The Resilient Earth: Science, Global Warming and the Fate of Humanity

by Simmons, Allen

  Galileo was born in Pisa, the eldest of six children. He was the son of Vincenzo Galilei, a famous musician of the day. As a young man, he seriously considered becoming a priest, but at his father's urging, he enrolled at the University of Pisa to study medicine. He never completed his medical degree, changing to mathematics instead.273

  In 1616, Galileo's support for the theories of Copernicus274 led him into conflict with the Catholic Church. Seventy years earlier, Copernicus had advocated a heliocentric view of the solar system. He placed the Sun, not Earth, at the center of the Universe, with the planets orbiting around it. Among the first to use a refracting telescope, Galileo became an advocate of the Copernican view of the solar system after viewing the previously undiscovered moons of Jupiter in orbit around that planet. During a hearing before the Inquisition, Galileo was ordered not to “hold or defend” the idea that Earth moves or that the Sun stands still at the center of the universe. Not an openly-rebellious person, as often portrayed in historical accounts, he stayed well away from the controversy for the next several years.

  After the election of Cardinal Barberini as Pope Urban VIII, in 1623, Galileo wrote a book presenting the evidence for and against a heliocentric solar system. Pope Urban, a friend and admirer of Galileo, had asked for the book to be published, but had warned Galileo to be careful with his pronouncments. The book, Dialogo sopra i due massimi sistemi del mondo (Dialog Concerning the Two Chief World Systems), was published in 1632, with encouragement from the Pope, and under a formal license from the Inquisition.

  Although the book presented both sides of the issue, there was no question which side got the better of the argument. For overstepping his bounds by clearly advocating the Copernican view, Galileo was again hauled before the Inquisition. This time, he was forced to recant his views, his book was banned, and he was imprisoned. An apocryphal embellishment to this conflict is the claim that, after his forced recantation that Earth orbited the Sun, he muttered, “Nevertheless, it does move.”275 The imprisonment was later reduced to house arrest but, for the rest of his life, Galileo worked quietly out of the public eye. He eventually went blind and died from natural causes in January, 1642. But not before completing his final and greatest work.

  The Discorsi e dimostrazioni matematiche, intorno a due nuove scienze (Discourses and Mathematical Demonstrations Relating to Two New Sciences) was published in 1638. Highly praised by both Isaac Newton and Albert Einstein, this book has resulted in Galileo being called “the father of modern physics—indeed of modern science.”276 Interestingly, Galileo was a practicing astrologer during most, if not all, of his career.277 Practicing astrology was still a normal activity for a mathematician in the early 17th century but, nowadays, is considered anything but modern or scientific.

  More than 300 years later, during the Pontificate of Pope John Paul II, there was a considerable effort for reconciliation between Church and science. In a speech given on 19 November 1979, for the centenary commemoration of the birth of Einstein, the Pope deplored the failure of the Church to perceive the legitimate autonomy of science. In 1992, a study by the Pontifical Council for Culture resulted in Pope John Paul issuing a statement expressing regret for the way the Church had responded to Galileo's theses.

  The first person known to have suggested that the Sun is a star was the Greek philosopher Anaxagoras (circa 450 BC). Around 1590 AD, Giordano Bruno suggested the same thing. For that, and other heresies, he was burned at the stake by the Roman Inquisition in 1600. During the 16th and 17th centuries, through the work of Copernicus, Kepler278 and Galileo, the nature of the solar system, and the Sun's place in it, slowly became apparent. Finally, in the 19th century, the distances to stars were accurately measured. After establishing how far away the stars were, their brightness could be fixed. This allowed scientists to finally prove that our Sun is a star—one of hundreds of billions in our galaxy.

  Stellar Evolution

  Stars come in a wide-range of sizes and colors; from massive super-giants to diminutive brown dwarfs, and from hot blue-white to cool deep-red. Their lifespans also vary widely, from a few tens of millions of years to several times longer than the age of the Universe. To a great extent, the color and lifespan of a star depends on its size. But, there are also exotic stars that do not fit the normal rules; novae, pulsars, X-ray binaries, and gamma-ray bursters, to name a few.

