EROEI and the Energy Cliff

Elements which support life are, by definition, critical to our survival. Ironically, such critical elements are often taken for granted. Take air (oxygen, to be more specific) for example. Without it, human life would simply not exist. Yet, we just assume that it is just there. In the space of reading the couple of sentences in this paragraph, did you notice that you just breathed in some oxygen?

In similar fashion, energy falls into the similar ‘critical element’ category when it comes to our society and economy. If we are to break down the concept of economy into the most basic, elemental blocks, we get this:

Energy + Raw Materials + Human Input = Products + Services

From transportation, agriculture, cooking, health services and communications to entertainment, energy is the enabler of all of the above and touches every facet of life. Not only that, the more complex a society is, the more energy it requires to fuel it.

In an agricultural society centuries ago where the main form of energy was derived from biomass, the amount of energy thus provided was limited, and life as a result was also very simple. Fast forward to recent generations, petroleum and electricity have unlocked a vast supply of energy at our disposal, thus allowing complex societies and economies to be built.

Energy Surplus and the concept of EROEI

Since it takes energy to obtain energy, it stands to reason that complex economies which require a large amount of energy to sustain them would need to have energy sources which would leave a large surplus of energy at the end of the extraction processes.

The concept of Energy Returned On Energy Invested, or EROEI, is simply a measure of how much energy you get in return after spending a certain amount of energy in order to get that energy:

EROEI = energy returned to society / energy used to get that energy

Take oil for example. If it takes a barrel of oil to run the pump at the oil well and it pumps out 100 barrels of oil, then the EROEI is 100 (100 divided by 1). The cost of extraction is 1% (one out of 100 barrels used), while the remaining (energy surplus) is available for us  to do ‘stuff’ with. In this case, a ratio of 100 is a pretty good deal – lots of net energy. Life’s good.

What is the minimum EROEI that a society must have?

A certain threshold of energy surplus must be met for society to exist and flourish. Equally importantly, for society to increase in complexity, a larger surplus of energy is required to support the infrastructure such complexity demands, as illustrated in the ‘hierarchy of energetic needs’ diagram below.

Hierarchy of energetic needsSource: EROI of Global Energy Resources

Here are the minimum EROEIs required to support various levels of activity, using oil as an example.

  • 1:1 – extract oil – you get to pump it out of the ground and see it.
  • 1.2:1 – refine oil – you can extract it and refine it.
  • 3:1 – transportation infrastructure – you need to build the road networks and maintain the trucks to deliver the refined fuel to the gas station.
  • 5:1 – grow food – additional energy is used to grow and process foods
  • 7 or 8:1 – support family of workers
  • 9 or 10:1 – education for workers and family
  • 12:1 – healthcare for workers and family
  • 14:1 – arts and social amenities

If you want to have a fairly basic subsistence by having just enough food to eat, then an EROEI of 5:1 would get you there. But if you want healthcare, education and other finer things of life like theatres, amusement parks or, gasp, YouTube/Facebook/internet, you are looking at a minimum EROEI of about 14:1.

The Energy Cliff

So what is the overall EROEI of our energy sources right now, you wonder? Before getting to that question, let’s do a refresher course on exponential math (Wiki definition) and talk about why it is relevant to the discussion at hand.

In a nutshell, an exponential function has 3 distinct phases. In the initial phase, the output values of the function increase imperceptibly slowly when the input values increase, and the function looks essentially flat when plotted on a linear chart. In the second phase, the output values of the function start to change significantly and noticeably as the input values increase. This is commonly known as the elbow of the curve. In the third and final phase, the values of the function increase by a huge amount for every small incremental increase in the input. In this final stage, the curve goes vertical (exponential growth).

exponential curveA typical exponential curve

We experience the phenomenon of exponential growth in many aspects of life. Population growth, bacterial growth and compound interest are a few examples.

As you recall, EROEI is a measure of the amount of energy society gets as a ratio of the amount of energy we put in to get that energy. Net energy, then, is the amount of energy left to do ‘stuff’ with after spending the initial energy to get that energy. By definition, net energy is very sensitive to the cost of energy extraction relative to the amount of energy thus extracted. The more it costs to extract the energy, the less net energy is left for use.

