Introduction
Human society has become a planetary force, approaching or even exceeding
natural dynamics . A great deal
of work has been devoted to measuring the scale of human society with respect
to the Earth system , especially after the
introduction of new concepts such as the “great acceleration”
, the Anthropocene or
“planetary boundaries” . Many studies assessing
the human impact on the Earth system focus on rates of change in a multitude
of parameters . Others define a natural background
against which the human impact should be measured, notably the Holocene epoch
, during which the climate was unusually stable
(and other environmental variables are argued to have been stable) compared
to the preceding Pleistocene epoch with its characteristic glacial cycles
. Suggested metrics of human impact on the Earth
system include changes in land use ,
bio-productive land capacity , human
appropriation of terrestrial net primary production
or
the impact of human appropriation of free energy on the capability of the
biosphere to generate free energy .
Here we propose an alternative approach to measure the human influence
against a natural background, following pioneering work by
, who first compared energy use in the biosphere and
in human civilization (where “biosphere” is taken here to be synonymous
with the biota, i.e. the sum total of all life on the planet). Our starting
point is the fundamental ability of all life forms, from archaea and bacteria
to human societies, to capture free energy and to use it for moving and
transforming matter in order to sustain an internal order. Building on Smil's
characterization of energy use in
the biosphere and human civilization, we expand the temporal dimension to
consider the full history of transitions in biospheric energy capture, and we
add a material cycling dimension, also partly inspired by Smil's work
. In both Earth and human history major revolutions
in energy capture have occurred, with each subsequent transition resulting in
higher energy input, altered material cycles and major consequences for the
internal organization of the respective systems.
In general, when a new biological mechanism of accessing under-utilized
resources evolves, this can lead to profound environmental change – as shown
by generic models capturing the co-evolution of life and its environment
. Indeed, in Earth history as new metabolic
waste products were created or the production of existing waste products was
scaled up, these waste products accumulated in the environment
. When step increases in free energy input to
the biosphere occurred, the environmental consequences were sometimes
dramatic and global – destabilizing nutrient and carbon cycles and the
Earth's climate . When past increases in free
energy input to human societies occurred, the resulting waste products also
disrupted the environment – initially on a local scale, but now globally.
Here we compare the order of magnitude of energy use by human societies with
the energy input to the entire biosphere throughout Earth and human history
based on a common framework. A clear distinction to note at the outset is
that the input of energy to the biosphere has thus far been dominated by
autotrophs harvesting sunlight, whereas humans are heterotrophs and our
current industrial consumption of fossil fuels is also essentially
heterotrophic.
We consider a series of six past revolutions, three in Earth history and
three in human history, each contingent on the previous one(s). In Earth
history, we focus on the origins of anoxygenic photosynthesis, of oxygenic
photosynthesis, and of eukaryotic photosynthesis, especially the colonization
of the land by plants. In human history we consider the Palaeolithic use of
fire, the Neolithic revolution to farming, and the Industrial revolution. In
each case we try to quantify the resulting increase in energy input to the
biosphere or to human societies, and discuss the consequences for material
cycling. Changes in energy input and material cycling in turn altered
limiting conditions for biological and cultural evolution and we highlight
some of the crucial biological and social consequences. We discuss
similarities and crucial differences among the six energy revolutions, their
underlying regulatory mechanisms and their impacts. For most of human
history, energy use by humans was but a tiny fraction of the overall energy
input to the biosphere, as would be expected for any heterotrophic species.
All major increases in energy input to human societies were contingent on new
technologies that shifted human energy and material use beyond the limits of
their biological metabolism. We show that the capacity of humans to push
energy inputs towards planetary scales only emerged with the industrial
revolution and that by the end of the 20th century human energy use reached a
magnitude comparable to the biosphere.
After revolutions in Earth history, long-term sustainability and stability
were only recovered when disrupted material cycles were closed again, through
global biogeochemical recycling mechanisms .
Equally, for humans to have a long-term sustainable future within the Earth
system will require both a shift to sustainable sources of energy and,
crucially, the closure of material cycles – amounting to a more autotrophic social metabolism.
We finish by advocating a research agenda that considers pathways towards a
renewable and decarbonized energy system in its ramifications for material
use and a prospective material cycle revolution.
Revolutions in Earth history
Anoxygenic photosynthesis
The first revolution in energy input to the biosphere was the origin of
photosynthesis. The earliest life forms were probably fuelled by chemical
energy stored in compounds in their environment, but the supplies would have
been small, except in unusual environments with concentrated
volcanic/metamorphic activity such as deep sea vents near mid-ocean ridges
(if plate tectonics started early on the Earth). Shortage of chemical energy
on a global scale would thus have severely restricted the spread of
chemolithoautotrophic life. The first truly global biosphere arose when early
life began to harness the most abundant energy source on the planet –
sunlight. Evidence for the photosynthetic fixation of carbon dioxide from the
atmosphere is coincident with the first putative evidence for life on Earth
> 3.7 Ga ,
and perhaps as early as 4.1 Ga . It takes the
form of small particles of graphite carbon, which have a likely biogenic
origin, and an isotopic signature consistent with carbon-fixation by the
enzyme RuBisCO.
