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Our civilization is based on energy. We will express global energy fluxes in units of terawatts. A Watt is a flux of energy flow, equal to Joule per second, where Joule is a unit of energy like calories. A hair dryer might use 1000 W of power, or 1 kilowatt, kW. A terawatt is 1012 W, and is abbreviated as TW.

The Sun bathes the Earth in energy at a rate of about 173,000 TW. Photosynthesis captures about 100 TW. Mankind is consuming about 13 TW of energy per year. Some of our 13 TW of energy use comes out of the 173,000 TW the Earth gets from the Sun. Windmills, hydroelectric dams, solar cells, and firewood all derive their energy relatively directly or indirectly from sunlight. But this is not enough; we are also taking advantage of other forms of stored energy, from fossil fuels and radioactive elements. Fossil fuels derive their energy from past sunlight, photosynthesis that took place millions of years ago.

Over the Earth's history, photosynthesis has preserved a sizable amount of reduced carbon, which could in principle be reacted with O2 to regenerate CO2 and energy. If we tried it on Earth, we would run out of oxygen in the air before we ran out of reduced carbon. As it turns out the vast majority of this reduced carbon is so diluted by rocks and clay minerals that it is unusable for fossil fuels. Only in special conditions do we find organic carbon concentrated enough to be useable for energy. The main forms of fossil carbon that we use for converting to energy are coal, oil, and natural gas.

Globally, most of the energy we use comes from fossil fuels such as petroleum, gas, and coal (Fig. 9.1). Methane is the most reduced form of carbon, so that when an atom of carbon from methane undergoes the transition to oxidized CO2, it gives off more energy than would an atom of carbon in coal, which has an oxidation state at the level

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Fig. 9.2 Natural gas emits less CO2 per Watt of energy than oil or coal.

Gas Oil Coal of carbohydrates (O). This means that a Watt of energy from methane releases less carbon than a Watt from coal or wood (Fig. 9.2).

Non-carbon-emitting sources, mostly nuclear and hydroelectric, make up about an eighth of the global energy supply. We use renewable sources of energy, defined by the US Department of Energy to mean geothermal, solar, wind, and wood and waste electric power, for about 0.8% of our energy.

The largest traditional fossil fuel reservoir is coal. Coal is solid, almost pure carbon when it is mature, the good stuff. Coal originates from land plants that deposited in swamps. Land plant deposits in swamps today, that might turn into coal someday, are

Coal use Oil use

I | Residential |?i?i] Transportation V//\ Commercial 17171 Electricity EvSl Industry

Fig. 9.3 How different forms of fossil energy are used in the United States.

called peat. The British Petroleum (BP) company, in their annual report on global energy (see Further reading) estimates the existence of 1000 Gton of carbon in coal reserves. Another source, an academic paper by Rogner (also listed in Further reading), estimates that the total amount ultimately available might be 10 times that. No matter what assumptions are made about future energy needs, there is enough coal to supply our needs for several centuries, if we choose to use it. In the United States, coal is mostly used for generating electricity (Fig. 9.3).

Oil and natural gas mostly come from ancient photosynthesis that took place in the ocean. Plants in water are generically called algae. There are microscopic algae in the ocean called phytoplankton. The phytoplankton in the ocean produce about as much organic carbon by photosynthesis as the terrestrial biosphere on land does. A tiny fraction of the dead phytoplankton ends up in sediments, while the rest gets eaten by somebody, animal or bacterial, in the water column or in the top layer of sediments. The sediments covering most of the sea floor do not contain enough organic carbon to ever make oil. You find organic-rich sediments nearby continents and under waters which have no oxygen dissolved in them, that is, in anaerobic conditions.

If organic-rich sediments are buried to a depth in the Earth between 7 and 15 km, they will be heated to temperatures of 500-700 K. This has the effect of converting some

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Fig. 9.4 The structure of normal octane, a hydrocarbon chain such as found in petroleum.

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North America

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Fig. 9.5 The amount of proven petroleum reserves, by region, from BP.

of the dead plankton into oil, consisting of chains of carbons with hydrogens attached (Fig. 9.4). The oil may just sit there in little droplets, unreachable by oil companies, or it may flow through the rock if it is porous enough, perhaps to collect in some upside-down pool on the bottom of an upside-down bowl in some impermeable layer of rocks above it. Probably only a tiny fraction of the oil ever produced is in a harvestable form today, that is to say mobile enough to have flowed together but not mobile enough to get squeezed all the way out of the Earth. Oil is perhaps the most convenient of the fossil fuels because it is easily stored and transported in its liquid form. In the United States, oil is mostly used for transportation (Fig. 9.3).

