Accepted by Natural Resources Research
Growth rates of global energy
systems and future outlooks
Mikael Höök1, Junchen Li2, Kersti Johansson1, Simon Snowden3
Contact e-mail: Mikael.Hook@fysast.uu.se
1 Uppsala University, Global Energy Systems, Department of physics and astronomy, Box 535,
SE-751 21, Lägerhyddsvägen 1, Sweden, http://www.fysast.uu.se/ges
2 China University of Petroleum – Beijing, School of Business Administration, 18 Fuxue Road,
Changping, Beijing, China
3 Management School, University of Liverpool, United Kingdom
The world is interconnected and powered by a number of global energy systems using fossil,
nuclear, or renewable energy. This study reviews historical time series of energy production and
growth for various energy sources. It compiles a theoretical and empirical foundation for
understanding the behaviour underlying global energy systems' growth. The most extreme
growth rates are found in fossil fuels. The presence of scaling behaviour, i.e. proportionality
between growth rate and size, is established. The findings are used to investigate the consistency
of several long-range scenarios expecting rapid growth for future energy systems. The validity of
such projections is questioned, based on past experience. Finally, it is found that even if new
energy systems undergo a rapid “oil boom”-development - i.e. they mimic the most extreme
historical events - their contribution to global energy supply by 2050 will be marginal.
Key words: Global energy systems, growth rates, energy forecasting, scenarios, long term
forecasting, evaluating forecasts
Energy is important to all sections of society and is the driving force behind modern civilization.
Mankind uses a number of different energy sources to provide the energy required for a
multitude of activities. Energy is usually defined as the capacity of a physical system to perform
work, whether the work is useful or extraneous to society. Energy exists in many forms, such as
heat, electricity, nuclear binding energy, kinetic and potential energy, electromagnetism as well
as many other forms. The use of energy is a key component in the development of society by
assisting it to control, and adapt to, the environment. In the modern world, utilization of energy
resources has become essential for agriculture, transportation, waste collection, industrial
production, information technology and communications systems that have become the pillars of
a developed nations. Consequently, energy represents an irreplaceable and non-substitutable
resource of paramount importance.
In prehistoric times, the only energy source available to mankind was energy that could
be obtained from physical labour, either from humans or animals. Firewood and primitive water
and wind mills complemented physical labour before industrialization. The invention of the
steam engine in late 18th century and the start of the industrial revolution rapidly led to an
increased demand for combustible fuels. Eventually, firewood and charcoal could not keep up
with demand and society turned to coal, witnessing the dawn of the fossil fuel era. The emerging
industries of the United Kingdom, Germany and the USA demanded more energy and this lead
to a rapid growth of domestic coal production in 19th century. Other nations followed suit and
coal became the dominant energy source for the industrialized nations. The discovery of oil in
Pennsylvania in 1859 signalled the start of the modern petroleum era. With new refining
methods and the introduction of the internal combustion engine, petroleum soon grew beyond
coal as the world’s most important energy source. Cars provided a mobility revolution for the
growing middle classes in the industrialized world and cheap energy boosted industrial output
significantly (Höök, 2010). In the background, oil sky-rocketed and soon outgrew all other
energy sources. One aim of this paper is to compile empirical data and present the nature of
historically occurring growth rates.
Today, over 80% of the world’s energy is derived from fossil fuels (IEA, 2010).
However, there have been a number of studies questioning the longevity of the world’s fossil
fuel endowment and suggesting that extraction would one day reach a maximum rate and begin
to decline. Science fiction author Jules Verne (1864) wrote a passage about coal fields and how
decades of accelerated consumption would lead inevitably to exhaustion unless the industrial
world devised a remedy. Later, Jevons (1866) foresaw the coming peak in British coal
extraction. Meadows et al. (1972) echoed the concerns of Malthus (1798) regarding the
interaction between mankind’s increasing exploitation of the earth and extraction of finite
resources. Hubbert (1956) has often been seen as the first to suggest that petroleum extraction
would peak. Campbell and Laherrère (1998) revisited Hubbert’s warning and later helped to
form the international Association for the Study of Peak Oil and Gas (ASPO).
Today, hundreds of scientific articles and studies have been performed regarding peak oil
and depletion of other fossil fuels. UKERC (2009) compiled over 500 studies on future oil
production and concluded that a global peak before 2030 appears likely and there is a significant
risk of peaking before 2020. Reconnecting back to growth rates, de Castro et al. (2009) found
that the attenuation of peak oil would require unconventional petroleum to grow by more than
10% annually over at least the next two decades. Proper data for global energy systems growth in
history is vital for understanding the underlying mechanisms and what to expect from the future.
In addition, current fossil fuel dependence also brings about another problem. Since fossil
fuels dominate our existing energy system, they will have to power any possible shift to
renewable or other energy sources (Moriarty and Honnery, 2009). The effect this has on fossil
fuel reserves will depend largely on the size and speed of the shift, since fossil fuel energy
required will be in addition to that already in use. Looking to the future, the growth of new
energy systems is needed to ensure the safe supply of energy for society once fossil fuels start to
become increasingly scarce and/or to minimize their negative environmental impact.
