World energy resources

• 1. Introduction

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  World energy resources are the largest amount of energy that Earth could in principle supply if every available source were counted together. The planet holds a total heat content of 13,000,000 YJ, or yottajoules, a number so vast that it makes human demand look tiny. Almost none of it can actually be tapped. The real story lives in the gap between what exists and what can be reached, paid for, and used safely. How much coal, gas and oil is truly left, and who can say for certain? Can uranium, sunlight, wind and tides carry the load instead? And are the limits that matter physical, or are they political?

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• 2. Counting What Remains

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  At the end of 2009, proven reserves stood at 19.8 zettajoules of coal, 36.4 of gas and 8.9 of oil. The real totals may be four or more times larger, and every one of these numbers is shaky. Estimating them requires a detailed picture of Earth's crust. Modern rigs can drill wells in up to 3 km of water to check the geology below. Half of the ocean is deeper than that, which leaves about a third of the planet beyond detailed analysis.

  Knowing that a deposit exists is not the same as pulling it out at a profit. Recovery depends on technology, economics and politics. Planners must weigh how reachable a deposit is, how much sulfur and other pollutants sit in the oil or coal, what transport will cost, and whether the producing region is stable. In general, the easiest deposits are taken first.

  Coal shows the pattern on the largest scale. It is the most abundant and most burned fossil fuel, and it launched the industrial revolution. In 2007, China, already home to many of the world's most polluted cities, was building about two coal-fired power plants every week. Coal's enormous reserves would make it a natural answer to global demand, if not for global warming and other pollution.

  Natural gas offers a quieter story. Recoverable reserves are put at 850,000 cubic kilometres, with at least as much again reachable by enhanced methods that free shale gas. Wide exploration and the arrival of shale fracking produced a major jump in what counts as recoverable. At present rates of use, gas could supply most of the world's energy needs for 100-250 years, depending on how consumption grows. Oil, the third fuel in that ledger, has proved far harder to pin down.

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• 3. The Oil Peak Debate

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  The best single estimate puts oil at 57 zettajoules, yet published figures run from a low of 8 to a high of 110. The low end counts only proven, recoverable reserves. The high end counts oil that is available but not necessarily recoverable, including optimistic guesses for oil sands and oil shale.

  Eighteen recognized estimates of supply profiles agree, on balance, that extraction will peak in 2020 at 93 million barrels per day. Current consumption runs at 0.18 zettajoules a year. That equals 31.1 billion barrels, or about 85 million barrels per day.

  Other forecasters expect the peak much sooner. A 2005 report from the French Economics, Industry and Finance Ministry sketched a worst case as early as 2013. The ASPO predicts 2010. Some theories say the peak could arrive in as little as 2-3 years, and others argue it already happened in 2005.

  The US EIA data give that last claim some support. World crude oil production, including lease condensates, fell from a peak of 73.720 million barrels per day in 2005 to 73.437 in 2006 and 72.981 in 2007. It then rose to 73.697 in 2008.

  Peak oil theory pictures production as a bell-shaped curve. Pushing output higher brings a faster collapse later, while falling output spreads the decline over more years.

  Prices added their own drama. They had fallen from a high of $147 per barrel to a low of $40. With a stated goal of lifting the price to $75, OPEC announced it would cut production by 2.2 million barrels per day beginning the 1st of January 2009. Governments watching such swings were already weighing a harder question, which was whether to leave fossil fuels before the fuels ran out.

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• 4. Sustainability and Policy

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  The Kyoto Protocol pushed some countries to act on carbon emissions, and further steps have been proposed. Concern over secure supplies, global warming and sustainability is expected to pull the world's energy use away from fossil fuels. A government making that move would most likely apply economic pressure through carbon emissions charges and green taxation.

  The European Commission offers a concrete example. It proposed that European Union energy policy should set a binding target to raise renewable energy in the overall mix from less than 7% in 2007 to 20% by 2020.

  Peak oil thinking adds urgency. It holds that about half of the available petroleum has already been produced, and it predicts that output will now decline.

  The opposite of sustainability has a nickname, the Easter Island Effect. It describes a disregard for limits that ends with natural resources used up. Some estimates, assuming current consumption rates, say today's oil reserves could be fully depleted by 2050. A far larger store of energy lies in uranium, and its numbers look very different.

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• 5. Splitting Atoms

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  The International Atomic Energy Agency estimates the world's remaining uranium at 2500 zettajoules. That figure assumes breeder reactors, which create more fissile material than they consume. The IPCC is far more cautious. It puts proved, economically recoverable deposits at only 2 zettajoules for reactors using the once-through fuel cycle. Ultimately recoverable uranium is estimated at 17 zettajoules for once-through reactors and 1000 with reprocessing and fast breeder reactors.

