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Bruno Ringeval (LSCE)

Titre : Interactions entre climat et émissions de méthane par les zones humides à l'échelle globale

Date et heure : Le 11-03-2011 à 14h00

Type : thèse

Université qui délivre le diplôme :

Lieu : Agroparistech (amphi Dumont), 16 rue Claude Bernard, 75005 Paris
Membres du jury :

François Ravetta (Examinateur)
I.Colin Prentice (Rapporteur)
Jed O. Kaplan (Rapporteur)
Agnès Ducharne (Examinatrice)
Gerhard Krinner (Examinateur)
Renato Spahni (Examinateur)
Pierre Friedlingstein (Directeur de thèse)
Nathalie de Noblet (Co-directrice)
Philippe Ciais (Co-directeur)

Résumé :

Methane (CH4) is an important atmospheric compound by virtue of its greenhouse gas effect and its role in atmospheric chemistry. Wetlands account for 20-40% of all global sources of CH4. To study the interactions between wetland CH4 emissions and the climate on a global scale, I modified the ORCHIDEE model to take into account such emissions. This modification was carried out in two steps to enable the simulation of (i) the dynamics of the extent of wetland areas and (ii) CH4 flux densities (per unit area); the product of these two variables gives wetland CH4 emissions. I then used this model to study the sensitivity of wetland CH4 emissions to the climate over two time periods, namely the current year-to-year variability (1990-2008) and the
1860-2100 period, by using the climate fields simulated by the IPSL general circulation model under the SRES-A2 scenario. I then focused on the effect of this sensitivity (i) on the variability of atmospheric CH4 concentration and (ii) on the climate through a feedback.

My results show that the interannual variability in the extent of wetland areas plays a key role in the year-to-year wetland CH4 emissions variability. For instance, it can explain up to 90% of the emission anomaly that was observed in certain years in the Tropics. This result points to the absolute necessity to be able to take into consideration the variations in wetland extent in bottom-up modeling of wetland CH4 emissions. The comparison between the wetland CH4 emissions simulated by ORCHIDEE and the estimations of the total sources given by top-down approaches underlined the large contribution of the wetland emissions to the 2006-2008 positive anomalies in the atmospheric CH4 growth rate.

With regard to the future evolution of wetland CH4 emissions, the using at the same time of a theoretical framework and ORCHIDEE simulations gives me the possibility to demonstrate that the gain of the climate/CH4 feedback from wetlands was low and negative (-0.016). However, there were some interactions between this feedback and the climate/CO2 feedback that had to be taken into account. These interactions were related to (i) the effect of atmospheric CO2 on methanogenic substrates by virtue of its fertilizing effect on plant productivity and (ii) the fact that a climate perturbation due to CO2 (respectively CH4) radiative forcing has an effect on wetland CH4 emissions (respectively CO2 fluxes at the surface/atmosphere interface). Thus, the consideration of both types of feedback and their interactions led to a 475 ppb increase in the 2100 atmospheric CH4 concentration predicted by the SRES-A2 scenario.

Université qui délivre le diplôme


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