The Inorganic Carbonate Carbon Sedimentary Sink for Fossil Fuel CO2

To see where carbonate rocks come into the greenhouse sink picture, we recap on the sequence of different fates that befall CO2 released to the atmosphere through anthropogenic activities such as the burning of fossil fuels and cement production (Fig. 6.4). Some of the added CO2 may be relatively quickly removed from the atmosphere and taken up by the terrestrial biosphere as a result of 'CO2 fertilization' of plant productivity (although nutrient limitation may limit the importance of this effect; see Hymus and Valentini, Chapter 2, this volume) as well as forest regrowth and changes in land use practice. Current estimates suggest that 100-180 Pg C may already have been removed in this way, equivalent to 28-50% of total emissions from fossil fuels and cement production (Sabine et al., 2004). The timescale for this CO2 sink to operate is years to decades (for the aboveground vegetation response) to centuries (for the soil carbon inventory to adjust). At the same time, CO2 dissolves in the surface ocean. If the ocean did not circulate or mix and if dissolved CO2 remained as CO2(aq) (see Box 6.1), the ocean surface would quickly come into equilibrium with the atmosphere without having absorbed much anthropogenic CO2. Fortunately, neither ocean dynamics nor CO2 chemistry is simple, making the ocean a powerful carbon sink (Sabine and Feely, Chapter 3, this volume). However, once fossil fuel emissions to the atmosphere have ceased, sequestration by this means, termed 'ocean invasion', cannot continue indefinitely. First, the ocean becomes less efficient at storing additional dissolved carbon at higher atmospheric CO2 concen-

(a)

CO2

CO2

f A, f---

H2O + CO2- + C°2(<q

->2HCO3 (

w f f c i •

Fossil fuel C02 released to the atmosphere dissolves in the surface waters of the ocean, equilibrating with a timescale of 10° years.

At the same time the terrestrial biosphere (currently) acts as a net sink for C02.

Fossil Fuel Nitrogen

On timescales of 101-102 years, C02 rich surface waters are mixed into the ocean interior.

However, as the Earth's surface warms, the terrestrial biosphere may turn into a net source of C02 to the atmosphere.

On timescales of 101-102 years, C02 rich surface waters are mixed into the ocean interior.

However, as the Earth's surface warms, the terrestrial biosphere may turn into a net source of C02 to the atmosphere.

In the absence of further fossil fuel emissions, the atmosphere will eventually equilibrate with the whole ocean, with about 34% of all anthropogenic C02 emissions remaining in the atmosphere. For a 4167 Gt C fossil fuel 'burn', this represents 641 ppm above the pre-Industrial C02 concentration.

Fig. 6.4. Mechanisms of carbon sequestration (I). Panels (a) through (c) illustrate the pathways of carbon uptake operating on timescales of years (101) to centuries (102) - ocean invasion and 'CO2 fertilization' of the terrestrial biosphere.

In the absence of further fossil fuel emissions, the atmosphere will eventually equilibrate with the whole ocean, with about 34% of all anthropogenic C02 emissions remaining in the atmosphere. For a 4167 Gt C fossil fuel 'burn', this represents 641 ppm above the pre-Industrial C02 concentration.

Fig. 6.4. Mechanisms of carbon sequestration (I). Panels (a) through (c) illustrate the pathways of carbon uptake operating on timescales of years (101) to centuries (102) - ocean invasion and 'CO2 fertilization' of the terrestrial biosphere.

trations. Secondly, once CO2-enriched surface waters have been mixed down to depth and throughout the ocean, outgassing of previously absorbed CO2 will then tend to balance the rate of uptake from the atmosphere. Once this happens, the removal of anthropogenic CO2 by the ocean will cease and the ocean and atmosphere can then be said to be in 'equilibrium'.

We use a computer model of ocean-atmosphere carbon cycling (see Box 6.2 for details) to illustrate the importance of ocean invasion and the processes discussed in Chapter 3. This will give us something of a benchmark with which to compare the relative importance of the geologic sinks. The predicted evolution of atmospheric CO2 in response to a future CO2 emissions trajectory (Fig. 6.5a) and the operation of the ocean invasion sink is shown in Fig. 6.5b. For a total release (burn) of 4167 Pg C (Fig. 6.5a) and with global climate held in the modern state (i.e. global temperatures are not allowed to respond to rising CO2), the final atmospheric CO2 concentration reached is 919 ppm. This is equivalent to 2007 Pg C; ~1400 Pg C more than the amount prior to the Industrial Revolution (in c. 1765). Thus, the ocean has taken up a little over 66% of the total release, storing it mainly in the form of bicarbonate ions (HCO-) (in Fig. 6.5c), with the remainder in the atmosphere. We have not taken into account any net uptake (or release) by the terrestrial biosphere in this calculation.