The first photosynthesis was not the familiar kind, which uses water as an
electron donor and produces oxygen as a waste product. Instead, molecular
phylogenies suggest that several forms of anoxygenic photosynthesis evolved
independently, early in the history of life, long before oxygenic
photosynthesis . This makes energetic sense as
there are several donor compounds from which it is easier to extract
electrons than water, requiring fewer or less energetic photons and simpler
photosynthetic machinery. Hydrogen gas (H2) gives up its electrons the
easiest and may thus have fuelled the first photosynthesis
. Other potential electron donors include
elemental sulphur (S0) derived from sulphur dioxide (SO2) gas, or
ferrous iron (Fe2+) dissolved in the ancient oceans
. The meagre supply of these compounds (relative
to H2O) limited the energy input to the early biosphere. For example, the
present-day flux of H2 emanating from volcanic processes can only support
∼0.1 EJ yr-1 (3 TgC yr-1) of anoxygenic photosynthetic net
primary production (NPP) , over 4 orders of
magnitude less than present marine biosphere (1800 EJ yr-1 or
48 PgC yr-1).
The challenge for the first photosynthetic biosphere would thus have been to
evolve the means of recycling the scarce materials that it needed to
metabolize, especially the electron donors for photosynthesis. The ease or
difficulty of evolving recycling has been examined theoretically by
simulating “virtual worlds” seeded with “artificial life” forms and
leaving the resulting ecosystems to evolve . In
these simulations, the closing of material recycling loops robustly emerges
, even if they incur an energetic fitness cost
. The empirical record of how and when recycling
emerged in the early Earth system is sparse, but there is some evidence for
biogenic methane production by 3.5 Ga . This would
have recycled hydrogen (and carbon) back to the atmosphere. If the early
biosphere was fuelled by anoxygenic photosynthesis based on H2, then
recycling of hydrogen via methane production and photolysis could have
boosted global NPP to 1.8 EJ yr-1 (48 TgC yr-1) or 0.1 % of
the modern marine biosphere . If volcanic activity
on the early Earth was elevated by an order of magnitude, a hydrogen-fuelled
biosphere might have approached 1 % of modern marine NPP
. Alternatively, if early anoxygenic
photosynthesis used the supply of reduced iron upwelling in the ocean then
its NPP, controlled by ocean circulation, might have reached
77–225 EJ yr-1 (2–6 PgC yr-1) or ∼10 % of modern
marine NPP
(Fig. ). A potential constraint on early biosphere productivity is
provided by the carbon isotope record of marine carbonate rocks, which is
conventionally interpreted as indicating that the proportion of carbon buried
in organic form (rather than inorganic carbonates) was around 20 % even as
early as 3.5 Ga. Given greater inputs of carbon from the mantle on the early
Earth, this would imply a marine organic carbon burial flux in excess of the
present 60 TgC yr-1, setting a lower limit on NPP at the time
(assuming no heterotrophic recycling, i.e. all organic carbon produced was
buried). This would likely preclude H2-based photosynthesis as the
dominant source of carbon 3.5 Ga onwards, suggesting instead an iron-fuelled
(or even oxygenic) biosphere. However, a more nuanced interpretation of the
carbon isotope record allows for the possibility that little organic carbon
was buried for large parts of the Archean Eon
.
Energy capture in
the biosphere and human society. Dates indicate beginning of the respective
revolution, energy estimates are given for dates where energy regimes had
matured. Data and sources are in Table S1.
The waste products of early metabolisms would have altered the environment.
The long-term burial of organic carbon, even if it was a small flux, would
have removed carbon from the atmosphere (and ocean) tending to cool the
planet. This cooling effect could have been profound given that today 15 ZgC
are stored as organic carbon in sedimentary rocks, compared to 38 EgC in the
ocean–atmosphere system. Somewhat counterbalancing the net removal of carbon
to the crust, the conversion of atmospheric CO2 to methane would have
increased radiative forcing, tending to warm the planet. As a crustal
reservoir of reduced carbon accumulated in sedimentary rocks, some organic
carbon would later be exposed on the continents, potentially supporting
heterotrophic productivity there. Relatively low estimates of global
productivity make it unlikely that the macro-nutrients nitrogen and
phosphorus became limiting, making them under-tapped resources in the ocean
environment.
Oxygenic photosynthesis
The next major revolution in energy input to the biosphere was the origin of
oxygenic photosynthesis, using water as an electron donor
. To split water requires more energy (i.e.
more high energy photons of sunlight) to be captured than in any of the
earlier anoxygenic forms of photosynthesis. It was contingent on the prior
origin of anoxygenic photosynthesis in that two existing photosystems –
derived from anoxygenic photosynthetic ancestors – were wired together in
the same cell . To be naturally selected,
oxygenic photosynthesis required an environment – plausibly freshwater
– where easier electron donors were absent or had
been drawn down to limiting concentrations. The resulting cyanobacterial cell
was the ancestor of all organisms performing oxygenic photosynthesis on the
planet today. It took up to a billion years to evolve
, with the first evidence of oxygen appearing
3.0–2.7 Ga .
Once oxygenic photosynthesis evolved, the productivity of the biosphere was
no longer restricted by the supply of substrates for photosynthesis, as water
and carbon dioxide were abundant. Instead, the availability of nutrients,
notably nitrogen and phosphorus, would have become the major limiting factors
on global productivity – as they still are today. Oxygenic photosynthesis
would have flourished wherever nutrients were available and anoxygenic
photosynthesis drew down its electron donors to limiting concentrations, or
where oxygen removed those electron donors by oxidizing them. Anoxygenic
photosynthesis might have flourished underneath oxygenic photosynthesis in
parts of the surface ocean if and when anoxic waters bearing Fe2+
extended up into the sunlit photic zone , and
this would have set up some competition for the nutrients nitrogen and
phosphorus.
Constraints on nutrient concentrations in the early ocean are scarce
. Nitrogen would initially have been in the
form of ammonium (rather than nitrate), but the advent of an oxygen source
plausibly triggered the onset of nitrification and denitrification
. Nitrification could have
produced small pools of nitrate in restricted surface ocean `oxygen oases'
with nitrogen in the form of ammonium elsewhere. Whether denitrification
could then have caused nitrogen scarcity ,
depends on whether nitrogen fixation had evolved and could counter-balance it
. Iron and vanadium-based nitrogen fixation were
plausibly already widespread , although
molybdenum-based nitrogen fixation may have evolved later
. Thus phosphorus was probably the ultimate limiting
nutrient, as it is today. Lower terrestrial weathering fluxes of phosphorus
(relative to present) have been predicted, due to a shift from terrestrial to
seafloor weathering to balance the carbon cycle earlier in Earth history, and
this would have tended to reduce ocean phosphorus concentration, because
seafloor weathering is not a source of phosphorus
. Initial work estimated only ∼10–25 %
of today's phosphorus concentration in the Late Archean ocean
, however subsequent studies have revised this
upwards to ∼1–4 times present-day phosphorus concentration
. Furthermore, nutrient recycling by the
microbial loop within the surface ocean was
conceivably more efficient than today because eukaryotic mechanisms of
exporting organic matter out of the surface ocean were absent. One model
suggests that marine NPP may have been ∼25 % of today's productivity
(450 EJ yr-1 or 12 PgC yr-1) in the Late Archean ∼2.7 Ga
.
