Whalen, H. A. J. Meijer, W. G. Mook and T. F.
Stocker (1999): Spatiotemporal patterns of carbon-13 in the
global surface ocean and the oceanic Suess effect. Global
Biogeochem. Cycles, 13, 307–335.
Gurney, K. R. et al. (2002): Towards more robust estimates of
CO2 fluxes: Control results from the TransCom3 inversion
comparison. Nature, 425, 626–630.
Indermühle, A., T. F. Stocker, F. Joos, H. Fischer, H. Smith, M.
Wahlen, B. Deck, D. Mastroianni, J. Tschumi, T. Blunier,
R. Meyer and B. Stauffer (1999): Holocene carbon-cycle
dynamics based on CO 2 trapped in ice at Taylor Dome,
Antarctica. Nature, 398, 121–126.
Inoue, H. Y., H. Matsueda, M. Ishii, K. Fushimi, M. Hirota, I.
Asanuma and Y. Takasugi (1995): Long-term trend of the
partial pressure of carbon dioxide (pCO2) in surface waters
of the western North Pacific, 1984–1993. Tellus, 47B, 391–
413.
Johnson, K. M., A. G. Dickson, G. Eischeid, C. Goyet, P.
Guenther, F. J. Millero, D. Purkerson, C. L. Sabine, R. G.
Schottle, D. W. R. Wallace, R. J. Wilke and C. D. Winn
(1998): Coulometric total carbon dioxide analysis for marine studies: Assessment of the quality of total inorganic
carbon measurements made during the US Indian Ocean
CO2 Survey 1994–1996. Mar. Chem., 63, 21–37.
Lamb, M. F., C. L. Sabine, R. A. Feely, R. Wanninkhof, R. M.
Key, G. C. Johnson, F. J. Millero, K. Lee, T.-H. Peng, A.
Kozyr, J. L. Bullister, D. Greeley, R. H. Byrne, D. W.
Chipman, A. G. Dickson, C. Goyet, P. R. Guenther, M. Ishii,
K. M. Johnson, C. D. Keeling, T. Ono, K. Shitashima, B.
Tilbrook, T. Takahashi, D. W. R. Wallace, Y. Watanabe, C.
Winn and C. S. Wong (2002): Consistency and synthesis of
Pacific Ocean CO2 survey data. Deep-Sea Res. II, 49, 21–
58.
Lee, K., S.-D. Choi, G.-H. Park, R. Wanninkhof, T.-H. Peng,
R. M. Key, C. L. Sabine, R. A. Feely, J. L. Bullister and F.
J. Millero (2003): An updated anthropogenic CO 2 inventory in the Atlantic Ocean. Global Biogeochem. Cycles (in
press).
McNeil, B. I., R. J. Matear, R. M. Key, J. L. Bullister and J. L.
Sarmiento (2003): Anthropogenic CO2 uptake by the ocean
based on the global chlorofluorocarbon dataset. Science,
299, 235–239.
Ono, T., Y. W. Watanabe and S. Watanabe (2000): Recent increase of DIC in the western North Pacific. Mar. Chem.,
72, 317–328.
Ono, T., T. Midorikawa, Y. W. Watanabe, K. Tadokoro and T.
Saino (2001): Temporal increases of phosphate and apparent oxygen utilization in the subsurface waters of western
subarctic Pacific from 1968 to 1998. Geophys. Res. Lett.,
28(17), 3285–3288.
Peng, T.-H., R. Wanninkhof and R. A. Feely (2003): Increase
of anthropogenic CO2 in the Pacific Ocean over the last
two decades. Deep-Sea Res. II, 50, 3065–3082.
Prentice, C. et al. (2001): The carbon cycle and atmospheric
carbon dioxide. In Climate Change 2001: The Scientific
Basis, Contribution of Working Group I to the 3rd Assessment Report of the Intergovernmental Panel on Climate
Change, ed. by J. Houghton et al., Cabridge University
Press, New York, U.S.A.
Quay, P. D., B. Tilbrook and C. S. Wong (1992): Oceanic uptake of fossil fuel CO2: Carbon-13 evidence. Science, 256,
74–79.
Quay, P., R. Sonnerup, T. Westby, J. Stutsman and A. McNichol
(2003): Changes in the 13C/12C of dissolved inorganic carbon in the ocean as a tracer of anthropogenic CO 2 uptake.
Global Biogeochem. Cycles, 17, 1004, doi:10.1029/
2001GB001817.
Sabine, C. L. and R. A. Feely (2001): Comparison of recent
Indian Ocean anthropogenic CO2 estimates with a historical approach. Global Biogeochem. Cycles, 15, 31–42.
Sabine, C. L., R. M. Key, K. M. Johnson, F. J. Millero, A.
Poisson, J. L. Sarmiento, D. W. R. Wallace and C. D. Winn
(1999): Anthropogenic CO 2 inventory of the Indian Ocean.
Global Biogeochem. Cycles, 13, 179–198.
Sabine, C. L., R. A. Feely, R. M. Key, J. L. Bullister, F. J.
Millero, K. Lee, T.-H. Peng, B. Tilbrook, T. Ono and C. S.
