In most discussions about CO2 and the greenhouse effect, volcanoes are mentioned as a rather minor source of gas emissions (page 124). Yet scientists have credited volcanic CO2 with ending the drastic snowball Earth ice ages of the Precambrian. Though biologic activity controls the short-term carbon balance, in the long-term volcanoes provide the majority of atmospheric carbon.165 This can be shown with some simple math.
If we assume a constant rate of ½ Gt (gigatons) of CO2 from volcanoes each year, as stated in Chapter 7, then the total emissions over a million years would be 500,000 Gt. This is an amount greater than all the carbon in all of the biologic sinks in the biosphere. If all of this gas remained in the atmosphere, the CO2 level would be more than 680 times modern levels. We know that there are multiple processes removing CO2 from the atmosphere, but scientists suspect that these processes might not always be sufficient to keep the CO2 buildup in check. There are two reasons for this.
First, the amount of volcanic activity was much higher on primitive Earth. The geologic record contains proof of many periods with extensive eruptions.166 Traditional paintings of dinosaurs, with smoking volcanoes as a backdrop, are geologically accurate.
Illustration 35: Lava at Kamaomao, Hawaii, photo by R. Hoblitt, USGS.
Second, the presence of huge ice sheets, covering most of the planet would have drastically curtailed biological activity; the very activity that works to remove CO2 from the atmosphere. Without large stretches of open ocean, CO2 would continue to accumulate in the air. The buildup that ended the Cryogenian Ice Age is thought to have had CO2 concentrations 350 times present levels. Atmospheric CO2 levels would have exceeded 10%, a concentration that would asphyxiate human beings.
The time of the Permian-Triassic extinction, 251 million years ago, is marked by massive floods of volcanic rock in Siberia that cover an area 1.6 million square kilometers, roughly the size of western Europe.167 ,168 This area, known as the Siberian Traps, contains rock deposits a mile thick in places.
The volcanic activity in Siberia did not resemble the spectacular, explosive eruptions of recent volcanoes like Mount St. Helens or Mount Pinatubo. Unlike cone-shaped mountains spewing out smoke and molten rock, the eruptions of the Siberian Traps were characterized by large amounts of lava oozing out of cracks in the ground. The volume of rock contained in these lava flows, called flood basalts, was so great it could have paved the entire Earth with a layer 20 ft (6 m) thick..
During this period of near constant eruptions, lasting a million years, Siberian volcanoes are thought to have pumped out 10,000 gigatons of carbon dioxide into the atmosphere, 14 times the amount present today.169 This injection of CO2 may have been the trigger that ended the Karoo Ice Age and there are those who think that volcanoes, not an asteroid collision or nearby supernova, were the cause of the Permian-Triassic extinction.170
These are examples of CO2 causing significant global warming events, but the rise in atmospheric levels involved are hundreds of times the amount generated by human sources today. Even more telling are findings that during the latest ice age, increased CO2 levels followed rising temperature. This implies that CO2 doesn't drive temperature, it follows temperature.
Further evidence of the nature of CO2 levels comes from a study of fossil birch tree leaves from the very beginning of the Holocene.171 About three centuries after the initiation of Holocene warming, a 150 year cooling period took place, called the Preboreal Oscillation. During this temperature dip, the reconstructed CO2 levels ranged from ~0.030% to ~0.034%. These levels contradict previous assumptions that Holocene CO2 levels were in the 0.027% to 0.028% range until the beginning of the Industrial Revolution. Atmospheric CO2 may reinforce change in global temperature, but it is not the reason the temperature changes, except under the most extreme circumstances.
A Species Shaped by the Ice Age
Modern humans first appeared on Earth between 130 and 150 thousand years ago. Our earliest ancestors probably began spreading out of Africa during the Eemian interglacial, but the worldwide human diaspora didn't occur until the Pleistocene Ice Age reasserted itself and the most recent glacial period began. Greatly lowered sea levels and barren wide open plains allowed humans to walk to Europe, Asia and North America, eventually reaching Australia and the farthest tip of South America.
Bands of hunter-gatherers spread across the face of the planet, collecting food during the shortened summers and living in caves for the nine month long winters. It was during this time that scientists think Homo sapiens acquired their full cognitive abilities.172 For 100,000 years, humankind was tested and molded by the ice age climate. More than 200 generations, most of our species' existence, was spent under these conditions. We are truly a species shaped by the Ice Age.
The intelligence, cunning and skill at making weapons that allowed our species to survive the harsh climate, would serve our kind well when the glacial period ended and the Holocene warming ensued. The warming climate caused a shift in human society, a change from a nomadic existence to farming and, eventually, civilization. The rest is, quite literally, history.
Just as climate helped shape our species, there is no question that humans have altered the climate. Agriculture, irrigation, and deforestation have had a significant impact on Earth's environment from ancient times. With the advent of industrialization, the impact has increased. It is understandable that the most successful species on Earth has had a dramatic impact on its environment.
In altering the environment to better suit our needs, we have also changed the climate. But at the same time, we may have lost some of the adaptability that our ice age ancestors depended on to survive. Archaeologist Brian Fagan has theorized that, while civilization has made people less susceptible to small variations in the environment, our technology has amplified the impact of the infrequent, but inevitable, large catastrophes. He believes that our environmental vulnerability is greater than we imagine.173 We will revisit these concerns in Chapter 16, The Worst That Could Happen.