With the advent of oxygenic photosynthesis there was thus an
order of magnitude increase in organic carbon production (Fig. ).
The extra flux of carbon sinking into the anoxic depths of the ocean would
initially have fuelled methanogenesis (as sulphate was yet to build up
significantly in the ocean,
). The resulting upward
flux of methane could support widespread methanotrophy near the source of
oxygen from oxygenic photosynthesis, consistent with very isotopically light
organic carbon from ∼2.7 Ga
. A large
flux of methane, equivalent to around 60 % of the primary production
sinking out of the surface layer of the ocean ,
would also escape to the atmosphere, warming the planet. However, if the
CH4 : CO2 ratio in the atmosphere approached 0.1, photochemical
production of an organic haze that scattered sunlight back to space would
have triggered cooling . This process would be
self-limiting, but might help explain the earliest glaciations ∼2.9 Ga
.
Oxygen remained a trace gas, O2 < 10-5 PAL (present
atmospheric level), until 2.45 Ga as indicated by the mass independent
fractionation of sulphur isotopes (MIF-S), preserved in sediments older than
this, which shows that the ozone layer was absent and high energy ultraviolet
radiation reached the surface (creating the signal), and sulphate had yet to
accumulate in the ocean (allowing the signal to be preserved)
. Elevated concentrations of methane in such a
reducing atmosphere would have supported an increased flux of hydrogen loss
to space, causing the long-term oxidation of the surface Earth system
. Stability broke down 2.45–2.3 Ga in the
“Great Oxidation” event . The MIF-S signature
disappeared indicating that oxygen rose > 10-5 PAL
sufficient to form an ozone layer. Massive deposits of oxidized iron appeared
in the form of the first sedimentary “red beds”, and oxidized iron also
appeared in ancient soils, indicating that oxygen increased to
> 10-2 PAL. Models suggest that once enough oxygen built up
for the ozone layer to start to form, this shielded the atmosphere below from
UV and slowed down the removal of oxygen by reaction with methane
.
This created a strong positive feedback explaining the abruptness of the
Great Oxidation
.
The Great Oxidation destabilized other environmental variables. As oxygen
rose, atmospheric methane concentration declined
,
which could help explain the series of Huronian glaciations
and the low-latitude Makganyene glaciation
2.32–2.22 Ga .
The reaction of oxygen with sulphide in continental rocks plausibly produced
sulphuric acid that dissolved phosphorus out of apatite inclusions in the
rocks and fuelled marine productivity . The
oxidizing power unleashed in the Great Oxidation could thus have made another
limiting resource, phosphorus, more available, boosting energy input to the
biosphere. One model estimates that marine NPP in the Proterozoic Eon after
the Great Oxidation was ∼1300 EJ yr-1 (34 PgC yr-1) or
∼70 % of today's value . This would have
supported increased organic carbon burial, which is inferred to have occurred
during the “Lomagundi” carbon isotope excursion, 2.23–2.06 Ga
, potentially triggering an “overshoot” of
atmospheric oxygen . However,
there were large crustal reduced sinks for oxygen at the time
, and after ∼ 150 Myr, excess buried organic
carbon was recycled through the crust back to the surface, consuming oxygen
and deoxygenating the ocean . After this
protracted interval of instability, the Earth entered an even longer period
of stability, known as “the boring billion”.
Marine productivity during this protracted interval of the Proterozoic Eon is
very uncertain. We know that the deep ocean remained largely anoxic and
“ferruginous” (with Fe2+ in solution), with euxinic waters
(SO4-reducing) at intermediate depths along some ocean margins, and
surface waters largely oxygenated
. Several authors have
argued for very low productivity partly on the grounds of a sparsity of
organic carbon rich shales, but largely based on theoretical arguments for
low nutrient conditions. Phosphate supply to the ocean could have been
reduced by scavenging onto iron oxides forming in freshwater and estuarine
environments . Phosphate could also have been
efficiently removed from ocean waters by the formation of mixed
Fe2+ / Fe3+ compounds such as “green rust”
. However, reducing deeper waters and sediments
(especially euxinic ones) should have been effective at recycling phosphorus
and shuttling it back to the surface ocean, consistent with high estimates of
phosphate concentration at 1.7 Ga . Nitrogen
limitation has been argued for on the grounds of a lack of molybdenum for
nitrogen fixation , but the existence of
alternative nitrogenases makes this unlikely .
Instead, heterogeneous ocean redox conditions could have supported a mixed
nitrogen cycle with ammonium in the predominantly reducing waters of the deep
ocean and small reservoirs of nitrate in oxygenated waters. In such a system
there would be large fluxes of denitrification along the extensive interfaces
between oxygenated and anoxic waters, counterbalanced by large fluxes of
nitrogen fixation in surface waters replenishing the nitrogen reservoirs.
Indeed the fact that the Great Oxidation was never reversed sets a lower
bound on Proterozoic productivity of ∼25 % of modern marine NPP in an
existing model .
The Great Oxidation increased energy consumption by the biosphere, even with
no change in energy input, because respiring organic matter with oxygen
(2870 kJ mol-1) yields an order of magnitude more energy than breaking
it down anaerobically (e.g. 232 kJ mol-1 for alcohol fermentation).
This greater energy source facilitated the evolution of new levels of
biological organization, in the form of eukaryotes. The ancestral
(heterotrophic) eukaryote is thought to have had mitochondria performing
aerobic respiration. The timing of eukaryote origins is deeply uncertain, but
with putative biomarker evidence 2.7 Ga now rejected
, and molecular
clocks suggesting a last common ancestor 1.8–1.7 Ga
, they may post date the Great Oxidation.
