Results from modeling of microscepage are presented that suggest that under proper conditions some thermogenic CH4 is transported to the atmosphere without complete consumption by methanotrophic bacteria in soils. These computer simulations are supported by direct measurement of CH4 fluxes in seasonally dry and/or cold climate sedimentary basins where losses to the atmosphere were postulated to occur. Measurements in four semiarid sedimentary basins where a thermogenic CH4 source is expected found small positive fluxes during cooler/drier seasons and negative fluxes during warmer/wetter seasons. Small positive fluxes up to 43 mg CH4 m-2 d-1 were found. A similar result was found for two geothermal areas in a semiarid climate where CH4 derived from the Fischer-Tropsch reaction operating in the reservoir provides a deep source of CH4. The results of the computer simulations, and direct field measurement of CH4 fluxes in dry and/or cold terrestrial environments suggest that a portion of the global atmospheric budget of CH4 may be from microseepage. ¿ 2000 American Geophysical Union