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Human domination of the biosphere: Rapiddischarge of the earth-space battery foretellsthe future of humankindJohn R. Schramskia,1, David K. Gattiea, and James H. Brownb,1

aCollege of Engineering, University of Georgia, Athens, GA 30602; and bDepartment of Biology, University of New Mexico, Albuquerque,NM 87131

Edited by B. L. Turner, Arizona State University, Tempe, AZ, and approved June 8, 2015 (received for review May 4, 2015)

Earth is a chemical battery where, over evolutionary time with a trickle-charge of photosynthesis using solar energy, billions of tons of livingbiomass were stored in forests and other ecosystems and in vast reserves of fossil fuels. In just the last few hundred years, humans extractedexploitable energy from these living and fossilized biomass fuels to build the modern industrial-technological-informational economy, to growour population to more than 7 billion, and to transform the biogeochemical cycles and biodiversity of the earth. This rapid discharge of theearth’s store of organic energy fuels the human domination of the biosphere, including conversion of natural habitats to agricultural fields andthe resulting loss of native species, emission of carbon dioxide and the resulting climate and sea level change, and use of supplemental nuclear,hydro, wind, and solar energy sources.The laws of thermodynamics governing the trickle-charge and rapid discharge of the earth’s battery areuniversal and absolute; the earth is only temporarily poised a quantifiable distance from the thermodynamic equilibrium of outer space.Although this distance from equilibrium is comprised of all energy types, most critical for humans is the store of living biomass.With the rapiddepletion of this chemical energy, the earth is shifting back toward the inhospitable equilibrium of outer space with fundamental ramificationsfor the biosphere and humanity. Because there is no substitute or replacement energy for living biomass, the remaining distance from equilibriumthat will be required to support human life is unknown.

energy | evolutionary biology | earth-space battery | sustainability | thermodynamics

As depicted in Fig. 1, earth is a battery ofstored chemical energy where the planet isthe cathode (stored organic chemical energy)and space is the anode (equilibrium). We callthis the earth-space battery. It took hundreds

of millions of years for photosynthetic plantsto trickle-charge the battery, gradually con-verting diffuse low-quality solar energy tohigh-quality chemical energy stored temporar-ily in the form of living biomass and more

lastingly in the form of fossil fuels: oil, gas,and coal. In just the last few centuries—anevolutionary blink of an eye—human energyuse to fuel the rise of civilization and the mod-ern industrial-technological-informational so-ciety has discharged the earth-space battery,inducing flow between the terminals, degrad-ing the high quality biomass energy to do thework of transforming the earth for humanbenefit, and radiating the resulting low-qualityheat energy to deep space.The laws of thermodynamics dictate that

the difference in rate and timescale betweenthe slow trickle-charge and rapid depletion isunsustainable. The current massive dischargeis rapidly driving the earth from a biosphereteeming with life and supporting a highlydeveloped human civilization toward a barrenmoonscape. Consider as an example that theenergy state of the earth is akin to the energystate of a house powered by a once-chargedbattery supplying all energy for lights, heating,

Humankind

Anode

Schramski/Delaney 2015

Cathode

Fig. 1. Earth-space battery. The planet is a positive charge of stored organic chemical energy (cathode) in the form ofbiomass and fossil fuels. As this energy is dissipated by humans, it eventually radiates as heat toward the chemicalequilibrium of deep space (anode). The battery is rapidly discharging without replenishment.

Author contributions: J.R.S. and D.K.G. designed research; J.R.S.

and J.H.B. performed research; J.R.S. and J.H.B. contributed new

reagents/analytic tools; J.R.S., D.K.G., and J.H.B. analyzed data;

and J.R.S., D.K.G., and J.H.B. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

1To whom correspondence may be addressed. Email: jschrams@uga.edu or jhbrown@unm.edu.

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cooling, cooking, power appliances, and elec-tronic communication; as the battery dis-charges, these services become unavailableand the house soon becomes uninhabitable.

