A World Without Ice - Henry N. Pollack (2009)
Chapter 5. NATURE AT WORK
The bright sun was extinguish’d, and the stars
Did wander darkling in the eternal space,
Rayless, and pathless, and the icy earth
Swung blind and blackening in the moonless air;
Morn came and went—and came, and brought no
—LORD BYRON “Darkness” (1816)
In early April of 1815 the city of Batavia, on the island of Java, began to hear sharp explosions that sounded much like distant artillery. But the city was not under attack, and no ship in distress was firing its cannons as a call for rescue. Batavia (now Jakarta) would later learn that it had been an aural witness to a violent eruption of the volcano Tambora, 1,200 miles to the east, along the Indonesian archipelago. For over a week Tambora erupted in a series of explosive events, creating sound waves heard even another 800 miles beyond Batavia. More than 70,000 people perished from the triple plague of a red-hot noxious gas and debris cloud rolling down the mountainside, a tsunami generated by the culminating explosion of April 11, and contamination of drinking water by the prolific ashfall. But if the devastating loss of life nearby was the immediate consequence, the eruption would continue to cause problems worldwide in the months and years to follow.
The explosions reduced the summit of the mountain from around 14,000 feet to 9,500 feet, blowing about 35 cubic miles of the volcanic edifice—an astounding volume of debris—into the atmosphere. In just a few months the atmospheric jet streams distributed this debris worldwide, blocking some of the sunshine from reaching and warming Earth’s surface. The atmosphere is not quickly purged of such a burden, and the climatic effects were very apparent the following year. Temperatures throughout the Northern Hemisphere were depressed well below normal, and crop failures were common in Europe and North America. Connecticut experienced snow in early June, lakes in Maine froze over in mid-July, the mountains of Vermont were snow-covered in August.51 Four killing frosts—one in June, one in July, and two in August—ensured that the New England harvest was meager. Around the world, the year 1816 became known as “the year without a summer.”
Ash, dust, and chemical aerosols injected into the atmosphere during volcanic eruptions form a veil that blocks some of the incoming sunshine and prevents it from reaching and warming Earth’s surface. The volcanic products have the effect of increasing Earth’s albedo for a few years, reflecting more incoming solar energy back to space. Eventually, the ash and dust fall back to Earth, clearing the atmosphere and allowing the Sun’s rays to once again warm the planet.
The eruption of Tambora was neither the first nor the last volcanic event to have a global impact on the atmosphere and climate. The diminished sunshine following an eruption in AD 536 on the island of New Britain, just east of Papua New Guinea, led to this description of conditions in the Middle East:
The Sun became dark and its darkness lasted for eighteen months. Each day it shone for about four hours, and still this light was only a feeble shadow. Everyone declared that the Sun would never recover its full light. The fruits did not ripen and the wine tasted like sour grapes.52
The dust from this eruption has been detected in well-dated ice cores in both Greenland and Antarctica. Tree-ring data from the European Alps, Scandinavia, and the Russian Arctic suggest that the cooling caused by this eruption may have been the most severe that the Northern Hemisphere has experienced in the last two millennia, even cooler than the effects from the 1815 eruption of Tambora.53
In August of 1883, another Indonesian volcano, Krakatoa, in one mighty explosion spewed more than six cubic miles of ash and dust into the atmosphere. The sound of the explosion was heard in Mauritius, an island located in the deep south of the Indian Ocean, some three thousand miles away, and the particles injected into the atmosphere soon led to spectacular red sunsets around the world. The colorful skies were captured in a series of sketches by the English painter William Ashcroft in 1884, and became known as the Chelsea sunsets. The official scientific report on the eruption of Krakatoa, published by Britain’s Royal Society in 1888, featured the Ashcroft sketches as its frontispiece. For years following the eruption, brilliant sunsets and brutal winters were experienced around the world. The legendary harsh winter of 1886-87 and the devastating blizzards of 1888—Krakatoa’s unwelcome gifts to the struggling settlers and ranchers in the Great Plains—brought cattle-grazing on the open range of the United States to an end.
