SIZE AND CONSEQUENCE OF THE GLOBAL METHANE HYDRATE ENDOWMENT

Keith A. Kvenvolden
Institute of Marine Sciences, University of California, Santa Cruz, California 95064, 
and U.S. Geological Survey, Menlo Park, California 94025

The global methane hydrate endowment is undoubtedly large, but just how large is the subject of continuing debate. This issue of size is important because many of the postulated consequences resulting from gas-hydrate occurrence are dependent upon how large the gas-hydrate reservoir is. Estimates of the amount of methane in gas hydrate have ranged from 7,600 x 10¹⁵ m³ to 0.2 x 10¹⁵ m³, with a current “consensus” value of 21 x 10¹⁵ m³. This last estimate has been vigorously challenged (Milkov, 2004, Earth-Science Reviews) with a new range of values of (1-5) x 10¹⁵ m³. The difference in the new and old estimates is about one order of magnitude, and both the consensus and new estimates are still speculative. Nevertheless, the smaller estimates, if more correct, make natural gas hydrate less interesting as a potential energy resource and as a factor in global change.

Instead of being 50 times larger than the global conventional gas resource (0.44 x 10¹⁵ m³), the new estimate suggests that the amount of methane in gas hydrate is only about 5 times greater. This comparison is questionable, however, because the underlying assumption is that the conventional methane is ultimately recoverable for human use, whereas, only an unknown, but likely small, portion of the hydrate methane will be available for exploitation. If only about 2 x 10¹⁵ m³ of hydrate methane constitutes the global endowment, the potentially exploitable portion is likely to be less than 50%, making the amount of hydrate methane for energy use at best about equal to or much less than the conventional methane fossil fuel resource.

A smaller gas hydrate reservoir containing only about 2 x 10¹⁵ m³ methane reduces significantly the impact of hydrate methane on global change processes. Arguments concerning changing ocean/atmosphere chemistry at the Lower Paleocene Thermal Maximum (Dickens et al., 1997, Geology) and during the Holocene (Kennett, et al., 2002, AGU) seem to require larger inputs of methane than are available from the global endowment suggested by the new range of estimates. It is premature to accept any of the global estimates of methane hydrate as representing the true global endowment. Clearly more information is needed to understand the amount of methane in the current and past global inventory, and the amount of hydrate methane required to effect the postulated global consequences of methane hydrate occurrence on past and present global change.