Mitochondrial respiration in turn allows eukaryotes to support a much larger
genome than prokaryotes, giving them the capacity to create more complex life
forms with multiple cell types , the first
evidence for which appears ∼1.2 Ga
.
Eukaryotic photosynthesis and land colonization
The next revolution in energy input to the biosphere involved encapsulating
an existing metabolism – oxygenic photosynthesis – in progressively more
complex, eukaryotic organisms and symbioses – algae, lichens and land plants
(with mycorrhizal fungi). This energy revolution involved increasing the
supply and utilization of limiting nutrient resources needed to perform
photosynthesis and increasing the area over which it occurred.
The lineage containing all extant photosynthetic eukaryotes arose
1.7–1.4 Ga , but eukaryotic algae only
became ecologically significant relative to cyanobacteria ∼740 Ma, when
biomarkers of algae become more prevalent in ocean sediments and the
diversity of eukaryote fossils starts to increase
. Larger eukaryote cells are better at
exploiting excess nutrients in polar surface oceans, but would also have
removed carbon and nutrients from the surface ocean more efficiently, thus
reducing recycling, with uncertain overall effects on productivity
. More efficient carbon export to sediments
plausibly increased phosphorus removal from the ocean, lowering global
productivity and tending to oxygenate the deep oceans
, and contributing to CO2 drawdown and
global cooling . CO2 drawdown by
silicate weathering might have been enhanced by the arrival of eukaryotes
(fungi and algae) in microbial ecosystems on the land
. Estimates of the productivity of global microbial
mats, based on a simple area-scaling of modern desert crust
, suggests only 3–11 % of today's
terrestrial NPP, comparable to today's cryptogamic cover, which achieves
1–6 % of terrestrial NPP . However, deserts
are unproductive environments and modern cryptogamic cover is living in a
world dominated by vascular plants. Taking the ecophysiological model of
cryptogamic cover and considering higher
atmospheric CO2 and lack of competition from vascular plants, putative
Neoproterozoic-early Paleozoic land biota might have achieved ∼25 % of
today's terrestrial NPP. Whatever the cause(s), the Earth experienced two
low-latitude “snowball Earth” glaciations – the Sturtian (starting
715 Ma) and Marinoan (ending 635 Ma) – amidst a protracted interval of
instability in the global carbon cycle. Iron formations deposited during
these events apparently record high concentrations of phosphate in the ocean
, which could be explained by a shutdown of
biological uptake and removal to sediments. In the aftermath of glaciations,
high productivity could have been fuelled – at least temporarily – by
elevated phosphorus concentrations. There is evidence of at least partial
oxygenation of the deep ocean, and complex eukaryotic life including animals
began to flourish in the oceans. However, by the early Phanerozoic,
phosphorus concentrations were broadly comparable to today
, implying comparable
levels of marine NPP.
The key change in energy input to the biosphere and material cycling came
later with the rise of plants on land, starting around 470 Ma and
culminating in the first global forests by 370 Ma
. This roughly doubled global NPP, increasing it
by an order of magnitude on land and potentially indirectly in the ocean.
Terrestrial NPP is estimated to have exceeded today's value
(∼2100 EJ yr-1 or 56 PgC yr-1) on average during the
Phanerozoic (Fig. ), with peaks
potentially exceeding twice the present value
. To colonize the land new
nutrient acquisition mechanisms were required, achieved through symbioses with mycorrhizal
fungi and nitrogen-fixing bacteria. Plants and their associated mycorrhizal
fungi accelerated the chemical weathering of the land surface in search of
rock-bound nutrients, notably phosphorus. Ultimately, stunningly effective
recycling developed, such that the average terrestrial ecosystem today
recycles phosphorus ∼50 times through primary production before it is
lost to freshwaters .
Increased silicate weathering lowered atmospheric CO2 levels, plausibly
triggering the Late Ordovician glaciations ,
although others question the magnitude of early plant effects on the carbon
cycle . A more established
view is that the weathering effects of later plants, notably the first
deep-rooting trees forming forests, caused the later Permian-Carboniferous
glaciations. Increased phosphorus weathering supplied nutrient to the oceans,
increasing marine productivity and plausibly triggering oceanic anoxic events
. The increase in organic carbon burial
with the rise of plants also increased atmospheric oxygen, as revealed in the
charcoal record . Although ignition sources
(lightning, volcanoes) have always existed on Earth, there was little to burn
before land plants arose, and experiments show that
O2 > 15 % of the atmosphere is required for biomass
combustion to be sustained
. The first charcoal
evidence for natural fires coincides with the appearance of vascular plants
on drier land ∼420 Ma . Plants in
turn provided a new source of organic carbon for burial in sediments,
especially new structural carbon polymers (including lignin), which are hard
to biodegrade. Fungi evolved to recycle these, but a delay may have caused
atmospheric O2 to peak in the Carboniferous
. The continuous charcoal
record indicates O2 persistently > 15 % of the atmosphere
since 370 Ma .
The rise in atmospheric oxygen and increase in food supply brought about by
land plants has allowed a flourishing of animal complexity from aerobic
pathways – including the emergence of us humans. Today, the total global
energy flux through heterotrophic biomass, based on a 10 % conversion
efficiency of 100 PgC yr-1 with energy density 40 kJ gC-1, is
∼400 EJ yr-1, roughly half on land and half in the ocean. Natural
fires additionally consume ∼55 EJ yr-1 (1.4 PgC yr-1)
, and human-induced fires
∼45 EJ yr-1 (1.1 PgC yr-1)
, giving a total biomass burning flux today of
∼100 EJ yr-1 (∼2.5 PgC yr-1)
, or ∼2.5 % of the energy and carbon
captured in photosynthesis.