Nuclear energy and other alternative energy sources account for only a small share of
current energy supplies and understanding how fast they might grow in the future is necessary in
creating realistic outlooks and effective policies. Through the use of analogies and comparison
with the historical track record, a number of outlooks will be examined in greater detail and
scrutinized in terms of agreement with patterns of historical development.
1.1 Data considerations and statistical sources
Until the 1950s, energy systems were primarily local and their economic mechanism was local
rather than global (Häfele and Sassin, 1977). As development progressed, different national
systems became more and more integrated becoming the large, international systems seen today.
The layout of the worlds energy systems have changed significantly over time, however, there is
no clear academic definition of a 'global energy system' despite many mentions of this phrase.
Regardless, a global energy system should be something that stretches over country borders and
significantly influences the entire world’s energy supply. It may be an energy source used by
nearly all nations with suitable conditions, such as hydroelectric power or combustion of
biomass, but also something that is used by a smaller number of countries while making up a
significant part of the global energy system (i.e. nuclear power).
The world's primary energy supply in 2008 was 12 267 million tons of oil equivalents
(Mtoe) according to the IEA (2010), which is roughly the same order of magnitude as the 11 200
Mtoe of primary energy consumption reported by BP (2010). Oil, coal and natural gas dominate
with their 4073, 3312, 2588 Mtoe contributions respectively. Combustible renewable and waste
accounted for 1227 Mtoe, while nuclear energy provided 712 Mtoe with 270 Mtoe derived from
hydroelectric dams. These six energy systems should clearly be considered as global.
Other energy sources, primarily including renewable sources such as geothermal, solar,
wind or tidal energy, are commonly combined and simply denoted “others” in global statistics.
Together they accounted for 0.7% of the world's primary energy supply in 2008 or 86 Mtoe with
geothermal accounting for about 0.4% (IEA, 2007; 2010). In addition, wind power accounted for
approximately 30 Mtoe, 0.2% of the worlds primary energy supply as of 2009 (World Wind
Energy Association, 2010). Solar, tidal and other sources account for very minor shares.
For the sake of argument, this study has chosen to define the threshold for a global
energy system at an annual contribution of 100 Mtoe or more to the world's primary energy
supply. This would equal 1% of the global energy supply and may be seen as “significant”. It is
important to understand that this threshold could be set rather arbitrary at either higher or lower
values. However, setting it lower would necessitate the inclusion of many young and immature
systems where availability of global data sets could be problematic. Setting it higher would
practically exclude renewables. With the threshold at 100 Mtoe, the world has only six global
energy systems; namely oil, coal, natural gas, biomass, nuclear and hydro. A seventh energy
system, i.e. renewable energy (excluding hydro and biomass), is just below the threshold and
could be seen as on the verge of becoming a global actor.
The historical data was mainly taken from Ion (1974, 1979), Jenkins (1989), Mitchell
(2003) and BP (2010) and these different sources are generally found to be in good agreement.
World production of fossil energy was very small prior to 1800. Consequently, the time interval
to be studied is 1800 to present. Recent energy production data has been taken primarily from BP
(2010) and the IEA (2010) along with similar sources. Biomass is a significant contributor to
world energy supply, with a roughly 10% share of world primary energy (IEA, 2010), but it will
be omitted from detailed analysis since no good and reliable long-term data series are available.
The IEA data goes back to 1971.
It should also be remembered that energy production or consumption statistics commonly
only represent commercially traded amounts. Furthermore, there are problems with poor
reporting practice in some countries, and uncertainty surrounding the data is likely to increase
the further back in time we reach. Discrepancies regarding definitions can also be found between
various reporting agencies and authorities. To the best of our abilities, we have attempted to
combine the various sources and create time series as long and as reliable as possible.
2. Net energy production fundamentals
For obvious reasons, all energy sources must yield more energy than is required to extract the
energy. Otherwise, they would act as an “energy sink” and would not be able to deliver any
useful energy to users or society. In the fields of physics, energy economics and ecological
energetics these types of study are frequently performed and used for categorizing various
Every conceivable energy source requires some amount of energy to drive the energy
generation or extraction. For oil production some energy must be applied to power pumps, for
coal extraction different types of digging machines must be powered, wind power requires
energy for the production and installation of turbines before wind mills start producing energy
and so on. This can be called energy used in the supply process or simply “invested energy.”
Energy that is produced over the lifetime of an oil field, coal mine, wind mill or other type of
energy source may be seen as usable energy acquired or plainly “returned energy.” This leads to
the basic definition of energy-return-on- e n in Equation 1. investm nt (EROEI) as show
This measure, or various reformulations of it, is also known as Energy-Return-On-
Investment (EROI), energy profit ratio (EPR), or net energy gain. Furthermore, it is related to the
embodied energy (emergy) concept that is frequently used in energy economics and ecological
energetics. Cottrell (1955) and Odum (1971) were the first to point out the importance of net
energy for society. Naturally, there has been criticism of the net energy method and claims that
unclear system borders can lead to widely different conclusions. More comprehensive
discussions on EROEI-theory and how to perform calculations have been done by others
(Bullard et al., 1978; Cleveland et al., 1984; Spreng, 1988; Cleveland, 2005; Gately, 2007). The
net energy analysis methodology also has some inherent shortcomings (Learch, 1975).
