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Figure 5.6. If over ihe next century 1000 GtC arc released to the atmosphere as ihe result of iossil I net burning and deforestation,, rhen in the absence ot" signi Bean r greening of the terrestrial biosphere roughly 80% will remain in the atmosphere, raising its CO> content to about 750 ppm. If we are very lucky and greening driven by fixed nitrogen and carbon dioxide increases storage in the terrestrial biosphere by as much as 200 GtC, CO> would increase to about ppm. The only way by which the CO* content could be held below 500 ppm would be to capture and store on the order of half of the CO 2 generated.

Experiments conducted by MBARTs Peter Brewer et al. (1999) using a remotely controlled submersible confirm that because of its higher compressibility, liquid CO2 becomes more dense than seawater at depths below 3 kilometers. Furthermore, the Brewer et al (1999) experiments show that when in contact with seawater, liquid CO? is rapidly converted to solid hydrate form (—6 H?0 to 1 CO?). This solid is more dense than either seawater or liquid C02, Hence, if the liquid CO2 were to be discharged into the deep sea, it would fall to the sea floor, creating a pile of solid hydrate. This hydrate would gradually dissolve into the bottom water and from there would be spread into the vast deep-sea reservoir. In this way, the route to the deep sea could be short-circuited, allowing the capacity for CX>? uptake by this vast reservoir to be tapped on a time scale far shorter than the several hundred years required if the CO? enters the sea at the surface. Because the time scale for return to the surface is also measured in hundreds of years, this type of disposal would prevent the return of the CO? to the surface until well after the greenhouse crisis was behind us.

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One might ask how we can he certain that no short circuits exist via which the CO? transferred to the sea floor could escape to the atmosphere on a much shorter time scale. First of all, natural radiocarbon contained in the sea's dissolved inorganic carbon acts as a clock, recording the isolation time of the waters that have descended into the abyss from the high-latitude source regions. This clock tells us that the isolation times average several hundred years and range up to a millennium in the remote deep northern Pacific. In addition, we have an even better indicator, namely, the Tie released from the crests of mid-ocean ridges (mean depth 2500 meters). The distribution of this helium isotope in the deep sea tells us two things. First of alt, the ridge-crest He becomes rather well mixed throughout the vast volume of deep seawater before it reaches the surf ace. This observation suggests that the CO2 will very likely do likewise (Farley ct al, 1995). If so, it will be largely neutralized by reaction with the carbonate and borate ion contained in the deep sea before reaching the surf ace. Second, from the inventory of T ie in the deep sea and estimates of the rate at w hich it is being released from ridge crests, it appears that the average helium atom remains in the deep sea for many hundreds of years before reaching the surface and escaping to the atmosphere (and from there to outer space).

With regard to disposal on the continents, the volume of pore space in the 1- to 2-kilometer depth range is vast. Most of these pores are currently filled with w ater Much of this w ater is too salty for use in agriculture, A much smaller portion of the pore space is filled with petroleum and natural gas. The liquid CO2 pumped into such reservoirs would displace the less dense resident water. CO2 disposed of in this way would remain trapped for many thousands of years.

Were w e to sequester, let's say, 25% of the 1000 Ci tC likely to be converted to CO? during the next century in deep continental reservoirs, the volume of liquid CO2 would be about 1000 cubic kilometers. Taking the reservoir porosity to be 20%, the volume of reservoir rock required would be 5000 cubic kilometers. Were these reservoirs to average 1 kilometer in thickness, they would cover only 5000 square kilometers (ix\, if all in one place, a square only 70 kilometers on a side). Hence, there is plenty of space av ailable for CO? storage on the continents.

5.7 Summary

Unless there is a dramatic reduction i n the f raction of the energy produced from fossil fuels over the course of the next century, the only way to prevent the atmosphere's CO? content from rising well above 500 ppm will be to implement large-scale CO? sequestration. Although not negligible in their potential contribution, taken together, energy conservation and terrestrial carbon storage certainly won't do the job. Rather, we must launch a serious effort to develop the means to sequester CO? and the political mechanism necessary to make sequestration economically competitive,

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