Transient response to a change in the concentration of atmospheric C02

The problem contained in the title of this section is immediately related to the problem of prediction of potential climatic changes, the importance of which for mankind's economic activities it is impossible to overestimate. It is not without reason that this problem has drawn the attention not only of separate groups of scientists and the scientific community in general, but also of government organizations who have to plan and take decisions about the economic, social and technical development of human societies.

According to estimates by Keeling (1973), Marland (1984) and Rotty (1987), who analysed data on the consumption of coal, oil and natural gas in all regions of the world, the total incoming C02 has, for 125 years since the dawn of the industrial revolution, amounted to 187 x 109 tC. Of this 98% is related to the burning of fossil fuel, and 2% to the production of cement. Since the industrial revolution the annual mean emission of C02 has increased from 0.1 x 109 tC in 1864 to 5.3 x 109 tC in 1984. Judging by the most recent estimates of Marland (1989), and Marland and Boden (see IPCC, 1992), the cumulative release of carbon from fossil fuel use and cement production between 1850 and 1987 amounts to 200 ± 20 GtC, and the global annual mean C02 industrial emission equals 5.7 GtC/year in 1987 and 6.0 + 0.5 GtC/year in both 1989 and 1990. An additional release of carbon to the atmosphere is due to impacts on the terrestrial biota (deforestation and changes in land use). It is important to understand that the role of each of the above-mentioned anthropogenic factors, and primarily of the terrestrial biota, especially that enhancing the emission of C02 when burning fossil fuel and increasing the rate of photosynthesis and biomass are competing.

There are several methods for estimating changes in biomass of the terrestrial biota. The most representative is an inventory one based on the use of historical data on the nature of land tenure. Having systematized such data Houghton et al. (1983) singled out six types of anthropogenic changes in terrestrial ecosystems: (1) clearing of land for agriculture; (2) the use of natural ecosystems as pastures; (3) the growth of secondary forests after clearing; (4) halting of cultivation of agricultural lands; (5) halting of exploitation of pastures; and (6) reforestation (planting of trees in previously cleared areas). The state of each ecosystem was characterized by a change in its area, age and carbon content in the biomass and organic matter of soil. As has been shown by Houghton and Skole (see IPCC, 1992), the annual emission of C02 due to the transformation of terrestrial ecosystems exceeds the intensity of industrial effluents up to 1960, and the total emission of C02 from 1860 to 1985 amounts to 115 x 109 tC with a standard deviation of + 35 x 109 tC determined by uncertainty in assignment of initial values of forest biomass and inorganic matter of soil, and by inaccurate information on the dynamics of land use. This estimate, together with other inventory estimations of C02 exchange between the atmosphere and terrestrial biota is presented in Table 6.2.

All other methods of estimating the anthropogenic impact on the terrestrial biota are indirect. We mention, in particular, the method based on the use of data on changes of ratio between concentrations of stable carbon isotopes 13C and 12C in the annual rings of perennial wood plants and corals. As is well known, the isotope 13C is contained not only in the atmosphere but also in the terrestrial biomass and in fossil fuel, and in the latter its relative content is about 20% less than that in the atmosphere. Because of this, burning of fossil fuel or destruction of biota must result in a reduction in 13C/12C by a value <513C, where <513C = 103[(13C/12C) - (13C/12C)PDB]/ (13C/12C)pdb is the relative departure of 13C/12C from the so-called PDB-standard, defined as the relative content of 13C and 12C, in shells of the fossil mollusc Belemnitella americana discovered in Cretaceous sedimentary

Table 6.2. Carbon input into the atmosphere due to anthropogenic impact on terrestrial ecosystems according to various researchers

Author

Observed period

Total input (109 tC)

Annual mean input (109 tC year"1)

Revelle and Munk (1977)

1860-

-1970

70-80

0.6-0.7

Bolin (1978)

1800-

-1975

40-120

0.2-0.7

Siegenthaler et al. (1978)

1860-

-1974

133-195*

1.2-1.7*

Stuiver (1978)

1850-

-1950

120*

1.2*

Freyer (1979)

1860-

-1974

70

0.6

Hampicke (1979)

1860-

-1980

180

1.5

Chan and Olson (1980)

1860-

-1970

150

1.4

Moore et al. (1981)

1860-

-1970

148

1.3

Houghton et al (1983)

1860-

-1980

180

1.5

Peng et al. (1983)

1850-

-1976

240*

1.9*

Peng (1985)

1800-

-1980

144*

0.8*

Kobak and Kondrashova (1987)

1860-

-1983

60-85

0.7

Houghton and Skole

1860-

-1985

115 + 35

0.85 + 0.30

* Estimates obtained from data of isotopic analysis.

deposits. Estimates of the intensity of the biotic source of C02 obtained by this method (they are marked by asterisks) are also presented in Table 6.2. As can be seen they do not contradict the inventory estimates: both point to the fact that the carbon input into the atmosphere for the last 120 years, because of the anthropogenic impact on the terrestrial biota, must amount to (60-90) x 109 tons, that is, of the order of half the emission of C02 at the expense of burning fossil fuel.

