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Possible solutions
The foregoing considerations make it clear that the major anthropogenic changes in the natural carbon cycle have been made within the last 1000 years or so and have been focused on reduction of terrestrial sinks and expansion of geological sources through the exploitation of fossil carbon with the use of coal expanding towards the end of the "little ice age" (Perlin 1989). In fact in a review of forest use since Mesopotamian times (4,700 years ago) Perlin argues that western civilization has developed by exploiting forests directly for fuel and for construction of infrastructure and the decline of most earlier development has been directly related to the consequences of deforestation. Certainly current relic stands of cyprus and juniper forests on around the peaks of mountains on the northern side of the Persian Gulf in Iran examined by the author, suggest that the whole region in that part of the world was once completely forested.
Short term (< 50 years)
If the model presented herein is correct we may draw the following conclusions. First, the origin of the current problem in its present form dates from the little ice age and the associated instigation of systematic deforestation in Europe and elsewhere which together with previous reductions in plant biomass throughout the last 3,000 years or so covering the development of modern civilization, have today left a standing stock of 500 Gt of plant-based carbon (C)1 as opposed to the amount estimated to have been present prior to this date which has been estimated as 1080 Gt C (Sundquist 1985). The "original" plant-cover may be termed the "reconstructed standing mass". For the sake of comparison, the latter figure can be used as a crude estimate of the additional organic carbon that might be supported globally without extensive energy subsidies. This is obviously a serious overestimate in potential capacity since the displaced forest area now supports alternate uses such as agriculture etc. By this reasoning, a total of 1080 Gt - 500 Gt = 580 Gt C has been displaced during the 3,000 year period.
The displaced standing stock of plant organic carbon alone, if re-established, would be enough to absorb only 58 years of total C discharges at 1990 levels. This estimate is obtained by subtracting the present biomass C from the reconstructed estimate and dividing by the amount of carbon added annually at current rates as of 1990 (1080-500 = 580, 580/10 = 58, where 10 Gt C is estimated to have been added in 1990 consisting of 4 Gt from biomass burning plus emissions from land disturbance and 6 Gt from fossil fuel combustion). However, in order to merely stabilize the atmospheric increases that will be responsible for global warming, without considering the roughly 170 Gt C accumulated in the atmosphere since the 1800's from fossil fuel use plus the median of 275 Gt C from changes in land use (where the estimated range might might be between 240-310 Gt C), it would be necessary to completely absorb at least the 0.4% projected annual increase of the current atmospheric carbon load (Ashmore 1990). This would be 3 Gt C calculated as 0.4% of 751 Gt (where 751 Gt is the total mass of atmospheric C in 1990). This is approximately 30% of the assumed 1990 discharge (3.0/10.0 Gt = 30%). Based on current trends, this amount would also continue to grow at least at 0.4% for the following year to 3.01 Gt and so forth.
For the sake of preliminary analysis it may be suggested that any proposed global tree-planting program together with existing forests would have a potential upper limit of 1080 Gt C standing stock. This would coincide with the estimate for the reconstructed system of 3,000 years ago. That is, an additional 580 Gt C could be added to the earth's surface before this potential ceiling might be reached. This limit would be achieved while planting enough trees to hold the atmospheric load constant in just 143 years, without requiring any emissions reductions.
The land required just to store 3.0 Gt C in the tropics, using the mature tropical forest estimate for carbon storage provided by Brown and Lugo, 1982, of between 46 - 183 t C/ha (median 114.5), may be calculated as 26.2 million hectares (3,000 million/114.5), an area equal to 1.42% of the existing tropical forest estimated by the same authors at 1,838 million ha and containing only 229 Gt C. To store 580 Gt C the required area would be 5,065 million ha, an area equal to 2.75 times the existing global tropical forests and 1.6 times that of all the African continent (at 3,000 million ha). The above estimate for annual demand of land to absorb atmospheric carbon is however, based on mature stands. Such carbon densities cannot be achieved immediately requiring from 25-50 years or so for development. Annual additions in new Eucalyptus plantations may be around 5.9-8.8 t C/ha (Markham, 1990) meaning that a thirteen to twentyfold increase in land would actually be required to draw-down 3.0 Gt C in the first year that is, from 340.9 million to 508.4 million hectares. The land requirements on this basis would exceed 5,000 million ha in 10 to 15 years. In the case of actively accumulating plantations Davidson (1985) indicates a variety of primary succession species (including Eucalyptus) with maximum accumulations ranging from considerably less than 12.5 tons C/ha/yr up to around 37.5 tons C/ha/yr during the period of maximum growth which may be estimated as lasting from 15 - 30 years or so depending on the kind of tree involved. During this period of growth, the land area required annually for the capture of say 3.0 Gt C would be from 80 million ha to 240 million ha (3,000,000,000/37.5 = 80,000,000).