Revolutions in human history
Like all animals humans are heterotrophs. Our biological metabolism relies on
the products of photosynthesis. At the same time humans are exceptional among
animals in creating and maintaining a social metabolism via breeding and
cultivating plants and animals, in constructing buildings and large
infrastructure systems and in producing numerous artifacts
. The social metabolism inevitably extends total human
energy capture and material use beyond the biological requirements. In modern
industrial societies the amount of energy and materials used to produce and
reproduce domesticated livestock and all artifacts typically is 2 orders of
magnitude larger than the basic biological metabolism of the human population
itself. For the following comparison between human energy use and the primary
productivity of the entire biosphere, it is therefore important to keep in
mind the different trophic levels involved, autotrophs versus heterotrophs,
and the unique capability of human societies to extend their biological means
of energy and materials utilization through agriculture and technology.
A critical question in this regard is how to define the system boundary of
human society vis a vis its environment in terms of inputs and outputs of
energy and materials. For materials we apply the method implemented by the
European Statistical Office
.
According to this method all raw materials, except water and air, that serve
the production and reproduction of humans, livestock, buildings, built
infrastructure, durable and non-durable goods and services are accounted for
as socioeconomic input. The main raw material inputs to modern societies are
therefore plant harvest for food, feed, other energy uses and as material
input to industrial production, sand, gravel and crushed stone mainly for
construction purposes, metals and non-metallic minerals for various
industrial production purposes, and fossil energy carriers for both energetic
and material applications. The national indicator derived from this method is
domestic material consumption (DMC) defined as raw materials extraction plus
imported goods minus exported goods measured in tons per
year .
Regarding energy we deviate from the most common approach to account for
total primary energy supply (TPES) used in national and international energy
statistics, see, e.g. the annually published energy balances by the
International Energy Agency. TPES excludes plant biomass used for food and
feed which makes this indicator unsuitable for a comprehensive reconstruction
of the evolution of human energy use in a deep history perspective. Instead,
the method used here applies the same system boundary to the material and the
energetic dimension, taking into account the primary energy used in technical
conversion processes as well as the energy content of plants for human
nutrition and for feeding domesticated animals .
Energy capture by human societies involves trophic levels and specific
mechanisms which are different from those occurring during primary production
at the planetary scale. A comparison between the two is still warranted, as
human society inevitably operates within the thermodynamically closed Earth
system. The emergence and continued existence of human civilization is
conditional upon the stability of certain basic dynamics of the Earth system
which are vulnerable to metabolic changes in kind and scale, such as changes
in the overall energy balance, or changes in the chemical composition of the
atmosphere, oceans or soils, rather than the specific mechanisms that caused
them.
Palaeolithic fire use
During most of their existence humans lived as foraging societies in an
uncontrolled solar-energy system , simply
tapping into the existing energy and material cycles of the biosphere,
without deliberately controlling them by systematic land management, and
without introducing new biogeochemical pathways. The first human revolution
in energy input was the intentional use of fire, which set humans apart from
all other species. With it humans extended their energy utilization beyond
their biological metabolism towards areas outside the human body. This marked
the beginning of a social metabolism – a collectively organized extension of
energy and material use by human societies
.
There is robust evidence that Homo erectus could control fire from
790 ka in Africa and from 400 ka in Europe
. The ability to cook, which implies the
control of fire, may date as far back as 1.5 Ma .
Cooking provided higher food energy, higher food diversity through
detoxification, and a selective force to develop social abilities and large
brains, thus playing a key role in human evolution. The use of fire may also
have facilitated humans occupying colder climates ,
and developing increased abilities to cooperate , a
decisive element of their evolutionary success.
Use of fire for cooking increased energy input to approximately
7–15 GJ cap-1 yr-1, i.e. a factor of 2–4 above the average
physiological energy demand of 3.5 GJ cap-1 yr-1
.
Assuming a population of 2–4 million at the beginning of the Neolithic
, overall energy capture by humans amounted to
roughly 14–60 PJ yr-1, a factor of ∼1000 below the global human
energy input in 1850 and ∼10 000 below today's (Fig. ). In
foraging societies, biomass accounts for more than 99 % of material input.
Materials are used predominantly for energetic purposes, as fire wood or
food. Thus the energetic and the material social metabolism were practically
identical.
Based on their direct energy and material inputs, foraging societies had a
negligible impact on the global environment. However, the intentional use of
fire for hunting, clearing land and other purposes could have caused
significant environmental impacts – accepting that the empirical evidence
regarding frequency, scale and age for applying those intentional burning
techniques is highly contested. Potential impacts include extinction of large
Pleistocene land animals and ecosystem tipping events, including shift of
vegetation to desert shrub triggering a weak monsoon in Australia
, rapid landscape transformations in the mesic
environments of New Zealand , the wet tropical
forests of the pre-Columbian Amazon , and
across the savannas and woodlands of Africa .
Foraging societies need large areas. Although the energy density of natural
vegetation ranges over 0.1–1 W m-2 (NPP of
3.16–31.6 MJ m-2 yr-1) , the bulk
biomass of the most abundant plants, grasses and trees, is not edible for
humans. The very small share of human-edible natural biomass restricts the
population density of foraging societies to no larger than
∼0.02–0.2 cap km-2 . Such low
population densities and the necessity to stay mobile prevent the
accumulation of artifacts and the development of complex institutions, e.g.
institutions to deal with conflict are prohibitively costly as long as moving
away is an attainable alternative. Therefore foraging societies are typically
portrayed as small egalitarian groups of low internal complexity, based
largely on a few extant foraging societies who have been pushed aside to
marginalized environments. In more favourable environments higher resource
intensities could have supported higher population densities and
significantly more complex social structures, including settlements,
handcraft, trade and social stratification
.
The Neolithic revolution
By the beginning of the Holocene, 11 700 BP, humans had successfully
inhabited all continents. Then, within a few thousand years a fundamentally
new socio-metabolic energy regime emerged on all continents except Australia,
involving the domestication of wild plant and animal species and the control
of their reproduction via husbandry. Agriculturalists greatly enhanced the
area productivity of edible species at the expense of non-edible species and
of food competitors. In contrast to pre-agricultural societies they lived in
a controlled solar energy system .