Energy in Physics and BiologyThe laws of thermodynamics are incontro-vertible; they have inescapable ramificationsfor the future of the biosphere and human-kind. We begin by explaining the thermo-dynamic concepts necessary to understandthe energetics of the biosphere and humanswithin the earth-space system. The laws ofthermodynamics and the many forms of en-ergy can be difficult for nonexperts. However,the earth’s flows and stores of energy can beexplained in straightforward terms to un-derstand why the biosphere and human civ-ilization are in energy imbalance. Thesephysical laws are universal and absolute, theyapply to all human activities, and they are theuniversal key to sustainability.Energy is how far a property (e.g., tem-

perature, chemical, pressure, velocity) is fromequilibrium. This distance, or gradient, canbe harvested to perform work, in the processmoving the property closer to equilibrium.Thus, whereas the capacity to perform workis often used as the simplest definition ofenergy, ultimately this capacity requires an

out‐of‐equilibrium system, a gradient, whichis available to be harvested. For example, theearth is out of chemical equilibrium withrespect to nearby space; as we burn fossilizedchemical energy to get work output, the earthloses the resultant heat and moves closer toequilibrium. Similarly, when we burn livingbiomass faster than the earth can replenish it,the earth again moves closer to equilibrium.The first Law of Thermodynamics assuresthat, although energy is transformed be-tween solar, chemical, work, and heat inthese transactions, it is neither created nordestroyed; it changes forms, but the totalquantity is conserved. The Second Law ofThermodynamics assures that as energychanges forms, all of this energy is even-tually degraded to low-quality heat energyand lost from the planet. These physical lawsnot only have allowed the evolution of life,they also have allowed the development ofhuman civilization. Living things use photo-synthesis to convert diffuse but reliable sun-light into energy-rich organic compounds,and they use respiration to break down thesecompounds, release the stored energy, and dothe biological work of living (1, 2). For humansthis means consuming food and respiring tofuel biological metabolism. However, humansalso use technological innovations to burn

organic chemicals and use this extrametabolicenergy to do the additional work of fuelingcomplex socioeconomic activities.Over the millennia of evolutionary time, as

living things evolved, they gradually trans-formed the earth from a barren planet into abiosphere teeming with life. Until the originof life, there were no significant stores of or-ganic chemical energy, and the surface of theplanet was not far from the chemical equi-librium of the adjacent outer space. Then, asliving things evolved and diversified, theydeveloped new biochemical pathways forconverting solar energy into biomass. It tookon the order of 1 billion years for the firstphotosynthetic and chemosynthetic prokary-otes to exploit the small energy gradientsavailable and synthesize enough biomass tobegin to charge the earth‐space’s chemicalbattery. Ancient unicellular organisms cre-ated a modest chemical energy gradient thatpersisted for billions of years. Then startingabout 600Mya, with the Cambrian explosionof diversity of large multicellular organismsand the subsequent colonization of land byplants, the biosphere acquired a large store ofliving biomass, mostly in the form of forests(3). In the Carboniferous, Permian, and Ju-rassic periods (350–150 Mya), remains ofdead plants and animals were preserved inthe earth’s crust to create the reserves ofcoal, oil, and gas. Since then, the earth hasmostly been in an energetic quasi‐equilibrium,continually perturbed by asteroid impacts,tectonic activity, glaciations, and climaticfluctuations, modestly adding to or subtract-ing from the stores of fossil fuels, but alwaysreturning to an approximate balance betweensolar input and heat loss, photosynthesis, andheterotrophic metabolism.Everything changed when anatomically

modern humans appeared and expandedout of Africa to colonize the entire Earth.The most important milestone was the de-velopment and spread of agriculture, whichbegan about 12,000 y ago. Before this, hunter-gatherer societies had been in approximateequilibrium, relying on photosynthetic en-ergy to supply plant and animal foods andfuels for cooking and heating and barelyaltering the Earth’s surface. With the ad-vent of agriculture, humans began system-atically to harvest the stored biomass gradientand to increase chemical energy discharge.Initially, human and animal labor and fires ofwood and dung were used to do the work ofmanufacturing tools, clearing land, tillingfields, and harvesting crops. However, evermore inventive societies developed newtechnologies based on harnessing new energysources. Most importantly, the industrialrevolution used wind and water mills to do

19 ZJ

≈ 2000 ZJ

0 ZJ = Equilibrium (Outer Space)

≈ 500 ZJ

23 ZJ 35 ZJ 2 ZJ

(e.g., Fossil Fuel, Nuclear, etc.)

35 -

23 -

19 -

< 40 -

≈ 500 -

≈ 2000 -

2 -

35 ZJ

All Bioenergy Potential in 2000

Net Primary Production in 2000

All Bioenergy Potential in 1900

All Bioenergy Potential in year 0

Recoverable Fossil Fuel Potential

Recoverable Nuclear Potential

Not Recoverable Potential

< 40 ZJ

Fig. 2. Earth chemical and nuclear energy storage (distance from equilibrium) (10, 11, 38, 39). Where necessary,biomass is converted to energy assuming 1 t carbon ∼35 × 109 joules. ZJ = zeta joules = joules × 1021.

