Tom Murphy’s Energy and Human Ambitions on a Finite Planet in Essential Diagrams

Peter Wurmsdobler
10 min readApr 28, 2023

Tom Murphy’s Energy and Human Ambitions on a Finite Planet¹ presents global energy production and consumption with all its consequences in an educational manner, with explanations, graphs and diagrams. Most importantly, however, using first principles of physics, this book conveys the order of magnitude of all quantities involved as well as their relationships: population growth, fossil fuels, climate change, renewable energy and the limits of growth in a finite world are addressed. As such, this book is a must-read for all people in power, politicians, government officials and business leaders. The purpose of this story is to relay its message in a few essential diagrams in a logical sequence as an incentive to study the whole book.

The rise and optimistic fall of fossil fuel consumption, the resulting accumulated CO₂ in the atmosphere followed by the delayed atmospheric temperature increase (due to thermal inertia), without taking into account positive feedback loops such as loss of albedo (less ice and vegetation), more water vapour in atmosphere (through higher temperature), release of methane in perma-frost region, etc.

Population Growth

In Chapter 3: Population, Tom Murphy uses historical data to show the evolution of the world population in various graphs with exponential growth (excellent introduction to exponential functions in Chapter 1: Exponential Growth). When looked at in a logarithmic plot, the growth rates become more apparent as seen in the following graph.

Global population estimate, over recent centuries. On the logarithmic plot, lines of constant slope are exponential in behaviour. Four such exponential segments can be broken out in the plot, having increasing growth rates (Section 3.1, page 31).

World Population Growth on Our World in data demonstrates that the annual growth rate peaked in 1982 at 2.2%, stands currently at about 1.1% and is predicted to come down to 0.1% by 2100. The growth model presented in Murphy’s book, assuming a certain carrying capacity, results in a logistics curve for the evolution of the world population (section 3.2) which is expected to level off at 11.8 billion people by 2200.

Energy Consumption

Linked to population growth, energy and resource consumption have both grown exponentially, too. The worlds power consumption currently stands at approximately 18 TW³ in 2018, i.e. 18·10¹² Watts or 158,000 TWh/year, at a growth rate of 2.3%, with no sign of slowing down. Energy can also be seen as a proxy for other resources and material growth.

Population (red) and energy demand (blue) on the same plot, showing how much faster energy demand (power) has risen compared to population, which translates to increasing per-capita usage. The vertical axes are scaled so that the curves overlap in the nineteenth century (Section 3.1, page 30).

Note that the global power consumption grows faster than the population due to an increasing power and resource expenditure per capita². Chapter 2 expands on the economic growth limits, with decoupling and substitution being found not to be enough (aligns with findings in Tim Jackson’s Prosperity without Growth).

The majority of energy needed to power our civilisation is derived from fossil fuel, notably coal, gas and oil. This happens either directly, e.g. through combustion in vehicles powered by oil derivatives, or indirectly through fossil fuel power plants. Section 7.2, Global Energy, gives an overview of the current energy consumption adding up to 178,000 TWh/year or about 20TW in 2020.

Recent history of primary energy consumption in the world. The three fossil fuels and nuclear are shown separately, while renewable sources are grouped together (Section 7.2, page 109)

Fossil Fuel Consumption

The fossil fuel part of the global power consumption amounts to 128,550TWh/year or 14.7TW in 2020. The global average energy rate of use of fossil fuel per person is a little over 2kW per person and actually remains nearly constant (10kW for US citizens). However, due to population growth, energy consumption still rises. Section 8.2 Overview: Coal, Oil, and Gas provides all the details.

Historical use of fossil fuels worldwide, which may be viewed as a zoom-in of the left-hand side of the peak in Figure 8.1. The three types are stacked on top of one another, so that gas makes the smallest contribution, not the biggest. On the left is the raw usage rate expressed in terawatts, while the right is a per-capita measure showing that the left-hand rise is much more than just a reflection of population growth (page 140).

So how long can that continue, irrespective of the effects on the climate? Section 8.4: Timescales elaborates on the percentage of resources already used and years left; not too long after all.

The world has already consumed 1.5 trillion barrels of oil, which is nearly the same amount as the 1.7 trillion barrels of proven reserves — indicating that we are roughly halfway through the resource (page 126).

Climate Change

The chemistry of combustion is well understood: every carbon atom (C ) combusted with oxygen (O₂) creates a CO₂ molecule in an exothermal reaction (see Section 8.3).

The amount of CO₂ released per mass of fuel as well as per energy unit for fossil fuels.

