3.1. Long-term changes in pCO2air and pCO2sea
The time series data for pCO2air showed similar, linearly increasing trends with little interannual variability at all latitudes along 137°E in both winter and summer, whereas pCO2sea showed substantial increasing trends with a larger interannual variability over the entire area, from the subtropical to equatorial regions, in both winter and summer (Figs 4a and c). The values for pCO2sea were lower than pCO2air in winter because of lower SST at higher latitudes, whereas, in summer, pCO2sea values were higher than pCO2air because of the effects of high SST exceeding those of lowering DIC. The magnitude of pCO2sea interannual variability was relatively large at latitudes south of 20°N in winter and north of 24°N in summer. Furthermore, the interannual variability of pCO2sea in summer was larger than that in winter, especially owing to larger SST variability from 25°N to 31°N.
Figure 4. Time series of pCO2air, pCO2sea and SST at six latitudes along 137°E. (a) pCO2sea and (b) SST in winter; and (c) pCO2sea and (d) SST in summer. Red circles, 3°N; violet triangles, 10°N; orange diamonds, 15°N; green squares, 20°N; light blue triangles, 25°N; and blue circles, 30°N. In (a) and (c), time series of pCO2air are also shown by dotted lines ranging from black (30°N) to pale grey (3°N).
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3.2. Long-term changes in pH at ambient SST
The estimated pH (Fig. 5a) was higher at high latitudes than at low latitudes in winter because of a large latitudinal gradient of SST (Fig. 4b). By contrast, pH normalized to a constant SST of 25 °C, pHT25, was lower at high latitudes than at low latitudes because of higher DIC concentrations at higher latitudes (Ishii et al., 2001). In summer, the latitudinal differences in pH disappeared because of a much smaller North–South SST gradient. The time series pH data in both winter and summer at all latitudes exhibited distinct long-term decreasing trends with notable interannual variation but without readily recognizable periodicity. The interannual variability of pH was also large in summer.
Figure 5. Time series of surface-water pH estimated for ambient SST at six latitudes along 137°E in winter (a) and summer (b). Symbols are defined as in Fig. 4.
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We calculated the average rate of pH decrease for each 1° of latitude along 137°E for the entire study period using the linear least-squares method. The rates of decrease ranged from (mean ± 1 standard deviation [1σ]) 0.0015 ± 0.0002 to 0.0021 ± 0.0002 yr−1 (average, 0.0018 ± 0.0002 yr−1) in winter (Fig. 6e) and from 0.0008 ± 0.0004 to 0.0019 ± 0.0005 yr−1 (average, 0.0013 ± 0.0005 yr−1) in summer (Fig. 7e). These trends were significant at most latitudes south of 33°N: p < 0.001 at all latitudes in winter; p < 0.05 at 26 latitudes and 0.05 < p < 0.1 at five latitudes in summer. The range of these decreasing pH rates in the present study area (3–33°N) was comparable to that (0.0017 ± 0.0005 yr−1) directly measured at 25 °C at ESTOC (29°10′N) in the North Atlantic (González-Dávila et al., 2007) and those calculated at ambient SST for the HOT (22°45′N; 0.0019 ± 0.0002 yr−1; Dore et al., 2009) and BATS sites (31°50′N; 0.0017 ± 0.0001 yr−1; Bates, 2007). These similar trends in different regions of the ocean could reflect the rapid response of pCO2sea to changes in pCO2air, and furthermore, suggest no intrinsic difference in the changes in the surface carbonate system between these subtropical regions.
Figure 6. Latitudinal distributions of long-term trends for (a) SST, (b) salinity, (c) pCO2sea, (d) n-DIC, (e) pH, and (f) buffer factor in winter. In (c), pCO2sea (blue circles) is presented along with n-pCO2sea (pCO2sea normalized to average temperature and salinity; red squares) and pCO2air (open squares). In (e), pH estimates are presented for ambient SST (blue circles) and normalized to 25 °C (pHT25; red squares). Error bars represent 1σ values for the rates of change at each latitude. In (f), buffer factor (∂ ln pCO2sea/∂ ln DIC) was calculated at observed SST and SSS and calculated TA (from constant n-TA).