  Most stars are what are called main-sequence stars, for reasons we will explain. There are three main phases in the evolution of a star: the pre-main-sequence phase, the main-sequence phase, and the post-main-sequence phase. In the pre-main-sequence phase, gravitational contraction provides most of a star's energy. During the main-sequence phase, energy is provided from thermonuclear fusion of hydrogen into helium, a process called nucleosynthesis. As a star's hydrogen runs out, it moves into the third post-main-sequence phase where hydrogen burning moves away from the star's core. This phase continues through the fusion of successively heavier elements until the star runs out of nuclear fuel.279

  Stars are formed out of interstellar clouds of gas and dust, though scientists are not quite sure what causes a cloud to collapse into a new stellar system. Possible causes include radiation pressure from exploding supernovae and the shock waves that form at the leading edges of the galaxy's spiral arms. Since the arms seem to be stellar nurseries, and also contain the large hot stars that tend to explode as supernovae, both mechanisms may play a part in stellar formation. Regardless of the cause, once the gravitational threshold is crossed, a potential new star enters the pre-main-sequence phase of its life, powered by the conversion of gravitational potential energy into heat.

  If the object being formed has enough mass, its internal density and temperature will become great enough for nuclear reactions to begin. The needed amount of mass is about 0.08 times the mass of our Sun. Below this limit, the nuclear fires won't ignite. The giant planet Jupiter, the largest planet in our solar system, is considered a failed star by many astronomers. Above the limit, enough energy is produced from fusion to halt the star's gravitational collapse.280

  When a star has stabilized, due to the nuclear burning of hydrogen, it has entered the main-sequence, shown in Illustration 86. How long a star remains on the main-sequence depends on how massive it is. A star like our Sun will be on the main-sequence for 10 billion years or more. Stars below 0.8 solar masses will last for as long as 1,000 billion years, a hundred times the current age of the Universe. At the other end of the weight scale, a star massing 30 times that of the Sun will burn up its hydrogen in about 5 million years. No stars heavier than 50 solar masses have been seen on the main-sequence.281

  Illustration 86: The Hertzsprung-Russel main-sequence diagram. Source NASA.

  The mass of a star also affects the frequencies of light that it produces. Stars emit light in a temperature-dependent frequency distribution scientists call black-body radiation. This means that the electromagnetic radiation a star emits follows the Plank curve. We will explain more about the significance of this later in the chapter. Heavy stars are bluer, hotter and shorter-lived than our Sun.

  The fate of a star when it leaves the main-sequence, after exhausting its supply of hydrogen, also depends partly on its mass. Once a star's hydrogen is spent, it will start burning helium in a nuclear reaction that forms carbon and oxygen. A side effect of this change in nuclear fuel is swelling and cooling of a star's outer layers. When this happens, the star becomes a red giant.

  Once helium is exhausted, a large star will burn carbon and oxygen, proceeding with successively heavier elements until iron and nickel are produced. Stars massing less than eight solar masses are not big enough to burn carbon so their careers as red giants ends when their helium supply is gone. With no ongoing thermonuclear reaction to provide heat to support it, a star will collapse.

  Illustration 87: Life Cycle of the Sun (images not to scale), Source: original images from NASA, composition by D. L. Hoffman.

  When a star the size of the Sun runs
out of fuel and collapses, the sudden contraction of matter causes one or more explosions. These explosions blow off the outer layers of the star, forming a planetary nebula. Many of the more spectacular photographs from the Hubble Space Telescope are of planetary nebulae. Over time, the matter in the nebula will disperse, leaving the burned-out remains of the star behind—a white dwarf.

  Since a white dwarf is no longer supported against gravitational collapse by the heat of fusion, its further collapse must be resisted by other forces. Main-sequence stars, with medium or smaller masses, are supported by electron degeneracy pressure. Matter in this state is highly compressed, and therefore extremely dense. A star with mass comparable to the Sun ends up shrinking down to a volume the size of Earth. Becoming a white dwarf is thought to be the final state of over 97% of the stars in our galaxy.282 This is the expected fate of our Sun, as shown in Illustration 87.