9512 | www.pnas.org/cgi/doi/10.1073/pnas.1508353112 Schramski et al.

work and burned first wood, then charcoal,and finally fossil fuels to mine and smeltmetal ores and to manufacture tools andmachines. These developments led to ever-larger human populations with ever-morecomplex economies and social systems, allfueled by an ever-increasing rate of chemicalenergy discharge.

The Paradigm of the Earth-Space BatteryBy definition, the quantity of chemical energyconcentrated in the carbon stores of planetEarth (positive cathode) represents thedistance from the harsh thermodynamicequilibrium of nearby outer space (negativeanode). This energy gradient sustains thebiosphere and human life. It can be modeledas a once-charged battery. This earth-spacechemical battery (Fig. 1) trickle charged veryslowly over 4.5 billion years of solar influxand accumulation of living biomass andfossil fuels. It is now discharging rapidlydue to human activities. As we burn organicchemical energy, we generate work to growour population and economy. In the process,the high-quality chemical energy is trans-formed into heat and lost from the planet byradiation into outer space. The flow of en-ergy from cathode to anode is moving theplanet rapidly and irrevocably closer to thesterile chemical equilibrium of space.Fig. 2 depicts the earth’s primary higher-

quality chemical and nuclear energy storagesas their respective distances from the equi-librium of outer space. We follow the energyindustry in focusing on the higher-qualitypools and using “recoverable energy” as ourpoint of reference, because many deposits offossil fuels and nuclear ores are dispersed orinaccessible and cannot be currently har-vested to yield net energy gain and economicprofit (4). The very large lower-quality poolsof organic energy including carbon com-pounds in soils and oceanic sediments (5, 6)are not shown, but these are not currentlyeconomically extractable and usable, so theyare typically not included in either recover-able or nonrecoverable categories. Althoughthe energy gradients attributed to geothermalcooling, ocean thermal gradients, greenhouseair temperatures, etc., contribute to Earth’sthermodynamic distance from the equilib-rium of space, they are also not included asthey are not chemical energies and pre-sumably would still exist in some form on aplanet devoid of living things, including hu-mans. Fig. 2 shows that humans are currentlydischarging all of the recoverable stores oforganic chemical energy to the anode of theearth-space battery as heat.The organism-generated earth-space bat-

tery consists of two kinds of organic chemical

compounds. The first are fossil fuels. Thesefossil fuels are primarily hydrocarbons, con-taining mostly carbon and hydrogen, almostno oxygen, and often small but significantamounts of other elements, such as sulfur,vanadium, iron, zinc, and mercury, whichcan be toxic when released into the envi-ronment and taken up by humans and otherorganisms. The reserves of fossil fuels, mostdeposited hundreds of millions of years ago,are finite and rapidly being depleted. Oil,gas, and coal, which account for more than85% of current global human energy con-sumption, are burned to produce the goodsand services for our industrial-technologi-cal-informational economy. Despite someexcellent sobering analyses of the presentuse and future prospects of fossil fuels (4, 7,8), the magnitude of the remaining eco-nomically recoverable hydrocarbon energystore is subject to much debate. In Fig. 2 weacknowledge the uncertainty by assigning aconservative value of <40 zeta joules (ZJ).

The Critical Importance of LivingBiomassHere we focus on the second kind of chem-icals comprising the earth-space battery, theorganic compounds in living biomass. Ourwork suggests that the two smallest values, 19and 2 ZJ, on the bar chart in Fig. 2 are themost important. The 19 ZJ represents thecurrent chemical potential energy stored inthe form of living biomass, most of it asphytomass in terrestrial plants and most ofthat in forests. These chemicals are the car-bohydrates, lipids, proteins, cellulose, lignins,and other substances that make up the bodiesof living organisms. Unlike fossil fuels, themagnitude of this energy storage gradient(i.e., its distance from equilibrium) is main-tained by a steady flow of solar energy (9).The 2 ZJ is the energy flow due to the netannual primary production (NPP) of theplanet, which is the quantity of energy con-verted each year from solar energy to biomassby the process of photosynthesis. Global NPPis the Earth’s yearly renewable energy budgetwithin which all living things operate andwithin which our hunter-gatherer humanancestors previously operated. Therefore,an input of 2 ZJ/y of photosynthesis maintainsa standing stock of 19 ZJ of stored biomass.This stored biomass is essential to modern

humans, because its chemical energy sustains ahabitable biosphere away from the chemicalequilibrium of space. The NPP and storedliving biomass of the biosphere maintain bio-diversity and regulate climate andbiogeochemicalcycling. Themetabolic energy that powers ourbodies and sustains our population is derivedfrom NPP, because all of our food is living

biomass produced by the plants and animalsin the earth’s diverse ecosystems: agriculturalfields, grazing lands, oceans, and fresh waters.Furthermore, biomass is essential for humansto access all other forms of energy, includingwind, hydro, fossil, nuclear, etc.

