GREENHOUSE EFFECT AND FUEL CONSUMPTION

The greenhouse effect is presently one of the main concerns of governments and international institutions connected with environmental problems. Estimates made about two decades ago indicated that it would cause global warming between 1 and 40 C in the 21st century. A better knowledge available now lowered the estimate (1 to 1.5 0C) and permitted to better envisage the possible consequences of this effect.

The greenhouse effect is a consequence of the absorption of solar radiation by the earth’s atmosphere. Since the absorption coefficient is a function of the radiation wave length, the visible solar spectrum range is slightly absorbed while the radiation in the infrared range is strongly absorbed, resulting in a sort of trap for the solar radiation. As any meteorological phenomenon, the effect has many sources and sinks, with feedback circuits that complicate its study extraordinarily. According to several authors, some of these mechanisms can have a positive feedback for the effect and result in catastrophes with unpredictable results. On the other hand, the greenhouse effect provides the temperature that is adequate for biological processes and therefore it is connected with the evolution of the species as Darwin has described it ( it could be inserted here some reflections about the future of the self-designated Homo sapiens species).

Given the correlation between energy production and use and the contribution of fossil fuels to the world energy supply (77% in 1990 and up to 88% in the next two decades) and carbon release, it is understandable the resistance of rich countries, specially the United States, to the implementation of a program for stabilizing the emission of the so called greenhouse gases, notably CO2, CH4, CO, N2O and water vapor.

There is large interest in quantifying the greenhouse effect gases emitted by the different sources, as well as the absorption capacity of sinks in order to improve the temperature variation forecast. However, the Biosphere is an open thermodynamic system that is not in the steady state, what makes it notably difficult to forecast its behavior. For example, the ocean, the largest modern carbon sink, has already had its phases of atmospheric carbon source in glacial eras. Part of the greenhouse effect gases emissions are inherent to organic phenomenon (biotic carbon) and part due to economic activity (anthropogenic carbon) and it is very important to know the latter. Basically, it means to quantify a weak signal superposed on a strong noise., what justifies the large effort that has been dedicated to the matter. Simple correlation , even though an approximation, between emissions and concentration of greenhouse effect gases in the atmosphere would facilitate the study. The aim of the present work is to present a comparison between fossil fuels consumption and the concentration of CO2 and CH4 in the atmosphere.

Results of concentration measurements from 1750 on, obtained from the analysis of air contained in layers of polar ice ad information about the forest stock, obtained from growth rings of trees, allowed for the establishment of an information base from which one can deduce the contribution of fossil fuels to atmospheric carbon (1). From the XVIII century on mineral coal started to be used in iron production, first in England, and in the beginning of the XIX century it had a marked participation in the fuel market. The rise of mineral coal is shown in Graphic 0, obtained from ref. 1 (mentioned in ref. 2). Therefore, the energy accounting of our interest begins in 1820.

Graphic 0

So we start the present work by developing a simple calculation that permits the non-specialized reader (including the author) to understand the process in its general lines. I intend to present in other e&e, issues considerations concerning the repercussion of the greenhouse effect on the energy and entropy flux in the Biosphere and on the physical economy of production. Simplified physical models will be used, comparing the results with those of more sophisticated models used by the Intergovernmental Panel on Climate Change (IPCC) and from other sources.

In this first article we will evaluate the anthropogenic emissions using the projections of Marchetti, C and Nakicenovic, N (“The dynamics of Energy Systems and the logistic substitution model” - Laxenburg, Áustria, IIASA, Research Report R -R-79-13) for energy use (2).

ENERGY USE AND THE CORRESPONDING CARBON EMISSION

Marchetti and Nakicenovic have proposed a simplified model for evaluating energy use in the form of a logarithmic graphic from which one deduces that between 1850 and 1990 the use grew on the average 2,38% annually. Marchetti presented also in several publications a logistic model of commercial fuel competition. Electricity of hydraulic origin is not included in the model because it represents less than 5% of the energy used in the period and because it does not directly contribute to carbon emission (excluding the decomposition of the submersed forests as is the case of Tucuruí). Extrapolating the value used in 1900 (1,000 MWyear/y) one obtains the energy used year by year, as shown in the graphic below (the corresponding spreadsheet is at the end of the article).