Given the amount of fossil fuels burnt per year, and therefore knowing the amount of CO₂ released as a total mass per year (~40·10¹²kg/year, or 40 Gt in 2020), the addition in parts-per-million (ppm) to the atmosphere can be calculated with an estimate of the total mass of the air in the atmosphere (about 5·10¹⁸kg). Since only half of the CO₂ will be added to the air, while the other half is absorbed into land and sea, the annual contribution to CO₂ in the atmosphere can be derived (section 9.3).

Estimated CO contributions from known fossil fuel expenditures based on chemistry and the assumption that half of CO stays in the atmosphere, while the rest is absorbed by the ocean and land. Units are parts per million by volume. The left-hand panel shows the annual addition, adding to 2.6 ppmv per year and accounting for the slope in Figure 9.1. The right-hand panel is the cumulative emission to date as a function of time — essentially adding up all the annual emissions from the left-hand panel (page 142).

Using these models and the “Keeling Curve”, the CO₂ measurements by Dr Keeling since 1958, a decent overlap can be obtained. This is a convincing proof of the anthropogenic nature of the CO₂ rise.

Fossil fuel contribution toCO₂ (red) on top ofCO₂ measurements (blue). The red curve uses a starting point of 285 ppmv and has 49% ofCO₂ emissions staying in the atmosphere (Page 142).

Section 9.2, Warming Mechanism, provides a detailed explanation of the physics of global warming where CO₂ is an important contributor in the thermal equilibrium of the planet. The importance of “radiative forcing” as a function of the CO₂ concentration is explained, which is the specific power contribution that shifts the Earth’s equilibrium temperature from the expected value based on pure radiation balance. The following graph shows the relationship of CO₂ concentration in the atmosphere and the global warming that can be expected.

As CO2 concentration increases, the radiative forcing (left axis) increases, driving the temperature (right axis) up.We are now at 420 ppmv, corresponding to a radiative forcing of 2.2W/m2 and 1.7◦C of ultimate temperature increase (red star). Presently, the temperature has only increased by 1.0◦C (red circle), but will catchup to a new equilibrium once oceans warm and ice melts (page 147).

Given the historical data on fossil fuel consumption, several extreme scenarios can be construed to predict the effects on climate, from maintaining current level scenario, over several weaning off scenarios to a complete and immediate stop of coal scenario. These should demonstrate the boundaries of possible outcomes, details in section 9.3, Possible Trajectories to give an idea of the order of magnitude of global warming.

Renewable Energy

A certain amount of power is needed to at least maintain our civilisation. Given the detrimental effects of using fossil fuels, alternative sources are urgently needed. Tom Murphy makes an important step back and assesses the magnitude of the true and only independent power source for Earth, the sun, and all its derived potential power sources in section 10.3, Renewable Energy Budget.

Energy inputs to the earth, ignoring the radiation piece (since that is an output channel). About 70% of incoming solar energy is absorbed by the atmosphere and land, while about 30% is immediately reflected back to space (mostly by clouds). About half of the energy absorbed at the surface goes into evaporating water, while smaller portions drive winds, photosynthesis (land and sea), and ocean currents. Additional non-solar inputs are geothermal and tidal in origin (Page 167).

The sun sends energy toward the earth at a rate of 1,360 W/m². Multiplying this by the projected area of the earth (π·R² ≈ 3.14·(6.371·10⁶)² = 1.28×10¹⁴ m²) results in 174,000 TW of solar power intercepted by planet Earth, so about 8,700 times the 20TW. This energy gets absorbed and distributed along the surface; a fraction is available in various form for harvest.

  • Solar Energy, the atmosphere reflects approximately 29.3% of the total solar irradiation, the atmosphere plus land absorbs ~960W/m². Given that the surface of the Earth is π·R², the effective insolation is only ¼, i.e. 240W/m². Taking other aspects into account, the average insolation at the ground is ~200W/m². At an average efficiency of photovoltaic panels at 15%, the average output is about 30W/m². (about 0.4% of all land covered with solar panels would produce the 18 TW³ required).
  • Hydroelectric Energy, accounts for about 9% of energy, or 16% of electrical production. Detailed assessments hydroelectric potential globally estimate a technically feasible potential around 2 TW, but only half of this is deemed to be economically viable. 477 GW, or about 0.5 TW, is delivered globally, which is therefore about half of what we believe to be the practical limit of∼1 TW.
  • Wind Energy, accounts for about 2.6% of energy, or 4.8% of electrical production. Some estimates of practical global wind installations come in as low as 1 TW — well below our 18 TW³ demand. Wind alone is unlikely able to replace the energy currently derived from fossil fuels, at least on a global scale. Some regions may well be able to obtain a significant portion of their power consumption from wind energy.
  • Nuclear, as of 2019, the world has about 455 operating nuclear reactors, amounting to an installed capacity of about 400 GW. 35 The average produced power — not all are running all the time — was just short of 300 GW. The thermal equivalent would be approximately three times this, or 1 TW out of the 18 TW³ we use in the world. Fusion is still just around the corner.
  • Biofuels, are often considered to be carbon-neutral, as the carbon released upon burning was taken in from the atmosphere in the process of photosynthesis, making it a cycle (not quite true as the land is cultivated using fossil fuels). According to my own estimates, the power obtained from crop on land is about 0.5W/m², a fraction of solar panels.
  • Small Players, globally, roughly 10 GW of electricity is produced from geothermal energy [107] , and an estimated additional 28 GW of direct heating. As for tidal capture, France and South Korea have a tidal power capacity of about 500MW, i.e. several orders of magnitude from a meaningful contribution.