Living Biomass Is Depleting RapidlyAt the time of the Roman Empire and thebirth of Christ, the earth contained ∼1,000billion tons of carbon in living biomass (10),equivalent to 35 ZJ of chemical energy,mostly in the form of trees in forests. In justthe last 2,000 y, humans have reduced this byabout 45% to ∼550 billion tons of carbon inbiomass, equivalent to 19.2 ZJ. The loss hasaccelerated over time, with 11% depleted justsince 1900 (Fig. 3) (11, 12). Over recent years,on average, we are harvesting—and releasingas heat and carbon dioxide—the remaining550 billion tons of carbon in living biomass ata net rate of ∼1.5 billion tons carbon per year(13, 14). The cause and measurement of bio-mass depletion are complicated issues, and thenumbers are almost constantly being reeval-uated (14). The depletion is due primarily tochanges in land use, including deforestation,desertification, and conversion of vegetatedlandscapes into barren surfaces, but also sec-ondarily to other causes such as pollution andunsustainable forestry and fisheries. Althoughthe above quantitative estimates have consid-erable uncertainty, the overall trend andmagnitude are inescapable facts with direthermodynamic consequences.

The Dominant Role of HumansHomo sapiens Is a Unique Species. Thehistory of humankind—starting with hunter-gatherers, who learned to obtain useful heatenergy by burning wood and dung, andcontinuing to contemporary humans, whoapply the latest technologies, such as fracking,solar panels, and wind turbines—is one ofinnovating to use all economically exploitableenergy sources at an ever increasing rate (12,15). Together, the biological imperative ofthe Malthusian-Darwinian dynamic to use allavailable resources and the social imperativeto innovate and improve human welfare haveresulted in at least 10,000 y of virtually un-interrupted population and economic growth:from a few million hunter-gatherers to morethan 7 billion modern humans and froma subsistence economy based on sustainableuse of plants and animals (i.e., in equilibriumwith photosynthetic energy production) to themodern industrial-technological-informationaleconomy (i.e., out of equilibrium due to theunsustainable unidirectional discharge of thebiomass battery).

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Fig. 4 depicts the multiplier effect of twolarge numbers that determine the rapid dis-charge rate of the earth‐space battery. Energyuse per person multiplied by population givestotal global energy consumption by humans.According to British Petroleum’s numbers(16), which most experts accept, in 2013,average per capita energy use was 74.6 × 109

J/person per year (equivalent to ∼2,370 W ifplotted in green in Fig. 4). Multiplying this bythe world population of 7.1 billion in 2013gives a total consumption of ∼0.53 ZJ/y(equivalent to 16.8 TW if plotted in red inFig. 4), which is greater than 1% of the totalrecoverable fossil fuel energy stored in theplanet (i.e., 0.53 ZJ/40 ZJ = 1.3%). As timeprogresses, the population increases, and theeconomy grows, the outcome of multiplyingthese two very large numbers is that the totalrate of global energy consumption is growingat a near-exponential rate.To put these numbers in perspective,

consider a point of reference. An individualhuman requires on average 8.4 MJ/d (2,000kcal/d) in the form of food to support a bi-ological metabolic rate of about 100 W. Tofuel their diverse activities, contemporary hu-mans supplement biological metabolism withextrametabolic energy derived from othersources, principally fossil fuels. Therefore, the

current per-capita consumption of 2,370 Widentified above for an average person is about24 times that of a hunter-gatherer ancestor.Furthermore, this average value does not in-dicate the wide variation in per capita energyconsumption as a function of socioeconomicconditions, which ranges from only slightlymore than the biological metabolic rate in thepoorest developing countries to more than11,000 W in the most developed countrieswith their energy-demanding industrial-technological-informational economies (8, 17).Compared with humankind’s metabolic needsand the remaining chemical stores in theearth-space battery (distance to thermody-namic equilibrium), the rate of net dischargeis very large and obviously unsustainable.The earth is in serious energetic imbalance

due to human energy use. This imbalancedefines our most dominant conflict withnature. It really is a conflict in the sense thatthe current energy imbalance, a crisis un-precedented in Earth history, is a directconsequence of technological innovation. Thedetrimental effects of discharging the organicchemical energy stored in the battery extendfar beyond the depletion of stored livingphytomass and fossil fuel energy. Considerminerals. Energetically overpowered hu-mans have discovered and mined most of

