Biofuels built Rome! The motive force behind Rome and every other preindustrial civilization came almost entirely from muscle (human and animal) and was thus fueled by agricultural food production. Cooking and heating were mostly accomplished by directly burning wood and other plant matter. During the Industrial Revolution we gradually moved away from biological energy sources, initially for practical reasons related to the greater energy density offered by coal and petroleum. Coal, for example, provides about twice the energy contained in an equal weight of biomass, and about eight times as much energy by volume (Figure 5.1). Oil and gas offer even larger advantages. Fossil fuels also reduced the need for human and animal labor, since higher quality fuels make mechanization more practical.

Despite its historical decline in popularity, energy derived from recently living plants continues to offer two potentially key advantages over fossil fuels. First, living plants continuously recycle carbon dioxide from the atmosphere, a process that in principle means their use should cause no net change in greenhouse gas abundance. Second, plant biomass is renewed with each new growth cycle, implying that biofuels should always be available (with certain important caveats). Increasing concern over both the future availability of fossil fuels and their inevitable net addition of CO2 to the atmosphere has therefore led to renewed interest in biofuels.

The most immediate questions for biofuels are whether their potential advantages can be achieved without sacrificing the well-known advantages of fossil fuels, and whether industrial-scale biofuels production can be accomplished without threatening food supplies or the environment. These questions are by nature complex, and informed debate requires a deeper grasp of the inner workings of biological energy systems and their interactions with the Earth.

Picture a herd of woolly mammoths grazing amid grass-covered, rolling hills on an autumn morning. They are moving slowly along a river that is muddy with silt, derived from melting of the last remnants of a glacial ice sheet that once covered half the continent. There is a chill in the air, with a strong breeze blowing in from the northwest. A group of fur-clad hunters stalks the mammoths from the opposite direction, discussing details of their attack plan with hand signals and low guttural grunts. If the hunt is successful they may survive through the long winter ahead; otherwise their future is in doubt.

Although incredibly primitive by modern standards, this scene could actually have played out as recently as ten thousand years ago, hardly a blink of the eye in geological terms. Agriculture began to spread through the eastern Mediterranean region only about two thousand years afterward, and the earliest pyramids in Egypt were built at the approximate midpoint in time between the disappearance of woolly mammoths and the appearance of cell phones.

The historical details have been lost to the ages, but it seems clear that massive environmental change, brought on by a respite in continental glaciation, set the stage for early human civilization. If Stone Age and earlier humans had any thoughts at all about the future of energy, they would almost certainly have been along the lines of “How can I keep warmer?” The idea that global warming might eventually become a problem would have been utterly unimaginable.

Since that time, the cutting edge of technology has advanced from stone axes to lasers, and our dominant energy sources have evolved from human muscle to fossil fuels. The historically recent emergence of the latter has transformed the world at unprecedented rates, leading many to worry about how long we can maintain the standard of living that we have come to accept as our birthright.

Perhaps the most iconic image of the bounty of fossil fuels was the well drilled at Spindletop Hill in east Texas, which began to gush oil in the opening days of the 20th century (January 10, 1901; Figure 9.1). Oil spewed more than 150 feet in the air, at an initial rate estimated at approximately 100,000 barrels per day. This flow rate translates to a power output of about 6 gigawatts, roughly equivalent to the rated capacity of three Hoover Dams (power is defined as the rate at which energy is transferred). Remarkably, the liberation of this staggering energy resource only required drilling 1,139 feet into the Earth, a depth less than that of many municipal water wells. Once unleashed, it took nine days to bring the gusher under control. With such immense power waiting literally to leap from the ground, it is small wonder that oil went on to reshape the world.

