Here are some energy prices per million-btu (MMbtu), cheapest energy first:
Coal (Powder River Basin) = $0.45/MMbtu
Coal (Central Appalachia) = $2.60/MMbtu
Natural gas = $6.97/MMbtu
Oil = $9.14/MMbtu
We can form various diffences from these prices, like Oil - Coal, or Oil - Natural gas. Those differences can either grow or shrink as time goes on, and Greenspan's view is that those differences will narrow over time.
These differentials may be a good indicator of energy stress. For example, if oil grows scarce, and causes the price difference to widen between coal and oil, then we can expect coal and natural gas to be converted to oil. It's a form of arbitrage, and exploitation of the difference will cause it to shrink in size. That is Greenspan's idea.
The first sign of energy stress is when the differences between Oil - Coal and Oil - Gas begin to narrow. This is the period we are about to enter where coal liquefaction and gas-to-liquids are the stopgap solution. The difference narrows because we are propping up an oil plateau by doing a massive raid on coal and gas supplies. At the same time, a related trend is companies and people stepping down the energy ladder to flee high prices, gradually switching to cheaper, more inconvenient, dirtier fuels. The
ladder looks like this:
Electricity
Gasoline
LPG
Distillate fuel oil
Other (asphalt and road oil, aviation gasoline, kerosene, lubricants, petrochemical feedstocks, petroleum coke, special naphthas, waxes, and miscellaneous petroleum products)
Jet fuel
Natural gas
Residual fuel oil
Wood and waste
Coal
Nuclear process heat (?)
These phenomena should narrow the differences, just like Greenspan says.
D-Day is the day when the differences shrink too much, and per btu prices of all forms of energy are too close together. If this ever happens, EROEI will become a very real and useful concept. All EROEI<1 oil production activities will cease because the equation money=energy will be true. Coal liquefaction and gas-to-liquids will stop. This is interesting because it's a plausible reason why oil production might drop off a cliff one day for economic reasons.
There are reasons for optimism, though. The table referenced above shows that the difference between coal and oil has been steadily widening since 1970. This means we aren't really stressed yet. We're still leaving the coal alone.