The size of methane hydrate reservoirs is poorly known and estimates of its size has decreased by roughly an order or magnitude per decade as more information is discovered.
The highest estimates (e.g. 3×1018 m³) were based on the assumption that fully dense hydrates would be found on the entire floor of the deep ocean.
Recent estimates constrained by direct sampling suggest the global inventory lies between 1×1015 and 5×1015 m³ (1 quadrillion to 5 quadrillion).
If this estimate is accurate, it corresponds to 500-2500 gigatonnes carbon (Gt C), which is smaller than the 5000 estimated for all other fossil fuel reserves, but substantially larger than the 230 Gt C estimated for other natural gas sources.
In the Arctic, permafrost reservoir has been estimated at about 400 Gt C, but no estimates have been made of possible Antarctic reservoirs.
For comparison, the total carbon in the atmosphere is around 700 gigatons.