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3.3. Contribution of changes in SST to those in pH and DIC
The rates of decreasing pHT25 were similar in winter (average, 0.0015 ± 0.0003 yr−1) and summer (average, 0.0014 ± 0.0004 yr−1). The steeper decreasing trends of pH at ambient SST in winter were attributed to the effects of a relatively greater rate of SST increase in winter (range, −0.03 to 0.06 °C yr−1; average, 0.02 ± 0.02 °C yr−1; Fig. 6a), compared with those in summer (−0.07 to 0.04 °C yr−1; average, −0.01 ± 0.02 °C yr−1; Fig. 7a), although these SST trends were insignificant. These results are consistent with similar trends in the calculated n-DIC (DIC normalized to a salinity of 35) in winter (range, 0.6–1.4 μmol kg−1 yr−1; average, 0.96 ± 0.16 μmol kg−1 yr−1; Fig. 6d) and summer (0.5–1.5 μmol kg−1 yr−1; average, 0.92 ± 0.26 μmol kg−1 yr−1; Fig. 7d).
The rate of increase in pCO2sea (average, 1.58 ± 0.12 μatm yr−1 in winter and 1.37 ± 0.33 μatm yr−1 in summer; Figs 6c and 7c) was not significantly different from that of pCO2air (average, 1.65 ± 0.05 μatm yr−1 in winter and 1.54 ± 0.08 μatm yr−1 in summer). However, the time series pCO2sea data in each latitude range normalized to the mean temperature and salinity for the entire period, n-pCO2sea, exhibited a pattern of lower rates of n-pCO2sea increase in latitudes showing increasing trends in SST, and higher rates in latitudes with decreasing SST trends, yielding a latitudinal distribution with relatively lower rates of increase in the southern region of the study area (especially, Fig. 7c).
These characteristics were the same in the trends of n-DIC and pHT25 (Figs 6 and 7) calculated on the assumption of a constant n-TA. These results suggest that the long-term response of the surface carbonate system to the anthropogenic CO2 increase in the atmosphere could be predominantly through increases in pCO2sea, in response to the changes in pCO2air. Therefore, in latitudes with increasing SST trends, larger temperature effects could allow pCO2sea to keep up with the pCO2air level even in areas with a smaller rate of n-DIC increase, and consequently, further depress the n-DIC increase rate; decreasing SST trends would conversely induce higher rates of DIC increase.
These temperature effects could also extend to the magnitude of pH rate of decrease; in latitudes with increasing SST trends, a smaller rate of n-DIC increase would produce a more gradual decreasing pHT25 trend. For pH at ambient SST, larger effects of increasing SST trends, acting to decrease pH, could make up for those of the lower rates of n-DIC increase, leading to significant decreasing pH trends.
For the individual rate at each latitude, the effects of changing SST accounted for −25 to 44% (average, 15%) in winter and −85 to 37% (average, −10%) in summer of the decreasing pH trends (Figs 6e and 7e). Compared with the rates of pHT25, the trend of increasing SST elevated the decreasing trend in pH at ambient SST in most latitudes (27 latitudes) in winter and the decreasing SST reduced the rates of decreasing pH at ambient SST in the northern subtropical region (a maximum of 46% against pHT25 at 25°N) in summer.
Conspicuously low rates of change in pHT25 were found around 23–24°N in winter, together those of n-pCO2sea and n-DIC, corresponding to high rates of SST increase (Figs 6 and 7). Midorikawa et al. (2006) indicated that the close coupling of interannual variations between the surface DIC concentrations and SST in these latitudes could be attributed to the influence of the interannual North–South movement of the Subtropical Front, which forms the boundary between the northern cold, DIC-rich water mass and the southern warm, DIC-poor water mass (Ishii et al., 2001), in addition to inverse DIC–SST anomaly relationships derived from winter vertical mixing. The position of the Subtropical Front, denoted by a surface potential density σθ of 24 (or SST of 22 °C) in winter, exhibited a long-term tendency to shift to the north: 0.5 ± 0.3° of latitude per decade during 1983–2007 (p= 0.10) and 0.29 ± 0.15° per decade during 1967–2007 (p= 0.07), based on hydrographic observations (JMA, 2001, 2008), and 0.22 ± 0.09° per decade during 1951–2007 (p= 0.01) based on SST reanalysis (Ishii et al., 2005). The long-term northward shift of the Subtropical Front could lead to the trends described earlier for the carbonate system properties and SST in these latitudes.