Graphic 1

The carbon emission was calculated from its content in the fuel, where petroleum is represented by hexadecane (C16H34 ), derived from the molecular mass next to the average, and natural gas (CH4 ). The contents are 0.46 for firewood, 0.90 for coal (fixed carbon), 0.85 for petroleum and 0.75 for natural gas. The results are in the graphic below.

Graphic 2

The annual emission can be expressed by a logistic law as shown in the following graphics that illustrate the traditional path of modeling and one can notice that the correlation coefficients are quite adequate. The maximum emission rate found is 11,460 MtC/year; in 2000 the emission was 5,400 Mt, what shows that the inflexion point of the logistic curve has been reached. As in other examples shown in articles of e&e, taking into account the emphasis presently given to the greenhouse effect, it seems that the system spends half of its resources learning how to administrate the other half. In the expressive metaphor of Ayres (“Resources, Environment and Economics” R.U. Ayres- John Wiley, 1978) we are moving from the cowboy economy to the astronaut economy (please, warn George W. Bush).

Graphic 3

The following graphic shows the distribution observed in the annual emission superposed on the logistic distribution (approximated by a polynomial). From the results it seems proper to consider that carbon emission can be represented by the logistic function. The graphic below shows the fitted logistic curve and the calculation results.

Graphic 4

CARBON RETENTION IN THE ATMOSPHERE

The carbon emitted .by biotic sources and by the use of energy is partially absorbed in the natural sinks (biomass and oceans) and the rest remains in the atmosphere. The latter is the cause of short-term concern. Naturally it is impossible to “stamp” carbon according to its origin so that a comparison between emission and retention in the atmosphere will refer to the total circulating carbon. Besides this discrimination difficulty, carbon is emitted in the form of several gaseous compounds such as CO2, CO and hydrocarbons such as CH4. In the medium term, the less stable compounds will decay to CO2 according to different processes with distinct kinetic characteristics. Some decay reactions are induced by radiation, allowing for new feedback mechanisms. Comments on the particular importance of certain compounds and certain mechanisms will be presented in future articles.

Using the polynomial representation of the annual emission (Graphic 4) one can calculate the integrated emission and compare it to the atmospheric stock. This stock is calculated from CO2 and CH4 concentrations measured in air bags contained in glaciers. Graphic 5 shows the evolution of concentration of these gases since 1750 (JI Vargas, ref 12).

Graphic 5

Calculating the mass of atmospheric air by means of the pressure at sea level (1kgf/cm2) and taking into account the relative air density and that of CO2 (28/44) one obtains the mass of atmospheric C. The results are in Graphic 6, together with the integrated emission.

Graphic 6

One can notice that carbon emission due to fuel use corresponds to 2.5 times the accumulation in the atmosphere from all effects (biotic and anthropogenic). The small contribution from firewood could be subtracted from the total if one could be sure of the maintenance of the forest stock, which seems improbable. Therefore, the fossil fuels contribution is dominant.

The change in the fossil fuels contribution, with de probable decline of charcoal and rise of natural gas, reduces the emission by unit of energy used to the average geometric rate of 0.6% annually, insufficient to compensate for the increase of energy used, namely, 2.38% annually. It is concluded that the emission decrease will demand considerable world effort. In a future work we will discuss the strategies that seem to be probable.

REFERENCES.

1- Speth, J. G. “Energy policy and environmental pollution: a look to the future” International Journal of Global Energy Issues, 1 (1-2):5-17, 1989

2- Vargas, J. I. “The Brazilian Energy Scenario and the Environment : An Overview” CBPF-CS-003/92

3- Marchetti, C. e Nakicenovic, N. “The Dynamics of Energy Systems and the Logistic Substitution Model” IIASA, Research Report R-R-79-13

4- Hémery, D, Debier, J. C., Déleage, J-P- “Uma história da energia” Trad. Sérgio de Salvo Brito-Ed. Universidade de Brasília, 1993.

5- “Energy for Tomorrow’s World” WEC Commission-1993

6- Sundquist, E. T. “The Global Carbon Dioxide Budget” US Geological Survey-1993

7- “IPCC Special Report on Emission Scenarios” 2001

Graphic Edition/Edição Gráfica:
MAK
Editoração Eletrônica

Revised/Revisado:
Tuesday, 11 November 2008.

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