These examples make clear that great volumes of volcanic debris sent high into the atmosphere during an eruption, and soon thereafter distributed around the globe by the atmospheric circulation, can affect the global climate for several years. Volcanism is but one of the arrows in nature’s quiver of climate-changing mechanisms that have played a role in Earth’s climate in the aeons before humans populated the globe. Let us now look at some of the other processes that have altered Earth’s climate prior to the appearance of humans.
WHEN THE CLIMATE is not changing, there is an equilibrium between the incoming energy absorbed by Earth’s surface and the outgoing energy radiated back to space from the surface. All factors leading to climate change disrupt this balance between energy deposits and withdrawals from Earth’s surface. Disruptions to the equilibrium include changes in the amount of sunshine arriving from the Sun, changes in the fraction of that energy that Earth reflects back to space, and changes in the atmosphere that cause it to capture some of Earth’s heat instead of allowing it to radiate back to space unimpeded.
THE SUN DELIVERS
The amount of radiant energy Earth receives from the Sun changes over time, and not just because of variations in the amount that leaves the Sun. The periodic changes in the ellipticity of Earth’s orbit around the Sun and in the tilt and precession of Earth’s rotational axis—the Milankovitch cycles described in chapter 3—affect both the distance of Earth from the Sun and how Earth is oriented with respect to the Sun. Being closer to or farther from the Sun will have obvious effects on Earth’s temperature, and the changes in the tilt and orientation of Earth’s rotational axis affect the strength of the seasonal variation in temperature.
But these Milankovitch cycles, with seemingly long periods of one hundred thousand, forty-one thousand, and twenty-three thousand years, are mere flutters on a very slow increase in solar luminosity that has been occurring since the beginning of our solar system four and a half billion years ago. This long-term increase in the Sun’s radiance is a common evolutionary characteristic of millions of stars similar to the Sun. At the birth of the solar system the primeval Sun displayed only 70 percent of the luminosity it displays today. A dimmer Sun in that very early history of our solar system would imply that Earth was much colder in its early days, and ice much more common. Calculations of Earth’s surface temperature with only 70 percent of today’s solar radiation warming its surface inevitably translate into an ice-covered early Earth. And yet, geologists have identified widespread sedimentary rocks—rocks deposited in water—that are almost as old as Earth itself, suggesting that liquid H2O was present early in Earth’s history. This apparent contradiction was named the “faint young Sun paradox” by astronomers Carl Sagan and George Mullen.54
The paradox can be resolved with an atmosphere that also evolved in response to the slowly increasing solar radiation. Although nitrogen has probably always been the principal chemical component of Earth’s atmosphere, oxygen has not. Oxygen in the atmosphere today is the principal waste product of photosynthesis, the process by which plants use sunlight to produce biomass—in other words, to grow. But photosynthesis did not become an important source of oxygen in the atmosphere until green plants evolved later in Earth’s history. Initially there was very little oxygen in the atmosphere, because, absent photosynthesis, the only other process producing oxygen was a weak mechanism called photodissociation, in which radiative energy from the Sun broke the chemical bonds between the oxygen and hydrogen atoms in some of the water vapor molecules in the early atmosphere, thus freeing up a little oxygen. Even today, photodissociation yields much less oxygen than does photosynthesis, and under conditions of a dimmer Sun early in Earth’s history, it would have been an even less efficient process.
With little oxygen available, carbon in the early atmosphere joined hydrogen to form methane, CH4, in the atmosphere. Methane, however, is a potent heat-trapping gas, some twenty times stronger in its heat-trapping capability than its oxidized cousin carbon dioxide, CO2. Earth’s early atmosphere therefore acted as an extraordinarily effective blanket. As oxygen slowly became more abundant over geologic history, carbon dioxide gradually replaced methane, and the heat-trapping ability of the atmosphere slowly declined. When the Sun was weak, Earth’s atmospheric heat-trapping blanket was strong, and as the Sun grew more radiant, the blanket grew weaker. The result is that Earth’s average surface temperature has remained in the range of liquid H2O throughout most of its history. The gradual oxidation of the atmosphere is now recognized as the resolution of the “faint young Sun paradox.”