Uncontrolled eruptions of oil like the one at Spindletop are of course hugely wasteful and pose great danger due to their flammability and to the contamination of the surrounding area. The petroleum industry and government regulators therefore go to great lengths to prevent such catastrophes, which have ironically become all the more shocking for their rarity. The fundamental geologic truth of conventional oil fields remains unchanged more than a century after Spindletop. All such accumulations comprise fluid hydrocarbons that have been highly concentrated by natural geologic processes. These concentrated fluid accumulations lend themselves to comparatively easy (and controlled) extraction, encouraging an analogy to the cream that can be skimmed from the top of a pail of raw milk.

In 1902 the English author W. W. Jacobs penned a short story entitled “The Monkey's Paw”, about a shriveled artifact with the magical power to grant its owner three wishes. Unfortunately those wishes resulted in unexpected and terrible consequences that far outweighed their benefits, to the bitter regret of the wisher. The cautionary message of this tale is that we should be very careful any time something valuable is offered without apparent cost.

Biofuels promise to grant only two wishes rather than three, but they are good ones: reduced greenhouse gas emissions and energy renewability. However, does the granting of these wishes carry a dark side that might outweigh their apparent benefit? The most obvious concern is the potential for biofuels to compete with food production. Part of the reason we can seriously consider biofuels at all is the “Green Revolution”, a dramatic increase in global farm output that occurred during the latter half of the 20th century. According to data published by the U.N. Food and Agriculture Organization, world production of rice, wheat, and corn roughly tripled between 1961 and 2007. Some individual crops or countries experienced even larger increases. For example, average corn yields in the United States increased roughly fivefold between 1950 and 2000, after having been essentially flat during the first half of the century (Figure 6.1). Total wheat production in China increased more than sixfold during 1961–2008.

This agricultural bounty offers the hope that we can burn our cake, and eat it too. However, increased crop yields carry their own price, in the form of nonrenewable Earth resources needed to support them. Leading the list are fossil fuels, used for everything from powering tractors to producing nitrogen fertilizer. Other vital fertilizers are also derived from limited geologic deposits.

At daybreak on July 4, 1054, a Chinese astronomer named Yang Weide noted the appearance of a new “guest star” in the eastern sky, near the more familiar reference star Tianguan (now known as Zeta Tauri). The appearance of a guest star was an exceedingly rare event considered to carry great geopolitical significance; the emperor Renzong therefore was duly notified. The new star remained visible during the daytime for 23 days, and finally disappeared from the night sky after 642 days. At the time, the Song dynasty was a world technological leader, having introduced paper money, gunpowder, and use of the magnetic compass for marine navigation. They did not yet have the telescope, however, and so could not know that the new guest star had been replaced by a much dimmer but still luminescent interstellar cloud, the Crab Nebula (Figure 13.1).

Yang's guest star was in fact a supernova, which has since been named SN 1054. It formed by the collapse of an aging star, which originally had a mass many times that of our own Sun. As it exhausted its core fuel supply, the star eventually reached a point where the energy released by nuclear fusion could no longer counteract the unimaginably large gravitational forces pulling inward. A violent implosion ensued, immediately followed by an explosion that temporarily made SN 1054 one of the brightest objects in the galaxy. Within seconds, the energy of this explosion also spawned a suite of new chemical elements, which were immediately hurled outward into interstellar space. Among them was one of the rarest naturally occurring elements of all, uranium.

Born of extreme violence, uranium remains inherently unstable or “radioactive” for the rest of its life, fated to spontaneous and inexorable decay to lighter elements. Some of these second-generation or “daughter” elements are themselves unstable, experiencing further decay until the process eventually reaches its end with drearily dull but reliably stable lead.

As planets go, the Earth is a party animal. Its continents wander endlessly, sometimes clustering together in temporary cliques, other times splitting up and disbanding. Their surfaces may plunge 10 km down or more, buried beneath thick blankets of sediment, or sky-rocket 8 km upward, shaking violently the whole while. Fireworks are on perpetual display in the form of volcanoes, which emit anything from ropy lava flows to giant dust clouds that blot out the Sun. Glacial ice appears, disappears, and returns again with alarming speed and frequency. The sea rolls relentlessly in and out, creating beachfront property in St. Louis or land bridges from Alaska to Asia.