The net energy of fossil fuel extraction has been shrinking in many parts of the world for
some time and this has been pointed out by several studies (Cleveland, 1992; Brown and Ulgiati,
2002; Gately, 2007). This is as a consequence of depleting the best reservoirs and coal fields.
Mankind is now forced to use less energetically favourable formations. This reinforces the
conclusions made by others that the era of easy fossil energy is coming to an end (Campbell and
Laherrere, 1998; Aleklett and Campbell, 2003; Höök and Aleklett, 2009).
The net energy from fossil fuels has proven superior to many other energy sources (Table
1). Based on this, one can conclude that other energy sources require a much greater investment
in terms of energy to provide mankind with the energy demanded. This higher energy investment
requirement also implies that greater financial investments and more physical resources must be
used to provide the same amount of useful net energy for society.
Table 1. EROEI for different energy sources. Compiled from Cleveland et al (1982), Cleveland
(2005) and Kubiszewski et al. (2009).
Energy source EROEI
Non-renewable energy sources
Oil & natural gas (wells from 1930s) >100
Oil & natural gas (wells from 2000s) 20
Coal (mines from 1950s) 100
Coal (mines from 2000s) 80
Nuclear power 9.1
Synthetic fuels from coal 0.5–8.2
Renewable energy sources
Solar heating 1.9
Wind power 19.8
* average values. See Kubiszewski et al. (2009) for closer discussion.
There is no perfect way to measure EROEI and there are major differences between
various studies. However, the net energy obtained from an energy source is important, as it is
only this amount that becomes available for society to use in non-energy-related activities such
as powering hospitals and agricultural machines or transporting goods and extracting non-energy
minerals. Ultimately, one may also see required energy investment as the underlying factor —
triumphant over all others. Hubbert (1982) wrote that: “there is a different and more fundamental
cost that is independent of the monetary price. That is the energy cost of exploration and
production. So long as oil is used as a source of energy, when the energy cost of recovering a
barrel of oil becomes greater than the energy content of the oil, production will cease no matter
what the monetary price may be.”
3. Historical development of global energy systems
To understand the growth of global energy systems, one can begin by studying the exceptional
expansion of fossil energy in the last 200 years (Figure 1). The development of society,
population, and the world economy during this brief period of time has been remarkable. Nearly
all of this growth has been powered or driven by fossil energy in its various forms. The
development of society surged and mankind both prospered and multiplied, all thanks to cheap
and abundant fossil fuels that provided useful net energy for a multitude of activities. In fact,
Hubbert (1974) described this as a “transition from steady state to transient state due to fossil
fuels.” The rapid growth of fossil energy output coincides with the green revolution and its use
of high yield agricultural techniques based on synthetic fertilizers and high energy use. In fact,
fossil energy has been described as the principal raw material of modern agriculture (Pimentel et
al., 1973; Green, 1978; Pfieffer, 2006; Pimentel, 2007; Johansson et al., 2010).
The phenomenal economic boom that occurred after the Second World War goes hand in
hand with the increased use of fossil energy. This is hardly surprising as physical theory shows
that energy is needed for economic production (Stern, 2004). Economic growth and development
is intimately linked to energy utilization. Akinlo (2002) found that energy consumption has a
significant positive long run impact on economic growth, while others found connections
between energy and real output (Hondroyiannis et al., 2002; Payne, 2010a, 2010b).
Figure 1. Historical production of fossil energy from 1800 to 2010, divided into coal, oil and
gas. The rapid expansion of oil and natural gas during the 20th century is remarkable. A
stagnation period can be seen in the time period between the two world wars before the oil boom
It is obviously necessary to maintain an eye on absolute terms and not stare blindly at
relative growth rates when planning for the future. High growth rates do not necessarily imply
that large amounts of energy being delivered to society. For example, the average growth rates in
petroleum production in the 1950s (9.9%) look spectacular when compared to the average
growth rate in 1970s (3.5%), but does not reflect the fact that world petroleum output was
roughly 4 times smaller than in the 1950s (cumulative output 7358 Mtoe) compared to the 1970s
(28 345 Mtoe). If weighed against absolute energy output, the growth rates of the 1950s are
approximately worth only 25% of the growth rates of the 1970s. Furthermore, Prieto (2008)
pointed out that world fossil energy consumption increased 23 times faster than the expansion of
wind power in terms of produced energy, despite the impressive growth rates of 20% for wind
3.1 Oil, gas and coal
The historical changes in the growth rate of fossil energy production are shown in Figure 2.
Extremely high growth rates of 20-30% occurred in the 19th century when the fossil energy
industry was young and underwent a pioneering era of booming expansion. Naturally, the two
World Wars and the Great Depression of the 1930s reduced growth for some time.
Figure 2. Changes in growth rate of annual fossil energy production from 1800-2010. A trend
towards decreasing growth rate with decreasing size is clear, although there are smaller
Figure 3. Changes in growth rate of oil energy from 1870-2010. The oil boom occurred at
annual growth rates of less than 10% on average before slowing down after the oil crises.
The post-war economic boom saw a growth in fossil energy production of over 5%
annually but this decreased and flattened out to only 2% by the 1980s. Despite technological
breakthroughs in petroleum exploration and extraction, coal mining and similar disciplines, this
rate of growth has diminished. However, this is as expected and reflects an increasing scarcity of
attractive hydrocarbon reservoirs, an increase in the challenge to continued expansion, as well as
a social disinclination to increasing dependence on fossil fuels.
