Energy Comers. Mgmt Vol. 36, No. 6-9, 923-926, 1995 pp.

Copyright 0 1995 Elsevier Science Ltd

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SIGNIFICANCE OF AFFORESTATION OF DESERT ITS EVALUATION AS A COUNTERMEASURE AGAINST CARBON DIOXIDE PROBLEM

AND

TOSHINORI KOJIMA? YOSHITAKA KAKUBARI*

AND HIROSHI .KOMIYAMA**

Department of Industrial Chemistry, Seikei University, Musashino, Tokyo 180, Japan

*Shizuoka University, Shizuoka 422, Japan

*University of Tokyo, Tokyo 113, Japan

Abstract - The surface plant is thought to be one of the most feasible final sinks. In the present paper, its role in the carbon cycle is summarized, and sig- nificance, scale and advantage of greening of arid and semiarid lands in the carbon dioxide problem are demonstrated. We also present the methodology of the evaluation of the effects of the greening on the problem, considering both the energy input and the amount of fixed carbon.

1. INTRODUCTION

The surface plant or surface ecosystem is thought to be one of the most feasible final sinks because it converts carbon dioxide into carbon only using the solar energy which is difficult for us to use effectively and economically. It can fix CO2 from atmosphere, i.e., fix CO2 from dispersed sources, e.g., homes and vehicles. Greening of arid and semiarid lands may be promising because they have a large potential to fix carbon and the change in land management is expected to be acceptable locally and globally.

First, the important role of greening of arid and semiarid lands in the carbon dioxide problem is demonstrated by showing the data on carbon cycle and stock in this planet.

For more than two years, a project to assess the above possibilities has been conducted under the sponsorship of RITE, Research Institute of Innova- tive Technology for the Earth, and The Japan Gas Association[l]. In the course of the activity, several technological problems have been found and discussed[2].

In the present study, the methodology of the evaluation of the effects of the greening on the problem, which has also been discussed in the project activity, is explained and discussed.

2. SIGNIFICANCE OF AFFORESTATION OF DESERT

Carbon balance ________~

+ TO whom all correspondence should be addressed.

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924 KOJIMA et al.: SIGNIFICANCE OF AFFORESTATION OF DESERT

Table 1. Carbon amount in various ecosystems [3-61 fossil fuel (55) deforestation(l5-20)

ecosystem area alive dead

/unit 108ha tC/ha tC/ha

trop.forest 18-25 150-190 60- 70 temp.forest 11-13 110-150 90-130 bush 8- 9 25- 50 90-110 trop.grass 13-15 5- 20 70- 90 farm land 13-15 4-10 50-70 (semi)desert 42-45 o- 2 o- 20

LAND(tota1) 149 40- 60 70- 90 SEA (total j 361 0.05 25 -----_ GLOBAL 510 12-16 41

ocean (max. 20-25) missing 0 more into ocean sink: @ north afforestation

@ stock inc. in plant by COa cont. inc. @ stock inc. in soil by COz cont. inc. @ stock & photosynthesis inc. in ocean biota

by COz cont. inc. & @I by inflow of nutrients 8 inc. in weathering rate of silicate rock

Fig. 1. Global carbon balance

(unit: lo8 t-C/y) [3].

The carbon balance between 1978 and 1988 is given in Fig. 1 [3]. The amount of carbon dioxide from fossil fuel utilization is easily estimated from the energy data. The amount of carbon dioxide accumulated in the at- mosphere is also easily calculated from its concentration change.

The amount of carbon dioxide released by the destruction of forest was estimated from the deforestation rate, more than 10 Mha/y, and the dif- ference between the carbon in tropical forest and that in grass land or desert, around 150-200 t/ha as shown in Table 1 [3-61.

Significance of afforestation The significance of surface plant system in carbon cycle is summarized

as follows. First, the surface biosphere has played an important roll of source of C02, since the appearance of man, and still now. The cumulative contribution of surface biosphere is almost double of that from fossil fuel 171.

In Fig. 1, accumulation of CO2 in the atmosphere is thought to be around 3.5 Gt-C/y. If we could stop the destruction of the tropical rain forest, and afforestation progressed at the same rate as above, 10 Mhaly, the ac- cumulation would be stopped, under the condition that the missing sink works as it is.

Advantaqe of afforestation of desert For the performance of afforestation, the arid and semiarid lands are

thought to be most promising. The global arid land area including desert, grass semiarid land, salt affected land, and unused land is more than 6 bil- lion ha, which is equivalent to 600 years' measures based on the above area of desired afforestation per year. Furthermore, the conversion of carbon dioxide in atmosphere is performed by using the sufficient solar energy which has not been used artificially and the produced carbon is automati- cally stocked in the plant. This means that the present measure enables the recovery of carbon dioxide from atmosphere, i.e., CO2 fixation from dis- persed sources, e.g., homes and vehicles essentially without artificial energy which produces CO2 or is used as an alternative of fossil fuel.

3. EVALUA$ION METHODOLOGY OF AFFORESTATION

The detail evaluation method of the present measure is discussed here. Though we mentioned that essentially no energy is needed for afforestation, the supply of water is essential for greening of the arid and semiarid lands. In addition, the critical evaluation on the required man power is necessary. Furthermore, it is true that no net accumulation is expected after stopping of the growth of wood.