3.4. Long-term changes in pH since 1969
Paired data for SST, SSS, TA, and pCO2sea was acquired at five latitudes from 9°N to 30°N around 158°E in February 1970 (Inoue et al., 1999). The n-TA values ranged in 2297 ± 3 μmol kg−1, which is similar to those estimated for 1994–2007 (Fig. 2). Considering the effects of SST variations, the pHT25 values from these 1970 data were lower by 0.002 ± 0.006 to 0.030 ± 0.009 than those estimated for 1970 from the extrapolation of the pHT25 trends in the same latitudes for the past 25 yr in the present study. The rate of pHT25 decrease for 1970–1983 was calculated as 0.0009 ± 0.0006 yr−1, which is smaller than those for the more recent trends. The difference between SST observed in 1970 and the average at the same latitude for 1983–2007 was 0.12 ± 0.82 °C, which was smaller than 1σ value (0.66 °C, the average in the entire area) for the interannual variability of SST during the past 25 yr, indicating that 1970 was not an anomalous year. Moreover, data set of SST, SSS, and pCO2sea at latitudes from 3°N to 30°N around 138°E in February 1969 (the average anomaly, −0.17 ± 0.60 °C for 1969; Inoue et al., 1999) also gave a low average rate of pHT25 decrease (0.0008 ± 0.0006 yr−1,) for 1969–1983, assuming TA of 2295/35* SSS μmol kg−1. Although a determination of acidification based on snapshot observations from two specific years is difficult because interannual variations must also be considered, these data suggest an acceleration of acidification after the 1980s.
3.5. Estimation of changes in pH during the next 50 yr
The observational records of pCO2sea and the time series of pH estimated on the basis of these data in the western North Pacific for the past 25 yr were in agreement with long-term trends calculated from the atmospheric CO2 increase. We therefore estimated the future trends of ocean acidification in the western subtropical North Pacific Ocean on the assumption that only the changes derived from the atmospheric CO2 increase could continue to dominate changes in the surface carbonate system in these regions without altering the ecosystems and other factors related to the carbon cycle, excluding SST. Thus, the magnitude of future ocean acidification attributable to thermodynamic changes in the surface carbonate system of the western subtropical North Pacific over the next 50 yr was tentatively evaluated for the following two cases. Case 1 used the same rate of pCO2sea increase as that observed for the past 25 yr, that is, the extrapolation of the recent trend of pCO2sea increase. Case 2 assumed a rapid response of pCO2sea to the pCO2air changes predicted in accordance with future CO2 emission scenario IS92a in IPCC (2007).
Calculations were performed using the results of winter observations averaged in two representative realms: the northern (25–28°N) and southern (11–14°N) subtropical regions. The northern region is located near the centre of the highest sea surface dynamic height in the western subtropical gyre, and the southern region is located in the southern half of the North Equatorial Current (Fig. 1). For the past 25 yr, SST showed an insignificant trend (0.009 ± 0.018 °C yr−1, p= 0.6) in the northern region, whereas a trend of increasing SST (0.020 ± 0.013 °C yr−1, p= 0.12) was observed in the southern region, although this trend was not statistically significant. For Case 1, two sets of calculations, with and without SST changes, were compared (Table 1).
Table 1. Estimates of thermodynamic changes in the surface carbonate system over the next 50 yr for Case 1 (extrapolation of recent rates of pCO2sea increase) and Case 2 (CO2 emission scenario IS92a in IPCC )
|Estimation period||n-DIC change||pH change||Buffer factorc|
|Magnitudea (μmol kg−1)||Rate (μmol kg−1 yr−1)||Ratiob (%)||Magnitudea||Rate (yr−1)||Ratiob (%)|
| 11–14°N||23 ± 3||0.92 ± 0.10|| ||−0.042 ± 0.003||−0.0017 ± 0.0001|| ||8.8 ± 0.2|
| 25–28°N||28 ± 4||1.12 ± 0.14|| ||−0.049 ± 0.004||−0.0020 ± 0.0002|| ||9.3 ± 0.1|
|Case 1 for 2040–2060|
| 11–14°N without SST trend||45 ± 6||0.79 ± 0.34||86||−0.073 ± 0.010||−0.0013 ± 0.0006||79||9.5 ± 0.3|
| with SST trend||36 ± 11||0.63 ± 0.51||68||−0.074 ± 0.010||−0.0013 ± 0.0006||79||9.3 ± 0.6|
| 25–28°N without SST trend||50 ± 4||0.85 ± 0.26||76||−0.085 ± 0.008||−0.0015 ± 0.0005||78||10.1 ± 0.2|
| with SST trend||46 ± 11||0.79 ± 0.55||71||−0.085 ± 0.008||−0.0015 ± 0.0005||79||10.0 ± 0.7|
|Case 2 (with SST trend) for 2040–2060|
| 11–14°N||60 ± 10||1.24 ± 0.48|| ||−0.114 ± 0.010||−0.0024 ± 0.0006|| ||9.8 ± 0.6|
| 25–28°N||70 ± 11||1.40 ± 0.52|| ||−0.129 ± 0.008||−0.0027 ± 0.0005|| ||10.5 ± 0.7|
In Case 1, the magnitudes of n-DIC and pH changes from calculations ignoring SST increases were 45–50 μmol kg−1 for n-DIC and −0.073 to −0.085 for pH, respectively, over the next 50 yr (Table 1, Fig. 8). Under a constant SST, the rates of n-DIC increase in the two regions for 2040–2060 were 14–24% lower, and the rates of pH decrease were 21–22% lower, than the rates for 1983–2007. We attribute this decrease in the rates of change to the increase in the buffer factor (Revelle and Suess, 1957) from the increase in DIC.
