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Why don’t lions chase mice?

This is the introduction to Tim Watkins’ latest book: Why Don’t Lions Chase Mice?

No, it is not a trick question. Indeed, the answer you gave is probably the correct one. A lion is a very large and powerful animal whereas a mouse is a very small but surprisingly nimble creature. Unless a mouse were foolish enough to walk into a lion’s mouth or at least stand beneath its paw, it is doubtful, that a lion – which spends around 18 hours a day resting and sleeping – could be sufficiently roused to attempt the chase.

This is easy enough to understand, but beneath this superficial answer is a piece of physics which is profoundly important to explaining almost every aspect of our way of life. A simple or small calorie is the amount of energy required to raise the temperature of a gram of water (at normal atmospheric pressure) by one degree centigrade. A K-cal (i.e. a thousand calories) – the measure of energy that we typically call a calorie when applied to food – is thus the amount of energy required to heat a kilogram of water by a degree. The average human in a developed economy requires between 2,000 and 3,000 K-cals per day (although, sadly, we tend to consume many more). A lion requires some 8-9,000 K-cals per day. And unlike humans who have access to high-calorie starchy foods, lions must get their calories from animal protein (muscle) and fat. A reasonably well-fed mouse contains around 30 K-cals; and so a lion would have to catch and consume more than 280 mice a day to sustain itself. While not entirely impossible in ideal conditions, in ordinary circumstances the lion would find itself burning more calories chasing the mice than it would receive in return.

A small domestic cat – itself a far nimbler creature than a lion – might fare better on a diet of mice. But lions are better off working in teams to kill large prey such as water buffalo which contain up to 500,000 K-cals; more than enough to share around a pride and have left overs for scavengers like hyenas and vultures.

Only plants have the luxury of obtaining the energy to maintain and grow themselves directly from sunlight. Every other living being must first expend energy in order to obtain energy:

This is expressed in several (more or less synonymous) ways:

  • E.R.O.E.I. – Energy Return on Energy Invested
  • E.R.O.I. – Energy Return on Investment
  • ECoE – Energy Cost of Energy

Each is a measure of the value of an activity.  An energy return of 1:1 is not normally[1] worth the expenditure.  Expending more energy than is obtained is positively life-threatening:

Just gaining some additional energy may not be sufficient either:

Remember how modern humans need between 2,000 and 3,500 K-cals per day? Only a half or less of this can be devoted to the jobs we do to get the currency with which to buy life’s essentials. Around 1,000 K-cals is required just to allow a human body to continue existing in a healthy condition. And some K-cals at least have to be expended on various non-work activities (including procreating the species). It might be that a minimum E.R.O.I. of 4:1 is needed just to sustain human life:

In addition to this limitation on our energy needs, we must also come to terms with the laws of thermodynamics. In particular, the second law of thermodynamics is that when energy is converted from one form to another, a proportion of that energy is always lost as waste heat. When we work or exercise, our bodies heat up and we have to produce sweat in order to evaporate the heat into the surrounding atmosphere. The same used to happen when our ancestors hunted animals or worked the fields to grow crops:

Accounting for the additional energy lost as waste heat, the minimum E.R.O.I. needed to sustain human life might be 8:1 or more; depending upon how efficiently we are able to use the energy available to us.

For most of the 250,000 years or so that humans have been around, our calorie intake has come from the direct sunlight that fed the plants at the base of the food chain.  We may have supplemented that energy with renewables like wind and water power – which are also a product of sunlight – but the annual growth of plants provided a hard limit upon the energy we could utilise.  What makes the modern world different is that we broke into planet Earth’s massive store of fossilised sunlight; coal, oil and gas.  For the first time in history, humans were able to escape the vagaries of the seasons.  The modern economy is more complex, but we could not escape the limitations of E.R.O.I. and thermodynamics[2]:

Fossil fuels provide vastly more energy than the starches and sugars in the foods we eat.  A kilogram of coal, for example, contains around 6,000 K-cals – enough to sustain 2-3 humans if only we could consume it directly.  Mining that coal, however, requires that we expend energy up front. And so coal – along with any other fuel source – has an E.R.O.I. of its own. Here, however, we encounter a problem seldom encountered by lions; or mice and men for that matter. Not all coal is equal. There is a massive difference, for example, in the coal seams which used to jut out of the sides of Welsh hills prior to the industrial revolution and the fractured and tortured seams which remain thousands of feet beneath the floors of today’s Welsh valleys – too difficult and too expensive to ever be recovered. The difference is that the E.R.O.I. of fossil fuels tends to decline because humans extract the easiest deposits first.

When the E.R.O.I. of coal dropped precipitously around the beginning of the twentieth century it had catastrophic consequences. The leading political world powers of the day – Germany, Great Britain and France – were each built around coal-powered economies. But as the E.R.O.I. of the coal they consumed fell, so too did the rate of growth across their economies. And while this is not a sufficient explanation for the outbreak of war in August 1914, it is an often overlooked factor which generated greater competition and more intense rivalries.