The most phenomenal feature of human utilization of fossil fuels is the rapid expansion
of oil production and consumption, often known as the oil boom. The cheap and easy oil of the
Middle East, Texas, California and many other places all over the world flowed to markets and
consumption rocketed. Petroleum energy output grew from virtually nothing in 1870 to nearly
3000 Mtoe in the 1970s. However, annual growth rates were only around 7% during most of that
period (Figure 3).
Similarly to oil, it is possible to calculate the growth rate behavior of coal and natural gas
(Figure 4 and 5). Despite being abundant and subjected to strong demand, the growth rates have
seldom been higher than 5-10%. Coal has the steadiest growth rate of all fossil fuels but that can
largely be explained by how rapidly increasing coal extraction in Asia managed to offset or fully
compensate declining output in other parts of the world over recent decades. Furthermore, coal is
also a largely a local or regional fuel where most consumption occurs in the same country as it
was produced. In summary, none of the fossil fuels have been able to grow much faster than at
most 10% over any protracted time period. Not even the oil boom and the dramatic rise in global
oil use corresponds to sustained growth rates higher than 7%.
Figure 4. Changes in growth rate of natural gas energy from 1890-2010. Growth rates of over
5% could only be sustained at low energy outputs and later decreased as the global gas system
grew larger and larger.
Figure 5. Changes in growth rate of coal energy from 1860-2010. Sustained growth rates of
over 5% have rarely occurred in the history, despite an increasing coal energy output.
3.2 Biomass, nuclear and hydropower
Global energy production statistics for biomass, combustible solids and waste only go back to
1971 (IEA, 2010). In this time interval, the global growth rate has varied from 0.5% to almost
3% while average growth rates were around 1.9%. It can be argued that a longer and more
comprehensive data set for global bio energy output should be used, but unfortunately no such
dataset was found.
Growth rate studies can also be performed on hydropower and nuclear energy time series
(Figure 6 and 7). Once more, a decreasing growth rate with increasing size can be seen. This
results from saturation caused by lack of undeveloped rivers, economic constraints, social
acceptance and many other causes. In other words, this reflects the increased challenges of
continued expansion. Interestingly enough, hydropower and nuclear energy have grown at a
generally slower rate than fossil energy. The growth rate of fossil fuel averaged well over 5% up
to energy output levels of 1000 Mtoe per year — a rate that neither nuclear nor hydropower have
Figure 6. Changes in growth rate of global hydropower generation from 1900-2010.
Figure 7. Changes in growth rates for global nuclear energy production from 1965-2010. The
growth rate significantly diminished after 1985 due to social rejection connected to Chernobyl,
proliferation and security questions.
In summary, the six global energy systems (oil, gas, coal, biomass, hydropower and
nuclear) covered in this study account for over 95% of the world's primary energy (IEA, 2010).
Furthermore, they have all shown a similar growth rate behaviour over time with the growth rate
decreasing as energy output increased. Thus, generic growth behaviour for global energy systems
can be established. Finally, the empirical growth rates obtained also reinforce the conclusion that
energy systems with high net energy yield tend to grow faster than energy systems with
relatively lower net energy gain.
3.3 EROEI and growth rates
From the historical data sets it appears as if energy sources with high EROEI values have been
able to grow faster than those with lower EROEI values (Table 1, Figure 3–7). One should also
be aware that there are political and socio-economic aspects involved in the growth process.
However, the development of global energy systems studied took place over a long period, in
many places with widely different economic and political conditions. It is reasonable to assume
that these differences even out on the larger global scale. In summary, empirical studies of
historical data support the concept that high EROEI-energy sources have been able to grow faster
than those with lower net energy yield.
Why did this happen? In many ways, fossil fuels were a dream given form — highly
concentrated energy sources enclosed in vast subsurface storage where they could be easily
extracted and utilized by a needy society. Oil has flowed up from the subsurface reservoirs for as
little as 1 dollar/barrel in the most extreme cases (Al-Malood, 2004), making it a virtually free
source of energy. Naturally, there are some exceptions but the general picture is clear, especially
with regard to older oil and gas fields such as the gigantic fields of the Middle East. The high net
energy yield combined with the low production costs simply made fossil energy the most
attractive fuel for consumers. It would simply not have made any sense to invest in the
development of less optimal energy sources and this can qualitatively explain why many non-
fossil energy systems have grown less during the time periods examined.
Fossil fuels hold an energetic advantage over many other energy sources (Table 1). In
essence, they are already concentrated and accumulated by nature while many alternative energy
sources must first be concentrated by mankind to provide useful energy. It should also be
remembered that some of the technologies presented in Table 1 are at the low end of the learning
curve. Future technological breakthroughs may improve their EROEI significantly, allowing
them to be more feasible substitutes for fossil fuels (Grübler et al., 1999). However, the “belief”
in major technological advances in alternative energy can sometimes be very one-sided,
especially if similar technological advances are not perceived as possible for fossil energy and it
is wise to avoid internal contradictions with assumed technological breakthroughs for some
energy sources, while using a status quo view for others (Odell, 1999). We can only believe that
“technological surprises” or “silver bullets” that can dramatically alter the general picture shown
in Table 1 are unlikely.