the richest deposits of copper, iron, zinc,gold, and silver, used these metals to sup-port the industrial economy, and dispersedthe unused “wastes” to landfills and un-recoverable pools. Consider nitrogen andphosphorus, critical ingredients of fertilizerbecause they are essential for plant growth.Global deposits of nitrate and phosphatehave been drastically depleted. Nitrogenfertilizer can be synthesized from atmo-spheric nitrogen gas, but this chemical pro-cess requires a large input of exogenousenergy, usually in the form of fossil fuel (18).More ominously, there is no substitute foror mechanism for artificially synthesizingphosphorus. Consider water. By dammingrivers and streams and digging wells intosubsurface aquifers, humans currently usemore than 56% of all accessible fresh water.Most of this water is used for irrigation ofcrops, so that human activities account forabout 26% of the water lost by evapotrans-piration from terrestrial ecosystems (19, 20).Consider impacts on global ecosystems (21)and biodiversity (22). To produce plant andanimal products for human consumptionand to house our growing population, wehave transformed ecosystems and land-scapes on approximately 83% of Earth’sice-free land area. We have replaced forestsand other native ecosystems with agricul-tural crops, pastures, forestry plantations,buildings, and pavement, pre-empting about40% of terrestrial NPP and reducing thestanding stock of living biomass on theplanet by an estimated 45%. Additional hu-man-caused changes have substantially re-duced the stocks of ocean fisheries, alteredglobal biogeochemical cycles and climate,and caused extinction of species at 100–1,000times the average prehuman extinction rates.Finally, consider that 15–30% of currentglobal energy consumption is used to simplysupply food for 7.2 billion people (23, 24).Most of this energy comes from fossil fuelsand is used for the supplemental inputs ofwater, fertilizer, pesticides, and machinelabor that enable modern agriculture toachieve high yields (25–27). Therefore, thehuman population is sustained by the NPPof agriculture, but the capacity of this agri-culture to feed the global population requiresmassive discharge of the earth-space battery.The unidirectional dissipation of living

biomass and fossil fuel energy from the bat-tery has provided our species unprecedentedpowerful domination over the biogeochemicalcycles and other organisms of the planet.Others have chronicled these changes andtheir consequences (18–22, 28–30), but theirwarnings have failed to arouse sufficient publicconcern and motivate a meaningful response.

0

5

10

15

20

25

30

35

40

0 500 1000 1500 2000 2500

Phyt

omas

s St

ores

, ZJ

Year AD

Fig. 3. Global phytomass stores. Calculated from table 2 of Smil (11), assuming 1 t carbon ∼35 × 109 joules.ZJ = zeta joules = joules × 1021.

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Ironically, powerful political and marketforces, rather than acting to conserve the re-maining charge in the battery, actually pushin the opposite direction, because the perva-sive efforts to increase economic growth willrequire increased energy consumption (4, 8).Much of the above information has beenpresented elsewhere, but in different forms(e.g., in the references cited). Our synthesisdiffers from most of these treatments in tworespects: (i) it introduces the paradigm of theearth‐space battery to provide a new perspec-tive, and (ii) it emphasizes the critical impor-tance of living biomass for global sustainabilityof both the biosphere and human civilization.

Humans and PhytomassWe can be more quantitative and put thisinto context by introducing a new sustain-ability metric Ω

Ω=PBN

[1]

which purposefully combines perhaps thetwo critical variables affecting the energystatus of the planet: total phytomass andhuman population. Eq. 1 accomplishes thiscombination by dividing the stored phyto-mass chemical energy P (in joules) by the

energy needed to feed the global populationfor 1 y (joules per year; Fig. 5). The de-nominator represents the basic (metabolic)energy need of the human population; it isobtained by multiplying the global populationN by their per capita metabolic needs for 1 y(B = 3.06 × 109 joules/person·per year ascalculated from an 8.4×106 joules/person·daydiet). The simple expression for Ω gives thenumber of years at current rates of con-sumption that the global phytomass storagecould feed the human race. By making theconservative but totally unrealistic assumptionthat all phytomass could be harvested to feedhumans (i.e., all of it is edible), we get an ab-solute maximum estimate of the number ofyears of food remaining for humankind. Fig. 5shows that over the years 0–2000, Ω has de-creased predictably and dramatically from67,000 to 1,029 y (for example, in the year2000, P = 19.3 × 1021 joules, B = 3.06 × 109

joules/person·per year, and N = 6.13 × 109

persons; thus,Ω= 1,029 y). In just 2,000 y, oursingle species has reduced Ω by 98.5%.The above is a drastic underestimate for

four reasons. First, we obviously cannot con-sume all phytomass stores for food; the pre-ponderance of phytomass runs the biosphere.