  Electron degeneracy pressure is a quantum mechanical force, resulting from the Pauli283 exclusion principle, which limits how tightly matter can be squeezed together (electrons don't like to be crowded). If a star weighs more than 1.4 solar masses, its collapse cannot be halted by forming electron-degenerate matter. This mass is called the Chandrasekhar Limit, named after Indian-American physicist, Subrahmanyan Chandrasekhar. It is the maximum non-rotating mass that can be supported against gravitational collapse by electron degeneracy pressure. More massive stars will continue to collapse because the degeneracy pressure provided by electrons is weaker than the inward pull of their gravity.

  When a star heavier than the Sun contracts, it ejects its outer layers. If it ejects enough mass to slip under the Chandrasekhar Limit, its core will become a white dwarf. But, when the star remnant contains more than 1.4 times the mass of the Sun, it will continue to collapse. Its electrons will be forced to combine with protons in the star's atoms, forming a strange form of matter called neutron-degenerate matter.284 The star becomes a neutron star.

  Stars with 10-20 solar masses become unstable after they exhaust their helium and begin burning carbon. Such stars can blow off their outer layers in incredibly violent explosions called a supernova. Even more massive stars, after exploding as supernovae, can become black holes.285

  Black holes are thought to form from heavy stars which start off with masses more than 20 or 25 times that of the Sun. How black holes form is still an area of active research so exact numbers are not available. When one of these stars expires in a supernova explosion and its core collapses, gravity wins out over any other force that might be able to hold the star up. Eventually, the star collapses to such an extent that it is contained within its Schwarzschild radius, or event horizon, from which even light cannot escape.

  Albert Einstein's286 1915 theory of general relativity provides the theoretical foundation for black holes. Einstein showed that gravity can bend the path of light just as it bends the path of any other moving object. When a massive star collapses, its gravitational pull becomes strong enough to prevent light from escaping. Though black holes can be extremely massive, their size is relatively tiny; a black hole with the mass of the Sun would have an event horizon the size of a small asteroid.

  It is now thought that all large galaxies contain gigantic black holes at their centers. These galactic black holes can be millions or even billions of times more massive than the Sun. Some black holes are among the most violent and energetic objects in the universe—forming quasars, quasi stellar objects, which shoot off jets of x-rays and gamma-rays as they suck in surrounding gas. Others, like the black hole at the center of the Milky Way, are more docile, quietly consuming surrounding stars.

  This information about stellar evolution and supernovae is important to fully understand our solar system: where it came from and how it will end. When the Universe began, 14 billion years ago, almost all normal matter was in the form of hydrogen.287 As mentioned, elements heavier than hydrogen, from helium through iron, are created as by-products of thermonuclear reactions—the ash of fusion. When stars explode, at the end of their main-sequence lifetimes, their matter is redistributed. Even heavier elements, up through uranium, are created by nucleosynthetic alchemy when massive stars erupt in supernovae explosions.

  This is where the material that makes up Earth and the other planets comes from. The oxygen and silicon in Earth's crust, the iron in its core and all the other elements, so essential to life, are all the remnants of dead stars. The nearby supernova that threatened the start of life on Earth may have contributed to its ultimate emergence instead.

  Our Star, the Sun

  The Sun is a fairly average, middle-aged star of the type astronomers call a type G dwarf. It is often said that the Sun is an “ordinary” star, but there are many more smaller stars than larger ones. The median size of stars in our galaxy is probably less than half the mass of the Sun, with the Sun in the top 10% by mass. It is currently composed of approximately 72% hydrogen, 26% helium and 2% of all other chemical elements in gaseous form. The source of the Sun's brightness, and that of other stars, is due to thermonuclear reactions occurring in their innermost parts. The temperature inside the Sun can reach 27,000,000°F (15,000,000°C).

