Our society can't exist in its current form without energy, and is likely to collapse if our access to energy becomes insufficient to maintain it, and yet, our energy supplies are limited.

Most of the energy that we use comes from the fossil fuels; oil, coal, and natural gas; and from nuclear fuel, uranium.

All of these materials exist in finite quantities and are non-renewable.  For fossil fuels, they represent energy (as carbon and hydrogen compounds) stored away by life on Earth over hundreds of millions of years.  For nuclear fission fuels, they represent energy that came from supernovae that existed before the solar system came into existence.

If we keep using all of these energy resources eventually we must use them up.  Currently, there are still a lot of these energy resources available, so we won't use them up in the very near future (although, as our use of energy grows with our exponentially growing economy, we will use them up much sooner than may be expected.)

As well as the limited quantity of energy available to us, there are other issues associated with our use of energy resources that will constrain our use of them:

• They are getting more expensive to obtain
• They are taking increasingly more energy to obtain
• There are increasing consequences to the changing processes of obtaining them.
• There are increasing incidental consequences to our use of them.
• For many oil consuming countries, oil is imported from politically unstable parts of the world.

When oil was first used by humans it was simply collected from natural seepage at the surface of Earth.  When the modern use of oil began in the later part of the 19th century, large quantities of oil were easily obtained by drilling shallow wells on land – the first commercial oil well, in 1859 in Pennsylvania USA, struck oil at a depth of twenty-one metres.  Modern oil wells are much more expensive because they have to be drilled much deeper – commonly to one to two kilometres; although some wells go down twelve kilometres.

During the 20th century huge quantities of oil were obtained cheaply across the USA from land-based wells by simply drilling straight down into oil deposits.  Most of that oil was extracted by the mid 'seventies.  This sort of drilling is still happening in the Middle East, although it too must eventually run out; current estimates give another 40 years at the current rate of production, which means it will run out much sooner than that, when the growth in oil use resulting from exponential global economic growth is taken into account.  

Oil supply development is now heading in two broad directions: unconventional oil, that is, oil that is obtained from oil-like substances such as oil shale and oil sands; and deep-water production, which involves drilling kilometres into the sea bed, kilometres below the surface, and then turning the drill to drill sideways for kilometres.  Unconventional oil involves extremely complex processing to obtain oil from the raw materials that are mined.  Deep-water drilling involves extremely complex exploration and production drilling.  These complexities mean that these methods are much more expensive than conventional methods for the same amount of oil produced.  

When coal was first used by humans it was simply found on the surface, or extracted near the surface from seams exposed by erosion or geological uplift, by the simple process of digging into a hillside, which is called drift mining.  

Modern coal mining involves the expensive removal of huge quantities of overburden to reach coal seams perhaps hundreds of metres below the surface.  This photo shows a coal mine under development in Queensland, Australia; this mine hasn't reached the coal yet; the overburden is still being cleared. 

To give you an idea of the size of the hole, the yellow trucks in the photo are sixteen metres long, and have a gross vehicle mass of 500 tonnes.

In many places, particularly in the eastern United States, plateaux containing coal seams have eroded into mountains.  Coal is mined by removing the mountain tops to get to the coal seams, and dumping the removed material in the adjacent valleys.  

While these mining processes are very expensive, they are not currently so expensive as to not be worth doing.

As with any resource, we use the cheapest sources first and leave the expensive sources for later.  While there may be a lot of fossil fuels left on Earth now, it's mostly from difficult sources and is therefore more expensive to get.  Increasingly, this expensive and hard-to-get fossil fuel will be all that is available to us.

It may be argued that the increasing cost of getting to fossil fuels won't be a problem as long as we have a strong economy and can afford the high cost.  However, as energy underpins all of our economic processes, rising energy cost has a recursive effect making all of our economic processes more expensive, including the processes of obtaining fossil fuels.

There is a far greater problem than the rising cost of oil, and it's a problem that inherently can't be resolved.  

It takes energy to obtain oil: energy to build equipment, energy to drill the wells, energy to pump the oil, and energy to process the raw material to a product that is ready to use.  As the easy-to-get oil is used up and we need to use more and more of the oil that is hard to get, so the amount of energy that it takes to get oil increases.  

When USA oil production was at its peak in the 1930s, the amount of the energy available from oil was 100 times greater than the amount of energy that was used to get it.  That means that to get 100 barrels of oil the energy equivalent of one barrel of oil had to be used.  This ratio is usually called energy returned on energy invested (EROEI).   

Currently, the oil production process has an EROEI of 20, and the EROEI of ultra-deep-water oil and oil sands is less than 10.  

Obviously, when the EROEI for an oil source reaches as low as 1, that is, for every single barrel of oil that is obtained the energy equivalent of one barrel of oil had to be used to get it, there is no point in getting that oil for its energy value (however, the oil's value in other uses, such as for plastics manufacturing or road making, may still make the process worthwhile.)  

In practice, the EROEI of an energy resource must be significantly higher than 1 to make the effort of getting it worthwhile.  The bare minimum EROEI that makes the process worthwhile has been calculated to be 3. 

As with any resource, we use the easiest sources first and leave the difficult sources for later.  This means that progressively, our energy resources are going to give less and less return on the energy that it takes to obtain them.  Strictly speaking, we won't run out of any of our energy resources; it will just take more energy to get them than they are worth.  We may yet reach the ridiculous situation where we are using renewable or nuclear energy to access and process our fossil fuels, and using more of that energy that the amount of energy that we get when we use those fossil fuels.

There are many indirect negative consequence of our use of energy.  

As we process or burn any of the fossil fuels a range of directly toxic pollutants, such as particulates, sulphur, arsenic, benzene, formaldehyde and nickel are released into the atmosphere.   

The collection and processing of unconventional oil, tight oil, oil shale, and oil sands, cause massive land surface disruption and use vast quantities of water.

The open cut mines and mountain-top removals that we use to mine coal represent huge environmental damage.

Deep-water oil production and Arctic offshore oil production present a much increased risk of accidents and much greater difficulty in responding to those accidents and cleaning up after them due the extreme conditions that they operate under.  The BP Deep-Water Horizon oil spill disaster is a clear example of this, with massive damage to the surrounding sea floor, wildlife, immediate environment, and adjacent coastlines. 

As we collect and use fossil fuels, large quantities of greenhouse gasses are released into the atmosphere, which cause global warming and consequently climate change.  Hydraulic fracturing in particular releases methane, and as we burn all fossil fuels carbon dioxide is released.

By chance, large deposits of oil formed in what has become the Middle East, which has become a politically and socially unstable part of Earth, perhaps in part because of the influence of the revenue from the oil.  

Most modern states import oil from the Middle East.  Economic dependence on oil means that those states depend on getting that oil from The Middle East, so their own stability depends on the political and social stability of the Middle East.

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