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Website Foreword (French)
Extracts : Introduction - Conclusions for Action

See also on this website : « Effet de serre : n’oublions pas le méthane »

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Website Foreword (French)

Il y a quatre ans, dans un article publié par le journal “La Recherche” puis dans le numéro 24 des Cahiers de Global Chance et enfin dans la publication d’une étude engagée pour le compte de l’AFD, Hervé Le Treut, climatologue actuellement directeur de l’Institut Pierre Simon Laplace et deux membres de Global Chance, Benjamin Dessus et Bernard Laponche, tentaient d’alerter, sans beaucoup de succès, la communauté scientifique et l’administration sur la sous-estimation chronique de la prise en compte des réductions d’émission de méthane dans la lutte contre le réchauffement climatique.

La note suivante, rédigée en anglais par les mêmes auteurs, reprend et complète l’analyse des métriques utilisées aujourd’hui pour comparer l’intérêt d’une action sur le méthane avec une action sur le CO2. Elle insiste en particulier sur l’inadaptation des métriques actuelles, en particulier le Pouvoir de Réchauffement Global d’une émission ponctuelle, à rendre compte des conséquences des programmes pérennes ou pseudo pérennes de réduction d’émissions qui sont la réalité des politiques proposes par les différents pays impliqués dans la lutte contre le réchauffement climatique.

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Introduction

The anthropogenic emissions of methane (CH4) are ranking second after the anthropogenic emissions of carbon dioxide (CO2) in terms of potential impact on the global Earth radiative budget. They both produce an important greenhouse effect, with an associated global warming. Reducing the anthropogenic emissions of methane is then an important stake for climate change mitigation.

Compared to CO2, CH4 has a much higher radiative efficiency and a much shorter life-time in the atmosphere. As a consequence, the climatic impact of the relative emissions and emission reductions of these two gases are very different, depending on the period of time considered for their assessment.

In order to define a common unit for the emissions of all greenhouse gases, the Intergovernmental Panel on Climate Change (IPCC) has proposed an index named Global Warming Potential (GWP). Its last review an definition can be found in the IPCC AR4 (IPCC1, 2007). The GWP of CH4, or any other gas, is the time-integrated radiative forcing of a pulse emission of 1 tonne, relative to that of CO2, chosen as a reference gas. The integration is carried out over a time span which starts on the emission year and is limited by a time horizon chosen for its relevance for climate impact studies. The time horizon of 100 years is almost universally retained, leading to adopt the equivalence of 1 ton of CH4 to 21 tons of CO2, or 21tons of “CO2 equivalent”.

There is currently a debate on the ability of this simple GWP concept to describe adequately the effects of the different gases under different mitigation strategies. Public action to curve down greenhouse gases may target different time scales, and different climate parameters. A recent IPCC expert meeting (IPCC, 2009) has concluded that, while GWP is a useful index, defined in a clear and physical manner, and improved by a continuous review process, it has not been designed to address the whole range of policies and is not adequate to deal with some of them. There is a concern about the best way to include the role of short-lived pollution, which is reflected in many of the papers presented at the IPCC Expert Meeting (IPCC, 2009). Focusing again on methane, and depending on the context, the CO2 equivalent value of 21 may appear too large or too small, with foreseeable consequences on the negotiations and policies. On one side, the surface temperature of the Earth, at a given time, is largely at equilibrium with the concentration of all greenhouse gases, including methane. As the concentration due to a pulse of methane declines very rapidly with time, the integration concept associated to GWP does not provide a correct assessment of this temperature. It is the reason why a different concept, the Global Temperature Potential (GTP) has been proposed as more suitable for this purpose (Shine et al, 2005). A detail computation of the GTP would involve a model of surface temperature evolutions, but the GTP of CH4 at year T may be approximated as the radiative forcing from the CH4 concentration that would result from a pulse emission of 1 tonne after T years, compared to that of a pulse of 1 tonne of CO2 after the same T years. On one side, we may think that GTP is better suited to estimate the impact of methane policies, because one of the reasons why we need short-term actions to curb down methane is precisely the fear of seeing the global earth temperature rising above the 2°C level. On the other side, instantaneous surface temperature is not the only parameter describing climate change. There are other parameters in the climate system that may depend more critically on the integration of the radiative forcing than its instantaneous value. This is probably the case of the deeper ocean thermal expansion, or large icecap melting, two processes which are essential in producing irreversibility in the climate change.