Agriculture had multiple independent origins; in the Near East
(∼ 10 000 BP), Peru (∼ 10 000 BP), South China (8500 BP),
North China (7800 BP), Mexico (4800 BP), East North America (4500 BP), and
possibly sub-Saharan Africa (4000 BP)
.
Archaeological evidence from several sites at the shoreline of the Persian
Gulf has revealed a rapid colonization of this area by advanced agricultural
and urban societies at around 7500 BP. As sea-level rise from the last
glacial low stand was only completed in the Persian Gulf 7000–8000 BP,
there could be even older agricultural sites in areas that are now beneath
the Indian Ocean . Explaining the relatively rapid
transition to agriculture is one of the most controversial topics in
universal history. The puzzle is that early agriculture, especially farming,
was not obviously superior to foraging. Ethnological studies have shown that
early farmers spent more hours to exploit their food base, relied on a less
diverse and less stable diet, were more prone to diseases, and even had lower
productivity in terms of calorific return on labour investment
. The Neolithic
revolution therefore tends to be explained as a necessity driven transition,
fostered by population pressure ,
deterioration of resources (extinction of Pleistocene megafauna), or climate
change. Whatever the reasons for switching from foraging to farming, it
creates a lock-in once population densities exceed the natural carrying
capacity of the surrounding ecosystem. Then reverting to foraging cannot
occur without substantially reducing population numbers.
After ∼7000 BP complex agrarian civilization emerged
. Extant biomass was still the energy source
for almost all energy uses: food, fodder, heat, mechanical power and chemical
transformation (metallurgy). Wind and water power used by agrarian
civilizations (sailing ships and mills) were locally important but
contributed only marginally to the energy input. Despite huge variations in
agrarian land use systems, a defining condition is that energy supply is
tightly coupled to productive land and (human and animal) labour working on
the land. Without any external energy subsidies in the form of mechanical
power and synthetic fertilizers, a larger usable energy output from extant
biomass typically requires more land or more labour input on existing land,
thus putting relatively strict limits to the possibility of increasing energy
supply per capita . Higher yields
could be achieved by various improvements in agricultural technology but
those improvements were typically population driven and lead to absolute
growth in energy capture per area of land while per capita energy
availability stagnated or even declined .
Estimates of global average energy input to agrarian societies are
45–75 GJ cap-1 yr-1 roughly a factor of 5 greater than in
foraging societies . With the
estimated population rising to ∼450 million in AD 1500 (when the
agrarian mode of subsistence dominated the global population), overall energy
capture by humans may have reached ∼20 EJ yr-1
, a factor of 300 above the
foraging regime, but 30 below today. When the industrial revolution took off
around 1850 human population was ∼1.3 billion and energy capture had
reached ∼60 EJ yr-1 (Fig. ).
The increased population and energy flows due to farming increased the
material inputs to, and waste products from, societies. The resulting
environmental effects began early in the Holocene, but their scale is much
debated . Irrigation began
around 8000 BP in Egypt and Mesopotamia, leading to some localized
salination and siltation of the land, reducing crop yields and encouraging a
shift in agricultural crop from wheat to more salt-tolerant barley
. The use of manure as fertilizer may have begun as
early as 9000 BP in SW Asia and 7000 BP in Europe
. The clearance of forests to
create agricultural land and supply biomass energy and wood from 8000 BP
onwards, reduced the carbon storage capacity of the land, transferring CO2
to the atmosphere . Cumulative carbon emissions
may have approached 300 PgC by 500 BP
contributing ∼20 ppm to atmospheric CO2 levels. The biogeophysical
effects of forest clearance also affected the climate, regionally and
remotely . Anthropogenic sources of methane
started around 5000 BP with the irrigation of rice paddies and have
contributed to changes in atmospheric CH4 concentration over the past
∼3000 years .
The energetic surplus generated by agrarian societies first allowed cities to
become a widespread phenomenon ∼5000 years after the beginning of
agriculture. This led to more complex social organization with increasing
division of labour, technological innovations, social stratification and
written language . This in turn requires
re-integration via exchange, trade and redistribution creating mutual
dependencies which increased the potential for conflict, prompting the
inception of social institutions to deal with such conflicts (e.g. priests,
judges). Additionally, stockpiling and concentration of resources in cities
attracted predators stimulating institutions of defense (military). Social
complexity has costs as well as benefits and a number of early complex
societies collapsed when those costs became prohibitively high
.
Agrarian societies are faced with relatively severe constraints regarding the
energy surplus they can achieve. On average around 90 % of the population
is required to work in agriculture. This limits the urban population engaged
in non-food producing activities to no more than 10 %
, although locally urbanization levels could be much
higher. The outstanding role of bio-productive land as the main factor of
production also explains the important political role of territory in
agrarian societies. The intrinsic connection between social stratification
and territory in agrarian societies can be illustrated by the role of land in
medieval European feudalism, where the power of the nobility was strongly
connected to the control over productive land. Economic growth was only
possible through land expansion and increase in area productivity. Both have
inherent practical limits and both require a growing population. Combined
with hard constraints on transportation in pre-industrial societies, this
leads to relatively fast local negative feedbacks in the energetic and
material social metabolism and renders sustained material growth impossible
on a per capita basis – making the distribution of material wealth a zero
sum game.
The Industrial revolution
Fossil fuels, especially coal and peat had been used for hundreds of years in
China, Burma, The Netherlands and England . However,
their contribution to the social metabolism always remained small. The key
energy transformation of the industrial revolution came with the ability to
massively scale-up fossil energy use . Unlike the Neolithic
revolution, the Industrial revolution was a historical singularity. Its
inception in 18th century England was followed by a worldwide expansion of
the new energy regime, which is still ongoing. The fossil energy regime
eventually surmounted the inherent thermodynamic constraints of agrarian
societies that had existed for millennia by decoupling socially usable energy
from bio-productive land and human labour
. Within 150 years, from 1850 to 2000,
global human energy use increased tenfold from 56 to 600 EJ yr-1
(estimates based on
),
the world population went from 1.3 billion to 6
billion, and global GDP increased from 800
to 6600 intGK$. Thus by 2000 the annual global energy flux through human
societies was one third of the global terrestrial NPP
and one third above the total global energy
flux through all non-human heterotrophic biomass (Fig. ).