EARTH AS A GREENHOUSE
However much energy the Sun delivers to Earth, that energy can be diminished or enhanced by processes within our atmosphere. Explosive volcanism (as described earlier in this chapter) can block some of the solar radiation from reaching Earth’s surface, and greenhouse gases in the atmosphere impede the escape of Earth’s heat back to space.
How do the greenhouse gases trap heat trying to leave Earth? For a decent analogy, think of the microwave oven in your kitchen, in which microwaves (electromagnetic waves larger than infared but shorter than radio waves) are generated within the oven and absorbed by the food you want to heat. More specifically, the microwaves are absorbed by water molecules contained in the targeted morsels. Because the microwaves are absorbed within the food, the energy they carried as traveling waves is converted to another form of energy, heat, as required by the first law of thermodynamics.
Let’s now scale the concept up to solar system dimensions to get a feel for Earth’s natural (and long-standing) greenhouse effect. The dominant radiation the Sun generates is in the visible part of the electromagnetic spectrum, that band of wavelengths that our eyes have evolved to be sensitive to. These wavelengths include all the colors of the rainbow—red, orange, yellow, green, blue, and violet. The Sun sends off a little energy outside of the visible range—some shorter ultraviolet waves and some longer infrared waves—but most of the energy arrives in the visible wavelengths.55
Our atmosphere is essentially transparent to the visible wavelengths, so this energy from the Sun passes through the atmosphere unimpeded, to be absorbed at Earth’s surface and warm it. But Earth cannot continually absorb energy and keep on heating up forever, at least not without serious consequences, such as melting. It must have a way of sending heat back into space to avoid continual warming. It accomplishes this balancing act by reradiating the energy received from the Sun back to space, but not in the visible wavelengths. The wavelengths that a body employs to radiate energy away depend on the temperature of the surface, with hotter bodies such as the Sun radiating shorter waves, and cooler bodies such as the planets radiating longer waves.
Energy comes to Earth as visible radiation from a very hot (~11,000º Fahrenheit) Sun, but departs as invisible infrared radiation from a 60º Fahrenheit Earth. But now comes the hooker—the atmosphere, which is transparent to incoming visible radiation, is not fully transparent to the outgoing infrared waves. Several gases in our atmosphere, present in only tiny amounts, absorb infrared radiation and convert the radiant energy into heat. This is what we call the “greenhouse effect”—the process by which Earth takes in a little more heat than it sends back, and accordingly it must warm up a bit and radiate a little more, in order to restore the balance between incoming and outgoing energy.
The greenhouse effect is not simply some theoretical scientific construct—it is a very real observable and measurable phenomenon, and one we should be thankful for, because Earth would be much colder and inhospitable without it. The principal gases in the atmosphere that absorb infrared radiation are water vapor (H2O), carbon dioxide (CO2), and methane (CH4); together they add up to less than 1 percent of the atmosphere. For every million units of atmospheric volume, only a few hundred parts are CO2, and less than two parts are CH4—but these minuscule amounts give a lot of “bang for the buck.” One sometimes hears incredulity that such tiny concentrations can have any impact, let alone a major one. But these trace gases in our atmosphere raise Earth’s surface temperature by more than sixty Fahrenheit degrees from what the surface temperature would be if Earth had no atmosphere. This natural greenhouse effect is what makes Earth the water planet, the blue planet, rather than just another of the many icy bodies of the solar system.
The absorption of infrared radiation by CO2—an atmospheric process that would become so discussed in the second half of the twentieth century—was first measured by John Tyndall in 1859. It is no small historic irony that 1859 was the same year that petroleum was discovered in Pennsylvania by Edwin Drake. Little did anyone imagine that a century later the CO2 from the combustion of petroleum would be warming the atmosphere by the mechanism first measured by Tyndall.