One caveat to this assessment is that the fraction of fossil fuel CO2 that is sequestered by ocean invasion actually declines with increasing total burn. In other words, if we had chosen a smaller quantity of carbon than 4167 Pg C, the proportion taken up by ocean invasion would be greater than 66%. Ocean invasion was found to account for 80.7% when the fossil fuel release was just 874 Pg C, but 69.7% when the fossil fuel release was 4550 Pg C in an ocean carbon cycle general circulation model (GCM) (Archer et al., 1997, 1998). The results of the high-end CO2 release of Archer et al. (1998) are thus comparable to what we obtain here, with the ~3% difference reflecting variations in the representation of ocean circulation and marine carbon cycling between the two models. A second caveat is that no change in ocean circulation, marine biological productivity or surface temperatures has yet been taken into account.

Because CO2 solubility decreases at higher temperatures (Zeebe and Wolf-Gladrow, 2001), warming of the surface ocean due to a

Box 6.2. The 'genie' carbon cycle model.

The details of the computer model used to illustrate the different pathways and mechanisms for CO2 sequestration are not central to the discussions in this chapter: 'To retain respect for either models of sausages, one must not watch too carefully what goes into either of them' (Ridgwell, 2001, adapted from a remark attributed to Otto Von Bismarck, 1st Chancellor of the German Empire, 1871-1890). However, we include a brief overview of the model for completeness.

To quantify the uptake of atmospheric CO2 by the ocean we use the GENIE-1 (Ridgwell et al., 2006a) coupled carbon-climate model, developed as part of the 'genie' Earth system modelling initiative (www.genie.ac.uk). The climate model component is fully described in Edwards and Marsh (2005, and references therein). In a nutshell, it is a coarse (i.e. low) resolution of a 'frictional geostrophic' general circulation model (Edwards and Shepherd, 2002) coupled to a 2D energy/moisture balance model atmospheric component (Weaver et al., 2001) including a simple thermodynamic and dynamic representation of sea ice. An 'ensemble Kalman filter' has been used to calibrate this model and thereby achieve a reasonable simulation of the modern climate (Hargreaves et al., 2004).

The (ocean) biogeochemical component of the GENIE-1 model calculates the (mainly vertical) redistribution of tracer concentrations occurring rapidly relative to transport by the large-scale circulation of the ocean. This happens through the removal from solution of nutrients (PO4) together with dissolved inorganic carbon (DIC) and alkalinity (ALK) in the sunlit surface ocean layer (euphotic zone) by biological activity. The resulting export of particulate matter to the ocean interior is subject to remineralization processes, releasing dissolved constituent species back to the ocean (but at greater depth). Further redistribution of tracers occurs through gas exchange with the atmosphere as well as due to the creation and destruction of dissolved organic matter. An ensemble Kalman filter is also used to calibrate the biogeochemical model (and reproduce the observed 3D distributions of phosphate and alkalinity in the ocean) (Ridgwell et al., 2006a).

To quantify the importance of carbonate burial and the role of the 'geologic' carbon sink, the GENIE-1 model is further extended by including a representation of the geochemical interaction between the ocean and deep-sea sediments (Ridgwell, 2001). This extension calculates the fraction (if any) of CaCO3 reaching the ocean floor that is preserved and buried in the sediments, and described in full in Ridgwell and Hargreaves (in press). It also calculates the amount (and rate) of carbonate previously buried in the sediments that can be dissolved to neutralize fossil fuel CO2.

The GENIE-1 model is uniquely suited for the analysis of the long-term fate of fossil fuel CO2 because it can simulate over 1000 years in less than 1 h of CPU time, and achieves this speed on a 'normal' Linux-based PC. (Some much higher-resolution and more detailed climate models would literally take a year of supercomputer time to do this.) Another important feature is that climate can interact with the carbon cycle (i.e. climate is responsive to changes in atmospheric CO2), allowing the importance of 'feedbacks' to be quantified (e.g. Ridgwell et al., 2006b). For instance, if the positive feedback between increasing atmospheric CO2 and sea surface temperature warming is not taken into account, the amount of CO2 taken up from the atmosphere by the ocean could be overpredicted by ~10% (see Fig. 6.5).