Carbon storage capacity in temperate coniferous rain forests might reach 400 t/ha, open conifer woodland 97 t/ha and temperate broad-leaved deciduous mixed forest 170 t/ha C (Adams et al., 1990). Reconstituted forests usually contain significantly less carbon than the stands they replace. A figure of 100 t C/ha for tropical plantations has been suggested (Palm et al., 1986) although lower figures are also reported, see Chart 4. Furthermore, if the growth rate of atmospheric CO2 increases to just over twice the current rate or 1.0% per year the potential upper limit proposed above (1080 Gt C) for mature forest storage would have to be achieved in just 57 years rather than 143 years. The situation will worsen as deforestation continues unabated. The residual soil often being extremely poor creates additional difficulties for replanting programs.
We may however, look at the problem from the viewpoint of finding mechanisms to absorb atmospheric carbon which exceeds the current estimated rate (temporary or long term) at which it is currently sequestered. This will be necessary if fossil-fuel economies contunue to be developed in exactly the same manner as has been the case over the last 100 years or so.
A global reforestation/aforestation program immediately implemented to absorb the excess atmospheric carbon could be expected to buy a window of time amounting to between 25 years in the tropics to 50 years in the temperate zones before growth in biomass would start to be offset by destruction and decay. In order to stabilize the input responsible for global warming, as a first approximation without considering the roughly 170 Gt C accumulated in the atmosphere since the 1800's from fossil fuel use plus the 275 Gt C from changes in land use (estimated 240-310 Gt C total) it would therefore be immediately necessary to completely absorb the 1%, high (0.4%, low), of projected annual increase of the current atmospheric carbon load, say 1% of 751 Gt = 7.51 Gt C (bracketed figures represent lower atmospheric CO2 growth estimate of 0.4% of 751 Gt = 3 Gt ). This is approximately 75% (3%) of the assumed 1990 discharge (7.51/10.0 Gt = 75%). Based on current trends, this figure would grow at 1% (0.4%) for the following year to 7.58 (3.01) Gt and so forth. A planting program that might have as a potential upper limit of 580 Gt C standing crop could therefore achieve this capacity while holding the atmospheric load constant in just 57 (143) years without any emissions reductions.
Annual land clearance in Southeast Asia alone has been estimated (Wiggers n.d.) as 4.2 x 106 hectares of which (0.8-1.4) x 106 ha. were due to direct forest removal. The potential for positive benefits in controlling atmospheric carbon through aforestation and reforestation on a global scale is clear.
Biomass yields a heating value of 20 gJ./t (20 gJ./0.5 tC) and the weighted world average for coal is 25 gJ/t (Sundquist 1985). Fossil carbon subsidized croplands may yield 15-35 t/ha/yr dry biomass (7.5-17.5 t/C/ha/yr) and general growth much less, say 3 t/C/ha/yr after discounting fertilizer and carbon penalty costs due to management etc. and taking into account that existing agricultural production is required to support global populations thus denying access to productive land. Considering only the 6 Gt C/yr produced by fossil fuels, an approximate energy substitution might be made for the 6 x 109 t C at 3.0 t/C/ha/yr by planting 2,000 x 106 ha of land with energy crops. Additional land will have to be planted to offset the carbon cost of management, harvesting, processing etc., not to mention the added biomass required to balance the higher calorific content of oil and natural gas with respect to carbon content. Under present energy consumption growth rates, the planted area would also have to accelerate at around 2.7% per year (Davis 1990). Thus the area initially required might be as much as 3,000 x 106 ha/yr and would rapidly grow to exhaust available productive land on a global scale (doubling time 27 years). Note: the area of the USA is 900 x 106 ha.