Figure 8. Estimated trends for the next 50 yr for Case 1. Trends based on the extrapolation of rates of increase for pCO2sea in winter (closed circles), averaged in the northern (25–28°N, blue) and southern (11–14°N, red) regions along 137°E over the past 25 yr, were compared with trends based on the extrapolation of the trends over the past 25 yr (dotted line 1). (a) pCO2sea, (b) n-DIC, (c) pH, (d) buffer factor. Future trends were estimated ignoring (broken line 2) and including SST changes (bold line 3). In (d), buffer factor (∂ ln pCO2sea/∂ ln DIC) was calculated at constant SST (average values for 1983–2007) for broken line 2 and at SST with the same trends as those observed for the past 25 yr for bold line 3 as well as constant SSS and TA (average values for 1983–2007).
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When the same trend of increasing SST for the past 25 yr is assumed for the future, the rates of n-DIC increase are reduced an additional 5–18%, whereas there is almost no change to the projected rates of pH decrease in either region for 2040–2060 (a reduction of <1 to 1%). Under these conditions, the effects of increasing SST compensated for the slowing of the rate of pH decrease resulting from the reduction in the rate of DIC increase from the increase in the buffer factor.
In Case 2, under the future CO2 emission scenario IS92a, larger amounts of oceanic CO2 uptake were predicted considering the past SST trend, leading to an additional n-DIC increase of 24 μmol kg−1 and subsequent additional pH decrease of 0.04 (total pH decrease of 0.11–0.13 over the next 50 yr; Table 1, Fig. 9), compared with Case 1. The rates of both n-DIC increase and pH decrease for 2040–2060 were notably higher in the two regions, compared with those for the past 25 yr and those in Case 1. The predicted rates of pH decrease for 2040–2060 were about 40% greater than those from recent years. It is therefore possible that future ocean acidification could be accelerated, depending on increases in CO2 emissions. The magnitudes of the total pH decreases estimated in the present study are less than that predicted for the average over the global ocean in previous reports (a decrease of approximately 0.2 by the year 2050) (Caldeira and Wickett, 2003; Orr et al., 2005).
Figure 9. Comparison of future trends between Case 1 (extrapolation of past pCO2sea trend; broken line C1) and Case 2 (CO2 emission scenario IS92a [IPCC, 2007]; bold line C2) with consideration of past SST change in the northern (25–28°N, blue) and southern (11–14°N, red) regions. (a) pCO2sea, (b) n-DIC, (c) pH and (d) buffer factor. Dotted line denotes the extrapolations of recent trends. In (d), buffer factor (∂ ln pCO2sea/∂ ln DIC) was calculated at SST with the same trends as those observed for the past 25 yr as well as constant SSS and TA (average values for 1983–2007).
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Our calculations in the present study are confined within the equilibria among the dissolved and ionic carbonate species, as mentioned in the section of Methods. Recent studies have indicated possible future changes in marine ecosystems resulting from global warming, ocean acidification, or both. Recent reports on the effects of ocean acidification on the structure and function of marine microbial communities under high CO2 conditions (e.g. Doney et al., 2009) suggest significant feedbacks to the upper ocean carbon cycle through related biogeochemical processes. It is important to monitor the surface carbonate system through time series observations, including high-precision pH measurements as well as the information on the formation of CaCO3 and organic matter, to determine the future trends of acidification in the upper water column, and to detect changes in the surface carbonate system. Future studies using time series data over extensive regions are required to determine the effects of changes in marine ecosystems on the upper-ocean carbon cycle.