One important metric to assess the viability of any alternative energy source is EROI, Energy Returned on Energy Invested, a measure of how profitable an energy source is in terms of energy, expressed as a ratio.

The table speaks for itself. One important note has to be made on EROEI for Biofuels: let’s say that corn ethanol provides an EROEI of 1.2:1 — in the middle of the estimated range. This means that in order to get 1.2 units of energy out, one unit has to go in. Or for every 6 units out, 5 go in. If we use that same resource as the energy input — in other words, we use corn ethanol as the energy input to grow, harvest, distil, and distribute corn ethanol — then we get to “keep” one unit for external use out of every 6 units produced.


For a long time, as human civilisations evolved from hunters and gatherers to peoples toiling the land, development was limited by what land and labour could yield. Then came along the industrial revolution, followed by the green revolution, both made possible with fossil fuels. This nearly limitless source of energy afforded civilisation a tremendous rise as shown in Tom Murphy’s The Most Important Plot Ever.

Energy over the ages, in the form of fossil fuels. Up until the present, fossil fuels capture the bulk of the human energy story. We know what it must look like in the long term as well. The huge question is how the second half of human history looks,after fossil fuels are depleted or abandoned.The yellow star is a guess as to our current position (page 116)

There are only a few issues with powering growth using fossil fuels, apart from the fact that these and other resources are limited on the planet: side effects such as climate change. Given that energy consumption is still rising, and countries are still permitting the construction of more coal power plants, even the assumption of fossil fuel consumption being maintained at the current level appears optimistic, let alone curtailing the consumption by 2050. That said, the temperature rise is likely to be higher than the predicted 3.4◦C globally, which could mean even higher temperature changes in certain areas, e.g. in Europe parts of it may become uninhabitable. Other areas like northern parts of the northern hemisphere might actually start to thrive.

Personally, I have come to terms with the inevitable outcome that the world will most likely be different in 100 years, with a reduced habitable area and much reduced bio-diversity. Life on Earth will continue in one form or the other as it adapts. Electrification of transport, renewable energy from sun, energy storage, demand response and large grid networks will all help a bit to mitigate the effects of mankind on the planet but do not constitute a change in paradigm. Rather, looking at the commercial success of silly and unnecessarily large electric vehicles (SUVs), my impression is that the worlds continues with business as usual.

There is an issue at the core of our society: the belief and the pursuit of continued growth on a finite planet. Rather than treating the symptoms, we need to address this root cause. Decoupling, substitution and efficiency gains are not enough; we need to evolve towards steady-state economics. Rather than building a spaceship to Mars, we need to think of Earth as a spaceship whose precious resources we have to make do with while evolving in non-material terms. Mankind has to learn to live within the means and not squander our inheritance in a few generations.

References & Footnotes

  1. Thomas W. Murphy (2022): Energy and Human Ambitions on a Finite Planet — Assessing and Adapting to Planetary Limits, University of California, San Diego.
  2. The fact that the energy demand grows at a higher rate than the population is due to increasing energy/capita or affluence which is exemplified by the I-PAT model: the intensity of any growing resource approximated by the product of population, affluence and technology with their respective growth rates. Consequently, the energy/resource growth rate is the sum of all contributing growth rates; in order to obtain a steady state, in a finite world with the amount of resources limited, something will have to give at some point.
  3. The quoted 18 TW refer to the amount of primary power on average, or its equivalent in TWh/annum. Since most of the current final power or energy is created using some for of combustion, converting the chemical energy in fossil fuels over thermal energy into mechanical/electrical energy, only a fraction of about 1/3 of the stored energy can be extracted, see Energy Flowchart. This limitation is due to the principles of thermodynamics. Therefore, the average power requirement would be perhaps of the order of 6–8TW if all was produced as electrical energy by wind & solar.



Peter Wurmsdobler

Works on the technological foundations of autonomous vehicles at Five, UK. Interested in sustainable mobility, renewable energy and regenerative agriculture.