Perhaps because such special conditions are required to produce harvestable oil, the distribution of oil deposits on Earth is very spotty (Fig. 9.5). Most of the oil is in the Middle East, although there is a smattering scattered throughout the rest of the world. We don't know precisely how much oil there is available to harvest. The BP report tallies the proven reserves of oil, stuff that has been discovered and documented already (Fig. 9.5). They say 1,150,000 million barrels of oil, which we can translate into 135 Gton C. The amount of oil that is likely to be ultimately extractable is almost certainly higher than this. Technological advances in oil extraction can double the amount of oil you can get from an oil field. More oil fields will certainly be discovered, although there are limits to how much more oil we can expect to discover. Half of the world's oil reserves today are in the 100 biggest oil fields, and most of the largest ones were discovered decades ago.

There is a class of carbon deposits for which the geological conditions were never quite right to produce oil, but from which oil can be extracted if the rocks are mined and

Year

Fig. 9.6 The value of the production to reserves ratio over the last 25 years, from BP.

Year

Fig. 9.6 The value of the production to reserves ratio over the last 25 years, from BP.

cooked. These nontraditional oil sources are called oil shales and oil sands. The German government during the Second World War tried desperately to produce oil from coal but never succeeded. The Canadians are mining and processing oil shales now. If future discoveries, technology progress, and all these effects are counted together, the estimates are all over the map, but some of them begin to reach 500 Gton C or so.

How long will the oil last? This is a question that is starting to be discussed in the newspapers. One way to calculate a number with units of time would be

production rate year

BP calls this the reserves to production ratio. The R/P ratio tells us how long it would take to use up a resource if we continued using it at the rate we're using it now. Notice that the units on both sides of the equation balance. The funny thing about this ratio is that its value hasn't changed in 15 years (Fig. 9.6). According to BP figures, we have 40 years of oil left, and it looks like we always had 40 years left, and we could have 40 years left forever. The clock seems to be ticking very slowly. Actually, what is happening is that oil is being discovered, and extraction efficiency is increasing, about as quickly as oil is being used. The rate of consumption is growing with time, but we have been discovering new oil quickly enough to keep pace with that growing demand.

Another way to estimate the lifetime of the age of oil was developed by geologist M. King Hubbert (see the Deffeyes books listed in Further reading). Hubbert pointed out that the rate of extraction of a limited natural resource such as oil tends to follow a bell-shaped curve. This is an example of an empirical observation; there is no theoretical underpinning to the curve, but it sure does fit nice. The rate of extraction has a spinup time at the beginning, gets going faster and faster, then slows down as the resource starts to run out. Resource extraction histories are not required to follow this curve. One could imagine an extract-the-juice-from-a-popsicle-on-a-hot-day curve. The popsicle consumption rate starts off slowly at the beginning because the o <n œ 0C

Bell curve

Popsicle curve

Bell curve

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Fig. 9.7 A bell curve and an eating-a-popsicle-on-a-hot-day curve.

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Fig. 9.7 A bell curve and an eating-a-popsicle-on-a-hot-day curve.

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Fig. 9.8 A Hubbert curve for the rate of sperm whale oil production. With permission from U. Bardi, Università di Firenze.

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Fig. 9.8 A Hubbert curve for the rate of sperm whale oil production. With permission from U. Bardi, Università di Firenze.

popsicle is too cold to eat, then at the very end you have to swallow the last half of the popsicle in one gulp to keep it from hitting the sidewalk (Fig. 9.7). The crucial feature of the bell curve is that the maximum rate of extraction, the peak, occurs when half of the resource is still in the ground. The peak of the popsicle-eating curve comes at the very end. It could be that global oil will follow a popsicle curve instead of a bell curve, but a bell curve seems to have worked in the past, for individual oil and coal fields. Figure 9.8 shows a Hubbert's peak for sperm oil harvesting back in the whaling days.

Hubbert's theory gains credibility from the fact that Hubbert used it, in 1956, to forecast the peak extraction of oil in the United States. He predicted the peak to be sometime between 1965 and 1972, and in fact it came in 1970. The solid line in Fig. 9.9 shows the time up to Hubbert's forecast, and the dashed line a prediction like his for the future (benefiting a bit from hindsight, I confess). Hubbert's prediction came at a time when oil production in the United States seemed endless.

Fig. 9.9 Oil production in the United States follows a bell curve.

Year

Fig. 9.10 Global oil production, history so far plus two Hubbert's peak projections.

Year

Fig. 9.10 Global oil production, history so far plus two Hubbert's peak projections.

