I don't find the topic of fossil fuel depletion, e.g. Peak oil, and the post-peak period which I have dubbed the "Oil-Dearth Era" anything other than depressing, and yet in my more dejected moods, mulling over the subject, I have tended to think along the lines of, "well, at least there's still plenty of coal. We need to use it cleanly, but at least it's there to dig-up and not about to run-out any time soon." Indeed it is sometimes said there is 10 trillion tonnes of coal lying in the ground for us, which could keep the world going for hundreds of years, even if we fended-off the direst consequences of falling oil-production by turning large quantities of it into fuel by coal-to-liquids (CTL) processes.
I have always acknowledged the difference between a resource and a reserve: that reserve is how much of a material there is within acknowledged holdings of it and that as prices go-up, some of the resource (that's the lot, even theoretically estimated amounts) can pass onto the reserve, but it is generally accepted that there are around one trillion tonnes of coal in accessible reserves, worldwide. The rest of that 10 trillion tonnes, including the huge 3 trillion tonnes of coal under the sea off Norway, will probably not be so readily exhumed. At any rate, we still have that trillion tonnes to rely on ...or do we?
Demand for coal rises relentlessly. China is the world's greatest producer of coal (and also of zeolites, interestingly) but became a net importer in 2007, as its own provision could no longer keep pace with demand. 80% of all energy in China comes from coal and within that overall mix, 80% of its electricity component is derived from burning coal. It is often said that China opens one new coal-fired electricity plant every week, and these are about twice the size of a typical such power station in the U.K. In 2006, China increased its electricity production by 102 gigawatts, which is more than twice that used in total in the UK, and three times enough to power California. There are many examples, but overall, the world is using more and more coal, with the effect that estimates of "how many years worth we have left" have fallen dramatically.
Rather as is the case for oil, it is generally conceded that there are unlikely to be any new major discoveries of coal, and Energy Watch made the forecast that there will be a world peak in coal production in 2025. This is also in accord with the predictions of some analysts in Germany, reported a year or so ago. Thus, if the quantity of the reserve is contained and finite, it is in principle possible to apply a Hubbert Peak analysis as he of the same name did for U.S. oil in 1956, and which when applied to world oil production suggests a peak any time now, and certainly within the next 10 years. This doesn't mean the world is about to run out of oil - far from it - but that the commodity of cheap oil will fall into decline at close to 3% per year, and what remains in the oil-wells will become more difficult to extract and refine so that the price of fuel and all else will soar, while the world stock markets oscillate unpredictably.
To an economist, how much of a particular commodity can be produced is a matter of its price and as prices increase, more reserves will emerge to be costed in the marketplace. Thus coal seams (resources) that are, at present prices, too thin, too far below ground or distant from major consumers, can thence become economic and reclassified as reserves. However, prices of mineable commodities tend to reflect rather more pragmatic aspects, namely how easy or not it is to extract the raw materials themselves and coal is no different in this respect than oil, say, or platinum for that matter. David Rutledge at the California Institute of Technology (CIT) has applied a "Hubbert Linearization" to the matter of world coal reserves.
The original Hubbert peak analysis involves drawing a symmetrical "bell-shaped" (logistic) curve, enclosing production/year plotted as a function of time, and hence the area under the curve requires an estimate of the total amount of oil that will ever be produced from the given resource. Hubbert also applied the method to coal production. The upshot is that the peak is reached when half the resource has been produced while the other half is still in the ground, but to get an accurate forecast, it is necessary to have a good idea of how much of the material there is ultimately recoverable. Now this is the tricky part, since estimates of the latter, particularly for coal, tend to be quite unreliable, but here is where the Hubbert linearization comes in.
In the linearization, rather than plotting the annual production (P) vs the increasing number of years (e.g. from 1930 - 2010, say), (P) is expressed as its ratio (P/Q) to total (cumulative) production to date (Q) on the y-axis, and plotted vs total production (Q) on the x-axis: (y is vertical and x is horizontal). Since the denominator (Q) increases over time, i.e. as the resource is extracted, (P/Q) decreases monotonically and ergo the line has a negative slope. The point at which the straight line ("linearization") crosses the horizontal axis corresponds to the total amount that will ever be recovered, when P = P/Q = 0, i.e. there is no more to be recovered and production has ceased entirely.
Rutlege attempted to validate the model for coal by applying it to UK coal production since 1855, which actually peaked in 1913. Not only does it seem to work, but there are some alarming conclusions. Potentially the result sets some historical excuses to rights, namely that the fall in coal production happened because of Winston Churchill's decision to switch the British navy to oil instead of coal, the various miners strikes, the switch from town-gas (made by heating coal in giant retorts) to natural "North Sea" gas, etc., and explains that the real cause is a fact of geology. This is highly disconcerting, since it suggests we actually have a lot less cheap and readily recoverable coal under these islands than is often claimed. Indeed, it is reported that the UK reserve of coal amounts to 1.5 billion tonnes in existing mine holdings, but that another 190 billion tonnes lies elsewhere if you include areas under the North Sea within British waters. This may be so but the analysis, as in Hubbert's original case for oil, refers to cheap coal which can be extracted relatively easily.
Obviously, if it is necessary to dig a new and enormous mining infrastructure and to access deep seams of coal (half the coal produced in the UK comes from near-surface mines), the process must inevitably incur higher energy and fiscal costs; hence however much coal we might "own" in the UK, whether we can afford to get to it is another matter, and this begins to make coal-gasification look attractive since actual digging is avoided, that is if the technology can be made to work here. [There was, in fact, a pilot study done in Derbyshire in the 1950's with success but the National Coal Board concluded the process was uneconomic. Now it might prove profitable, after all for the UK].
As applied to world coal production, the result of the Hubbert Linearization is not rosy either, since it predicts that there are around 450 billion tonnes of it in readily accessible locations, or less than half the trillion tonne estimate I was banking on. For sure, more coal will be dug, but at increased costs and the resources (oil and gas and indeed coal itself) to provide the energy to do so must be found. Given that coal production must increase by 70% by 2030 to power predicted economic growth, which is coincidentally when "peak coal" is to be expected (2025 according to the linearization model, and in agreement with other forecasts), the world is on its way to an even bigger energy crunch (gap) than is expected to follow peak oil.
If the "Coal Dearth Era" is to provide another seam of depleting energy resources, running parallel with and compounding the "Oil Dearth Era", then the world simply cannot rely on fossil fuels to underpin human societies, and without some new "energy" technology to match the combined scale of oil and coal, or some form of agriculture that can enhance crop yields above the maximum carrying capacity of the planet at 3 billion, what other scenario is there but a die-off from the existing population of 6.5 billion (let alone an increase to 9 billion by 2050, as growth-enthusiasts predict)? I am even less happy than when I began writing this. The good news is, as I have commented before in "Chemistry World", that the inevitable fall in accessible fossil resources will necessarily result in a decline in the world's CO2 emissions, whether we implement deliberate carbon reduction strategies or not - however, we may not have much of a civilization left by then.
 "The great coal hole", by David Strahan: http://www.davidstrahan.com/blog/?p=116
 Chris Rhodes: www.rsc.org/chemistryworld/restricted/2007/March/letters.asp.