Economic conditions in the past have also largely favoured fossil fuels and together these
two factors may explain why the most extreme growth rates have been found for fossil energy.
Proposed carbon taxes and rising extraction costs due to depletion may very well significantly
decrease the overall competitiveness of fossil fuels. In a similar way, various subsidies and other
support mechanisms may help to make non-fossil energy sources more favourable, but whether
this is enough to compensate for the EREOI-disadvantage is yet to be determined. In conclusion,
we can only assume that the growth patterns seen for fossil energy appears reasonable for long-
term projections of future world energy system developments.
4. Growth rate analysis
Analytically, growth may be mathematically defined as the change from one point in time to the
next. If the growth rate at time is denoted and the energy output by an arbitrary global
energy system at time is denoted , one obtains the basic expression in Equation (2):
Using a difference approximation o eri ws rewriting as follows: f the d vative allo
Consequently, growth rate analysis does not mathematically add any new information
when compared to studying energy production directly. However, it is sometimes a useful way to
alternatively present information. Especially when dealing with sustainable development
patterns. Transforming the expression in Equation 3 by introducing y = f(t) yields:
Thus, growth is mathematically proportional to the inverse of the global energy systems
output. Such facts were also highlighted by Bartlett (1993; 1999; 2004), who studied the
arithmetic of growth. A 7% annual growth implies a doubling in 10 years and in just ten
doubling periods the output will be more than 1000 times larger. Furthermore, the growth in any
doubling period is greater than the total of all preceding growth. Taking oil as an example, the
energy output by 1970 was nearly 2000 times larger than in 1870. From pure arithmetic, the
practical challenges with sustaining growth for an extended period of time become rather
significant. In addition, can scaling laws be found that can help analysts to understand how
global energy systems behave?
Similar scaling behaviour is already widely known from many fields. The basic concept
is that a rescaling of the arguments shifts the relationship up or down on a numerical level by
some proportionality relationship, but leaves the basic shape unchanged, i.e. the basic shape is
scale invariant. Scaling behaviour is common in many systems and a powerful tool for deeper
understanding due to their universality and tractability. Identifying the underlying relationships
makes it possible to study how rescaling affects, and what will happen over, shifts in magnitude.
Studies have performed detailed analysis on corporate growth and found scale behaviour
over more than 7 orders of magnitude, regardless of what type of goods the company
manufactures (Stanley et al., 1996; Amaral et al., 1997, Buldyrev et al., 1997; Amaral et al.,
2001; Axtell, 2001; Fu et al., 2005). The discovery of this scaling behaviour in economic and
financial systems has shed new light on economics and has led to the establishment of a new
scientific field that bridges economics and physics (Kaizoji, 2003). Most energy production is
done for commercial purposes and it is only reasonable that similar scaling laws, as found in the
economy, are also present in global energy systems.
In the fields of ecology and the study of biological systems, scaling behaviour and power
laws are also present. Allometric scaling laws are particularly well-studied. A ¾-power law of
metabolic rate as a function of body mass is observed over 27 orders of magnitude (West et al.,
2002) and was closely analyzed by others (Farrell-Grey and Gotelli, 2005). Robust scaling
behaviour has also been found in ecology and agriculture (Taylor et al, 1988) as well as in
species-area relations (Martin and Goldenfeldt, 2006) and in couplings between plants, water,
energy, and terrain (Milne et al, 2002). Furthermore, West and Brown (2004) have described
life’s universal scaling laws and argue that properties of biological networks allow for a
quantitative theory of growth in any living systems. Human society is also a living system and its
energy resource exploitation may be seen as an equivalent to the metabolic processes studied in
biological systems or in ecology. Consequently, there are also reasons to expect scaling
behaviour from this perspective.
Power law scaling is the universal property that characterizes collective phenomena that
emerge from complex systems composed of many interacting units (Kaizoji, 2003). In addition,
power law correlations have been found in a multitude of other systems, ranging from base-pair
correlations in DNA, lung inflation and interbeat intervals of the human heart to complex
systems involving interacting subunits that display “free will” such as city growth and economics
(Stanley, 1995). Statistical physics are filled with examples of scaling phenomena and how
widely different systems can be characterized by the same fundamental laws, independent of
“microscopic details” (Stanley et al., 1996; Clauset et al., 2009).
4.1 Empirical considerations
For practical reasons, growth rates cannot be sustained forever without resulting in absurdities.
One of the first analysts to highlight this was William Stanley Jevons (1866) who used historical
coal production data and showed that British coal extraction had grown at a relatively consistent
rate of 3.5% per year. By extrapolating that growth trend and combining it with estimates of
available coal reserves he reached the conclusion that the growth would have to decline. At some
point, production would also simply hit a peak due to the inherent finiteness of coal. Jevons
(1866) stated the following: “In the increasing depth and difficulty of coal mining we shall meet
that vague, but inevitable boundary that will stop our progress.” Increasing depth and difficulty
as the easy reserves are mined out can be one of the practical challenges associated with
sustaining growth. Such models where size, and therefore corresponding growth rates, is time
dependent on some upper limit has resulted in a wide number of growth curve models used in
descriptive or predictive energy system analysis (Höök et al., 2011).