Wong (2002): Distribution of anthropogenic CO 2 in the
Pacific Ocean. Global Biogeochem. Cycles, 16(4), 1083,
doi:10.1029/2001GB001639.
Slansky, C. M., R. A. Feely and R. Wanninkhof (1997): The
stepwise linear regression method for calculating anthropogenic CO2 invasion into the North Pacific Ocean. p. 70–
79. In Biogeochemical Processes in the North Pacific, ed.
by S. Tsunogai, Proceedings of the International Marine
Science Symposium on Biogeochemical Processes in the
North Pacific, 12–14 November 1996, Mutsu, Japan, Japan
Marine Science Foundation.
Sonnerup, R. E., P. D. Quay, A. P. McNichol, J. L. Bullister, T.
A. Westby and H. L. Anderson (1999): Reconstructing the
oceanic 13C Suess effect. Global Biogeochem. Cycles, 13,
857–872.
Takahashi, T., R. T. Williams and D. L. Bos (1982): Carbonate
chemistry. p. 77–83. In GEOSECS Pacific Expedition, Volume 3, Hydrographic Data 1973–1974, ed. by W. S.
Broecker, D. W. Spencer and H. Craig, National Science
Foundation, Washington, D.C.
Takahashi, T., J. Olafsson, J. G. Goddard, D. W. Chipman and
S. C. Sutherland (1993): Seasonal variation of CO2 and
nutrients in the high latitude surface oceans: A comparative
study. Global Biogeochem. Cycles, 7, 843–878.
Takahashi, T., S. C. Sutherland, C. Sweeney, A. Poisson, N.
Metzl, B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely,
C. Sabine, J. Olafsson and Y. Nojiri (2002): Global sea-air
CO2 flux based on climatological surface ocean pCO 2, and
seasonal biological and temperature effects. Deep-Sea Res.
II, 49(9–10), 1601–1623.
Takahashi, T., S. C. Sutherland, R. A. Feely and C. E. Cosca
(2003): Decadal variation of the surface water pCO 2 in the
western and central equatorial Pacific. Science, 302, 852–
856.
Tsunogai, S., T. Ono and S. Watanabe (1993): Increase in total
carbonate in the Western North Pacific Water and a hypothesis on the missing sink of anthropogenic carbon. J.
Oceanogr., 49, 305–315.
Tsurushima, N., Y. Nogiri, K. Imai and S. Wanatabe (2002):
Seasonal variations of carbon dioxide system and nutrients
in the surface mixed layer at Station KNOT (44°N, 155°E)
in the subarctic North Pacific. Deep-Sea Res. II, 49, 5377–
5394.
Wakita, M., S. Watanabe, Y. W. Watanabe, T. Ono, N. Tsushima
and S. Tsunogai (2003): Temporal change of dissolved inorganic carbon in the subsurface water at station KNOT
(44°N, 155°E) in the subpolar western North Pacific. J.
Oceanogr. (submitted).
Wallace, D. W. R. (1995): Monitoring global ocean inventories. OOSDP Background Rep. 5, 54 pp., Ocean Obsrv. Syst.
Dev. Panel, Texas A&M Univ., College Station, TX, U.S.A.
Wanninkhof, R., T.-H. Peng, B. Huss, C. L. Sabine and K. Lee
(2003): Comparison of Inorganic Carbon System Parameters Measured in the Atlantic Ocean from 1990 to 1998
and Recommended Adjustments. ORNL/CDIAC-140, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
Watanabe, Y. W., T. Ono and A. Shimamoto (2000): Increase in
the uptake rate of oceanic anthropogenic carbon in the North
Pacific determined by CFC ages. Mar. Chem., 72, 297–315.
Watanabe, Y. W., T. Ono, A. Shimamoto, T. Sugimoto, M. Wakita
and S. Watanabe (2001): Probability of a reduction in the
formation rate of the subsurface water in the North Pacific
during the 1980s and 1990s. Geophys. Res. Lett., 28(17),
3289–3292.
Winn, C. D., F. T. Mackenzie, C. J. Carrillo, C. L. Sabine and
D. M. Karl (1994): Air-sea carbon dioxide exchange in the
North Pacific Subtropical Gyre: Implications for the global
carbon budget. Global Biogeochem. Cycles, 8, 157–163.
Winn, C. D., Y.-H. Li, F. T. Mackenzie and D. M. Karl (1998):
Rising surface ocean dissolved inorganic carbon at the Hawaii Ocean Time-series site. Mar. Chem., 60, 33–47.
Wong, C. S. and Y.-H. Chan (1991): Temporal variations in the
partial pressure and flux of CO 2 at ocean Station P in the
subarctic northeast Pacific Ocean. Tellus, 43B, 206–223.
Wong, C. S., N. A. D. Waser, F. A. Whitney, W. K. Johnson and
J. S. Page (2002): Time-series study of the biogeochemistry of the North East subarctic Pacific: reconciliation of the
C org/N remineralization and uptake ratios with the Redfield
ratios. Deep-Sea Res. II, 49, 5717–5738.
Temporal Evolution of the North Pacific CO 2 Uptake Rate
15