So why don't we notice? Sometimes we do, particularly during major earthquakes and volcanic eruptions. Generally though we experience an illusion of stability, because we are relatively short-lived animals inhabiting a very long-lived planet. Most of the action happens too slowly for us to perceive directly. To get a better sense of the vast discrepancy between geologic and human timescales, it helps to compare the history of the Earth to a single calendar year. Marked on January 1 of this calendar is the notation “Earth condenses from the presolar nebula”. The earliest life-forms appear sometime in late March, but complex animals do not evolve until November 11. The dinosaurs appear on December 10 and disappear the day after Christmas. Modern humans (Homo sapiens) show up on New Year's Eve and are quite late to the party, arriving only twelve minutes before midnight. The entire 20th century is encompassed in the last tick of the second hand before midnight.

The Birth of the Earth

The Earth originally condensed about 4.55 billion years ago, a time so distant that even the Earth itself can't easily recall the details. No intact rocks from the Earth's first ∼500 million years have been found, leaving a substantial gap in the early record of the planet's evolution.

There is something deeply satisfying about burial in the Earth. On the one hand, it can symbolize hope for the future, for example, if we sow the seeds for a new crop or bury something of value that we plan to dig up later (treasure perhaps, or a time capsule). On the other hand, burial is wonderfully effective for concealing that which we wish to permanently forget, such as the inevitable decay of our deceased relatives or the mountains of household trash we produce every day. The former we reverently inter in cemeteries; the latter we dump unceremoniously into landfills. The net result is the same in both cases, however; a problem has been eliminated and we can move on with our lives.

It is tempting to believe that energy wastes can be eliminated the same way, and perhaps they can. Saline water (brine) that has been produced together with crude oil or natural gas has been routinely re injected into deep disposal wells for decades. If released untreated at the surface, subsurface brines can harm vegetation, wildlife, and livestock and contaminate freshwater supplies. Returning them to their point of origin provides an expedient, low-cost solution that generally causes little trouble. In a relatively small percentage of cases this solution may create new problems of its own, however, if high-pressure fluid injection unlocks previously stable earthquake faults.

Burial might also be used to dispose of unwanted CO2, to limit its buildup in the atmosphere. While not yet routine, this approach is currently being tested at various sites worldwide. Ironically, one of the main challenges is obtaining large volumes of concentrated CO2 to bury. Its concentration in the atmosphere is very small, currently about 400 parts per million. We would like to bury just CO2 and leave the rest of the atmosphere where it is, but separating the two would be very expensive.

Water is without doubt the most vital natural resource on Earth. Humans are mostly made of water, and without continual replenishment we cannot expect to live more than a few days. Even if we could, the plants that we depend upon for sustenance would quickly wither and die without water. Fortunately water is abundant on Earth, so much so that we tend to overlook its importance. The other planets in the solar system are not so fortunate, and that probably accounts for the apparent absence of life there. There is a chance that Mars did harbor life in the distant geologic past, and much of our exploration of that planet has focused on its history of water. Numerous canyons, dry river channels, and layered sedimentary deposits attest to the importance of water in originally shaping the Martian landscape. It has since become a hyperarid planet, however, where water appears to be confined to limited areas of polar permafrost.

The geological record of the Earth's first few hundred million years has almost entirely vanished, but what little evidence remains suggests that liquid water was already present at the Earth's surface as early as 4.4 billion years ago. Fortunately for us the Earth's overall water supply appears to be stable. By far the largest water resource is the ocean, which presently covers 71 percent of the planet's surface to a mean depth of nearly 4,000 m. As noted in Chapter 2, over the past 541 million years the depth of the ocean has remained surprisingly consistent, fluctuating only a few hundred meters at most.