EARTH’S GEOLOGICAL THERMOSTAT
Earth has a “thermostat” that prevents the surface temperature from straying too widely. It does not, however, make adjustments daily, as in our homes. Rather, the adjustments take place over millions of years and are related to geologic processes that are temperature dependent. The conceptualization of this thermostat originated with Jim Walker, a very broad-based earth scientist at the University of Michigan. Walker synthesized perspectives from atmospheric science, oceanography, geology, and geochemistry to envision the way this geological thermostat works.56
Imagine an Earth that is a little too warm because of an atmosphere with an above-average concentration of the greenhouse gas CO2. How does Earth turn down the thermostat? Some chemical reactions that decompose rock—a process that geologists call weathering—are more effective at higher temperatures, and so when Earth is warmer, the rivers that drain the continents carry a bigger load of dissolved chemicals to the sea.
One element that weathers from continental rocks is calcium, which, when delivered to the sea, combines with carbon dissolved in seawater to produce calcium carbonate, which ultimately is deposited on the seafloor as limestone. As carbon is removed from the seawater through limestone deposition, the sea pulls more CO2 from the atmosphere, thus diminishing the greenhouse effect and cooling the planet. But as the surface cools, less weathering takes place, the supply of calcium to the sea slows, limestone deposition diminishes, and once again CO2 builds up in the atmosphere to warm the planet..
Earth’s temperature oscillates between warmer and cooler through fluctuations in the effectiveness of the natural atmospheric greenhouse,
which in turn modulates the availability of calcium to form limestone. But the functioning of this thermostat is dependent on oceans full of water in which to absorb CO2 from the atmosphere and deposit limestone on the ocean floor. Without water, Earth’s thermostat would be broken.
VENUS—A PLANET WITHOUT A THERMOSTAT
Earth’s closest neighbor in the solar system is Venus, the second rock from the Sun. Venus is similar to Earth in many ways: it is about the same size, its gravity field is about the same strength, its chemical composition parallels that of Earth, and it has an atmosphere. Its greater proximity to the Sun, at about only three quarters of Earth’s distance from the Sun, would suggest a surface temperature warmer than Earth’s, but still in the range that, if there were any H2O present, it would be liquid water rather than ice or water vapor. So it was quite a surprise when planetary scientists discovered that the surface temperature of Venus was in excess of 860º Fahrenheit. This temperature is high enough to melt lead, and way too warm for water to exist at the surface of the planet.
What has happened on Venus? The clues emerge from the composition and mass of its atmosphere—a gaseous envelope around the planet nearly one hundred times more massive than Earth’s atmosphere, and composed almost entirely of CO2. In short, Venus has a thick greenhouse blanket that has trapped enough heat to raise the planet’s surface temperature more than eight hundred Fahrenheit degrees higher than it would have without such an atmosphere. Comparatively, both Venus and Earth have a similar amount of carbon, but on Earth only a tiny fraction of the carbon is in the atmosphere. Most of Earth’s carbon resides in deposits of coal, petroleum, natural gas, and limestone. In other words, Earth has stored most of its carbon underground, whereas carbon on Venus resides almost entirely in its atmosphere. What a difference that makes in the surface temperature!
Why is Venus unable to regulate its surface temperature in the way that Earth’s geological thermostat has maintained a water-compatible temperature on Earth? The likely reason is that Venus is closer to the Sun, at a distance where it receives almost twice as much solar energy as Earth does. There, more intense evaporation led to a complete depletion of surface water, and without water there can be no biosphere to create coal, no oceans in which to deposit limestone—in short, Venus had no way to sequester carbon in solid form, and so carbon simply accumulated as gaseous carbon dioxide in the atmosphere, creating an intensely effective greenhouse blanket.