The forecast for world petroleum depends entirely on what you call oil and how much is ultimately out there. Figure 9.10 shows world oil production in the past, fit to two bell curves, one for 200 Gton C in oil that will ultimately be extractable and one for 500 Gton C. We have already extracted 117 Gton C. If 500 Gton C is ultimately right, then the peak comes in the 2030s sometime. If 200 Gton C is right, consistent with BP, the peak should be now. The significance of the peak for the world economy is that demand and consumption of petroleum is growing exponentially, along with other social factors like population and GDP. If the constant or dwindling oil supply is unable to keep up with the exponentially rising demand, that's what they call shortage. The bottom line is that the time to watch out for is when the oil is half gone; things won't be fine until the last drop.

Natural gas is another basic type of fossil fuel. Natural gas comes out of the Earth as a gas instead of a liquid. It is mostly methane, CH4, with a few other slightly larger carbon molecules thrown in. Oil and oil source rocks ultimately convert to natural gas if they are buried deeply in the hot interior of the Earth. Methane can also be produced from organic carbon by bacteria. Methane sometimes comes out of oil wells, or it can be extracted from coal beds. Methane is more difficult to transport than liquid petroleum because it is a gas and must be held under pressure. For this reason, methane associated with oil in remote locations is sometimes just burned, giant dramatic flares into the sky, rather than collected. In Siberia, for example, natural gas is worth nothing. The ultimate extractable amount of natural gas is probably even more poorly known than it is for petroleum because it hasn't been explored for as intensively. Industry estimates are about 100 Gton C as natural gas in proven reserves. Just like for oil, the ultimately available natural gas reservoir will certainly be higher than that.

There is also a 363 kg gorilla in the picture, hiding in near-shore ocean sediments. Methane hydrates are methane gas molecules frozen into soccer-ball cages of water ice. Water can form these cages around any gas; all the gas is required to do is hold the little soccer-balls open. CO2 forms hydrates on Mars. It turns out that there are thousands of Gtons of carbon in methane hydrate deposits on Earth, in mid-depth sediments of the ocean and in permafrost soils.

There are two required ingredients for methane hydrates: lots of methane and cool temperature. It is unusual to find methane hydrates very close to the sea floor because methane concentrations tend to be low in surface sediments. The temperature sets a lower limit to the depth range where methane hydrates are found because it gets hotter with depth in the Earth. Beneath the hydrate melting depth, methane often exists as bubbles of gas within the sediment. This layer of bubbles can be seen in sediments around the world in the reflection of seismic waves, echoes of explosions set behind ships. The reflective layer of bubbles tends to parallel the sediment surface, and so is called a bottom-simulating reflector. The bottom-simulating reflector in seismic data is the best indication we have of how extensive the methane hydrate deposits are.

Methane clathrate deposits seem like the most unlikely and precarious of things. The ices themselves would float in seawater if they were not held down by the sediment above. The bubbles make the deposits even more unstable. They decrease the average density of the sediment column. If the mixture of solid, seawater, and bubbles (called a slurry) starts to rise up, the gas bubbles would expand causing the slurry to rise even faster. Methane regularly escapes catastrophically from the sediment to the ocean, leaving behind explosion craters called pockmarks. Methane may also be released by submarine landslides. We will hear a prognosis for methane clathrates in Chapter 12.

Returning to Fig. 9.1, we see how different countries get their energy. India and China, the largest of the developing world, use a lot of coal whereas Brazil uses mostly petroleum and hydroelectric power. Denmark is remarkable for windmills, which fall into the category "renewable" on this plot. France has invested heavily in nuclear energy (not renewable). The US energy sources are similar to the global average. In part this is because the United States is a big consumer of energy, accounting for about a quarter of energy use globally (Fig. 9.11).

If we divide the energy use of a country by the number of people, we get the energy use per capita. We see in Fig. 9.11 that Americans use five times more energy than the global average citizen of the world, ten times more than the average Chinese or Indian, and twice as much even as the average European or Japanese.

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Fig. 9.11 (a) Energy consumption, (b) energy consumption per person, and (c) energy consumption per dollar gross domestic product, GDP, globally and from various countries.

Energy use is closely related to economic productivity, which is measured by the total value of everything produced for sale in a country in a year, the gross domestic productivity or GDP in units of dollars per year. We can divide energy use by GDP to derive the energy intensity, a measure of the energy efficiency of an economy, not only reflecting waste but also the difference between heavy industry such as steel production versus high-tech industry. We see in Fig. 9.11 that the Europeans and the Japanese can make money using less energy than we can whereas the Chinese and Indians are more energy intensive (less efficient).

Carbon emissions from these countries look similar to the energy story (Fig. 9.12). The United States is responsible for about a quarter of global CO2 emissions. Per person, the United States releases five times more than the global average, and more than twice as much as the Europeans or the Japanese, ten times more than the developing world. Per GDP, the United States. is less than half as efficient as Europe or Japan.

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Fig. 9.12 (a) Carbon release in CO2 per year, (b) carbon release per person, and (c) carbon release per dollar gross domestic product, GDP, globally and from various countries.

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