Fossil resources.

The following is a list of energy resources that are diminishing at various rates.
  1. Carbon and hydrocarbons in the Earth's crust,
  2. Oxygen in the Earth's atmosphere,
  3. Primeval forest,
  4. Nitrate deposits such as guano,
  5. Water in the aquifers, if it is not being replaced by rainfall,
  6. The radioactive energy of the Earth's mantle, which provides geothermal power,
  7. The rotational energy of the Earth, relative to the Moon, which provides tidal power.

We usually do not consider (6) and (7) as actually fossil resources. But it is radioactivity that powers all the tectonic phenomena, including the fact that the Earth is not simply a solid ball of iron and rocks.
That being the case, nuclear power should not be classified as fossil power.

Coal, "Natural Gas", and Petroleum

We tend to regard only the carbon and hydrocarbon as fossil. Coal seams were originally luxuriant forests, capturing solar energy by photosynthesis, until they were inundated, isolated by water and mud from ordinary bacterial and fungal decay, and turned into carbonaceous rock. We know that methane, "natural gas" is a common bacterial product of organic decay. It is generally presumed that petroleum deposits are derived in some similar, possibly subsequent, but unknown process.

Oxygen

Hardly anybody notices that oxygen is a fossil "fuel". When we burn hydrogen, methane, or carbon, we also consume oxygen. Almost all of that oxygen was produced from carbon by the photosynthesis that created the coal seams and most if not all of the other hydrocarbon deposits. Some might have come from direct solar decomposition of water vapor, but so far as can be ascertained, oxygen is a deadly poison to the earliest micro-organisms, so it can hardly have been present in the primeval atmosphere.

Twelve pounds of carbon combines with thirty-two pounds of oxygen to make carbon dioxide. So if we permanently "sequester" carbon dioxide instead of releasing it to the atmosphere, we are also removing oxygen from the atmosphere. I suspect that the difference would be measurable in a couple of centuries, but it might not matter for 10,000 years. At high temperatures, carbon will even remove the oxygen from iron oxide, and leave the pure iron. It is fiendishly difficult to dissociate oxygen from carbon dioxide.

Primeval Forest

Primeval forest is an immensely valuable renewable resource, having evolved to extract for its own purposes the maximum energy from solar radiation. It probably represents the most efficient natural way of carbon sequestration on land.
It is a resource that has been and is being destroyed almost everywhere it occurs.
It is probably not a good environmental idea to cut down Brazilian jungle to cultivate sugar cane for E85 ethanol.

Nitrates

It was found by the Incas that the fertility of their fields could be wonderfully increased by adding huano, a sort of powder or rock that is actually the dried deposit of seabird excreta. It is now called guano. Its importance is that it supplies the plants with usable nitrogen, which is required by every amino acid in every protein found in living organisms.
When the European powers discovered the stuff, a vast market grew up which has wiped out all of the accessible ancient reserves.

Since that time, industrial chemists have found a way to manufacture nitrate fertiliser (and nitrogen-containing explosives like TNT) from the nitrogen of the air, using hydroelectric or fossil sources of hydrogen to combine with the nitrogen.

Organic Farming

It was reckoned by Western agricultural scientists that the most productive technique on Earth of sustainable agriculture was that of the traditional Chinese. They recycled everything, including human excreta. They could support about ten people per acre, without additions of artificial fertiliser. That's 6,400 people per square mile of good land, or about a million square miles of it for a global population of 6,400 million. Some authorities have estimated the global availability of "good land" as being no more than that, and possibly less. If true, it follows that a global population significantly bigger than what we have at present cannot be supported "organically", but will need a new energy supply to provide nitrate fertiliser.

Aquifers and Fossil Water

An aquifer is a sort of underground river. Any well that you can dig near a hill, or spring that comes out of a mountain-side, is fed by an aquifer. Ideally, your supply of water is being continually replenished by rainfall on the hill or mountain-side.
If it's a well, and you pump the water up from it, you may have a reservoir that goes up when it rains, and down in the dry season. But if it goes down year after year, your water resource is not being sustainably used, and before it's all gone, may not be fit to use.
If there were an independent energy supply, you might be able to desalinate seawater, and have it supplied to you.

Ancient Radioactivity

One of the most difficult challenges ever made to Darwin's theory of evolution was a calculation by the greatest thermodynamics expert of the time, William Thomson, Lord Kelvin, professor of Physics at Glasgow University.
With impeccable mathematics and the best available information on the melting points, specific heat, and thermal conductivity of the Earth's rocks and metals, Kelvin estimated the length of time it would take that mass of molten material to cool to its present apparent temperature, as given by the rate of conduction of heat from below into the deepest chambers of various mines.
The figure he obtained was between 20 million and 100 million years.

This was in stark contrast to the estimates of the time it would take for evolution to produce vertebrates, or even insects. It was also far too short to explain the presence of limestone rocks made from sea-bottom deposits, in the heights of the Himalaya mountains.

The explanation is that the radioactivity of various very long-lived isotopes, including uranium, thorium, and potassium, has kept the Earth's mantle and core molten for a prodigiously long time.

These isotopes can hardly be called fossil fuels, because they were in existence since before the Sun's thermonuclear activities began. They were part of the Earth when it was part of the cloud of dust and gas that condensed to form the Solar System.

Tidal Power

J.B.S Haldane, in one of his flights of fancy, reckoned that harnessing the tides on a global scale might be what humanity would eventually have to do. This was before the advent of nuclear power.
The key to evaluating the endurance of this supply is a fact known to astronomers and encapsulated in their calculation of the calendrical "leap-second".

The most accurate clocks count what are equivalent to vibrations in the structure of atoms of a certain element. They are called "atomic clocks". The Scientific International (SI) standard second is defined by these clocks.

To a very good approximation the Earth orbits the Sun in an ellipse. Relative to the Sun and the distant stars, any fixed point in that orbit can be used to mark the beginning of a year. The time that the Earth takes to orbit the Sun, i.e. the year, is a constant number of seconds.
But it turns out that if a solar day is defined as the time it takes the Earth to make one revolution relative to the Sun, and the length of a year is 365 solar days plus a number of seconds, then that number of seconds is slowly diminishing. The period of rotation of the Earth, the "day", measured in atomic clock seconds, is increasing.

The reason for this is that the Moon's tidal interaction with the Earth transfers angular momentum from the Earth to the Moon, which is very gradually being flung farther away.
The adjustment for this is called the "leap second".
I could equally well have used the sidereal day, which is measured relative to the stars, and gives a year different by one from that of the solar day, as the night sky appears to rotate with the seasons.

The Earth's energy of angular momentum is diminishing.
We could calculate, given the mass of the Earth and its diameter, and whatever we can glean about the distribution of mass, how much energy is being exchanged by this gravitational process.
I'm not at all sure that anything less than wildly heroic international projects could tap into very much of it. Nor am I optimistic about the environmental impact.

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