SHORTER-TERM FLUCTUATIONS IN CLIMATE
The slow evolution of the Sun over billions of years and the geological thermostat regulating temperature over millions of years cannot explain significant changes in climate over decades or a century. Those long-term natural processes change so slowly that effectively one century looks pretty much like the next. To explain the relatively fast contemporary warming in the twentieth and twenty-first centuries, we need to look for other causes. The variability of the energy radiated from the Sun has always been recognized as an important natural factor driving changes in Earth’s climate.
Solar physicists and astronomers have learned from years of research that the Sun is a very active body, and that the amount of radiative energy leaving the Sun varies over many time scales—minute by minute, day by day, year by year, decade by decade. All of these rapid fluctuations ride along on the three long Milankovitch cycles, which create a slowly changing backdrop for the changes in solar output that occur on shorter and less reliably periodic time scales.
Short-term variations in solar output can impact Earth in a variety of ways. Occasional solar flares leaping upward a million miles above the Sun can be so intense that they disrupt radio transmissions on Earth for several days and even damage the electronics of orbiting communication satellites. These solar outbursts are a serious military concern, because they can blind battlefield surveillance from space and interrupt the remote control of the pilotless drones that are playing an increasingly important role in military operations.
The decadal variability of the solar output can be seen in the abundance of sunspots, dark irregular patches that appear on the face of the Sun. Sunspots have been observed astronomically for more than four hundred years.57 Their numbers wax and wane with an apparent period of about eleven years—give or take a year. However, the strength of each cycle, as indicated by the number of sunspots at a cycle’s peak, show upward or downward trends extending over centuries.
Generally speaking, the more sunspots there are, the more energy the Sun radiates. Conversely, when the Sun shows fewer spots, its radiative output is correspondingly diminished. The dark patches are actually colder than the brighter areas of the Sun that surround them, and so one might imagine that a darker Sun—one with more spots—would be radiating less energy than a brighter Sun. But the real meaning of more spots is that the Sun is churning more vigorously, and bringing more energy to its surface for radiation into space. When the Sun is “quiet,” the spots are few, and outbound radiant energy is less.
In the period 1650-1715 there were very few spots on the face of the Sun, a period known as the Maunder Minimum, named for the nineteenth-century English astronomer Edward Maunder, who first pointed out the paucity of spots. On Earth it also coincided with a particularly cool interval within the broader climate downturn known as the Little Ice Age (mentioned in chapter 4), during which glaciers advanced in their valleys and the growing season grew shorter.
In the first half of the twentieth century, the peaks of the sunspot cycle grew for several cycles in a row. These decades of an increasingly active Sun probably contributed to a climb in the global average temperature of almost 0.9 Fahrenheit degree from 1910 to 1950. But in the last four decades of the twentieth century, the Sun has undergone a modest decline in radiative output. The minimum of the sunspot cycle in 2008 was the lowest in the past half century.
Since 1978, scientific satellites orbiting above our atmosphere have measured the incoming solar radiation in great detail—not only in the visible light of the electromagnetic spectrum, but also in the shorter ultraviolet and longer infrared wavelengths. These observations provide much more comprehensive information about the variability of solar radiation than does the simple counting of sunspots. The essential story the satellite radiometers tell, however, is the same as the sunspots: solar radiation has been declining in the latter decades of the twentieth century. Despite that, Earth’s temperature has continued to climb over the same interval. Apparently, the Sun is sharing the stage with other factors that are affecting Earth’s climate and causing it to warm.
VOLCANIC DIMMING IN THE TWENTIETH CENTURY
The eruptions of Tambora in 1815 and Krakatoa in 1883 were the signature volcanic events of the nineteenth century. The twentieth century was barely under way when the Santa María volcano in Guatemala erupted in 1902. This eruption blasted away much of the 12,000-foot summit of the mountain, sending some 1.3 cubic miles of volcanic ash high into the stratosphere and from there around the world. Although somewhat smaller than Tambora and Krakatoa, Santa María was estimated by volcanologists as probably one of the five biggest eruptions of the past few centuries. It was followed a decade later by the even bigger eruption of Novarupta, on Mount Katmai, in Alaska, which delivered more than four cubic miles of ash to the atmosphere.