The one saving grace was that by 1914 a superior fossil fuel – oil – was beginning to make its presence felt. In addition to having a higher energy-density, the fuels derived from oil came in liquid form and so could be easily stored and transported. As a consequence, many of the industrial processes which had been powered by coal were switched to oil; leaving the remaining coal to power the remaining – largely heavy – industrial processes like steel manufacture and railways.

In switching from coal to oil, we massively increased E.R.O.I. across the economy. That is, there was far more energy left over after we had subtracted the energy required to extract more energy, to allow a massive expansion of non-energy economic activity. Nevertheless, by the end of the twentieth century we were experiencing the same “E.R.O.I. crunch” that had occurred with coal a century earlier. All of the cheap and easy oil fields were depleting, and the smaller and more expensive deposits that we were replacing them with provided less energy return on the energy invested.

To return to lions and mice, we find ourselves in a situation in which plant growth has failed and the animals we might normally eat have died from starvation. We wouldn’t normally eat mice, but now we will attempt to catch any animal that we can, even one as small as a mouse.  But while this might keep us alive for a little longer, only if we can find larger prey will we be able to guarantee the survival of our species.

In the modern world, it is less the species than the complex globalised life support systems which we developed back in the days when energy was plentiful which is at risk. In those days, manufacturing goods and growing food on the opposite side of the planet from where they are consumed seemed to make sense because the cost of transporting them using cheap oil-powered ships and aeroplanes was so low. Now, as the E.R.O.I. of oil itself falls, it is only a matter of time before the most energy-consuming goods and foods must either be re-localised or simply abandoned altogether. Other activities, too, will become increasingly strained. The morning commute – not just from suburbs into city centres, but from city to city – which became a normal feature of daily life in the late twentieth century is already decreasing. More and more people find that the cost of running a vehicle or using public transport is so expensive that they are better off taking a local job that pays less.

The question – which nobody has managed to resolve definitively – is what is the minimum E.R.O.I. required to maintain a modern economy? Some aspects of modern life, such as a basic secondary education, can be maintained at a relatively low E.R.O.I. whereas the arts and more specialist forms of medicine require a much higher E.R.O.I. to continue:

Unfortunately, there is little agreement over what exactly our combined E.R.O.I. for energy from all sources actually is. The remaining conventional oil remains above 20:1 whereas light oil from fracking comes in at around 5:1. Oil from bitumen sands is around 2.5:1 and corn ethanol (biofuel) is energy negative. Industrial scale wind turbines have an E.R.O.I. of 20:1, but only if we overlook the fossil fuels which have to be added to provide a reliable flow of energy and to maintain them over the course of their lifespan – the energy might be renewable; the technology most certainly isn’t. Nuclear from pressurised water reactors is around 15:1, while solar farms come in at around 5:1.

Prior to the Covid-19 pandemic at least, climate change was widely held as the crisis of our age. With the energy available to the economy already declining prior to SARS-CoV-2 embarking upon its world tour, however, a near-term energy crunch may prove far more deadly. That few people have even been aware of a looming energy problem is in large part because humans have never had to pay the full cost of the benefits we have derived from fossil fuels. For most people – including economists who ought to know better – energy is just another relatively cheap economic input. And since for the best part of three centuries we have enjoyed growing energy return on energy invested, most people take for granted that “clever people somewhere else” will come up with some other form of energy with which to unleash another round of economic growth.

At the time of writing, though, such alternative energy only exists in theory or in costly and unsustainable laboratory experiments. At the commercial/industrial scale, the only energy sources currently available to us necessitate a shrinking of the economy at a rate even worse than the self-inflicted economic shutdown in response to the pandemic

Energy is the life-force behind all human activity. As contrarian economist Steve Keen puts it, “Capital without energy is a statue; labour without energy is a corpse.” As the remainder of this book explains, the reason we should all care about why lions don’t chase mice is because it is the same reason why advanced globalised economies must collapse if the quantity of energy upon which they were built is no longer available. At stake is almost everything that we take for granted about our modern way of life.

As we shall see, the choice ahead of us is stark. We are not facing the kind of crisis which can be solved with a different economic theory, a change of political party in government or even a new system of government. The laws we must grapple with are not like man (and woman)-made social distancing or road speed laws. They are the physical boundaries of the universe itself. It matters not one jot whether the red team or the blue team is in office when E.R.O.I. drops below the ratio required to maintain industrial civilisation. The only questions left for us to resolve are:

  • Can we do anything to increase E.R.O.I., and if not,
  • What is the least damaging way to de-grow an economy?

By the end of this book, I hope to convince readers that there is still a window – albeit a rapidly closing one – of opportunity to achieve one or other of these outcomes. However, I would caution that so long as humanity continues to be preoccupied with greed, personal gain and the relative trivialities of the 24 hour news cycle, then we will hand the process of de-growth to Mother Nature; and we most certainly will not enjoy the outcome.

(Continue Reading)

[1] There are circumstances in a modern industrial economy where a negative E.R.O.I. might be of limited use.  For example, coal-rich but oil poor states such as inter-war Germany may opt to use more coal-based energy than they obtain in the form of oil-based energy.


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