Unlike the Neolithic revolution, the puzzle of the industrial revolution is
not that it began, but that it continued . Similar
innovation-driven growth periods in agrarian civilizations (e.g. the Dutch
golden age) could not be sustained, because they were sooner or later
counterbalanced by diminishing returns on energy investment in the
agricultural sector. Even for the classical British economists Adam Smith,
David Ricardo, Thomas Malthus, and John Stuart Mill, who witnessed England's
industrial take-off, there was no doubt that diminishing marginal yields in
the agricultural sector would eventually bring industrialization to a halt
. A key challenge was to feed a growing
industrial labour force with a controlled solar-energy based system of
agriculture (given that the agricultural sector did not industrialize until
the 1930s in the USA and the 1950s in Europe)
. England was in a specially favoured
position, because since the late 16th and early 17th century area yields,
total agricultural production and labour productivity had been growing
continuously . This allowed 18th century
England to support a growing industrial labour force in the initial phase of
the industrial revolution. When agricultural productivity gains eventually
came to a halt around 1830 – while the population was still growing rapidly
– England's hegemonic political position was instrumental to massively
increase food imports
.
The availability of technologies to overcome bottlenecks in energy
utilization also played a decisive role in the industrial revolution
happening in England. Notably, the coincidence of a domestic endowment of
coal with the emergence of a new technology complex consisting of the steam
engine and coke-based iron smelting. With this technological complex energy
constraints could be exceeded , which had
previously limited coal extraction, steel production, and long-distance
transportation.
The step increase in energy capture with industrialization is associated with
fundamental changes in global material cycles. Material inputs to societies
were transformed from biomass dominance to minerals dominance. Global average
per capita material use increased from 3.4 to 10 t cap-1 yr-1
from 1870 to 2000, and with roughly constant biomass use of
3 t cap-1 yr-1, the average use of mineral and fossil materials
increased from 0.4 to 7 t cap-1 yr-1
. In industrial
economies ∼80 % per weight of the total annual outflow of materials is
CO2, making the atmosphere the largest waste reservoir of the industrial
metabolism . Between 1850 and 2000 global CO2
emissions from combustion of fossil fuels and materials processing increased
125-fold from 54 to 6750 TgC yr-1 and reached 9140 TgC yr-1 in
2010 .
Year 2000 (a) material and (b) energy
use per capita and (c) total population (data.wordbank.org), by income groups.
Industrial societies require large physical stocks: buildings, transport
infrastructure, energy, water and waste infrastructure, production facilities
and durable consumer goods. For example, the material stock of
industrializing Japan has increased by a factor of 40 between 1930 and 2005
reaching 38.7 billion tonnes or 310 tonnes per capita
and the non-metallic minerals incorporated in
residential buildings, roads and railways in the EU25 was 75 billion tonnes
or 203 tonnes per capita in 2009 . In the
USA the amount of iron incorporated in durable products and infrastructure
increased from 100 million tonnes to ∼3200 million tonnes between 1900
and 2000 . Industrial societies also use a much
larger diversity of minerals. Almost all metals are now commercially used in
increasingly complex combinations . Overall
recycling rates (measured as the global average of the content of secondary
metal in the total input to metal production) of metals are uncertain.
Recycling rates are above 50 % for only three metals (Nb, Ru, Pb), between
20–50 % for another 16, and below 20 %, often less than 1 %, for all
the other ∼40 metals in wide industrial use .
A recent study estimated that only 6 % of globally extracted materials are
currently recycled within the socioeconomic system .
The global biogeochemical cycles of nutrients have also been transformed by
industrialization. Between 1860 and 2005 anthropogenic creation of reactive
nitrogen grew more than tenfold, from ∼15 to 187 TgN yr-1
. Furthermore, the creation of nitrogen
oxides as a waste product of fossil fuel combustion increased from ∼0 to
25 TgN yr-1 . The excess reactive
nitrogen was transferred to other environmental pools, partly denitrifying to
atmospheric N2, but also contributing to eutrophication and acidification
of terrestrial and coastal marine ecosystems, to global warming and to
tropospheric ozone pollution. Analogous human-induced acceleration affected
the P-cycle.
The industrial revolution also gave rise to entirely new metabolites. The CAS
Registry currently includes 92 million unique
chemical substances in commercial use of which only 320 000 are regulated in
key markets. It is unknown how many of these substances represent entirely
new chemicals and whether they are harmful to humans or the environment. With
15 000 new entries daily comprehensive in-vivo toxicity testing is
practically impossible .
The industrial revolution expanded to the European continent and to the USA
in the early 19th century, to Japan in the late 19th century and to large
nations like China, India, and Brazil in the last decades of the 20th
century. With the transition to an industrial mode of production the
socio-economic power of the nobility (based on control over productive land)
diminished and shifted towards the owners of the means of industrial
production (which Karl Marx called capitalists). Large differences in
consumption among countries persist until today
(Fig. ; data from
and the World Bank
Income Classification, ). If we consider high income
countries with an average energy use of 302 GJ cap-1 yr-1 as
fully industrial, and upper middle and lower middle countries, with an
average energy use of 140 and 74 GJ cap-1 yr-1 respectively as
transitioning to an industrial energy regime, then ∼15 % of the world
population lived in a mature industrial energy regime in 2000, ∼44 %
were in transition, and the remaining ∼40 % still lived under largely
agrarian conditions with average energy use amounting to
42 GJ cap-1 yr-1. The correlation between energy use and human
development appears to be highly non-linear. At high levels of human
development large increases in energy input have little or no effect on
further increases in standards of living. However, at low levels of human
development relatively small increases in energy input have large positive
effects , for example supplying ∼3.5
kilowatt per person can greatly increase life expectancy
.
