Control of long-lived radiatively active gases is the only important means of controlling emissivity.13 We focus here on CO2. Following the discussion in Section 10.2.2 above, we may usefully distinguish between (i) reduction in fossil fuel use, (ii) reduction in CO2 emission per unit of fossil carbon used, and (iii) control of CO2 by removal from the atmosphere. Following the discussion above (Section 10.2.2) we refer to these as conventional mitigation, carbon management, and geoengineering, respectively.
The distinction is sometimes made between technical and biological sequestration where the former is intended to label pre-emission sequestration and the latter post-emission. This labeling is imprecise, however, because there are proposals for non-biological capture from the atmosphere, and for pre-emission biological capture in engineered systems (Reichle, Houghton et al., 1999).
The use of intensive forestry to capture carbon as a tool to moderate anthropogenic climate forcing was first proposed in the late 1970s (Dyson, 1977). It is now a centerpiece of proposals to control CO2 concentrations under the Framework Convention on Climate Change, particularly under the Clean Development Mechanism. The focus of interest has moved beyond forests to other managed ecosystems such as croplands. There is an extensive literature on both the science and economics of such capture; the summary below aims to frame the issue with reference to geoengineering.
Four alternatives are considered for disposition of the carbon once captured. It may be (a) sequestered in situ either in soil or in standing biomass, (b)
13 There is little opportunity to modify surface emissivity (typically values are 85-95% in the mid IR) and in any case modification has little effect since only a small fraction of surface radiation is transmitted to space. The main gas controlling atmospheric emissivity is water, but no direct means for controlling it have been proposed.
harvested and separately sequestered, (c) harvested and burned as fuel, or (d) harvested and burned as fuel with sequestration of the resulting CO2.
In situ sequestration has been the focus of most of the FCCC-related analysis (Bruce, Lee et al., 1996; Watson, Zinyowera et al., 1996; Rosenberg, Izaurralde et al., 1998). Uncertainty about the duration of sequestration is crucial. For example, recent analysis has demonstrated that changes in management of cropland, such as use of zero-tillage farming, can capture significant carbon fluxes in soils at low cost, but continued active management is required to prevent the return of carbon to the atmosphere by oxidation (Rosenberg, Izaurralde et al., 1998). For both forest and cropland, uncertainty about the dynamics of carbon in these ecosystems limits our ability to predict their response to changed management practices or to climatic change, and thus adds to uncertainty about the duration of sequestration.
Sequestration of harvested biomass was considered in early analyses but has received little attention in recent work, perhaps because use of biomass as a fuel is a more economically efficient means to retard the increase in concentrations than is sequestration of biomass to offset fossil carbon emissions. Finally, biomass could be used to produce carbon-free energy (H2 or electricity) with sequestration of the resulting CO2 (IPCC95). This process illustrates the complexities of the definitions described above, because it combines pre- and postemission capture and combines biological and technical methods.
Recent studies of carbon capture in cropland have identified the possible contributions of genetically modified organisms to achieving increases in carbon capture, and have stressed the importance of further research (Rosenberg, Izaurralde et al., 1998). The US DOE research effort on sequestration currently includes genomic science as an important part of the sequestration research portfolio for both terrestrial and oceanic ecosystems (Reichle, Houghton et al., 1999).
Use of terrestrial ecosystems to supply energy needs with minimal net carbon emissions - via any combination of sequestration to offset use of fossil fuels or via the use of biomass energy - will demand a substantial increase in the intensity and/or areal extent of land use. Whether captured by silviculture or agriculture, areal carbon fluxes are or order 1-10 tC/ha-yr. If the resulting biomass were used as fuel the equivalent energy flux would be 0.2-2 W/m2, where the lower end of each range is for lightly managed forests and the upper end for intensive agriculture. Mean per-capita energy use in the wealthy industrialized world is ~5 kW. Thus about 1 hectare per capita would be required for an energy system based entirely on terrestrial carbon fixation, roughly equivalent to the current use of cropland and managed forest combined.
Is management of terrestrial ecosystems for carbon capture geoengineering?
