As always, the years supply is (almost) always given as if there were no change in the consumption rate. It is starting to dawn on some people that the exponential consumption time very quickly shortens the "actual" reserves left if on considers growth.

For example, that 200-250 year US reserve that is often quoted lasts but 80-87 years if you consider a growth rate of ~2.2% per year. Throw in a dramatic coal-to-liquids program and it's easy to demonstrate that the US coal reserves quickly drop to a couple of decades.

Yes, it's all "a blink of the eye" in time.

Resource and reserve calculations for coal are somewhat different to those for oil. You are still constrained in how far from a known (as in mined or drilled) site one can project that the deposit extends out. If it extends out less than a quarter of a mile it can still be called a reserve. Beyond that point it is only an "inferred resource", and that only out for a distance of up to three miles (as my memory recalls). Because coal does not flow like oil or gas, it is much more likely to be in place all the way between two boreholes, than is the likely case if oil and gas are found. Thus the final reserves, as they are proved, may, in fact, be larger than the currently defined resource.

On the other hand the presence of faults, washouts and other geological features can make what might otherwise be a highly desirable site (see Glenrothes in Scotland) uneconomic to mine. Plus I don't know the other constraints on the definition or reserve and resource in India (and these are, I suspect, their numbers not the USGS ones) so these are likely more ballpark than reliable numbers.