Ewrgy COIIWS. Mgmt Vol. 36, No. 6-Y. pp. 457-460, 19~5

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PREDICTING FUTURE VARIABILI?“Y OF DISSOLVED INORGANIC CARBON INTHEOCEAN

Kathleen Cole and Gilbert Stegen

Science Applications International Corporation, 13400B Northup Way Bellevue WA 98005

Robert Bacastow

Scripps Institution of Oceanography, Geological Research Division La Jolla CA 92093

Abstract - Estimates were made for the future uptake of excess COz across the air/sea interface. If CO2 emissions continue to rise as predicted in a moderate growth scenario, surface ocean inorganic carbon concentrations could rise as much as 400 pmol kg-’ (20%) over the next 300 years. Increases in CO2 uptake of this magnitude may have a significant impact on the surface ocean ecosystem. Furthermore, these high atmospheric and surface ocean concentrations of COz will significantly reduce the effectiveness of COz ocean storage, particularly for shallow depth discharges.

1. INTRODUCTION

Analysis of CO2 gas trapped within ice cores indicates that prior to the industrial revolution, the ocean and atmosphere were in equilibrium with respect to COz exchange. However since the industrial revolution, the level of COz in the atmosphere has risen from 280 ppm to in excess of 350 ppm. In response to rapidly rising CO2 levels in the atmosphere, the ocean is presently absorbing between l-3 Gigatons of excess carbon (GtC) from the atmosphere each year ‘. This flux represents an important control on atmospheric CO2 concentration, which is, in turn, linked to climate change.

Estimates of future emissions of anthropogenically-derived COz range from 2000 to 8000 GtC’. Such rapidly rising CO2 levels in the near surface ocean may present an environmental risk to the ocean. Estimates of the growth in COs emissions have two important applications that will be explored: 1) modeling ocean uptake over the next few centuries and 2) assessing the impact of atmospheric emissions on the efficacy of ocean storage. Estimates will be made of the extent of fossil fuel CO2 absorption by the ocean, based on simulations, and the distribution of this COz.

2. MODEL DESCRIPTIONS

Ocean carbon cycle model

The ocean carbon cycle model is a three dimensional model based on a model by Bacastow and Maier-Reimer4. The grid points of the model are separated by 2SQ, which is approximately 275 km near the equator. There are 10 vertically spaced levels of calculation. Ocean bathymetry is realistic, within the resolution limitations of the grid. The GCM is driven by observed surface winds, while salinity and temperature are held close to observational values at the ocean surface. Chemical species modeled include total dissolved inorganic carbon (DIC), alkalinity, dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved oxygen, and phosphate. Photosynthesis is limited to the surface box, where the food chain produces DOC and POC. An atmospheric box exchanges CO r with the surface ocean boxes with transfer coefficients that vary with location according to wind velocity and other factors. The rate of exchange for each surface box is proportional to the difference in the partial pressure of COz in the surface water and the partial pressure of CO2 in the atmosphere. The COz partial pressure is computed from temperature and salinity dependent equilibrium constants.

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458 COLEctal.: DISSOLVEDINORGANICCARBONINTHEOCEAN

Vertical difsusion model

The vertical diffusion model contains an atmospheric box, a surface ocean box, a deep ocean box and a sediment box. The deep ocean box is divided into 100 levels and the sediments are divided into 10 levels that mix vertically in a diffusion-like process. In addition to diffusive mixing, there is exchange between the atmosphere and surface ocean box (air/sea exchange), the surface and deep ocean boxes and the deep ocean and sediment boxes (calcite dissolution). The extent of calcite dissolution is controlled by the saturation state. The calcite dissolution rate is controlled by the sediment and deep ocean water mixing rates, that expose fresh calcite to undersaturated sea water 4.

Fossil fuel emissions model

Estimates of future fossil fuel CO2 production were derived by Hidy and Spencer3. Figure 1 shows the annual and cumulative emissions curves for anthropogenically derived C02. In this scenario, emissions will rise until 2100 and then decline. Between 2000 and 2200 AD, fossil fuels emissions will total about 4000 GtC for this model scenario with a maximum annual emissions rate of 30 GtC yr-1.

30

25

20

15

10

5

0

-7000

1900 2000 2100 2200 2300 2400 Year

Figure 1. Hidy and Spencer3 model of fossil fuel based emissions of CO2 in Gigatons carbon.

3. RESULTS

Increase in DIC from the natural uptake of excess CO 2

Near surface concentrations of dissolved inorganic carbon (DIC) are estimated to currently be rising at a mean rate of 1 pmol kg-’ y-i. Below the surface layer, the rate of increase in DIC drops off to about 0.05 pmol kg-’ y-’ at 1000 meters. However, the DIC concentration of the surface ocean varies with location primarily due to variations in the vertical mixing intensity.

In Figure 2, the modeled increase in DIC concentration in the top 1000 meters of the Pacific along 17O”W between 1970 and 1980 is shown for the OGCM-based carbon cycle model. Although COz exchange occurs over the entire ocean surface; ocean circulation and vertical mixing determine the depth of excess DIC penetration. Along 17O”W in the Pacific, the model predicts that penetration of fossil fuel-derived COz, was deepest between 25”s and 55”s and between 20”N and 35”N. Penetration is most shallow in the equatorial region where vertical density stratification is greatest.