  The Sun is 900,000 miles (1,400,000 km) in diameter and emits about 3.86x1026 watts of energy from its surface.288 This translates to about 1366 watts per square meter at Earth's orbit. It is the source for 99.98% of the energy that drives Earth's ecosystem, the other 0.02% coming from the cooling of Earth's molten interior.

  Prior to the discovery of the process of thermonuclear fusion, gradual gravitational contraction, commonly called Helmholtz Contraction, was thought the most likely way stars generated energy. Since the 1930s, however, astrophysicists have become convinced that thermonuclear fusion is responsible for lighting the stars. According to contemporary stellar models, the physical conditions within the core of a star make the fusion process unavoidable.

  Each second, more than 4 million tons of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. So far, during its life, the Sun has converted around 100 Earth-masses of matter into energy. Unlike the perfect disk of many ancient myths, we know the Sun is actually a violent, roiling mass of gas driven by nuclear fire. Flares leap from the solar surface that dwarf Earth in size.

  As a consequence of changes brought about by thermonuclear fusion, a slow increase in size and energy output is predicted over time. We stated earlier, when Earth was young, the Sun was 30% fainter than today. The Sun's estimated brightness four billion years ago would not have been enough to warm Earth's atmosphere to a comfortable level for life. Any liquid water exposed to the surface would have quickly frozen solid. But geological observations from that time, evidenced in sedimentary rock, required the presence of flowing liquid water. That evidence led to what astronomer Carl Sagan289 called the “Faint Young Sun Paradox.”

  Scientists have offered a number of possible explanations for this apparent contradiction. Noting that before the advent of abundant life, atmospheric oxygen levels were much lower than today and there was also a significant amount of methane (CH4) present. In today's atmosphere, oxygen interacts with methane, rapidly turning it into carbon dioxide. The absence of oxygen in the early atmosphere would have allowed methane concentrations much larger than currently observed. Methane, a much more potent greenhouse gas than CO2, could explain the anomalously high temperatures. Extensive volcanism could also have helped to raise the levels of greenhouse gases in the young Earth's atmosphere.

  In the very long term, astrophysicists believe that the Sun's output increases by about 10% per billion years. Some scientists speculate that, in about one billion years, the additional 10% will be enough to cause a runaway greenhouse effect on Earth. The rising temperature produced by the increase in solar radiation will cause more water vapor. This in turn will accelerate greenhouse heating to the point where it will evaporate the oceans and destroy life on Earth. This will take place long before the Sun swells into a red giant near the end of
its life.

  Sunspots and Solar Climate

  The slow increase in the Sun's output is not thought to be the cause of the short-term warming over the past several decades. But variation in solar output over shorter periods has been a subject of intense study. In particular, the observed link between sunspot activity and Earth's climate.

  Perhaps, the most interesting feature of sunspots is that their number increases and decreases in a regular rhythm over about a decade. This regular cycle was first noticed by the German astronomer Samuel Heinrich Schwabe in 1843. This has become known as the solar magnetic activity cycle, or sunspot cycle. The number of sunspots in each cycle is not constant; there have been periods where many sunspots were observed, and others when sunspots seem to disappear altogether. Sightings from China, Korea and Japan between 28 BC and 1743 AD averaged only six sunspots per year. None were observed between 1639 and 1700.290

  Illustration 88: Sunspot activity and solar output during the Maunder Minimum. Source John Eddy based on Science, no.192, 1189 (1976).

  The most famous of these periods is known as the Maunder Minimum, a period of low sunspot activity that lasted from 1645 to 1715. This coincides with the coldest portion of the famous Little Ice Age period discussed in Chapter . The Maunder Minimum is named after the English astronomer Edward W. Maunder (1851-1928). From studying historical records of sunspot counts, called the sunspot number,291 Maunder discovered that sunspots were virtually absent during this period, and disappeared altogether during the decade starting in 1670. Astronomers observed only about 50 sunspots during the 70 year period from 1645 to 1715. Normal sunspot activity would have produced 40,000 to 50,000 sunspots.