KOJIMA et al.: SIGNIFICANCE OF AFFORESTATION OF DESERT 925

Water supply and enerqy balance As to the water supply, an example of energy balance is shown in Fig. 2.

To build up a forest ecosystem, more than 600 kg-water/m*/y, or 600 mm/y of rainfall is thought to be essential. To produce the amount of fresh water from sea water, more than 30 MJ is necessary even when an energy saving membrane process is employed for it [8]. On the other hand, the biomass produced from the ecosystem is expected to be less than 20 MJ/m2/y. This means that the afforestation of arid land is negative measure and is not in- dependent if all needed water is produced by conventional methods.

In some areas of semiarid land, some water is supplied from the natural rainfall or some wells. Microcatchment of the water is also an expected tool to correct water in such areas. In general, with the increased energy input, the amount of fixed carbon is expected to be drastically increased first, while attain to a plateau with more energy. There should exist the optimum energy input.

Thus the net amount of fixation is to be evaluated as the amount of the fixed carbon from which the energy input is subtracted, as shown in the fol- lowing evaluation function of eqn. (1). where the amount of carbon stock in the system, C is to be the average value during the period of n years. When we cultivate a rapidly growing plant, the contribution appears from the ear- lier stage, while a slowly growing one, from the later stage. But at the final stage of the evaluation, a plant with higher stock of carbon should show higher contribution to the CO2 problem. Thus in our evaluation, the average value was considered. As to how long period should be considered, its value itself is one of the most important parameters. The period may not be longer than several hundred years, because the fossil fuel will be consumed, while it may not be shorter than several ten years, because alter- native soft energy passes will not become main energy sources in the term.

Energy balance (per unit area), E is given as

E = (C - Co + B - I) / I, (1)

where C is the average amount of carbon stock in the ecosystem during the period of n year, CO is the initial amount of carbon in ecosystem, B is the total amount of carbon accumulated as a result of the biomass produc- tion, I is the artificial cumulative energy input to the project for n years. The unit of all values should be used consistently: C-ton/m', which is easily converted to J/m2 by assuming the primary energy production from fossil fuel.

Positive values of the equation are necessary to realize the net carbon fixation and the higher, the more stable. The higher value of the numerator means the more effective one.

PI- -* 0 Greening 0; desert k + F)

Fig. 2. Energy balance in greening of desert: R.O. membrane

926 KOJIMA et al.: SIGNIFICANCE OF AFFORESTATION OF DESERT

To complete the above evaluation, all values included in eqn. (1) should be given as a function of time, especially the fixation rate of carbon in the ecosystem should be formulated as a function of time and energy input. These values should be evaluated for each area of desert, after determina- tion of most suitable ecosystem for the specified area.

Other factors to be evaluate In the previous section, the present measure is 600 years' one on the

basis of the carbon balance. However more detail evaluation is needed by considering the energy input and output of the system. The possible con- tribution of the present measure is to be evaluated by the following equa- tion.

K=A/[(C-CO+B-I)xS/n], (2)

where A is average amount of carbon accumulated in air per year, and S is the area of arid land suitable for a specified afforestation technique. When the value of K is larger than unity, the present measure has sufficient capacity to prevent increase in atmospheric CO2 concentration for n years.

The last factor we should evaluate is the ratio of average man power needed for afforestation to the world population, M, when the present measures are employed to prevent atmospheric CO2 concentration increment completely for n years.

M = (T x A x n) / [(C - CO + B - I) x P], (3)

where T is average number of staffs necessary for the project and P is the population in the world. M is equivalent to the ratio of conventional GNP reduction without consideration of additional production caused by the present technique.

CONCLUSION

The important role and various advantages of greening of arid and semi- arid lands in the carbon dioxide problem were demonstrated by showing the data on carbon cycle and stock in this planet. The methodology of the evaluation of the effects of the greening on the problem was also presented, which is exDected to lead to the direction of the research to be conducted in

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REFERENCES

H.Ozawa, Y.Omote, H.Komiyama and Y.Kaya, submitted to Energy Conversion and Management. S.Matsuda, T.Kojima and H.Komiyama, submitted to Energy Conversion and Management. T.Kojima, "Nisankatanso Mondai-Uso to Honto" (Question and Answer in Carbon Dioxide Problem, in Japanese), Agune Shofusha, Tokyo (1994). G.M.Woodwell, W.A.Whittaker, G.E.Linkens, C.C.Delwiche and D.B.Botken. Science, 199, 141 (1973). K.Yoda, Chikyu Kagaku (Geochemistry, in Japanese), l& 78 (1982). T.Tsutsumi, "Shinrin no Busshitsu Junkan" (Material Circulation in Forest, in Japanese), Tokyo Daigaku Shuppankai, Tokyo (1987). SCOPE29 (1986) in Y.Kaya, "Chikyu Kankyo Kogaku Handbook" (Handbook of Global Environmental Handbook, in Japanese) p.493, Ohm Sha, Tokyo (1991). K.Matsumura and T.Kojima, Journal of Arid Land Studies, 1, 17 (1991).