Economy & Energy

Year IX -No 64:

October - November 2007

ISSN 1518-2932


No 65 Em Português





other e&e issues

 e&e No 65



Main Page

Capital Stock in Brazil and Capital Productivity  by IBGE

Comparative  Costs of Thermoelectric Plants  and Angra 3 Plant

Revisiting Methane Concentration in the Atmosphere

The Importance of Capital Productivity to Growth

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Text for Discussion:


Revisiting Methane Concentration

in the Atmosphere


Carlos Feu Alvim
Omar Campos Ferreira

José Israel Vargas

1. Introduction

The Group 1 Fourth Report of the Intergovernmental Panel on Climate Change - IPCC[1] is available in its final version. The Technical Summary [ref 1] synthesizes results, pointing out what it has called “Robust Finds” and what it considers “Key Uncertainties”.

One of the main objectives of the Group’s study is to determine if changes that occurred in the atmospheric composition are or not due to human activities (anthropogenic causes). The report is categorical when considering the hypothesis of human activities as the cause of substantial global warming with a probability of 95% of being correct (extremely likely).

Specifically this report considers the evolution of methane concentration in the atmosphere and highlights two conclusions classified as robust:

·         The present levels of CH4 (and CO2) highly exceed the measurements made in glaciers from the last 650 thousand years.

·          In the last forty years the average concentration rate of GHG variation (radiative forcing) is higher that that observed in least two thousand years.

The report lists among “fundamental uncertainties” that should be elucidated the recent changes in the methane growth rate in the atmosphere.

2. The TAR Scenarios and the Conclusions of the Previous Report

Most of the scenarios considered in the Third Assessment Report – TAR (ref 2) indicated for the years subsequent to its publication (2201) a significant growth of methane concentration in the atmosphere, illustrated in Figure 1.

The scenarios represented different evolution hypothesis without any mitigation measures. Of these scenarios (Special Report on Emissions Scenarios - SRES) only Scenario B1, that can be considered the ideal one from the development point of view, predicted the reversal of methane concentration growth. However, this reversal would be reached only around 2030.[2]

Based on the historical behavior of methane concentration in the atmosphere until 1995, the authors of the present report have adjusted a logistic curve that implies the stabilization of this concentration at the 1900ppbv level[3]. This change would occur in 69 years, between the beginning of the process and its saturation (90%). Having started in 1940, this cycle would be ended in 2010. The details of this methodology are described in the № 55 issue of this periodical (ref. 3).

Complex phenomena that involve natural and social variables can be approached by this modeling adopted to describe them, including the evolution of methane concentration and it permits predicting its future. The methodology has been described in several articles by C. Marchetti and adopted by J. I. Vargas (ref 4) and others and has proved to be adequate to situations where the growth of an element introduced in the system is initially accelerated and subsequently self-limited, leading to saturation of the system evolution under examination.

It should be remembered again that a significant part of the methane concentration growth in the atmosphere has been imputed to human activities. On the other hand, methane concentration also depends on natural laws that regulate chemical reactions in the atmosphere or its eventual absorption in the soil or oceans. It should be observed that the logistic law used to describe the phenomenon in question is characterized by the occurrence of an inflexion point from which it decreases monotonically until it is practically completed (90%).

Besides the average values of methane concentration resulting from the TAR projection, as can be verified, Figure 1 also shows the fitting obtained by e&e. The data corresponding to the experimental values show the logistic behavior expected for the methane concentration augmentation from 1940 onwards.

The details of the lower part of Figure 1 shows what happened between 1985 and 2010, indicating the stabilization (already pointed out in our study) that has astonished the authors of IPCC’s previous projections. It seems to indicate a saturation value slightly lower than the 1900 ppbv obtained through the logistic fitting of data up to 1995[4].

3. Is Methane Concentration in the Atmosphere Becoming Stable?

The fundamental issue under discussion is not the saturation value (1750or 1900 ppbv of methane) but to know if the present path tends to growing values like those projected from the TAR scenarios or if they would follow a saturation level indicated by the present data.

The matter was object of intense discussions in Group I, summarized in the Technical Report of the Group that, as has been observed, included the theme among the “key uncertainties” to be carefully examined.

Examination of the time derivate of methane concentration shows the behavior trend in the atmosphere. This procedure was adopted in the Fourth Assessment Report of Group I. It was evident the sustained drop of concentration growth rate of CH4 as shown in Figure 2 (taken from that Report).