Six global energy systems are available for study to investigate the presence of scaling
behaviour. However, quantifying and analyzing the behaviour is complicated by socio-economic
factors such as recessions and depressions, two world wars, oil crises and similar. Handling such
events is hard and can easily lead to arbitrary results depending on what is included. Simply
plotting growth rates against energy output levels indicates an underlying relationship, although
the exact nature is less clear (Figure 8 and 9). Both linear and power relations can be established
between growth rates and energy output levels.
Figure 8. Both linear and power relationships are reasonable descriptions for the relation
between growth rate and energy output for coal.
Figure 9. Linear and power fits are also reasonable descriptions for hydropower growth rates
related to energy output.
Medium to large negative linear correlations between growth rate and energy output for
all energy systems are found. For fossil energy, the linear correlation coefficient is calculated to
be -0.51 for coal, -0.62 for oil, -0.57 for natural gas and -0.44 for all fossil energy. The
correlation for hydropower and nuclear becomes -0.68 respectively -0.94. Power relations appear
to be plausible for describing the observed patterns. For those interested in obtaining better
quantitative relationships, the methodology expressed by Clauset et al. (2009) can be used.
Why does history show these patterns? One reason is that the development of energy
systems is often constrained in some way, where bounded or bell-shaped growth is the only
realistic pattern on a longer timescale (Höök et al., 2011). Such behaviour can be mathematically
described by many different curves, such as S-shaped, Hubbert-type or more arbitrary
generalized Richards (Richards, 1959) or Bass models (Bass, 1969). Since such models describe
annual or cumulative energy output as a function of time, elementary mathematics of growth
(Equations 2–4) will automatically yield a corresponding time-dependent relation for the growth
rate. Analytical growth rate expressions for selected models can be found in Höök et al., (2011).
Therefore, the exact nature of a quantitative growth-size relation is no more meaningful than
attempting to formulate a model for energy system size as a function of time. No single model
gives a good description of the size–time relationship for all the present global energy systems
due to their complexity. Consequently, no good and general relationship for growth–time can be
expected to be found. As a result, the qualitative behaviour may be seen as the most important
thing to bear in mind.
Historical growth patterns are more or less inversely proportional to the energy output
and it is empirically concluded that scaling behaviour is present for global energy systems. This
is just as expected from the analysis, but also agrees with the findings in economic, ecological,
and many other systems. Such behaviour has been observed for all currently used global energy
systems. Growth rates for nuclear and hydropower show similar behaviour as for fossil fuels,
despite fundamental differences in technology. This may be seen as an indication of the
universality of the scaling behaviour and that similar development patterns may be expected for
other global energy systems. Consequently, it may also be argued that future global energy
systems will follow a path reasonably similar to historical experience.
In summary, empirical data reinforces the notion that growth of the global energy system
will become more challenging to sustain with increasing size. The existence of scaling behaviour
also implies that the rapid growth rates of small energy systems (such as wind) will decrease
with increasing size of the system, just as seen for all global energy systems covered in this
study. This also establishes a base for a “forecasting by analogy”-approach to depict how new
energy systems might develop.
5. Assumed growth patterns in other studies
Many studies have attempted to make long-term projections of global energy supply and/or
consumption in the future. Some examples are the IPCC Special Report on Emission Scenarios
(SRES, 2000), World Energy Technology Outlook (WETO, 2006), World Energy Outlook (IEA,
2008) and many others. In addition, a number of academic forecasts are also available, including
the renowned outlooks made by Hubbert (1956) and Meadows et al (1972).
There are always reasons to question underlying assumptions in certain long-term
outlooks based on available resources, assumed economic conditions, technological development
and future political climate. Incorrect assumptions have made many outlooks flawed. Hubbert
(1956) made a correct prediction for peak in US oil production, but his world outlooks turned out
to be in severe disagreement with reality. Despite accusations of pessimistic assumptions, the
“standard run”-scenario from Meadows et al. (1972) actually agrees well with the recent 30
years of data (Turner, 2008). The IEA World Energy Outlook 2008 has been shown to contain
unrealistic depletion rates as well as other inconsistencies regarding future oil production
(Aleklett et al, 2010). The IPCC emission scenarios have also been criticized for unrealistic
expectations of future fossil energy production growth (Höök et al., 2010).
The arithmetic of growth (Bartlett, 1993, 1999, 2004) and the apparent universality of
global energy system growth behaviour justify comparisons between empirical data and some of
these future outlooks. A selected number of long-term outlooks are now inspected from a growth
rate perspective to see whether they are consistent with theoretical and empirical data by chiefly
using history as a comparator.
5.1 Example 1, Shell Energy Scenarios
Firstly, the energy scenarios to 2050 from Shell (2008) are chosen for closer inspection. The two
scenarios in Shell (2008) are called Scramble, a reference case with a business-as-usual aura, and
Blueprints, which emphasizes energy security and planned local initiatives. The combined
energy output from fossil fuel reaches a peak around 2030-2040, while the contribution from
biomass and nuclear are more or less constant. Most new energy output is expected from the
renewable category, including solar, wind and other renewables (excluding biomass), in both
scenarios. Decreasing growth rate with increasing energy output is present in both scenarios and
this is consistent with theory and historical data (Figure 10–11).