Forward look: a solar-powered recycling revolution
Each revolution in Earth and human history involved a new mechanism to
capture free energy and the accessing of previously underutilized resources.
The resulting step increase in free energy input privileged the systems,
biological or social, using the new energy capture mechanism, making them
globally significant or even dominant. However, material constraints
ultimately became limiting to the expansion of energy innovators either
because the scale of waste products they generated disrupted their
environment, or because the material resources they depended upon became
scarce. The lesson for human society is that to have a long-term sustainable
future within the Earth system will require both a sustainable source of
energy and the closure of material cycles
.
A sustainable energy system is challenging but feasible from a purely
technological point of view. The technical potential for renewable energy
technologies, most of which ultimately rely on solar energy, exceeds current
and future global primary energy demand by several orders of magnitude
. However, the rate of de-carbonization of the global
energy system is constrained by a number of economic (e.g. economic viability
of renewable energy technologies, large up-front investments, devaluation of
investments in existing energy infrastructure), socio-cultural (e.g. public
acceptance of large-scale infrastructure projects, food security and various
other competing land uses), and technological (e.g. issues of transmission,
integration and storage) factors . Current
assessments of global development scenarios with ambitious climate mitigation
targets put the supply of RE between 250 and 500 EJ yr-1 in 2050
.
Depending on assumptions this corresponds to 25–75 % of the projected
(2050) global primary energy demand. The importance of other, more contested
energy technologies for achieving a sustainability transition of the global
energy system depends on the development of future energy demand. Assuming
ambitious energy efficiency improvements the transformation goals can be
achieved without nuclear fission, carbon-capture and storage, or high-tech
carbon sink management. With less progress on the demand side, one or more of
these technologies would be required in the energy mix
. Nuclear fusion might be an option in the
long-term, but is no attainable option in the coming decades when climate
mitigation measures must be implemented .
Significant additional investments and several decades of technology
development would be needed to bring nuclear fusion into large-scale
practical implementation .
Whilst energy generation for (post-) industrial purposes can be largely
de-carbonized, food energy production cannot. The carbon cycle linked to food
production can conceivably be re-closed, through a combination of reductions
in land-use change CO2 emissions, and land-based carbon dioxide removal
(CDR). However, the much larger (in a fractional sense) perturbations to
nutrient (N and P) cycling present a greater challenge, for two contrasting
cycles. Nitrogen is abundant in the atmosphere and returned there relatively
rapidly by natural biological recycling processes, hence with a sustainable
source of energy, nitrogen could be fixed indefinitely. Phosphorus, in
contrast, is a rock-bound, finite and non-substitutable resource likely
facing either economic or physical scarcity
within this century . For both nutrients
there is a need to minimize the harmful by-products of excess deposition. Yet
fertilizer N and (especially) P demand is set to increase significantly with
an ongoing shift to more meat-rich diets .
In addition to reversing this trend there is huge potential to counteract
this increase through more efficient phosphorus and nitrogen application to
crops (through e.g. better targeted fertilizer application), and reducing
losses from domestic animal (and human) excrement, crop residues and the
post-harvest life cycle .
The longevity of manufactured capital leads to considerable path dependency
and even lock in, and complicates its analysis and accounting. Recent studies
have investigated the material stocks of specific metals
and there are some signs of saturation for
specific material stocks in industrialized countries. However, it is unclear
to what extent a saturation of the stocks of any single metal are due to
material substitution . Even if stocks for
bulk materials (mainly for construction) were to saturate in industrialized
countries due to the projected stabilization of population and slow economic
growth, stock levels will need to increase dramatically in emerging and
developing countries. Careful design and implementation of these future
stocks holds huge potential to slow further growth of the industrial
metabolism and minimize lock in.
The explosive proliferation of new metabolites could be tackled by a shift
toward green chemistry that
encourages the design of products and processes that minimize the use and
generation of hazardous substances. However, given the immense amount of
newly introduced chemicals and the importance of material and chemical design
for many high-tech produces, additional strategies will be necessary. These
may range from new and faster toxicity screening tools, to environmental
design guidelines to regulations regarding recyclability and biodegradability
material components and final products, applying cradle to cradle principles.
The solar-powered material-recycling “revolution” that we have sketched out
demonstrates that the material dimension of the industrial metabolism is much
more complex, much more inert and inflexible, and at the same time much less
understood than its energetic dimension. Furthermore, such a revolution must
anticipate a level of social organization that can implement the changes in
energy source and material cycling without preventing present and future
generations from attaining similar achievements in standard of living and
individual liberation associated with industrial societies. With regards to
the lasting attention to Georgescu-Roegen's flawed fourth law of
thermodynamics it is important to note
that such a “revolution” does not contradict any established thermodynamic
laws
as is amply demonstrated by the biological evolution of the Earth's biota.
Pertinent large-scale systemic characteristics and relevant regional
ramifications of the material social metabolism are still poorly understood,
e.g. the structure and dynamic of complex global material supply chains,
path-dependency and potential lock-in created by the different components of
the manufactured capital, quantitative assessments of the technical and
economic potential to close materials cycles, or effective means to balance
the huge number of newly introduced chemicals with feasible tools to assess
their toxicity for humans and other species. Furthermore multiple barriers as
well as co-benefits between a solar-powered material cycling revolution and
other sustainability goals such as climate mitigation, adaptation, reducing
extreme poverty, reducing social inequalities, and increasing health are
severely under-researched.
Future societies might look back at the period of a globally expanding
industrial metabolism, with its characteristic exponential material growth,
as a necessary phase to transition from the inherently scarce agrarian
controlled solar energy system to a second generation controlled solar energy
that can provide “affluence without abundance”
at a much higher level than foraging societies could ever achieve. An
outstanding task therefore is to formulate a steady-state “Earth system
economics” that supports long-term human and planetary well-being. Two of
the most difficult problems to be solved along the way will be to find out
how desirable attributes of society, such as knowledge, can still grow while
resource input is constrained and how to organize a just distribution of
access to physical and non-physical resources in an economy that functions
physically as a zero sum game.