Human society, like any system composed of dynamicprocesses, depends on an external energy source. His-torically, that source was the Sun, which provides heat,light, and photosynthesis for food to support work energyby man and animal, and affects wind and water motion.Since the early 19th century, though, the discovery of andaccess to a vast supply of fossil fuels within Earth has en-abled the industrial revolution, near-exponential growthof population,1technologies, and wealth. That period couldwell be renamed the energy revolution (see figure 1). As we enter a new millennium, we are growing in-creasingly concerned about the limits of our fossil fuelsthat are driving the world’s economies. Many journal arti-cles, committee reports, and books have addressed this“energy problem”; they contain opinions, ideas, and sug-gestions from experts within their various subdisciplineson possible ways to improve our practices or innovate tech-nologically. But a complex interdependence exists amongthe technological, social, and environmental aspects of en-ergy use (see the articles in PHYSICS TODAY, April 2002).Furthermore, many of the ideas researchers propose can-not significantly impact the real magnitude of the energyproblem or may provide only short-term relief.Our basic choices are limited. Nature’s energy re-sources are confined to two categories: Earth-stored fossilresidues and nuclear isotopes, whose economic utility islimited by the finite amounts that exist on Earth, and theradiation flux of solar energy, whose economic utility islimited by the finite rate at which we can capture the Sun’senergy and by the land areas that societies can dedicate toharness it.The longevity of the fossil energy supply and the netrate of solar energy availability are both reduced by theenergy consumed through their conversion to a suitableenergy form and the technologies that accompany that con-version: storage, delivery, maintenance, and repair of en-vironmental damage. Solar-derived consumer energy,whether as electricity, biomass, or wind, represents aclean, alternative energy form. It isimportant to understand a basic law ofnature: Energy, once used, is not re-generable. So the public term “renew-able energy” is misleading. The following analysis examinesthe magnitudes of the world’s energysupplies and the basic constraints onour ability to support in the long termsociety’s demands using those finitesupplies. To put those magnitudes intoa human context for policymakers and the public, thelongevity of our resources will be expressed on the scale ofa human lifespan (where 1 human lifespan is approxi-mately 75 years).Energy demandsIn viewing overall societal energy issues, it is useful to ex-press energy magnitudes in units of the quad (Q), where1 Q ⊂ 1015BTU, roughly equal to 2.5 × 1014kcal or1.06 × 1018joule. Current US energy consumption isabout 100 Q/year, roughly a quarter of the world’s total demand.2Energy demand by humanity continues to rise. An in-crease of about 1.5% per year is projected in the US andworld demand is expected to increase by 1–2% per year formany decades, mainly due to continued populationgrowth. While total demand is, of course, influenced bypersonal demand, even unusually large (20%, say) conser-vation efforts would be nullified by population growth inless than 20 years.Earth-stored resources!!Petroleum. In 1956, petroleum geologist M. King Hub-bert correctly predicted that a peak and subsequent dropin US production would occur around 1970.3In fact, for-eign imports have since risen to 60% of current consump-tion. US dependence on foreign petroleum is certain to increase.In 2000, Jay Hakes of the Energy Information Ad-ministration presented a similar and extensive US De-partment of Energy assessment of the likely trend andpeak in the world petroleum supply.4Figure 2 shows thepredicted range of years when the peak is likely to occurfor demand whose growth rate may be between 0–2%. Be-cause growth rates due to population alone are anticipatedto be at least 1% per year for many decades to come, thepivotal event is expected to occur well within a humanlifespan. Moreover, the analysis was based on an opti-mistic estimate of the world oil resource of approximately2200–3900 billion barrels, nearly twice the proven re-serve.5That would place the anticipated time to reach thepeak well within a few decades.!!Natural gas. A natural gas shortage exists now in theUS. Yet the current growth rate of US demand is ap-proaching 3% per year.2,6As seen in figure 3, the proven©2004 American Institute of Physics, S-0031-9228-0407-030-9 July 2004 Physics Today 47Paul B. Weisz is an emeritus professor of chemical and bioengi-neering at the University of Pennsylvania and a retired senior sci-entist and manager at the Central Research Laboratory of theMobil Corp. He is also currently an adjunct professor of chemicalengineering at the Pennsylvania State University.Population growth and energy demand are exhausting theworld’s fossil energy supplies, some on the timescale of asingle human lifespan. Increasingly, sharing naturalresources will require close international cooperation,peace, and security.Paul B. WeiszBasic Choices and Constraints onLong-Term Energy SuppliesUS natural gas reserve would last very few years, even atconstant (year 2000) demand.Estimated gas reserves worldwide are relativelylarge. Geologists have good reasons to believe the sum ofour reserves and still undiscovered (but likely to exist) nat-ural gas could last roughly for another 45–60 years (seefigure 3).2,6However, those reserves are widely scatteredaround the world: 58% are reported to be located in Rus-sia, Iran, and Qatar, with small contributions in numer-ous other countries.2Clearly, their use will depend on vastinternational and intercontinental transportation bypipelines, transoceanic shipment as liquefied natural gas(LNG), or advance conversion to liquid fuels. Energy sac-rifice by basic thermodynamic requirements plus processefficiency loss will accompany advance conversion to liq-uid fuels. Geopolitical cooperation will be essential.!!Coal. The largest fossil fuel resource available in theUS is coal. The energy content of the current US reserveis