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Massive methane release due to decomposition of methane hydrate in the seafloor is a potential cause of global warming. However, the degree of global warming has not been estimated due to uncertainty over the proportion of methane flux from the seafloor to reach the atmosphere. Massive methane release results in the methane-saturated seawater, thus some methane would reach the atmosphere. However, methane bubbles passing through undersaturated seawater may potentially reach the atmosphere due to rapid vertical transport. We estimated the degree of saturation of methane in the water column necessary for methane bubbles to reach the atmosphere, and a ratio of methane released into the atmosphere to methane released from the seafloor, using a one-dimensional numerical model for predicting evolutions of methane bubbles and methane concentration in seawater. We found that some methane bubbles reache the atmosphere even when the methane saturation fraction in the water column is much lower than 100%. However, in order for most of the methane bubbles to be released into the atmosphere (i.e., a methane release ratio is > 80 %), it is required that methane saturation in the water column exceed 90 %. We compared the minimum amount of methane input from the seafloor necessary for methane to reach the atmosphere to the amount of methane in the sediment in the form of methane hydrate and free gas. In most cases, our results suggest that the typical amount of methane in the sediment (i.e., typical hydrate fraction of ~ 2 % and free gas of two-thirds of the amount of hydrate) is significantly lower than the minimum amount of methane input. It is, therefore, suggested that the massive quantity of methane bubbles released from the seafloor would not reach the atmosphere directly but would be dissolved in the seawater. The global warming suggested at the Paleocene / Eocene Thermal Maximum (PETM), at 55 Ma, due to massive methane release, should have been caused by the decomposition of methane hydrate from an unusually large hydrate fraction. With regard to global warming due to human activities, the release of methane bubbles due to methane hydrate decomposition may not be significant enough to accelerate total global warming.
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