Amplified Scale Showing the 1940-2005 Period

Figure 1: Projections of methane concentration

compared with the verified concentrations.

Methane Concentration in the

Atmosphere e Annual Growth of Concentration

Figure 2: Growth of methane concentration in the atmosphere and annual concentration values for two measurement series.

(Source: Fourth Assessment IPCC Report).

An analysis similar to that of Figure 2 was made in an article in the No 55 e&e issue with data from 1940 onwards (Figure 3). The annual variations represented in Figure 3 are five-years averages centered in each year. This procedure will give a better idea of the behavior in the temporal horizon of the phenomenon under study that will extend to several decades. It will also be shown the values of the derivate resulting from the concentration data fitting.


Figure 3: Methane concentration and annual variations

(movable five-year average).

Data of Figure 3 complement those of Figure 2 and demonstrate that the annual concentration growth (temporal derivate) has been progressing in a coherent way for decades – in the sense of reaching a saturation value or at least to substantially reduce the growth rate.

The Group I discussion is carefully presented in the report that with diplomatic prudence regarding the subject describes the different positions:

·         Stabilization of methane concentration can only mean that in the last years the rate of methane input in the atmosphere would have been equal to its elimination.

·         The best known mechanism of methane elimination in the atmosphere is the formation of water and carbonic gas. Incidentally these are the same products obtained in gas combustion. This exothermic reaction can be induced by different factors, and the predominant one is the reaction unleashed by the presence of the OH radicals in the atmosphere[5].

·         However, measurements of OH concentration (produced by photochemical reaction in the atmosphere) discard this hypothesis because it has not been recorded a substantial increase of its presence in the atmosphere.

·         Some members of Group I think that the concentration growth is due to stabilization of emissions and not to an increase of methane elimination rate. The pointed out causes are quite different but they consider that the present methane emissions evaluation is uncertain because it involves complex processes.

·         Explanations regarding peaks and valleys in concentration for specific years were presented. They would be associated with natural phenomenon like volcanic eruptions but they do not permit to explain the observations in the considered period.

4. Some Conclusions ?

The perplexity of Group 1 left open the explanation of what is happening to methane concentration. As expected, the doubts expressed did not affect the Groups’ studies concerning scenarios projections as well as the impact of mitigating measures.

As can be observed in Figure 4, the hypothesis concerning emissions evolution and methane concentration still admit emissions and concentration growth in the Fourth Report. Some projections are made similar to those of the Third Report that have not been confirmed by the measurement made in the subsequent years.

In these conditions the suggested mitigation measures are coherent, as expected, with the trend admitted in the reference scenarios and are naturally based on emissions erroneously projected. If the present trend is confirmed, the mitigation margin would therefore be merely limited to reducing the methane concentration reduction already verified.

The methane concentration evolution issue examined in Group I has of course economical and political implications since the methane emission reduction credits market is a world reality.

The methane emissions reductions, as published in the Group I report, can be contested regarding its efficiency since they would be permitting the continuation of carbonic gas emission about which there are no doubts concerning its growth. As the Group I report itself demonstrates, CO2 has a temperature change effect higher than that foreseen by the GWP equivalence because it remains in the atmosphere for a much longer time than methane.

The methane concentration projection issue and the evaluation of it emission became an interesting challenge for the scientific community. For the authorities that propose the adoption of mitigation measures as well as the organs that are willing to finance them, the Group I report has created an unexpected problem.



1 – IPCC, Fourth Assessment Report, Working Group 1 Report,

“The Physical Science Basis”, Technical Summary.

2 - IPCC, Third Assessment Report, Climate Change, “The Scientific Basis”, Technical Summary.

3 – Feu, C. et ali, “A Evolução da Concentração de Metano na Atmosfera”, N0 55 issue of the e&e periodical.

4 – Vargas, J. I., “A Prospectiva Tecnológica: Previsão com um Simples Modelo Matemático”, N0 44 and 45 issues of the e&e periodical.


[1] IPCC – Intergovernmental Panel on Climate Change

[2] The scenario of the Fourth Report are suggested by  TAR

[3] Part per billion in volume

[4] It would not be surprising , as can be observed in the values previously treated with the methodology that after a temporal oscillation, the path resumes the trend to saturation values previously studied

[5] Other mentioned reactions are free chlorine radicals, destruction in the stratosphere and soil absorption.


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Friday, 25 April 2008

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