  Supply, then, does not limit what nuclear power could contribute through the 21st century. Politics and environment do. Worries about nuclear safety and radioactive waste began to slow growth at the end of the last century, particularly after a number of nuclear accidents. Fear of proliferation, especially of plutonium produced by breeder reactors, explains why the international community actively discourages countries such as Iran and Syria from developing nuclear power.

  Thorium offers another path. Its reserves significantly exceed those of uranium, and it is estimated to be roughly three times as abundant in the Earth's crust. Many consider it easier to obtain. Uranium mines are enclosed underground and very dangerous for miners, while thorium comes from open pits. Since the 1960s, numerous facilities around the world have burned thorium.

  Research into a very different nuclear process, the fusing of hydrogen, began in the 1950s.

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• 6. Fusion's Hard Problem

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  No material can withstand the temperatures needed to ignite fusion fuel, so the fuel must be held in place without touching anything. Magnetic confinement and inertial confinement are the two main alternatives. Both were hot research topics in the early years of the 21st century, and Cadarache is one name tied to that work.

  Fusion is the process that powers the sun and other stars. It releases large quantities of heat by joining the nuclei of hydrogen or helium isotopes, which may be taken from seawater. In theory that heat can be turned into electricity, with vast supply and relatively little pollution.

  The obstacle is control. The temperatures and pressures needed to sustain fusion make it a very difficult process to manage.

  The United States, the European Union and other countries support the effort, for example by investing in the ITER facility. Even so, one report argues that inadequate research has stalled progress for the past 20 years. Against so difficult a prospect, renewable sources need no such leap, because they arrive afresh every year.

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• 7. Managed Renewables

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  A coal mine and a forest make the simplest comparison. Once the mine is exhausted, it is gone for good. A managed forest, in contrast, is a continuous supply of energy, though it could still be depleted by careless use.

  Most of Earth's available energy is renewable. Renewable resources account for more than 93 percent of total US energy reserves. To compare them fairly, annual renewable flows were multiplied by thirty years. If all non-renewable resources were used up evenly over those 30 years, they would supply only 7 percent of what developed renewables could provide each year.

  Biomass and biofuels are growing industries. Using waste products avoids a trade-off between food and fuel. Burning methane also cuts greenhouse gas emissions, because methane is 23 times stronger as a greenhouse gas than the carbon dioxide released in burning it. Biofuels are widely believed to be carbon neutral, but evidence suggests that fuels made with current farming methods are substantial net carbon emitters.

  Geothermal and biomass are the only two renewable sources that need careful management to avoid local depletion. Estimates for geothermal vary widely with assumed investment and geological guesses. A 1998 study suggested 65-138 GW of electrical capacity using enhanced technology, while other estimates range from 35 to 2000 GW, plus 140 EJ per year of direct use.

  A 2006 MIT report examined Enhanced Geothermal Systems. It concluded that the United States could affordably generate 100 GWe or more by 2050, for a maximum research investment of 1 billion US dollars over 15 years. It calculated the world's total EGS resources at over 13 YJ, with over 0.2 YJ extractable and potentially over 2 YJ with better technology. That is enough to meet all the world's energy needs for several thousand years.

  Hydropower tells a different tale. In 2005 it supplied 16.4% of world electricity, down from 21.0% in 1973, and only 2.2% of the world's total energy. Its slipping share leaves room for sources that are far larger in theory.

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• 8. Sun, Tides and Wind

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  About 89 PW of solar power falls on the planet's surface. Capturing less than 0.02% of it would meet current energy needs. The barriers are the high price of making solar cells and dependence on the weather. Solar generation also stops at night. That is a particular problem in high northern and southern latitudes, where demand peaks in winter just as sunshine is weakest.

  Importing power from countries nearer the equator in winter could ease the gap, as could inexpensive energy storage. Solar generation is the fastest growing source of energy, with annual average growth of 35% over the past few years. China, Europe, India, Japan and the United States are the major growing investors. Even so, solar's share of worldwide electricity use at the end of 2014 was just 1%.

  Tidal power produced 0.3 GW of electricity at the end of 2005. Tides come from the pull of the Moon, which accounts for 68%, and the Sun, which accounts for 32%, combined with Earth's rotation. Their fluctuations dissipate energy at an average rate of about 3.7 TW.

  The energy held in the oceans' tidal fluctuations is only about 0.6 EJ. Left alone, it would dissipate in about four semi-diurnal tide periods. Taking too much would disturb the tidal dynamics, so usable tidal energy is limited to around 0.8 TW, or 20% of the dissipation rate.

  Waves are a second-hand form of solar energy. They come from wind, which comes from sunlight, and each conversion drops the available energy by about two orders of magnitude. The total power of waves washing against Earth's shores adds up to 3 TW.

  Wind itself is estimated at 300-870 TW. Using the lower estimate, just 5% of the available wind energy would supply the world's current needs. The oceans cover 71% of the planet, and most of that wind blows across open water, where fewer obstructions let it blow harder.

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