CO2 flux variability

Over the last 40 years, the growth rate of CO2 in the atmosphere has experienced interannual variations as large as ±2 Pg C/ year (Francey et al., 1995; Keeling et al., 1996). There is an ongoing controversy on the relative contributions of this variability from atmosphere-land and atmosphere-ocean exchanges (Fig. 3.4). Time series measurements of atmospheric CO2, 13C and O2/N2 sources have suggested that

Table 3.1. Regional distribution of net sea-air fluxes (Pg C/year) for 1995 based on NCEP 41-year U-10 reanalysis of wind data and the Wanninkhof (1992) gas exchange relationship and areas represented in each region. (Modified from Takahashi et al., 2002; see http://www.ldeo.columbia.edu/res/pi/CO2/carbondioxide/pages/air_sea_

Table 3.1. Regional distribution of net sea-air fluxes (Pg C/year) for 1995 based on NCEP 41-year U-10 reanalysis of wind data and the Wanninkhof (1992) gas exchange relationship and areas represented in each region. (Modified from Takahashi et al., 2002; see http://www.ldeo.columbia.edu/res/pi/CO2/carbondioxide/pages/air_sea_

flux_rev1.html.)

Latitude band

Pacific

Atlantic

Indian

Southern

Global

Flux (Pg C/year)

North of 50°N

0.01

-0.31

-

-

-0.30

1 4°N-50°N

-0.49

-0.25

0.05

-

-0.69

14°S-14°N

0.65

0.1 3

0.1 3

-

0.91

1 4°S-50°S

-0.39

-0.21

-0.52

-

-1 .1 2

South of 50°S

-

-

-

-0.30

-0.30

Total

-0.22

-0.64

-0.34

-0.30

-1.50

% of uptake

15

42

23

20

100

Area (106 km2)

North of 50°N

4.93

9.23

-

-

14.16

1 4°N-50°N

42.59

24.41

2.12

-

69.1 2

1 4°S—14°N

50.93

1 5.91

19.83

-

86.67

1 4°S-50°S

53.58

25.01

31.03

-

1 09.62

South of 50°S

-

-

-

41.10

41.10

Total

152.0

74.6

53.0

41.1

320.7

% of area

47

23

17

13

100

Table 3.2. Regional distribution of net sea-air flux (Pg C/year) for 1995 based on NCEP 41-year U-10 reanalysis of wind data and the Wanninkhof and McGillis (1999) gas exchange relationship. (Modified from Takahashi et al., 2002; see http://www.ldeo.columbia. edu/res/pi/CO2/carbondioxide/pages/air_sea_flux_rev1.html.)

Table 3.2. Regional distribution of net sea-air flux (Pg C/year) for 1995 based on NCEP 41-year U-10 reanalysis of wind data and the Wanninkhof and McGillis (1999) gas exchange relationship. (Modified from Takahashi et al., 2002; see http://www.ldeo.columbia. edu/res/pi/CO2/carbondioxide/pages/air_sea_flux_rev1.html.)

Latitude band

Pacific

Atlantic

Indian

Southern

Global

North of 50°N

0.03

-0.37

-

-

-0.34

1 4°N-50°N

-0.59

-0.31

0.06

-

-0.84

1 4°S-1 4°N

0.53

0.1 1

0.1 3

-

0.77

1 4°S-50°S

-0.45

-0.28

-0.64

-

-1.37

South of 50°S

-

-

-

-0.39

-0.39

Total

-0.48

-0.85

-0.45

-0.39

-2.1 7

ocean flux variations must be in the order of 1-2 Pg C/year (Francey et al., 1995; Keeling et al., 1996; Rayner et al., 1999; Battle et al., 2000; Bousquet et al., 2000). However, ocean modelling and revised inverse models (Winguth et al., 1994; Le Quere et al., 2000, 2003; Obata and Kitamura, 2003; McKinley et al., 2004; Peylin et al., 2005; Wetzel et al., 2005) as well as empirical approaches (Lee et al., 1998; Park et al., 2006) have suggested a much smaller ocean variability of ~0.3-0.5 Pg C/year.

Of the few direct time series measurements made over large ocean regions so far, only the equatorial Pacific Ocean (Feely et al., 1997, 1999) and the Greenland Sea (Skjelvan et al., 1999) have shown large year-to-year variations in sea-air CO2 flux. However, there are not many data-sets with which to evaluate such flux variability directly. The variability observed in the equatorial Pacific and North Atlantic oceans is not sufficient to account for all of the variability estimates, but other regions including the Southern Ocean and subtropical regions have not been studied sufficiently to determine their contributions to oceanic variability. Recent ocean model results have suggested that after the equatorial Pacific, the Southern Ocean and the northern extratropical regions are also important regions showing significant interannual variability in sea-air CO2 flux (Peylin et al., 2005; Wetzel et al., 2005). Resolving this controversy and imposing stricter constraints on carbon cycle models will require more detailed observations of the magnitude and causes of variability in the sea-air CO2 flux and other carbon-related species in the ocean, as well as continued atmospheric measurements of temporal and spatial distributions of CO2, 13C and O2/N2.

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