In this paper we wish to address another concern, which has not received much attention while we think it has large implications : the “pulse” concept is clearly inadequate to describe current mitigation actions and the common practice of policy makers. In fact, almost all the emission mitigation programmes from international and national instances, whether already effective or considered for future action, correspond to emission reductions which should last at least several decades (building insulation, fossil fuel substitutions in power plants, etc.). Furthermore, the national programmes take as an implicit hypothesis that these reduction policies will perpetuate : at the end of a given action, a new reduction action is supposed to begin, at least equally effective, and hopefully more effective. So we have to consider that the mitigation measures we wish to evaluate will most generally perpetuate almost indefinitely. In addition, defining a methane (or CO2) reduction programme involving new initiatives every year (new plant rehabilitations for example) every year induces a continuous increase of this methane reduction.

Opening a debate on a new definition of the GWP is troublesome for at least two reasons : (i) it may undermine international agreements that have been slow to construct ; (ii) it may be seen in some cases as an incentive to substitute easier short-term strategies to the absolutely necessary reduction of CO2 emissions, which carry the dominant and lasting impacts on climate. But CO2 and CH4 reductions are not in competition one against the other : they generally concern different economic actors and should be encouraged simultaneously, because we know that we need all possible actions to curb down climate warming. Therefore whatever index we use, we should be fully conscious of its impact on a given type of policies. The aim of the paper is to analyse the consequences of using different metrics on CH4 mitigation policies.

In paragraph 2, we redefine GWP, GTP, and introduce a modified definition of those metrics to assess the relative role of lasting emissions (or reversely, lasting mitigation) rather than pulse emissions (or reversely, pulse mitigation) of CO2 and CH4, from a short (2030) to long-term (>2100) perspective. In paragraphs 3, 4 and 5 we review the different methane mitigation options which might be better targeted by a more specific definition of the methane CO2 equivalent.

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Conclusions for Action

The 2007 IPCC report has confirmed that a global warming above 2°C would constitute a danger for our environment. This implies that climate change is an issue for a much sooner future than was considered by policy makers a decade ago. The report highlights the very short-term need to reach a turning point around 2020 and a 30% to 40% reduction in CO2eq global emissions in 2030 compared to its 1990 level, whereas world emissions are currently growing at a rate of about 3% per year. This highlights the necessity to put in place urgent, ambitious, and continuously strengthened policies, which would target all possible options to turn down greenhouse gas emissions.

In this short term context, it is more appropriate, as explained above, to appreciate the efficiency of CH4 and CO2 reduction policies through global warming indices such as GTPPro or GWPPro rather than the GTP or GWP pulse indices. The GWP index value of 21 generally taken into account by policy makers for the comparison of CH4 and CO2 underestimates the role of methane emission reduction programmes by a ratio of as much as 4 or 4,6 at 20 years for respectively GTP or GWP estimations, and still 1,9 or 2,5 at 100 years (Table 11).

Using a single index to favour the most relevant policies at different time scales therefore appears as an oversimplification, which may help clarify complex international negotiations, but may be detrimental for other useful and cost-effective actions. One possibility, as emphasized by Daniel et al (2009), might be to have different baskets for the greenhouse gases of different time-life and to consider the relevant mitigation options within those baskets.

It would thus be very useful to promote an open debate on these physical basis in order to find a consensus on new equivalence coefficients for CH4 more suitable to assess real global warming consequences with time of real emission’s reduction programmes as they are envisaged by policy makers.

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Read the full paper (pdf, 800 Ko)

See also on this website : « Effet de serre : n’oublions pas le méthane »