Modelling methane fluxes from terrestrial and sub-aquatic permafrost in East Siberia: evaluation of potential impact on global climate
Abstract
The concept of "methane bomb" associated with the rapid release of methane from thawing permafrost has been discussed in the scientific literature. Particular concerns are associated with thawing Siberian wetlands, and with the East Siberian Arctic Shelf (ESAS). Observations indicate high concentrations of methane over ESAS, up to 7-8 ppm at selected locations over the Laptev sea, while the latitude-mean concentration equals 1.85 ppm. Some researchers attribute it to the recent increase in gas permeability of thawing sub-sea permafrost, destabilization of hydrates and enhanced venting of methane to the atmosphere through taliks. Other studies suggest that enhanced methane venting at selected locations over ESAS is not related to recent climatic warming. In this study we check both hypotheses using mathematical modelling and evaluate the contribution of methane sources in Russian terrestrial and sub-aquatic permafrost regions to global climatic warming. We compiled the data base containing contours of wetlands in Siberia. According to it, the total area of Siberian wetlands is approximately 0.7 million km2, of which ca 0.35 mln km2 are located in permafrost regions. Estimated net flux of methane from the frozen wetlands under the current climatic conditions is about 28.5 Mt/y. According to our model results, projected by the mid-21st century changes in the thaw depth and higher temperatures may increase the methane flux from Siberian frozen wetlands by 6-10 Mt/y, which is likely to increase the atmospheric concentration by 100 Mt and lead to ca. 0.01 °C global temperature rise. We simulated the dynamics of permafrost and the depth to the boundaries of hydrate stability zone (HSZ) at ESAS using a hypothetical climate scenario. It suggests that at the time of inundation (ca 8 Ky b.p.) the top sediment layer warmed by ca. 12 °C from -13.5 °C (mean annual air temperature) to -1.5 °C (bottom water temperature). Temperature was set to this constant value until 1985. Since then in accord with modern observations we imposed 0.09°C/year trend until 2100, and afterwards prescribed the temperature to constant value of 11.5 °C. The rate of temperature change in the 21st century in this schematic scenario by far exceeds all IPCC projections. By making such unrealistic settings we explore the likelihood of the so-called "methane bomb" concept under most favourable climate conditions. Our results indicate that enhanced concentrations of atmospheric methane at ESAS are not related to recent climatic changes. Analysis of paleo geological data indicate that enhanced venting takes place in the vicinity of the paleo river beds on the ESAS, where permafrost never existed. It is thus likely that methane from deeper sediment layers vent to the atmosphere through pathways at such localities, while elsewhere it is sealed by permafrost. Our model results indicate that in the following 1000 years permafrost layer characterized by near-zero gas permeability will remain at ESAS above the HSZ preventing the massive escape of methane to the atmosphere except for the fault zones and paleo river beds, where it seeps already under the current conditions. Acknowledgement. This study is supported by the German-Russian Otto Schmidt Laboratory, project OSL-12-02, and the Russian Foundation for Basic Research, project 11-05-12011.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B14D..08A
- Keywords:
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- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0442 BIOGEOSCIENCES / Estuarine and nearshore processes;
- 0702 CRYOSPHERE / Permafrost