Second, basing our estimate on human bio-logical metabolism does not include that highrate of extrametabolic energy expenditurecurrently being used to feed the populationand fuel the economy. Third, the above es-timate does not account that both the globalhuman population and the per-capita rate ofenergy use are not constant, but increasing atnear-exponential rates. We do not attempt toextrapolate to predict the future trajectories,which must ultimately turn downward asessential energy stocks are depleted. Finally,we emphasize that not only has the globalstore of phytomass energy decreased rapidly,but more importantly human dominanceover the remaining portion has also in-creased rapidly. Long before the hypotheti-cal deadline when the global phytomassstore is completely exhausted, the energeticsof the biosphere and all its inhabitant specieswill have been drastically altered, with pro-found changes in biogeochemical functionand remaining biodiversity. The very conser-vative Ω index shows how rapidly land usechanges, NPP appropriation, pollution, andother activities are depleting phytomass storesto fuel the current near-exponential trajecto-ries of population and economic growth. Be-cause the Ω index is conservative, it alsoemphasizes how very little time is left to makechanges and achieve a sustainable future forthe biosphere and humanity. We are alreadyfirmly within the zone of scientific uncertaintywhere some perturbation could trigger a cat-astrophic state shift in the biosphere and inthe human population and economy (31). Aswe rapidly approach the chemical equilibriumof outer space, the laws of thermodynamicsoffer little room for negotiation.

DiscussionThe trajectory ofΩ shown in Fig. 5 has at leastthree implications for the future of human-kind. First, there is no reason to expect a dif-ferent trajectory in the near future. Somethinglike the present level of biomass energy de-structionwill be required to sustain the presentglobal population with its fossil fuel‐subsidizedfood production and economy. Second, as theearth‐space battery is being discharged everfaster (Fig. 3) to support an ever larger pop-ulation, the capacity to buffer changes will di-minish and the remaining energy gradientswill experience increasing perturbations. Asmore people depend on fewer available energyoptions, their standard of living and very sur-vival will become increasingly vulnerable tofluctuations, such as droughts, disease epi-demics, social unrest, and warfare. Third,there is considerable uncertainty in how thebiosphere will function as Ω decreases fromthe present Ω = ∼1,029 y into an uncharted

Fig. 4. History of global growth in per capita energy consumption, population, and total energy consumption.Reproduced from ref. 30, with permission from Macmillan Publishers Ltd, Nature.

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thermodynamic operating region. The globalbiosphere, human population, and economywill obviously crash long before Ω = 1 y. IfH. sapiens does not go extinct, the human pop-ulation will decline drastically as we will beforced to return to making a living as hunter‐gatherers or simple horticulturalists. Also,the earth after the collapse of human civi-lization will be a very different place than thebiosphere that supported the rise of civiliza-tion. There will be a long-lasting legacy of al-tered climate, landscapes, and biogeochemicalcycles, depleted and dispersed stocks of fossilfuels, metals, and nuclear ores, and diminishedbiodiversity. The most powerful species in the3.5-billion-year history of life has transformedthe earth and left a mark that will endure longafter its passing.

Many of the organizations and authorswho have recognized the seriousness of thelooming energy crisis are suggesting the pos-sibility of achieving some level of sustain-ability of the global population and economyby implementing renewable energy technol-ogies (32, 33). We too recognize the impor-tance of solar and other renewables incushioning the ecological and socioeconomicconsequences as the biosphere returns towarda steady state between NPP and respiration.There is indeed a large supply of solar energythat has not yet been tapped for human use.As mentioned above, sunlight is highly dis-persed low-quality energy. Consequently,current technologies rely heavily on fossil fuelsto design, mine, build, and operate the col-lection and distribution systems (34) and

expand the yet to be designed but compulsorylarge-scale energy storage systems. Moreover,whereas some deployment of solar systems(e.g., over roofs, roads, and parking lots) causeslittle direct reduction of biomass, greaterdeployment will undoubtedly result in in-creasing indirect biomass consequences toboth fabricate and install solar collectors andother infrastructure. The earth-space batteryparadigm clarifies why the total upfrontand ongoing energy investments in solarand other renewables need to be balancedwith the energy produced, i.e., greater en-ergy return on energy investment (4, 35),and why their production and installationmust not negatively impact the remainingbiomass budget of earth.The logic presented above is indisputable,