The eruptions of Krakatoa, Santa María, and Novarupta in short order kept the atmosphere murky and the climate cooler for the better part of three decades. But after the 1912 eruption of Novarupta, there were no significant explosive volcanic events for a half century, which allowed the atmosphere to clear. The absence of volcanic dust also contributed to the climb in the global average temperature during the period 1910-50. But in the last half of the twentieth century, explosive volcanism returned. The eruptions of Agung in Indonesia in 1963, El Chichón in Mexico in 1982, and Pinatubo in the Philippines in 1991 kept the atmosphere dustier and the Sun dimmer than usual.
If these natural factors were the only ones at work in the last half of the twentieth century, a quieting Sun trying to penetrate a murkier atmosphere would have led to a slight cooling of Earth’s surface. But in fact the temperature has continued to climb nearly one Fahrenheit degree since the mid-twentieth century, indicating that natural factors alone were not in control of Earth’s climate. Indeed, for the first time in the history of Earth, other factors affecting climate—human factors—were growing in importance and beginning to overshadow the natural mechanisms.
Climatologists make a useful (albeit somewhat artificial) separation of the factors that cause changes in the climate, into natural and anthropogenic. Natural causes are those that are independent of human activity, whereas anthropogenic causes arise from human activity. It is safe to say that for most of Earth’s history the causes of climate change were entirely natural, simply because there were no humans present on the planet. Our human predecessors, various species of the genus Homo, first appeared on Earth some three million years ago. As their numbers grew and their technology improved, their impact on Earth and the climate has become increasingly apparent.
In their 2007 Fourth Assessment Report, the IPCC scientists concluded with 90 percent certainty that the rise in temperature in the latter several decades of the twentieth century was attributable mainly to human activities. This ascendancy of the anthropogenic component of climate change, surpassing the natural drivers, was a subtle and unheralded tipping point in the history of our planet.
THE SECOND TRENCH OF DENIAL
In the previous chapter I note that there are people skeptical of the instrumental record of Earth’s warming over the past century. These skeptics assert that the record misrepresents the true state of climatic affairs. They have argued that you can’t believe the thermometers, or the scientists who deploy them and interpret the readings. This rejection of the instrumental record of rising temperatures was the first trench of denial the climate contrarians dug. They defended that trench tenaciously, but one by one they abandoned it, slowly retreating in the face of overwhelming evidence from human and natural thermometers that Earth was indeed warming.
But these climate contras soon set up camp in a second defensive trench—grudgingly accepting that Earth may be warming, but then arguing that humans have had nothing to do with it. If Earth is warming, they argue, then it must be due to the Sun, or to cyclical changes in Earth’s climate associated with long-term variability in atmospheric and oceanic circulation. Even though the sunspot count and the direct measurements of solar radiation by satellites during the last half of the twentieth century both trend toward cooling, not warming, the skeptics have marshaled other arguments to support their belief that all climate change is solar in origin. Let’s pause to examine some of these contrarian arguments.
Other planets are warming. One argument the contras put forward as “proof” that solar activity is driving climate change on Earth stems from changes observed on other bodies in the solar system. If several bodies are indicating a warming, then surely, according to this line of reasoning, the common cause must be the central element of the solar system—the Sun. The favorite example that the contras cite is the apparent warming of the (former!) planet Pluto by about 3.5 Fahrenheit degrees over the past two decades. The evidence of warming comes from an observed tripling of the atmospheric pressure of Pluto, which implies that some of the nitrogen at the surface of Pluto has evaporated and returned to the atmosphere. But if Pluto—the most distant large body in our solar system—has warmed by 3.5 Fahrenheit degrees because of increased radiant energy from the Sun, then planets closer to the Sun should have warmed even more. In particular, Earth—forty times closer to the Sun than Pluto—should have warmed more than 18 Fahrenheit degrees, an amount clearly far greater than Earth has experienced. If such a solar explanation for the warming of Pluto were true, there would be no ice left on Earth. A better explanation of the warming of Pluto can be found in seasonal effects in Pluto’s 250-year orbital journey around the Sun, or possibly changes in Pluto’s albedo that have led to less sunshine being reflected from its surface. Similarly, an apparent warming of Mars is almost surely due to fewer dust storms and a more transparent Martian atmosphere.