Medium to long term (>50 to 100's of years or more)
Once the short term options for reforestation are taken up attention will focus on aforestation, finally shifting to long term storage. The half-life of wood products may extend from 100-200 years or so in the case of teak furniture to much less than 5-10 years where general purpose timber, particularly softwoods is involved in agricultural and commercial construction. Because of the variation in the life-span of wood products harvesting is equated with oxidation and release of stored carbon as CO2 for the purpose of this discussion. The fossil carbon cost of forest management is assumed to be approximately equal to or in excess of the advantage claimed for wood products as a carbon store over 4-5 harvest cycles in a managed plantation.
Medium-long term control of atmospheric carbon is an altogether different matter as it will depend on permanent or semi-permanent carbon sequestering apart from assumed steady oceanic uptake by surface absorption and diffusion over time-frames of hundreds to thousands of years. The broad potential of the oceans to fix carbon is constrained by actual nutrient availability. However, rapid transmission of carbon takes place (months to years) after initial fixation in the surface photic zone either by remineralization or photochemical degradation of organic detritus (1.6-7.3 Gt C/yr, median 4.45 Gt (Sundquist 1985)) into solution as it sinks or as direct deposition Sundquist 1985) into bottom sediments (.0063-0.75 Gt ) at depths above 1500 m (the oxidation depth). Since the calcium/magnesium carbonate route is restricted over the short term, and not withstanding proposals to fertilize polar waters to encourage productivity and then entrainment of carbon to deep waters, the only controllable carbon storage mechanism which would otherwise appear to be available involves terrestrial wetlands. Unfortunately wetlands have been drastically restricted in modern times due to agricultural expansion and at the same time their potential rate of accumulation may be low if the example of the coastal peat swamps of Southeast Asia is a useful guide. In fact one estimate (Schlisinger 1990) posits the present capacity of the terrestrial soil sink as not likely to exceed 0.4 Gt C per year. A "trial balance" for global carbon sinks is presented in Table 1, based on measured data or measured data plus estimates. No "model" derived figures are used and the balance is surprisingly good, however, the outline is based on median values and some of the available estimates are quite variable. In particular, the oceans have the largest part in the global carbon dioxide cycle and a clear understanding of their role either as a potential buffer or as a source (for instance if UV-B radiation were to negativly affect phytoplankton production on a global basis) cannot be obtained from the existing data.
In order to fix and hold the equivalent of 7.51 (3.0) Gt C annually, a tropical lowland peat forest system accumulating 20 cm/100 years of peat with a bulk density of 0.1 g/cm3 would need to cover a stunning 7,510 x 106 (3,000 x 106) hectares. This area is 375 (150) times the estimated 20 x 106 ha. of lowland peat presently estimated as existing in all of Southeast Asia, an area which moreover is currently slated for a variety of development schemes from peat mining for electrical generation to traditional expansion of agriculture. Such a system would however, be expected to function far into the future. Moreover it appears to be the only long term carbon storage solution available. One that will by necessity limit the quantity of fossil fuel and consequent emissions that can be tolerated on a global basis. The estimated total (original) area of peat lands - selected countries - is provided in (Euroconsult 1984) as 420.9 x 106 ha. which is 5.6% (14%) of the area required to store 7.51 (3.0) Gt C annually under the above assumptions. A lower estimate of 280 x 106 ha. peatland has been provided recently ((Adams et. al. 1990), a figure which is 66% of that given in (Euroconsult 1984). See chart 5.
Sustainability
The foregoing discussion makes it clear that a consideration of the potential limits in use of fossil carbon as an energy source must be figured into any definition of sustainable development. The following definiton in terms of "balance" may be suggested:
Environmentally sound and sustainable development results from human actions which permit continued development with the environment as the final arbitrator. It is development which permits further development without terminally closing off larger options. An ecologically sustainable biosphere will have the following properties: (1) will not depend on excessive fossil carbon subsidies; (2) will retain sufficient biodiversity to enable accommodation to all possible global change scenarios; (3) will provide food, fiber, shelter, recreation and appropriate chemical by-products to maintain human populations which do not exceed the carrying capacity; (4) can be sustained by energy subsidies which do not exceed those required to ensure a credible standard of living for the world's population.