because the laws of thermodynamics are ab-solute and inviolate. Unless phytomass storesstabilize, human civilization is unsustainable.The battery paradigm highlights the need tocontinue to refine estimates of the globalbiomass degradation (13) and its corre-sponding chemical energy contents and ofrecoverable fossil fuels. It emphasizes the needfor greater recognition of the central impor-tance of living biomass and the past, present,and future trajectory of decreasing Ω. Historyoffers a mixed message about the capacity ofhumans to innovate and act in time to avoidcollapse. At local and regional scales, manymultiple past civilizations (e.g., Greece, Rome,Angkor Wat, Teotihuacan) failed to adaptto changing social and ecological conditionsand crashed catastrophically. At the sametime, human ingenuity and technological in-novations allowed the global population andeconomy to grow at near-exponential rates.This growth has been fueled by exploitingnew energy sources, transitioning among an-imal, hydro, wind, wood, coal, oil, naturalgas, nuclear, photovoltaic solar, geothermal,and others. The implications of past localizedcollapses and global growth are of questionablerelevance to the current situation, however,because now, for the first time in history, hu-manity is facing a global chemical energy limit.The earth-space battery paradigm provides asimple framework for understanding the his-torical effects of humans on the energy dy-namics of the biosphere, including the unal-terable thermodynamic boundaries that nowpose severe challenges to the future of hu-mankind. Living biomass is the energycapital that runs the biosphere and supportsthe human population and economy. Thereis an urgent need not only to halt the de-pletion of this biological capital, but to moveas rapidly as possible toward an approxi-mate equilibrium between NPP and respi-ration. There is simply no reserve tank of

0

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0 500 1000 1500 2000 2500

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1960 1970 1980 1990 2000 2010Year AD

Fig. 5. Number of years of phytomass food potentially available to feed the global human population. Calculated fromthe total stored phytomass energy of the planet divided by the metabolic energy needs to feed the global population for 1 y(i.e., joules/joules per year = years) assuming an 8.4-MJ per capita daily diet for the entire year. Rapidly decreasing trend lineindicates increasing dominance of phytomass by humankind. For reasons given in the text, these values are very conser-vative. Little margin remains to safely continue the current trend.

9516 | www.pnas.org/cgi/doi/10.1073/pnas.1508353112 Schramski et al.

biomass for planet Earth. The laws of ther-modynamics have no mercy. Equilibrium isinhospitable, sterile, and final.

Materials and MethodsTo calculate omega in Fig. 5, we used the data on increase inpopulation N from the years 0 to 1950 and from 1950 to2000 from the US Census Bureau (36, 37). In all cases, if

there was a variation in population estimates for a givenyear, to be conservative, we used the lowest. Phytomassenergy content P required a continuous function to rep-resent all years from 0 to 2000. We used second-orderequations to fit the data points in Fig. 3. The first threedata points (years 0–1800) were represented by phyto-mass energy = [35 − 1.70 × 10−6 (year)2 − 1.801 × 10−3

(year) − 1.8031 × 10−3] zeta joules. The remaining datapoints (years 1800–2000) were represented by phyto-

mass energy = [35 − 3.386 × 10−5 (year)2 + 9.373−2

(year) − 67.770] zeta joules.

ACKNOWLEDGMENTS. The University of Georgia’s En-vironmental Engineering students are gratefully acknowl-edged for persistent engaging questions and deep dis-cussions that enhanced key points in this research. Thiswork was funded in part by National Science FoundationMacrosystems Biology Grant EF 1065836 (to J.H.B.).

1 Brown JH, et al. (2004) Toward a metabolic theory of ecology.Ecology 85(7):1771–1789.2 Sibly RM, Brown JH, Kodric-Brown A (2012) MetabolicEcology, A Scaling Approach (John Wiley & Sons, Ltd, WestSussex, UK).3 Payne JL, et al. (2011) The evolutinary consequences of oxygenicphotosynthesis: A body size perspective. Photosynthesis Res107(1):37–57.4 Hall CAS, Klitgaard KA (2012) Energy and the Wealth of Nations:Understanding the Biophysical Economy (Springer, New York).5 Woodwell GM, et al. (1978) The biota and the world carbonbudget. Science 199(4325):141–146.6 Whittaker RH (1970) Communities and Ecosystems (Macmilliam,New York).7 Hall CAS, Day JW (2009) Revisiting the limits to growth after peakoil. Am Sci 97(3):230–237.8 Brown JH, et al. (2011) Energetic limits to economic growth.Bioscience 61(1):19–26.9 Schneider ED, Kay JJ (1994) Life as a manifestation of thesecond law of thermodynamics. Math Comput Model 19(6-8):25–48.10 Bazilevich NI, Rodin LY, Rozov NN (1971) Geographical aspectsof biological productivity. Sov Geogr 12(5):293–317.11 Smil V (2011) Harvesting the biosphere: The human impact.Popul Dev Rev 37(4):613–636.12 Smil V (2013) Harvesting the Biosphere: What We Have Takenfrom Nature (MIT Press, Cambridge, MA).13 Houghton RA (2010) How well do we know the flux of CO2from land-use change? Tellus, Series B 62(5):337–351.14 Houghton RA, Hall F, Goetz SJ (2009) Importance ofbiomass in the global carbon cycle. J Geophys Res 114(G2):G00E03.

