Carbonclimate feedbacks

In addition to the direct impacts of elevated CO2 on the ocean carbon system, there are many possible indirect effects related to the climate changes associated with the atmospheric CO2 increase. These feedback mechanisms include: (i) reduced CO2 solubility due to the increase in sea water temperature; (ii) enhanced stabilization of the upper water masses of the water column that will lead to decreased exchange of DIC and nutrients from the ocean interior; and

(iii) enhanced productivity in high-latitude regions (Table 3.5). The potential magnitude of these carbon-climate feedbacks has been examined in several modelling studies (Sarmiento and Le Quere, 1996; Sarmiento et al., 1998; Joos et al., 1999; Matear and Hirst, 1999; Greenblatt and Sarmiento, 2004).

CO2 solubility has a strong inverse relationship with temperature. Greenblatt and Sarmiento (2004) estimate that, as the surface ocean warms over this century, ~9-14% of the CO2 that would have been stored in the ocean will be retained in the atmosphere by 2100 (a positive climate feedback). The thermodynamics of this process are well known and, consequently, the uncertainties are reasonably low.

However, there are other processes that are not fully understood. For example, increased stratification of the water column due to warming and changes in the hydrological cycle is expected to cause a decrease in the exchange of carbon and nutrients between water masses, particularly in high latitudes. The decreased carbon exchange makes it more difficult to move the anthropogenic CO2 into

Table 3.5. Cumulative oceanic uptake of CO2 (Pg C) due to different climate-induced feedback effects.

Scenario and

Climate

Solubility

Stratification

Biological

Net

years

baseline

effect

effect

effect

effect

1% CO2/yeara

554

-52

-1 1 7

+ 111

-58

for 100 years

-

(-9.4%)

(-21.1%)

(+20.0%)

(-10.5%)

IS92a-like,b

401

-56

-68

+ 108

-1 6

1765-2065

-

(-1 4.0%)

(-1 7.0%)

(+26.9%)

(-4.0%)

IS92a,c

376

-48

-41

+33

-56

1850-2100

-

(-1 2.8%)

(-1 0.9%)

(+8.8%)

(-14.9%)

WRE550,d

530

-68

-1 5

+33

-50

1765-2100

-

(-1 2.8%)

(-2.8%)

(+6.2%)

(-9.4%)

WRE1000,e

612

-58

-27

+36

-48

1765-2100

-

(-9.5%)

(-4.4%)

(+5.9%)

(-7.8%)

aFrom Sarmiento and Le Quere (1 996). bFrom Sarmiento et al. (1998). cFrom Matear and Hirst (1999). dFrom Joos et al. (1999). eFrom Plattner et al. (2001).

Notes: 'Climate baseline' refers to a simulation with anthropogenic CO2 emissions but pre-industrial ocean temperatures and circulation. 'Effects' refer to uptake changes for various climate feedbacks and are expressed relative to the climate baseline. 'Net effect' is the uptake change when all climate feedbacks are present (i.e. full climate change simulation, after Greenblatt and Sarmiento, 2004).

aFrom Sarmiento and Le Quere (1 996). bFrom Sarmiento et al. (1998). cFrom Matear and Hirst (1999). dFrom Joos et al. (1999). eFrom Plattner et al. (2001).

Notes: 'Climate baseline' refers to a simulation with anthropogenic CO2 emissions but pre-industrial ocean temperatures and circulation. 'Effects' refer to uptake changes for various climate feedbacks and are expressed relative to the climate baseline. 'Net effect' is the uptake change when all climate feedbacks are present (i.e. full climate change simulation, after Greenblatt and Sarmiento, 2004).

the ocean interior, thus decreasing the oceanic uptake efficiency and providing a positive feedback (Table 3.5). This increased stratification, however, also increases CO2 drawdown by biological activity in the Southern Ocean (negative feedback) where there is an excess of surface nutrients and the organisms are generally light-limited. The different model studies disagree on the magnitude of these two competing effects and, in some cases, do not even agree on whether the combination of these two effects will provide a positive or negative feedback (Table 3.5).

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