Figure 10. Changes in growth rate of global renewable energy production (excluding biomass)
from 2000-2050 in the Scramble scenario from Shell (2008).
Figure 11. Changes in growth rate of global renewable energy production (excluding biomass)
from 2000-2050 in the Blueprints scenario from Shell (2008).
Shell (2008) depicts a boom for renewable energy, more or less equal to the previous oil
boom, in both the reference scenario Scramble (Figure 10) and the alternative outlook Blueprints
(Figure 11). Shell (2008) also wishes for a 21st century dawning of an energy revolution and yet
explicitly states that there are no “silver bullets” for future energy challenges. Despite
significantly lower EROEI (Table 1), renewable energy is expected to grow more or less as fast
as fossil fuel use grew. Using the most extreme growth example in history, both scenarios appear
5.2 Example 2, Scenarios focused on meeting climate policy
The second example is taken from a family of scenarios aimed at meeting various CO2-
stabilization goals due to climate policy. Such scenarios are often based on complex energy-
economy models with numerous assumptions, parameters and internal feedbacks. The
framework for the outlooks studied was first presented by Azar et al. (2003), and later updated
(Azar et al., 2006). Similar projections have been made by van Ruijven et al (2007) or Calvin et
The 550 ppm scenario with carbon capture and storage (CCS) allowed from Azar et al.
(2006) also shows solar hydrogen growing from virtually nothing to 3500 Mtoe in 30 years
(Figure 12). Figure 13 displays the 350 ppm scenario without CCS from Azar et al. (2006) and
depicts how solar hydrogen, starting at an insignificant output in 2010, takes over 40% of the
global primary energy supply with its 4800 Mtoe output by 2050. In both cases, the inverse
proportionality between growth rate and size is consistently included. However, the magnitude of
the growth rate is something completely different.
Figure 12. Annual growth rates and energy output of solar hydrogen in the 550 ppm scenario
from Azar et al. (2006). The extreme growth rate surpasses anything in history with comparable
The corresponding growth rates here are extremely high and simply unmatched by any
historical experience (Figure 2 to 7). It might certainly be technically possible to achieve such
vast volumes of solar hydrogen but it would likely require a development unlike anything seen in
world history. Such extreme growth rates can possibly be justified by something similar to a
Manhattan project with massive public and governmental support. However, though such growth
rates are not outside the realm of extreme possibilities, they can hardly be seen as a reasonable
assumption in any form of business-as-usual sense. Consequently, such scenarios should be seen
as rather extreme or even dubious.
Figure 13. Annual growth rates and energy output of solar hydrogen in the 350 ppm scenario
without CCS from Azar et al. (2006). The growth rate is 2-3 times higher than the oil boom and
can only be seen as very optimistic compared to historical experience.
6. Future outlooks based on analogies
Global energy shifts have occurred throughout history, some by choice and others by necessity.
New global energy shifts will surely happen. The existence of a deus ex machina or some
revolutionary breakthrough that easily catapults society beyond oil or possibly even beyond
carbon cannot be excluded. On the other hand, such marvellous events should not be seen as
likely either. Exploration and innovation takes time and the implementation of new technologies
requires even more time. The slow technology shift in post civil war Southern USA may be seen
as a cautionary tale of how long massive reformation of society may take (Friedrichs, 2010).
Fossil fuels presently dominate the existing global energy system and are required to
power any possible shift to renewable or other energy sources (Moriarty and Honnery, 2009).
With decreasing supply of cheap oil and fossil energy it may even be likely that the world
economy will be hampered and that future global energy shifts will occur at lower growth rates
than seen in history. This further strengthens the use of historical growth experiences as
reasonable tools for understanding how future development may turn out.
The important factor is how fast global energy transitions can occur. Coal never grew
much faster than 5% annually, while oil seems to be the most extreme case with its 7% annual
growth rate for a century. Both Smil (2000) and Bezdek and Wendling (2002) note how
forecasters often have made vast overestimations of the potential of new energy technologies.
Therefore, applying extremely high growth rates on large scales should be seen as rather
questionable and requiring extraordinary justification.
Possible outlooks for future energy supply can be explored by using analogies. The
growth rate pattern for oil up to the 1970s (Figure 3) can be regarded as one of the most extreme
events in history, as it outperforms the growth of every other energy system of equal scale. Using
this extreme event as a development analogy for future energy transitions will naturally provide
an optimistic outlook by empirical standards. From history, one knows the kind of growth rates
that have occurred and what that may be seen as reasonable for future global energy shifts.
The “oil boom”-scenario features a sustained 7% annual growth rate for an arbitrary
global energy system, which basically implies a doubling of energy output every 10 years. No
apparent upper limit for the energy output must necessarily be assumed, in contrast to the
geological limitations imposed by nature on fossil fuels. Applying the “oil boom”-scenario to
wind power, presently at a level below 100 Mtoe (IEA, 2007), gives a future energy output of
roughly 500 Mtoe in 2050 and approximately as much as the present primary energy production
of the world at around 12 000 Mtoe in 2100 (Figure 12). Similar results would also be obtained
for solar power, solar hydrogen and other renewable energy sources.