15 Jevons WS (1866) The Coal Question (Macmillan and Company,London), 2nd Ed.16 BP (2013) Statistical review of world energy. Available at www.bp.com/content/dam/bp/pdf/statistical-review/statistical_review_of_world_energy_2013.pdf. Accessed June 26, 2015.17 Brown JH, et al. (2014) Macroecoloty meets macroeconomics:Resource scarcity and global sustainablity. Ecol Eng 65:24–32.18 Vitousek PM, et al. (1997) Human alteration of the globalnitrogen cycle. Ecological Applications 7(3):737–750.19 Postel SL, Daily GC, Ehrlich PT (1996) Human appropriation ofrenewable fresh water. Science 271(5250):785–788.20 Nilsson C, Reidy CA, Dynesius M, Revenga C (2005)Fragmentation and flow regulation of the world’s large river systems.Science 308(5720):405–408.21 Hooke RL, Martin-Duque JF, Pedraza J (2012) Landtransformation by humans: A review. GSA Today 22(12):4–10.22 Pimm SL, Russell GJ, Gittleman JL, Brooks TM (1995) The futureof biodiversity. Science 269(5222):347–350.23 Felix E, Dubois O (2012) Energy-Smart Food at FAO (Food andAgriculture Organization of the United Nations, Rome).24 Canning C, et al. (2010) Energy Use in the U.S. Food System(USDA, Washington, DC).25 Schramski JR, et al. (2013) Energy as a potential systems-levelindicator of sustainability in organic agriculture: Case study model of adiversified, organic vegetable production system. Ecol Modell267:102–114.26 Pheiffer DA (2006) Eating Fossil Fuels: Oil, Food, and the ComingCrisis in Agriculture (New Society Publishers, Gabriola Island, BC, Canada).27 Pimentel D, Pimentel M (2008) Food, Energy, and Society (CRCPress, Boca Raton, FL).28 Bai ZG, et al. (2008) Proxy global assessment of landdegradation. Soil Use Manage 24(3):223–234.

29 Sanderson EW, et al. (2002) The human footprint and the last of

the wild. Bioscience 52(10):891–904.30 Ehrlich PR, Kareiva PM, Daily GC (2012) Securing natural capital

and expanding equity to rescale civilization. Nature 486(7401):68–73.31 Barnosky AD, et al. (2012) Approaching a state shift in Earth’s

biosphere. Nature 486(7401):52–58.32 Jacobson MZ, Delucchi MA (2011) Providing all global energy

with wind, water, and solar power, Part I: Technologies, energy

resources, quantities and areas of infrastructure, and materials.

Energy Policy 39(3):1154–1169.33 IPCC (2011) Special Report on Renewable Energy Sources

and Climate Change Mitigation (Cambridge Univ Press,

Cambridge, UK).34 Palmer G (2014) Energy in Australia: Peak Oil, Solar Power, and

Asia’s Economic Growth, 1st Ed (Springer International Publishing,

Cham, Switzerland).35 Prieto PA, Hall CAS (2013) Spain’s Photovoltaic Revolution

(Springer, New York).36 US Census Bureau (2013) Historical estimates of world

population. Available at https://www.census.gov/population/

international/data/worldpop/table_history.php. Accessed June 26,

2015.37 US Census Bureau (2013) Total midyear population for the world:

1950-2050. Available at https://www.census.gov/population/

international/data/idb/worldpoptotal.php. Accessed June 26, 2015.38 Smil V (2008) Energy in Nature and Society: General Energetics

of Complex Systems (MIT Press, Cambridge, MA).39 Numerical Terradynamic Simulation Group (2004) Four years of

MOD17 annual NPP. Available at images.ntsg.umt.edu/. Accessed

June 26, 2015.

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