The catastrophic fires and floods around the world this year have focussed our attention more than ever on climate change and introduced a new sense of urgency to the issue. Ahead of the international COP26 Climate Change Summit in Glasgow in November, Professor Tom Davies offers his perspective on the debate, arguing that exponential population growth is the main cause of the problems we face.

In 1972 a bestselling book called The Limits to Growth described the impact of population growth on world resources. It predicted that increasing pressure on resources would eventually lead to the collapse of our civilization. As one reviewer noted at the time, ‘If this book doesn’t blow the mind of anyone who can read without moving their lips, then the Earth is kaput’. 

A bestseller in 1972

At the time of its publication, I was a member of a group of chemical engineers who set up and delivered a new undergraduate course on Environmental Chemical Engineering at the University of Exeter. With a degree in Fuel Technology, and after a lifetime of working on and teaching about carbon saving engineering systems, I would like to offer my perspective to the debate in the hope that some simple basic facts may help with understanding the very complex subject of climate change.


The human population on Earth is the cause of most of the problems we face. It has been growing exponentially ever since records began.

A simple exponential time series is 1,2,4,8,16 etc  where the quantity being measured (eg world population) doubles at regular intervals (the doubling time), so that the last in the series is always bigger than the combined total of all the previous values. We will all have seen the graphs showing the spread of the COVID virus which is a classic example of exponential growth.

For the population of the world the doubling time since the industrial revolution has been  around 30 years and so the future population of the world has been quite predictable. In 1968 it was 3.5 billion, in 1978 it was 4.3 billion, in 1989 it was 5.1 billion, in 1997 it was 5.9 billion and now in 2021 it is approaching 8 billion.

If this pattern continues it means that during the next 30 years there will be more people born on Earth than in the whole of the history of mankind put together which the Earth clearly cannot sustain. 


Every person on earth needs resources such as energy and food. The distribution and use of the Earth’s resources has always been unevenly shared between nations and always will be. Relatively small numbers of people use most of the resources and cause most of the damage to the climate (16% of the population use 60% of the resources). Up until recently the USA was by far the biggest consumer of resources but is now being challenged by China and Russia.

The Malthus doctrine is relevant to this scenario. Malthus was an 18th century mathematician who studied the behaviour of the banana fruit fly. He found that in a colony of fruit flies supplied with a controlled amount of bananas,  if he then provided more bananas the population of flies simply increased to consume the extra resource. So it is with the human race – supply more energy or food and the population will grow to consume it. It should therefore be unsurprising that the doubling time for resource exploitation is far less than for population growth.


To exploit any resource requires energy. The doubling time for global energy consumption is about 10 years, driven by an exploding world population with aspirations to a lifestyle enjoyed in the West. For example in 1972 the average American was using 12kW of continuous power. In the UK it was about 4kW whilst in most of the underdeveloped world it was the equivalent of a bowl of rice a day (about 200W). Hence the situation we now find ourselves in, where the world will need to find more energy in the next 10 years than has been used in the history of the world. The energy cost of supplying more energy is growing steeply. So, for example, to extract 10 barrels of oil from North American oil shale may take 9 barrels of oil and cause huge environmental damage.


The only significant large scale source of readily usable energy until recent times has been fossil fuel. It still is by far the largest resource, again unevenly spread around the globe (Russia has the largest hydrocarbon reserves, mainly coal).

The origin of fossil fuels (coal, oil,  gas, and wood in order of size of reserves) is the plants and plankton which grew  by photosynthesis whereby the sun’s energy is used to drive chemical reactions which combined carbon dioxide (CO2), water (H2O) and nitrogen (N2) all readily available in the atmosphere and on land or in the sea, to provide hydrocarbon materials, and releasing O2 into the atmosphere. 

This process has been going on for around 200 million years and the remains of the photosynthetic material (plants on land, plankton in the oceans) have accumulated and been transformed by geological actions into coal (from plants and trees) and oil (from plankton) with methane gas being associated with both kinds of deposits.

The human race has been pillaging these hydrocarbon reserves of (effectively) stored solar energy and burning them using O2 and in the process releasing the CO2 and H2O and oxides of nitrogen back into the atmosphere, effectively rapidly reversing the creation process. It has been estimated that the world uses at least 1 million year’s worth of stored solar energy every year.