Even if a new energy system undergoes a rapid “oil boom”-development, its contribution
will be marginal for global energy supply in 2050. Their present contribution is simply too small
to allow for significant contributions in the foreseeable future under growth scenarios equivalent
to historical growth of other global energy systems. The growth rates must be much higher than
anything seen in history to allow the new energy system to reach significant shares of the world
energy supply in the foreseeable future.
Future energy supply will be largely determined by what happens with fossil fuels, given
their present dominance. For example, annual decline in existing oil production has been
determined to be 4-8% (Höök et al., 2009), which is equal to a lost production capacity of 3–7
million barrels per day. Offsetting such massive volumes by unconventional oil (i.e. tar sands,
deep sea oil, polar oil, etc.) requires sustained growth rates of more than 10% over the next two
decades (de Castro et al, 2009), again, questionable. World ethanol production was at 1.27
million barrels per day in 2009 (RFE, 2010). Offsetting a single year’s annual decline in existing
oil production would correspondingly require world ethanol output to grow by approximately
300-600%, which is clearly not feasible. Offsetting even small amounts (in percentage terms) of
fossil energy would require absurdly large growth for any alternative energy source.
Alternative energy sources cannot make significant contributions prior to 2050 even if
they are developed as fast as the most extreme historical example. It simply takes time to
develop and construct new global energy systems capable of delivering significant amounts of
energy to society’s benefit. In other words, the development and expansion of alternative energy
sources has started too late to produce even a significant contribution over the next couple of
decades — much less dominating the energy generation scene. Dominance of renewable or
alternative energy in the foreseeable future can only be achieved by growth rates much higher
than ever seen before in history.
Figure 14. Energy output and growth rate of wind power in a future “oil boom”-scenario. In
2050, the energy output would only be equivalent to roughly 5% of the present world energy
Fossil fuel utilization soared in the wake of industrialization, and especially during the oil boom.
Even with their superior energy density and favourable economics, fossil fuel output has
generally grown by less than 10% per year with a decreasing growth rate as energy output
increases (Figure 2, 3, 4, 5 and 6). Not even the oil boom featured a sustained growth rate of
more than 7% for a period longer than a few decades and those growth rates occurred at energy
output levels below 3000 Mtoe (Figure 3). The growth rates future global energy systems might
experience is hard to predict accurately, but it is at least possible to find sensible values based on
historical experience. Naturally, the future can differ greatly from the past, but this should be
seen as a dramatic trend break in need of proper justification.
Based on both theoretical and empirical data, we find it unrealistic to expect that future
energy system growth patterns will be widely different from the past. It was also found that
certain future scenarios depict rapid development of new energy sources with growth rates
widely surpassing even the most extreme cases seen in history. Such spectacular growth rates
may occur, but they mostly appear to be wishful thinking. They strike us as computerized output
needed to fulfil a demand-driven model or to justify visions of a futuristic and post-fossil society
with a similar level of energy consumption as today. In essence, such outlooks should be
regarded as dubious and in need of much more justification to be regarded as realistic.
Ion (1975) warned of experts unable to check whether their computerized conclusions
made sense and Rotty (1979) stated that one should be able to make a more accurate analysis
than simply projecting continued exponential growth in attempting to estimate the energy
demands of the future. Smil (2000) summarizes how long range energy forecasters have missed
virtually every important shift of the past decades, how unexpected and dramatic changes or
deviation from trends suddenly occurred, and how they vastly overestimated the potential of new
energy and conversion technologies. Bezdek and Wendling (2002) also provides a long
compilations of errors, most notably many overestimations, and some valuable lessons from
long-range energy forecasts. In truth, many inaccurate forecasts were done in good faith with
state-of-the-art models, competent researchers and good funding, which shows how difficult
long-range energy forecasting is. Sometimes, it is insightful to fall back to empirical data and
study what a forecast or scenario would correspond to in real terms, using historical experience
The future will undeniable hold new global energy shifts, but the important factor is how
fast those transitions can occur. All historical experience shows that decades-long sustained
annual growth rates never exceeded just a few percent. Oil seems to be the most extreme case
with its 7% annual growth rate lasting a century. Therefore, extremely high growth rates on large
scales should be seen as rather questionable and in need of extraordinary justification. Based on
historical analogies, even rapid “oil boom”-development of new energy systems would not be
enough to dramatically change the global energy supply prior to 2050.
It is wise to regard promises of rapid growth and virtually endless energy supplies from
new energy sources with solid scepticism. The future may not be as bleak as some believe, but
on the other hand, it might not be as rosy as others proclaim either. In essence, one can only
agree with Schlesinger and Hirsch (2009) that some serious realism is needed in energy
planning. Is it reasonable and good judgement to take the most extreme historical event (the oil-
boom) as a reference case for the future? Should energy planning be based on the belief that new
energy sources can grow faster than anything ever seen before in history? Energy is essential for
a multitude of sectors and activities within society. Strategic and long-range planning must be
built on proper understanding of energy system growth, especially in the light of peak oil and the
challenges it brings (Campbell and Laherrere, 1998; Aleklett et al., 2010).
We would like to thank Sergey Yachenkov at the Kurchatov Institute in Moscow for constructive
discussions. Professor Al Bartlett has our appreciation for being an important source of
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