If one makes use of Marland's (1989) estimate for fossil fuel combustion and cement manufacturing and Houghton and Scoles's (see IPCC, 1992) estimate for the release of carbon from land-use changes, then the total release of carbon to the atmosphere during the period from 1850 to 1986 must be 312 + 40 GtC. This release of carbon must increase the concentration of atmospheric C02 by 147 ppm (1 ppm C02 of the global atmosphere equals 2.12 GtC). Thus, if the preindustrial concentration of atmospheric C02 was 288 ppm as reconstructed from ice core analyses (IPCC, 1990), then the global annual mean average concentration of atmospheric C02 in 1986 should be 435 ppm. However, measurements available from that time show a concentration of atmospheric C02 of 348 ppm corresponding to 41 + 6% of the cumulative release. The remaining fraction must be redistributed between ocean and terrestrial biota. In other words, this means that the global net uptake of C02 by the ocean and productivity of vegetation have increased since the beginning of the industrial revolution. The former is explained by increasing C02 partial pressure difference between ocean and atmosphere, while an increase in the latter is not so evident. It may be associated with the so-called fertilization effect (enhanced vegetative growth with increasing C02 levels).

Direct measurements show that the inter-hemispheric C02 concentration difference (currently about 8 ppm) is smaller than one would expect if nearly all of the fossil releases occurred in the Northern Hemisphere. This suggests that there is an unexpectedly large sink in the Northern Hemisphere equivalent to more than half of C02 release of the fossil fuel. Tans et al. (1990) partitioned the northern sink of carbon between ocean and terrestrial biota, using data on the C02 partial pressure in surface waters, whereas Keeling et al. (1989) presupposed that a large fraction of this sink was due to a natural imbalance in the Northern Hemisphere carbon cycle consisting of a net transport of carbon from the Northern to the Southern Hemisphere in the ocean and a return transport of carbon in the atmosphere. The last statement is equivalent to an assumption about the existence of a countergradient flux of C02 in the atmosphere: the inter-hemispheric C02 concentration difference mentioned above implies a continuous flux of C02 from the Northern to the Southern Hemisphere.

The second feature is a relatively small net input of carbon to the atmosphere in the tropics because of both outgassing of C02 from warm tropical waters and deforestation. According to Keeling et al. (1989), this may be explained by a significant (about 50%) reduction of the net flux of C02 from the tropics as a consequence of the fertilization effect. However, this conclusion, as well as the conclusions of Tans et al. (1990) and Keeling et al. (1989) concerning the origin of the large carbon sink in the Northern Hemisphere, needs to be tested.

In discussing forthcoming changes in the concentration of atmospheric C02 we note that to predict these it is necessary to know, first, the prediction of the development of power engineering, and, second, that fraction of C02 emission which remains in the atmosphere. The solution of the first problem is inseparably linked with the prospects for the development of society with its complex of economic, social, demographic and ecological problems. It is clear that such predictions or scenarios, as they are usually called, can only be approximate. Nevertheless, we introduce two of them, proposed by Legasov and Kuzmin (1981) and Working Group III of the Intergovernmental Panel on Climate (IPCC, 1990). The following considerations form the basis of the first prediction.

Energy consumption should provide a sufficiently high living standard for a population. When proposing that, by the end of next century, the population of the Earth will amount to 10-12 billion, and that by that time the specific energy consumption (consumption of energy per capita) will have reached the present-day level of 10 KW • year/year per capita in all the developed countries, then the total energy consumption will be equal to 100-120 TW-year/year (1 TW-years = 109 KW-year = 31.54 x 1018 J). But the growth in population and the process of industrialization in many of the developing countries tends to increase in energy expenses per unit of Gross National Product (GMP). Industrialization also contributes to the additional consumption of materials (metals, plastics, etc.) per capita and, accordingly, increases the specific energy capacity of production. Additional energy will be required for production of foodstuff for the increased population and this in turn will require the cultivation of lands with low fertility and new energy consumption to produce vast amounts of fertilizers for the amelioration of lands. It will be necessary to increase energy consumption to exploit new deposits of mineral resources, for the recovery of industrial waste, and to provide humanity with fresh water. Additional energy consumption will be required for the preservation of the environment (sewage disposal, air cleaning, etc.). All this leads to the fact that the specific energy consumption will increase up to 10- 20 KW • year/year per capita, or up to 100-200 TW • year/year. On the other hand, a transition from the policy of producing even more energy to one of increasing the efficiency of the use of energy and its maximum economy is inevitable. The resulting energy consumed by the Earth's population by the end of the twenty-first century may amount to 60 TW • year/year. Such a prediction was assumed by Legasov and Kozmin (1981) as the basis when estimating the intensity of the net emission of C02 of an industrial origin.