The fossil fuel epoch as a blip in the history of the world.


In the 1970s the World Health Organisation predicted that the population of the world would reach 8 billion by around 2020. It was also estimated that there were about 8 billion cultivable acres of land on the face of the Earth and that each human being would need about 1 acre to provide food for 1 year. After 2020 food shortages would begin followed by starvation in third world countries and as a consequence pandemics would erupt!


The Earth’s atmosphere is an extremely thin layer effectively less than 10 miles high. Were the Earth to be shrunk to the size of a billiard ball it would be smoother (including Mt Everest) and the atmosphere would be invisible. The pollution of the delicate atmosphere by the combustion of fossil fuels is now creating global warming caused by the so-called greenhouse gases. These asymmetric molecules (CO2, H2O etc) allow the passage of short wavelength radiation from the sun but trap the reradiated long wavelength radiation from the surface of the Earth (like the glass of a greenhouse) causing global warming and the well documented knock-on effects (polar ice melting, fires, floods, crop destruction).

Energy flow on a universal scale

Virtually all usable energy available to mankind comes from or originated from the sun. The sun is a giant nuclear reactor in which gravitational collapse drives gas fusion reactions at very high temperatures which emit colossal amounts of energy some of which has been and continues to be intercepted by the Earth (10-12   - a trillionth - of total energy emitted). The rest of the Sun’s energy is lost to outer space where it can be detected as background microwave radiation. 

So-called renewable energy is almost always solar in origin. The wind is driven by the sun and the wind produces waves on the sea so wind power and wave power are forms of solar power. The evaporation of seawater leads to rainfall and creates the opportunity for hydroelectric power generation. Geothermal power is available from the hot rocks in the Earth’s crust. The sun and the moon both create ocean tides which have the potential to provide immense quantities of power. We have the technology to harness all these renewable energy sources.

Virtually all useable energy originates from the sun.

The price of progress

The age of unrestrained extraction and combustion of fossil fuels will inevitably come to an end as reserves are depleted and the energy and environmental costs of extraction and conversion become too high. We have already passed peak oil and gas production but coal reserves are still huge (coal can be converted into oil and gas) and fuel combustion will continue unabated with the continuing damage to the climate leading to further increases in global warming. 

When each of us as an individual decides to buy something, we first consider the cost. Yet society at large has long bought the idea of continual growth in population and production without adding up the final cost.

During the recent rapid expansion of the Chinese economy it was reported that China was opening one new coal fired power station every week. Australia is not restricting the expansion of its coal mining industry and it is cheaper to mine coal in Australia and ship it to Newcastle than mine it in Newcastle.

Large scale migration of people escaping regions which are too hot or become flooded because of rises in sea level and food shortages seem inevitable.

The alternatives for the future are limited and are described brilliantly by David Attenborough in his latest book A Life On Our Planet.

Following on from this both the BBC Radio 4 series 39 Ways to Save the Planet ‍by Tom Heap, and The Earthshot Prize on BBC1 by Prince William and Sir David Attenborough have described some of the solutions which are needed to cut carbon emissions and halt climate change.

The biosphere is dangerously close to a tipping point beyond which irreversible and catastrophic changes will occur.  Whether the world is prepared to take the required drastic action to avoid disaster may be revealed at COP26. I am not hopeful.

Tom Davies is Emeritus Professor of Thermofluids Engineering 

His  vintage lecture course on Energy Conversion available as a series of self explanatory  PowerPoint slides and a large number of reference websites,  with a comprehensive coverage of the subject can be accessed here.


Recommended further reading

IPCC Headline Statements

Update to Limits to Growth. Comparing the World3 Model with empirical data by Gaya Herrington  In Journal of Industrial Ecology (2021)   

World scientists’ warning of a climate emergency 2021 -  a new paper published in the journal BioScience.

Global food supplies will suffer as temperatures rise- climate crisis report  by Fiona Harvey, The Guardian, 13 August 2021 

It’s now or never: scientists warn time of reckoning has come for the planet by Robin McKie, The Observer, 15 August 2021   

Oct 31, 2021

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