As discussed in the concluding section, the ambiguity of the answer provides insight into shifting standards regarding the appropriate level of human intervention in global biogeochemical systems. Considering the defining attributes of geoengineering described in Section 10.2.1, we can describe a land management continuum in which, for example, land management that considers in situ carbon sequestration as one element in a balanced set of goals forms one pole of the continuum, and the large-scale extraction and separate sequestration of carbon from intensively irrigated and fertilized genetically modified crops forms the opposite pole. The land-use requirements discussed above suggest that manipulation of carbon fluxes at a level sufficient to significantly retard the growth of CO2 concentrations would entail a substantial increase in the deliberate manipulation of terrestrial ecosystems. Put simply, enhancement of terrestrial carbon sinks with sufficient vigor to aid in solving the CO2-climate problem is plausibly a form of geoengineering.
Carbon can be removed from the atmosphere by fertilizing the "biological pump" which maintains the disequilibrium in CO2 concentration between the atmosphere and deep ocean. The net effect of biological activity in the ocean surface is to bind phosphorus, nitrogen, and carbon into organic detritus in a ratio of —1:15:130 until all of the limiting nutrient is exhausted. The detritus then falls to the deep ocean providing the "pumping" effect. Thus the addition of one mole of phosphate can, in principle, remove —130 moles of carbon.14 The possibility of fertilizing the biological pump to regulate atmospheric CO2 was discussed as early as the NAS77 assessment. At first, suggestions focused on adding phosphate or nitrate. Over the last decade it has become evident that iron may be the limiting nutrient over substantial oceanic areas (Watson, 1997; Behrenfeld and Kolber, 1999). The molar ratio Fe:C in detritus is —1:10 000, implying that iron can be a very efficient fertilizer of ocean-surface biota. Motivated in part by interest in deliberate enhancement of the oceanic carbon sink, two field experiments have tested iron fertilization in situ, and have demonstrated dramatic productivity enhancements over the short duration of the experiments (Martin, Coale et al., 1994; Monastersky, 1995; Coale, Johnson et al., 1998). However, it is not clear that sustained carbon removal is realizable (Peng and Broecker, 1991).
Ocean fertilization is now moving beyond theory. Recently, a commercial
14 This ratio includes the carbon removed as CaCO3 due to alkalinity compensation. This first order model of the biology ignores the phosphate-nitrate balance. Much of the ocean is nitrate limited. Adding phosphate to the system will only enhance productivity if the ecosystem shifted to favor nitrogen fixers. In many cases, nitrogen fixation may be limited by iron and other trace metals.
venture, Ocean Farming Incorporated, has announced plans to fertilize the ocean for the purpose of increasing fish yields and perhaps to claim a carbon sequestration credit under the emerging FCCC framework (Kitzinger and Frankel, 1998, p. 121-129).
Ocean fertilization may have significant side effects. For example, it might decrease dissolved oxygen with consequent increased emissions of methane -a greenhouse gas. In addition, any significant enhancement of microbiological productivity would be expected to alter the distribution and abundance of oceanic macro-fauna. These side effects are as yet unexamined.
On the longest timescales, atmospheric CO2 concentrations are controlled by the weathering of magnesium and calcium silicates that ultimately react to form carbonate deposits on the ocean floor, removing the carbon from shorter timescale biogeochemical cycling. In principle, this carbon sink could be accelerated, for example, by addition of calcite to the oceans (Kheshgi, 1995) or by an industrial process that could efficiently form carbonates by reaction with atmospheric CO2. I call this geochemical sequestration.
In either case, the quantity (in moles) of the required alkaline minerals is comparable to the amount of carbon removed. The quantities of material processing required make these proposals expensive compared to other means of removing atmospheric CO2.
The most plausible application of geochemical sequestration is as a means to permanently immobilize carbon captured from fossil fuel combustion. Integrated power plant designs have been proposed, in which a fossil fuel input would be converted to carbon-free power (electricity or hydrogen) with simultaneous reaction of the CO2 with serpentine rock (magnesium silicate) to form carbonates. Carbonate formation is exothermic; thus, in principle, the reaction requires no input energy. Ample reserves of the required serpentine rocks exist at high purity. The size of the mining activities required to extract the serpentine rock and dispose of the carbonate are small compared to the mining activity needed to extract the corresponding quantity of coal. The difficulty is in devising an inexpensive and environmentally sound industrial process to perform the reaction.
The importance of geochemical sequestration lies in the permanence with which it removes CO2 from the biosphere. Unlike carbon that is sequestered in organic matter or in geological formations, once carbonate is formed the carbon is permanently removed. The only important route for it to return to active biogeochemical cycling is by thermal dissociation following the subduc-
tion of the carbonate-laden oceanic crust beneath the continents, a process with a time-scale of > 107 years.