A cosmic ray connection. Another suggestion advanced by the climate contras relates to how the Sun might interact, via an intermediary mechanism, to change the amount of cloud cover over Earth, and thereby change Earth’s albedo. This very complex scenario runs along these lines: Earth is perpetually being showered with cosmic rays from space—streams of charged particles that emanate from the Sun and other nearby stars. The particle stream from the Sun is called the solar wind. Most charged particles are deflected around Earth by our planet’s magnetic field, or that of the Sun. But a few leak through the magnetic shield and are thought by some to promote clouds by serving as a “seed” around which water vapor will adhere and nucleate clouds. When the Sun is more active and generates a stronger solar wind, the magnetic shield contracts more tightly around Earth and becomes a better shield. Fewer particles leak into the atmosphere, and therefore there are fewer clouds. Thus a more radiant Sun would lead to lesser cloud cover over Earth, thereby allowing more sunshine to warm Earth’s surface. Conversely, when the Sun is quieter, Earth’s magnetic field relaxes a bit and allows more charged particles to enter the atmosphere and nucleate more clouds These reflect incoming sunlight back to space, which in turn will cool Earth’s surface. The net result, were this complex scenario actually taking place, is that Earth would warm when the Sun is more active, and would cool when the Sun is quieter. Earth’s temperature would rise and fall, tracking the ups and downs in solar activity. To the contras, this represents a possible mechanism that would reassert solar control of Earth’s climate.
However, almost every element of this complex series of feedbacks is conjectural and unsubstantiated. Indeed, the nucleation effect of cosmic rays has not been demonstrated under realistic conditions in the laboratory, and cloud cover over Earth has not been observed to have a strong correlation with variations in the solar wind or cosmic rays in general. This mechanism gets high marks for imagination, but has not earned a passing grade in the real world of observations. It is an interesting idea, but there is no evidence to suggest that it actually works.
Natural cycles. The skeptics frequently assert that the current warming of Earth is the result of “natural cycles.” They know that the geological record indicates swings of climate long before humans came to populate the Earth, and they suspect that maybe nature is again at work in the current warming episode. “Isn’t today’s climate change just one more example of these natural processes at work?” But the logic of this avenue of thinking is partially flawed, because the statement has an implicit but unfounded premise: the only factors influencing climate today are the same ones that have influenced climate in the geologic past.
This logical flaw can be easily seen with a simple analogy. Ask yourself: Did forest fires ever occur before there were people on Earth? The answer, of course, will be yes. Lightning strikes did start forest fires in the distant past. Then ask if that means that all forest fires today occur only because of lightning strikes. At that point the flaw in logic becomes clear: today, in addition to lightning, forest fires also result from arsonists, careless campers, and thoughtless smokers tossing cigarette butts from their cars. The takeaway lesson is that in addition to natural processes there is a new player in the forest fire arena today, the human population.
Credible climate scientists do not limit their inquiry into causes of climate change to those factors active in the prehuman past—they should and indeed do consider the possibility that over time, the causes of climate change may vary. Their task is to understand what has caused climate changes of the past, and what is causing contemporary climate change. The causes may or may not be the same, but scientists must evaluate the role of all possible causes, old and new, to decide which are the most important at a given time. And as the evidence does in fact indicate, human activities overtook natural factors in the twentieth century, to become the dominant force driving climate change today. Nature, long the conductor of the climate orchestra, has been displaced by the human population. In the next chapter we will see the great array of footprints we have placed on planet Earth.