The IPCC scenario (the so-called Business-as-Usual (BaU) scenario) covers the emissions of carbon dioxide (C02) and other greenhouse gases (methane (CH4), nitrous oxide (N20), carbon monoxide (CO), nitrogen oxides (N03) and chlorofluorocarbons (CFCs)) from the present up to the year 2100. Population is assumed to approach 10.5 billion in the second half of the next century. Growth of the economy is taken to be 2-3% annually in the coming decade in the OECD countries and 3-5% in the Eastern European and devyeloping countries. The economic growth levels are considered to decrease thereafter. The energy supply is coal and on the demand side only modest efficiency increases are achieved. CO controls are modest, deforestation continues until the tropical forests are depleted and agricultural emissions of CH4 and N03 are uncontrolled.

Since completion of the 1990IPCC scenarios, new information has become available. It includes revision of the population growth rate, re-estimation of sources and sinks of greenhouse gases and improved data on tropical deforestation and forest biomass, as well as the consequences of recent political events in the former USSR, Eastern Europe and the Middle East, affecting the level of economic activity and the efficiency of energy production and use. This requires an update of the 1990 IPCC scenarios. As a result, we have six alternative 1992 IPCC scenarios differing among themselves as to the population and economic growth rates, assumptions about use of fossil fuel and biotic carbon emissions.

In the first of the scenarios referred to as the IS92a scenario, the update population assumptions are about 10% higher than those in the 1990 IPCC BaU scenario: global population increases from 5.25 billion in 1990 to 8.41 billion in 2025 and to 11.31 billion in 2100, with about 94% of the growth taking place in developing countries.

The assumptions about economic growth from 1990 to 2000 are also higher than those used in the BaU scenario. In accordance with them, the annual average rate of increase of Gross National Product (GNP) is assumed to be equal to 2.9% from 1990 to 2025 and 2.3% from 1990 to 2100. The estimates of the GNP growth rate for the initial 35 years, from 1990 to 2024, are substantially lower than that experienced by most world regions in the past 35 years, from 1955 to 1989. Over the last 75 years of the coming century, GNP is assumed to be slowing due to expected decrease in the population growth. The future GNP per capita is assumed to be rising everywhere throughout the next century, but most rapidly in the developing countries where even in 2100 the income per capita will still remain below levels in the developed countries.

Other assumptions concern primary energy consumption, resource availability, land-use changes and emissions. In particular, it is suggested that the annual average decline in total primary energy requirements per unit of GNP will be 0.8% for 1990-2025 and 1.0% for 1990-2100, while the cumulative net fossil C emissions, total tropical deforestation and related cumulative net C emissions from deforestation will accordingly be equal to 28.5 GtC, 6.78 x 106 km2 and 42 GtC for 1990-2025 and 1386 GtC, 14.47 x 106 km2 and 77 GtC, for 1990-2100. Finally, the C02 emission rate from energy, cement production and deforestation is taken to be 7.4 GtC/year in 1990, 12.2 GtC/year in 2025 and 20.3 GtC/year in 2100.

The remaining 1992 IPCC scenarios were designed to examine the sensitivity of future greenhouse gas emissions to a wide range of alternative input assumptions regarding population and economic growth rates, oil and gas resource availability, resulting in higher or lower prices and promoting expansion of nuclear and renewable energy, additional use of coal-mine methane for energy supply, improvements in regional pollution control (by means of a 30% environmental surcharge on fossil energy use), changes in deforestation rates and carbon stored within the biomass, etc. Overall, the scenarios indicate that the emissions of C02 and other greenhouse gases might rise substantially in the absence of stronger control measures.

Now that the net C02 emissions of industrial or industrial plus biotic origin have been determined, the next task must be to estimate the fraction of these emissions which remains air-borne. The simplest way is to assume that the same fraction remains air-borne as was observed during the last decade, i.e. 46 ± 7%. A propos, it is this method that has been used to produce estimates of climate changes consequent on BaU and IS92a scenarios. One may also use the results of analyses of air samples in ice cores from Greenland and Antarctic ice sheets and data on variations of the annual average C02 concentration at monitoring stations, on the one hand, and estimates of industrial emissions, on the other hand. In the latter case, the average value of a for the period from 1760 to 1984 is found to be equal to 56% in the absence and (39 ± 5)% in the presence of the biotic source with an emission rate of (1.6 ± 0.8) GtC/year (see Bjutner, 1986) and, again, difficulties arise with the prediction of future atmospheric C02 concentrations. In practice the assumption of the constancy of a is not fulfilled: even for the relatively short period from 1963 to 1978 the value of a calculated without consideration of the biotic source increases from 49 to 60% (see Bjutner, 1986). It remains only to use the global models of the carbon cycle where the parameter a. is not fixed but, rather, is determined, together with all other variables sought.

First of all let us examine a model situation: a step-wise increase in the concentration of atmospheric C02. The appropriate transient response of the climatic system is described within the framework of a zero-dimensional model by the equation

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