In a global sense sustainability must encompass: (1) attainment of conditions permitting the continuity of human existence; (2) correspondence with an "environmentally appropriate" level of resource use. As used herein "environmentally appropriate" is not defined except that it must not contradict the first element and will somehow relate to carrying capacity.
Summary
Apart from the possibly uncertain and probably restricted capacity in immediate ocean solution, the actual carbon sink storage mechanisms handle relatively small throughputs compared to current anthropogenic discharges. Possible adjustments to the effects of atmospheric increases in carbon based gases and some of the associated nitrogen oxides produced from organic matter are also few in number. They include reforestation, aforestation, protection and expansion of wetlands for their carbon accumulation values, reduction in the disturbance and development of peatlands and of course drastic curtailment of the use of fossil energy. Although comparatively limited in their capacity to absorb carbon, as indeed are peat systems, coral reefs would be somewhat difficult to expand directly but should never-the-less be strictly protected to preserve absorptive capacity. Far from having to invoke mysterious "missing carbon" sinks (which depend on the values selected to perform the estimate) the overall balance appears to be good. This leads to the certain conclusion that either additional terrestrial sinks involving both new forests and expanded wetlands (carbon farms) must be established and enhanced to absorb the added carbon or emissions will have to be drastically cut to avoid excessive temperature rise. In this regard biomass energy sources might receive increased consideration since they can maintain a steady carbon budget between fixation during growth and release when oxidized. As with direct sequestering/buffering strategies however, the land requirements will be large for this strategy to effectively replace fossil fuel use and the threat to biodiversity consequently increased. The most obvious outcome is that all of the above mentioned actions will have to be implemented.
This in turn means that attention must now be fixed on the development of environmentally appropriate energy capture technologies with the major emphasis on those which do not involve long-term emissions of CO2. In order for this to be done, two things have to be taken into account and not necessarily in the order mentioned: (1) definition of maximum permissible energy fluxes consonant with the maintenance of a sustainable biosphere and the attainment of global development objectives; (2) definition of the appropriate or permissible mix of energy capture strategies that can be used to accommodate development objectives so defined.
An obvious conclusion is that since generation of energy using nuclear fission does not involve long-term CO2 emissions (this does not include the carbon penalties involved in site selection, plant construction, decommissioning etc.) this mode of energy capture is now due for a revised and re-concentrated environmental impact assessment from a global perspective. Solar energy and associated hydrogen transmission systems will also fall into this category particularly if the panels can be located on otherwise unproductive and barren ground.
At best, however, a strategy involving increased dependency on nuclear fission to supply usable energy will trade a problem that will mature in 50-100 years (increasing CO2) for a presently intractable and ever-increasing waste-disposal and management problem that will move into the bracket of hundreds, thousands and millions of years. For instance the half-life of Iodine-l29, the "perfect tracer" is 15.7 x 10 to the power of 6 years. This vagile and "inevitable" isotope has been characterized as the second most important radionuclide in the estimation of integrated effects to critical organs after Strontium-90 (Cowser et. al., 1967). Short of reducing populations, the nuclear option has one distinct advantage: it will buy time to consider the situation in total.
Here for Table 1 but return on 2
Note: In Table 1 it is assumed that new terrestrial growth and decay are at least approximately equal therefore no entry is made for this category. At the same time, it has been argued in this presentation that on historical grounds, terrestrial and also marine species should be able to respond to increased carbon dioxide with enhanced productivity. In any case growth of non-agricultural standing stock will not exceed current biomass destruction. Agricultural production is assumed to be totally oxidized. Also, marine dissolved inorganic* is taken as separate from the organic partitions but may involve double counting to a maximum of 1.9 Gt C. The median measured particulate organic flux to the deep oceans exceeds the estimate for dissolved inorganic carbon by a factor of 2. Plainly carbon must move through seawater before it can be taken up by plants or incorporated into carbonates, however, dissolved inputs are measured with living organisms present and it is not clear that the results are independent. In particular, the oceans have the largest part in the global carbon dioxide cycle and a clear understanding of their role either as a potential buffer or as a source (for instance if UV-B radiation were to negativly affect phytoplankton production on a global basis) cannot be obtained from the existing data.
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