10.4.3.4 Capture and sequestration of carbon from fossil fuels
The CO2 generated from oxidation of fossil fuels can be captured by separating CO2 from products of combustion or by reforming the fuel to yield a hydrogen-enriched fuel stream for combustion and a carbon-enriched stream for sequestration. In either case, the net effect is an industrial system that produces carbon-free energy and CO2 - separating the energy and carbon content of fossil fuels. The CO2 may then be sequestered in geological formations or in the ocean.
Because the status of carbon management as geoengineering is ambiguous, and because there is now a large and rapidly growing literature on the subject (Herzog, Drake et al., 1997; Parson and Keith, 1998), only a brief summary is included here despite its growing importance. Our focus will be on oceanic sequestration because it most clearly constitutes geoengineering (Section 10.2.2).
One may view CO2-induced climate change as a problem of mismatched timescales. The problem is due to the rate at which combustion of fossil fuels is transferring carbon from ancient terrestrial reservoirs into the comparatively small atmospheric reservoir. When CO2 is emitted to the atmosphere, atmosphere-ocean equilibration transfers —80% of it to the oceans with an exponential timescale of —300 years (Archer, Kheshgi et al., 1997). The remaining CO2 is removed with much longer timescales. Injecting CO2 into the deep ocean accelerates this equilibration, reducing peak atmospheric concentrations. Marchetti used similar arguments in coining the term geoengineering in the early 1970s to denote his suggestion that CO2 from combustion could be disposed of in the ocean (Marchetti, 1977). Oceanic sequestration is a technical fix for the problem of rising CO2 concentrations; it is a deliberate planetary-scale intervention in the carbon cycle. It thus fits the general definition of geo-engineering given above (Section 10.2) as well as the original meaning of the term.
The efficiency with which injected CO2 equilibrates with oceanic carbon depends on the location and depth of injection. For example, injection at —700 m depth into the Kuroshio current off Japan would result in much of the CO2 being returned to the atmosphere in —100 years, whereas injections that formed "lakes" of CO2 in ocean trenches might more efficiently accelerate equilibration of the CO2 with the deep-sea carbonates.
The dynamic nature of the marine carbon cycle precludes defining a unique static capacity, as may be done for geological sequestration. Depending on the increase in mean ocean acidity that is presumed acceptable, the capacity is of order —103-104 gigatons of carbon (GtC), much larger than current anthropogenic emissions of —6 GtC per year.
In considering the implications of oceanic sequestration one must note that - depending on the injection site - about 20% of the carbon returns to the atmosphere on the —300 yr time-scale. Supplying the energy required for separating, compressing, and injecting the CO2 would require more fossil fuel than would be needed if the CO2 was vented to the atmosphere. Thus, while oceanic sequestration can reduce the peak atmospheric concentration of CO2 caused by the use of a given amount of fossil-derived energy, it may increase the resulting atmospheric concentrations on timescales greater than the — 300 yr equilibration time.
10.4.4 Energy transport The primary means by which humanity alters energy transport is by alteration of land surface properties. The most important influence is on hydrological properties, particularly through changes to surface hydrological properties and the rates of evapo-transpiration, but additionally via dams that create large reservoirs or redirect rivers. A secondary influence is on surface roughness via alteration of land use.
Inadvertent alteration of local and regional climate has already occurred due to alteration of land surface properties via either the means mentioned above or by alteration of albedo (Section 10.4.2.4). In addition, deliberate alteration of local microclimates is a common feature of human land management. Despite the long record of speculation about the alteration of surface properties with the intention of altering regional or global climate, it seems highly unlikely that geoengineering will ever play an important role in land management given the manifold demands on land use and the difficulty of achieving such large-scale alterations.
Other means of altering energy transport are more speculative. Examples include weather modification and redirection of ocean currents using giant dams. In principle, the direct application of mechanical work to alter atmospheric motions offers an energetically efficient means of weather modification, however, no practical means of applying such forces is known. Alternatively, weather modification may be accomplished by cloud seeding. Despite very large cumulative research expenditure over its long history, cloud seeding has demonstrated only marginal effectiveness. Accurate knowledge of the atmospheric state and its stability could permit leverage of small, targeted perturbations to effect proportionately larger alterations of the atmospheric dynamics.
The small perturbations could be effected by cloud seeding or direct application of thermal or mechanical energy. The increasing quality of analysis/forecast systems and the development of effective adjoint models that allow accurate identification of dynamic instabilities suggest that the relevant predictive capability is emerging.
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