CO2 is added to the atmosphere in the model to simulate anthropogenic CO2 emissions. We chose a hypothetical time-history of fossil fuel consumption (and combustion) of 4167 Pg C (Fig. 6.5a), similar to the '4kfast' scenario of Lenton (2000). This follows IS92a 'business as usual' to 2100, followed by a linear decline in emissions to use up all 'conventional' fossil fuel reserves (coal, oil, gas) of ~4000 Pg C. The total amount of carbon released to the atmosphere is 4167 Pg C, with 3 784 Pg C released from year 2000 onwards. This scenario falls midway between the 'A22' (3028 Pg C) and 'A23' (4550 Pg C) scenarios analysed by Archer et al. (1998), and is slightly less than the 5270 Pg C scenario employed by Caldeira and Wickett (2003). If 'exotic' fossil fuels, including methane clathrates, are exploited, total fossil fuel release could be as much as 15,000 Pg C (Hasselmann et al., 2003). Fossil fuel CO2 emissions to the atmosphere up to year 2000 are estimated from the increase in ocean + atmosphere carbon inventory in the experiment when atmospheric CO2 was forced to conform to the observed CO2 concentration trajectory - Enting et al. (1990) up until 1994, and Keeling and Whorf (2005) thereafter.

1400-

1200-

1000-

1400-

1200-

1000-

1800

-h-4 2000 2200

2400 2600 2800

Fig. 6.5. Model analysis of the uptake of fossil fuel CO2 by the ocean. (a) Time-history of the rate of CO2 emissions to the atmosphere prescribed in the model (see Box 6.2). From 1 765 to 2000, the emissions trajectory is deduced to be consistent with the observed trajectory of atmospheric CO2 (Enting et al., 1994; Keeling and Whorf, 2005). Note that these calculated anthropogenic emissions are net of any uptake by the terrestrial biosphere (i.e. if the terrestrial 40 biosphere acts as a carbon sink over this interval, the CO2 emissions will be underestimated). (b) Model-predicted trajectory of atmospheric CO2 (assuming no change in the terrrestrial biosphere). The solid line shows predicted atmosphere CO2 with climate (and ocean surface temperatures) held constant. The dotted line shows the impact of allowing the carbon cycle and climate to interact and produce 'feedback' between CO2 and surface temperatures. (c) Predicted evolution of the different components of the ocean DIC reservoir: CO2(aq), HCO- and COii-. Note the different y-axis

1800

-h-4 2000 2200

2400 2600 2800

3000 Year

4k 5k 6k 7k 8k 9k 10k 20k 30k 40k

Fig. 6.5. Model analysis of the uptake of fossil fuel CO2 by the ocean. (a) Time-history of the rate of CO2 emissions to the atmosphere prescribed in the model (see Box 6.2). From 1 765 to 2000, the emissions trajectory is deduced to be consistent with the observed trajectory of atmospheric CO2 (Enting et al., 1994; Keeling and Whorf, 2005). Note that these calculated anthropogenic emissions are net of any uptake by the terrestrial biosphere (i.e. if the terrestrial 40 biosphere acts as a carbon sink over this interval, the CO2 emissions will be underestimated). (b) Model-predicted trajectory of atmospheric CO2 (assuming no change in the terrrestrial biosphere). The solid line shows predicted atmosphere CO2 with climate (and ocean surface temperatures) held constant. The dotted line shows the impact of allowing the carbon cycle and climate to interact and produce 'feedback' between CO2 and surface temperatures. (c) Predicted evolution of the different components of the ocean DIC reservoir: CO2(aq), HCO- and COii-. Note the different y-axis

scales used for CO,,aql and CO2- (bottom of panel c) and HCO- (top of panel c).

stronger greenhouse effect will also render ocean invasion a less effective sink (Plattner et al., 2001; see Chapter 3). This effect creates a 'feedback loop' (see Berner, 1999; Ridgwell, 2003); higher temperatures result in more CO2 left in the atmosphere, which drives a stronger greenhouse effect causing yet higher temperatures, and so on. In this case the feedback has a 'positive' sign, and acts to amplify the impact of an initial perturbation (in our example, the release of fossil fuel CO2). If we now take into account feedback between CO2 and climate in the model, the final (steady-state) fraction of total emissions taken up by the ocean declines to 60%, with a residual 1037 ppm remaining in the atmosphere (Fig. 6.5b). The 2050 weakening of the ocean invasion sink due to feedback between CO2 and climate in the GENIE-1 model used here (Box 6.2) is ~13%, compared with ~24% found by Plattner et al. (2001), most likely reflecting differences in the ocean circulation response to surface warming (and freshening).

With no other carbon sinks operating, our long-term future is thus looking decidedly on the warm side; the mean global ocean surface temperature is 23.1°C, ~4.5°C warmer than the pre-Industrial state of the model (18.6°C). To put this into some perspective, the year 2005 value is 19.2°C in the model; just 0.6°C above the pre-Industrial estimate. This situation would persist indefinitely. There are important implications of the residual atmospheric fossil fuel fraction and degree of long-term greenhouse warming for stability of Greenland and Antarctic ice caps and of methane hydrates present in continental margin sediments (Archer and Buffett, 2004), as well as for the timing of the onset of the next ice age (Archer and Ganopolski, 2005).

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