The sensitivity of net energy to EROEI is best illustrated by a chart using EROEI as the input and the net energy (expressed as a percentage of the total energy available) as the output. This graph, as depicted below, exhibits the behavior of a classic exponential curve except in this case the curve happens to be upside down.

energy cliff 1Source: EROI of Global Energy Resources

The dark gray area represents the amount of energy available to do ‘stuff’ with, whereas the light gray area represents the cost of energy extraction. When the cost of extraction is low, the percentage of net energy out is very high. Take, for example, the left edge of the chart where the EROEI is 50:1. There the net energy available is 98%.

Moving along the curve, when EROEI falls to 30:1, the net energy left is 97%. A significant drop in EROEI from 50:1 to 30:1 only results in a tiny drop in net energy. Similarly, when EROEI goes down to 20:1, the net energy falls to 95%. Still pretty good.

Things start to behave differently the further the EROEI decreases. At 10:1, the net energy falls 90%. At 5:1, the net energy falls to 80%. From there onwards, the net energy drops off rapidly. At 2:1, 50% of the energy is used to extract energy. And at 1:1, you need to put in the same amount of energy just to get that energy out.

Around an EROEI of about 10:1, you start the rapid journey down what is known as the energy cliff.

There are a couple of observations worth noting regarding the energy cliff:

  • High EROEI energy sources deliver a high amount of surplus energy to society. When EROEI is high, shifting from a very high EROEI to an EROEI which is less high (for example, from 50:1 to 30:1) has a very negligible effect on society. Conversely, when EROEI is low, shifting from a low EROEI to a lower EROEI would have a dramatic effect on the net energy available.
  • As we cruise down the flat phase of the exponential energy curve (such as what we experienced from the dawn of the petroleum age to about now), the decrease in EROEI has been more or less inconsequential and almost imperceptible. As we enter the ‘elbow’ phase of the curve, however, the cost of extraction becomes visibly high and the net energy return visibly diminished. Once we hit the vertical phase of the energy cliff and barring any success in finding energy substitutes, the current construct of our society begins to break down.

Where Are We on the Curve?

It would neither be easy nor useful trying to come up with a specific value of EROEI for the world as a whole, as the value of EROEI varies from one source of fuel to another and also from one country to another for the same fuel source, as outlined in the table below. However, one thing is unmistakable. That is, except for that of coal, the EROEIs of fossil fuels such as oil or natural gas have been steadily declining from over 100:1 at the dawn of the petroleum age into the teens by 2010 across all oil producing regions.

EROEI by sources and regionsPublished EROI values for various fuel sources and regions

Source: EROI of Global Energy Resources

Everywhere you look, you can see we are either already inside or entering the ‘elbow’ area of the energy curve.

As conventional oil production worldwide continues to decline and the shortfall is made up by non-conventional oil sources such as tight oil, tar sands and deepwater oil fields, the downward pressure on ERORI will only increase, as the EROEIs of non-conventional oil are nowhere near those of conventional oil (e.g. tight oil around 8 to 10:1, tar sands less than 5:1).

The EROEIs of renewable sources such as solar and wind are generally low. In addition, they produce electrical energy which is no replacement for oil which powers over 95% of the world’s transportation needs, although these power sources will be playing a critical role as our society evolves from petroleum towards electrification.

All in all, the writings on the wall are pretty clear – we are not at the precipice of the net energy cliff just yet, but we are definitely entering the ‘elbow’ area of the energy curve. In order to avoid going off the cliff, we will have to either find high quality (high EROEIs) alternative energy sources or produce energy from low quality sources (with low EROEIs) in large quantities.

A high priority should be placed on the search for alternative energy sources, as energy transitions typically take decades to complete. Conservation and judicious use of the remaining fossil fuels would buy us for time and make the energy transition less disruptive.

References and further reading


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  1. John

    There is only one solution. Reduce the population by 6 Billion. We were out of time for solutions 10 years ago. Now we are on borrowed time. Dead man is walking and he’s about to realise he’s dead.

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