At the current rate of uptake, the concentration of DIC in the surface ocean would rise about 200 pmol kg-’ in the next 2 centuries. However, as the rate of fossil fuel emissions rise in the future, the rate of excess COa uptake by the surface ocean will also increase despite changes in the buffer factor. As is presently the case, mixing between the surface and deep oceans in the future will only transfer a small fraction of the surface ocean excess to the deep ocean. These circumstances suggest a large increase in the concentration of DIC in the surface ocean in the future. In Figure 3, model predictions are summarized for changes in the inventories and distributions of inorganic carbon in the

COLE ef al.: DISSOLVED INORGANIC CARBON IN THE OCEAN 459

atmosphere, ocean and sediments over the next 3 centuries. These changes are shown for a scenario of moderate future COz production (4000 GtC between 2000 and 2200 AD) and a second scenario that includes a 40% reduction in atmospheric emissions. This reduction could result from conservation and/or the storage of CO2 in aquifers and depleted reservoirs. In both scenarios, maximum atmospheric and surface ocean COz concentrations will occur around 2180. Maximum increase in surface ocean DIC of 450 and 350 pmol kg-’ were calculated for the high and low emissions cases, respectively. In the high emissions case, this represents an increase in DIC of more than 20%. In both cases, sufficient excess CO2 has been transferred to the deep ocean to initiate calcite dissolution.

0

1000 90s Latitude 90N

Figure 2. Modeled increase in DIC in pmol kg-’ between the surface and 1000 meters from 1970 to 1980. This figure represents a vertical section of the Pacific along 17O*W.

1860 1940 2020 2100 2180 2280 2340

Year

1860 1840 2020 2100 2180 2280 2340

Year

Figure 3. Modeled impact of future CO2 production on the atmosphere, ocean and sediments. The COa production estimate is a moderate emissions growth scenario3. The upper graph shows the total carbon inventories of the atmosphere and sediments over time. The lower graph shows the dispersion of dissolved COz with depth in the ocean as a function of time.

460 COLE et al.: DISSOLVED INORGANIC CARBON IN THE OCEAN

The simulations are for (a) none of future CO2 production is sequestered, “business as usual”. (b) 40% of future CO2 production is assumed to be conserved or permanently stored by means other than deep ocean disposal.

Impact of rising DIG levels on the sequestration of CO 2

Figure 4 illustrates the impact of rising atmospheric CO2 concentrations on the effectiveness of COz storage in the ocean. Simulations were prepared for the disposal in the ocean of the output of a 1 Gigawatt coal-fired power plant for 100 years. In figure 4a, 3 pairs of curves are shown for atmospheric emission of COz or the discharge of COz at 575-850 m, 1500-2500 m depth off Japan. In all cases the fraction of discharged COa remaining in the atmosphere or lost to the atmosphere (in the case of disposal) increases when future emissions of COz are included in the model calculations. The loss of sequestration effectiveness appears greatest for shallow disposal options. Since more of the disposal COz is transported into the surface ocean when a shallow disposal option is used the greater loss of efficiency should be expected.

2000 2050 2100 2150 2200 2250 2300 2000 2050 2100 2150 2200 2250 2300 Year Year

Figure 1. Simulations of the disposal of the CO2 output of a 1 GW power plant for 100 y into the ocean. (a) The loss of COz to the ocean from 575-850 m (curves 2 and 5), and from 1500-2500 m (curves 3 and 6) with and without including increases in atmospheric COz emissions in the future. Curves 1 and 4 are for the emission of this CO2 directly into the atmosphere.

4. CONCLUSIONS

The DIC concentration at the sea surface is currently rising at about 1 pmol kg-’ y-l due to rising COr emissions. If COz emissions continue to rise in the future as predicted in a moderate growth scenario, surface ocean inorganic carbon concentrations could increase to 450 l.tmol kg-i (>20%) over the next 200 years. These results suggest that additional effort needs to be expended in analyzing the potential for the adverse environmental impact of future atmospheric emissions. Furthermore, these high atmospheric and surface ocean concentrations of CO2 will significantly reduce the effectiveness of ocean storage of COz. Due to increased contact between disposal CO2 and the atmosphere, the loss of sequestration efficiency is greatest for shallow disposal options.

REFERENCES

1 Houghton J., G. Jenkins and J Ephrans, 1990. CZimate Change, the IPCC Scientific Assessment, Cambridge Univ Press.

’ Hidy, G and D Spencer, Climate alteration - aglobal issue for the power industry in the 21st century, Ann Rev of Energy and Envir., in press.

3 Cole, K and G Stegen, 1993. The capacity of the oceans to absorb COz. Energy Conversion Mngt, 34, 991-998.

4 Bacastow, R and E Maier-Riemer, 1990. Ocean circulation model of the ocean carbon cycle, Clim. Dyn., 4, 95-125.