Modern CO2 uptake rates

Several independent approaches have been used to estimate the modern oceanic uptake rate of anthropogenic CO2. Table 3.3 shows a summary of the ocean observations

Nitrogen Deposition Galloway

- Mean inversion ----- Rayner inversion SLAVE model

Fig. 3.4. Comparison of (a) atmospheric mean annual growth rate, (b) land CO2 flux anomalies and (c) ocean CO2 flux anomalies between 1980 and 1995 (in Gt C/year). The grey zone denotes the range of the inversion models, and the dark line denotes the mean. The coloured lines show the ocean models. (After Peylin et al., 2005.)

- Mean inversion ----- Rayner inversion SLAVE model

Fig. 3.4. Comparison of (a) atmospheric mean annual growth rate, (b) land CO2 flux anomalies and (c) ocean CO2 flux anomalies between 1980 and 1995 (in Gt C/year). The grey zone denotes the range of the inversion models, and the dark line denotes the mean. The coloured lines show the ocean models. (After Peylin et al., 2005.)

and model estimates of the anthropogenic CO2 uptake in the 1990s (in Pg C/year). Most of the models are in reasonably good agreement with the flux estimates from the observational data when corrected for the pre-industrial carbon flux.

Models suggest that anthropogenic CO2 uptake occurs everywhere in the surface ocean, even in areas that have a total net CO2 flux out of the ocean. For example, the pCO2 of sea water in the eastern equatorial Pacific is much higher than atmospheric pCO2. This sea-air difference in pCO2 causes the ocean to release CO2 to the atmosphere. As atmospheric CO2 concentrations rise, the difference between the sea water pCO2 and atmospheric pCO2 is decreased and the rate of CO2 loss drops. The additional CO2 that is stored in the sea water, which would have been lost to an atmosphere with lower pCO2 values, is referred to as the anthropogenic uptake.

It is important to note that there is a difference between CO2 uptake and CO2 storage. While there may be a large anthropogenic CO2 uptake in the equatorial Pacific, the water is quickly transported off the equator and the anthropogenic CO2 is actually stored in the subtropical gyres (Gloor et al., 2003). Ocean carbon models suggest that the high-latitude Southern Ocean is also a region with large anthropogenic CO2 uptake, but the anthropogenic CO2 is stored further to the north where mode and intermediate water masses are formed. Water mass formation regions are areas in which

Table 3.3. Estimates of oceanic anthropogenic CO2 uptake in Pg C/year.

Method

Carbon uptake (Pg C/year)

Reference

Measurements of sea-air pCO2 difference 2.1 ± 0.5 Inversion of atmospheric

CO2 observations 1.8 ± 1.0 Inversions based on ocean transport models and observed DIC 2.0 ± 0.4 Model simulations evaluated with CFCs and pre-bomb radiocarbon 2.2 ± 0.4 OCMIP-2 model simulations 2.4 ± 0.3 Based on measured atmospheric O2 and CO2 inventories corrected for ocean warming and stratification 2.3 ± 0.7

GCM model of ocean carbon 1.93

Takahashi et al. (2002) Gurney et al. (2002)

Matsumoto et al. (2004) Orr (2004)

Bopp et al. (2002) Wetzel et al. (2005) McNeil et al. (2003)

Notes: Fluxes are normalized to 1990-1999 and corrected for pre-industrial degassing flux of -0.6 Pg C/year.

water is moved from the surface into the ocean interior. Once the waters leave the surface, the anthropogenic CO2 is isolated from the atmosphere and stored until these waters return to the surface.

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