EVALUATION OF THE POTENTIAL IMPACT OF METHANE CLATHRATE DESTABILIZATION ON FUTURE GLOBAL WARMING

Future global warming due to anthropogenic emissions of greenhouse gases has the potential to destabilize methane clathrates, which are found in permafrost regions and in continental slope sediments worldwide, resulting in the release of methane gas. Since methane is a strong greenhouse gas, this could produce a potentially important positive feedback. Here, the coupled heat transfer and methane destabilization process in oceanic sediments is modeled in a series of one-dimensional, vertical columns on a 1 degrees x 1 degrees global grid. Terrestrial permafrost is divided into 11 columns based on mean annual surface air temperature. Our base case clathrate distribution results in about 24,000 Gt C as methane clathrate in marine sediments and about 800 Gt C in terrestrial sediments, only a small fraction of which could be destabilized by future global warming. Scenarios of anthropogenic CO2( )and CH4 emission are used to drive a simple model, of the carbon cycle, yielding scenarios of CO2 and CH4 concentration increase. These increases drive a one-dimensional coupled atmosphere-ocean climate model. Globally averaged temperature changes as a function of time and ocean depth are used as upper boundary conditions to drive the heat transfer/methane clathrate release models. Three versions of the ocean model are used which result in different temperature perturbations at the sediment-water interface: a purely diffusive ocean model, an upwelling-diffusion ocean model with fixed temperature of bottom water formation, and an upwelling-diffusion ocean model with a feedback between surface temperature and the upwelling velocity. Methane release from clathrate destabilization is added to the anthropogenic CH4 emission, leading to stronger increases in both CH4 and CO2 concentration. Based on a wide variety of parameter input assumptions and anthropogenic emission scenarios, it appears that the potential impact on global warming of methane clathrate destabilization is smaller than the difference in warming between low and medium, or medium and high anthropogenic CO2 emission scenarios, or arising from a factor of two variation in climate sensitivity. Global warming increases by 10-25% compared to the case without clathrate destabilization for our scenarios using what, in many respects, are worst case assumptions.