Ice ages are among the most radical climate changes that occur on Earth. The fact our planet has transitioned several times from very warm conditions to very cold conditions, is an indication that there are large forces at work. We can ignore the Snowball Earth ice ages that occurred before Earth gained its complex biosphere, and concentrate on the more recent ice ages. We have shown that climate variation during ice ages is not driven by CO2 levels. In fact, temperature does not follow CO2 levels, CO2 levels follow temperature. So, what does cause ice ages and, of more immediate concern, what causes the alternating glacial and interglacial periods of the Pleistocene Ice Age?
Most scientists think that it is the periodic variation in Earth's orbit around the Sun. According to Orrin H. Pilkey, Professor Emeritus of Geology at Duke University, and Linda Pilkey-Jarvis, a geologist with the Washington State Department of Ecology:
“The fundamental causes of the ice ages and their huge sea-level changes are various Earth orbital changes, such as the tilt of the axis of spin and the eccentricity of the orbit around the sun. These changes are responsible for changes in the location and intensity of solar radiation on the surface, which in turn determines global climate.” 174
The orbital changes referred to are called the Milankovitch Cycles, and we will examine them in detail in Chapter 9. But, as we have said, Earth's climate is very complex—there are other contributing factors.
Other drivers of ice ages, and by inference, climate, have been proposed. These include: changes in Earth's atmosphere, shifting continents, geologic uplift of mountain ranges, variation in the energy received from the Sun, and possibly radiation from exploding stars in the form of cosmic rays. We will discuss the prominent scientific theories for each of these topics in separate chapters. Before we investigate the more periodic sources of change, we will examine mass extinctions and the seemingly random events that cause them.
Ancient Extinctions
“What a book a devil's chaplain might write on the clumsy, wastefu
l, blundering low and horribly cruel works of nature!”
— Charles Darwin
When looking at the history of life on Earth, the most dramatic events are the great mass extinctions. These ancient ecological incidents may seem to be terrible, wasteful, pointless catastrophes, but consider the following statement from the Smithsonian Institution Department of Paleo-biology:
“Extinction is the complete demise of a species. It takes place when all individuals of a species die out. Extinction has occurred throughout the history of life on Earth. It is the ultimate fate of all species. In fact, it has been estimated that 99.9% of all species that have ever lived on Earth are now extinct.”175
A sobering thought, that the ultimate fate of all species is extinction. Since the threat of mass extinction is often cited as a consequence of global warming we will investigate the causes of previous extinction events. When did science first come to suspect that some species have vanished from Earth forever? That is an interesting story involving a pair of French naturalists, an American president, and some ancient bones found by the Ohio river.
The Ohio Animal
Thomas Jefferson, revered as the author of the American Declaration of Independence, and third president of the United States, was also an avid amateur scientist and noted naturalist. A man of the Age of Reason and a product of the Enlightenment, Jefferson was considered an expert in architecture, civil engineering, physics, mechanics, meteorology, anatomy, and botany.
He could read and write Greek, Latin, French, Spanish and Italian. A mathematician and astronomer, he made suggestions for improving almanacs and accurately calculated the eclipse of 1778. He was recognized as a pioneer in ethnology, geography, anthropology and, as our story will show, paleontology..
As John F. Kennedy said in 1962, to a White House gathering of 49 Nobel Prize recipients, “I think this is the most extraordinary collection of talent, of human knowledge, that has ever been gathered at the White House—with the possible exception of when Thomas Jefferson dined alone.”
Illustration 36: Thomas Jefferson, painted by C.W. Peale, 1791.
During his presidency, Jefferson commissioned the Corps of Discovery, led by Meriwether Lewis and William Clark, to explore the American interior. They were instructed to collect and send back samples of plants, animals, and rocks along the way. Jefferson filled rooms in the White House, and later his home, Monticello, with the specimens they sent back.
Jefferson had such an interest in paleontology that he commissioned, and personally paid for, William Clark to explore the famous fossil deposits at Big Bone Lick, Kentucky.176 Because of his efforts to establish scientific stratigraphy in the US some have dubbed him the “Father of American Paleontology,” but Jefferson never claimed to be more than an enthusiastic amateur.
When the influential French naturalist Georges-Louis Leclerc de Buffon spoke disparagingly of new world animal life in his 36 volume treatise, Histoire Naturelle, Jefferson was less than pleased. Jefferson assessed Buffon's pronouncements this way:
“The opinion advanced by the Count de Buffon, [Buffon, xviii. 100, 156.] is 1. That the animals common both to the old and new world, are smaller in the latter. 2. That those peculiar to the new, are on a smaller scale. 3. That those which have been domesticated in both, have degenerated in America: and 4. That on the whole it exhibits fewer species. And the reason he thinks is, that the heats of America are less; that more waters are spread over its surface by nature, and fewer of these drained off by the hand of man. In other words, that heat is friendly, and moisture adverse to the production and development of large quadrupeds.”177
In Notes on the State of Virginia, written in 1781, Thomas Jefferson vigorously defended the wildlife of North America against these insults. Buffon claimed, wrote Jefferson, “that nature is less active, less energetic on one side of the globe than she is on the other ... as if both sides were not warmed by the same genial sun.” Jefferson's analysis of Buffon's hypothesis continued.
“I will not meet this hypothesis on its first doubtful ground, whether the climate of America be comparatively more humid? Because we are not furnished with observations sufficient to decide this question. And though, till it be decided, we are as free to deny, as others are to affirm the fact, yet for a moment let it be supposed. The hypothesis, after this supposition, proceeds to another; that moisture is unfriendly to animal growth. The truth of this is inscrutable to us by reasonings a priori. Nature has hidden from us her modus agendi. Our only appeal on such questions is to experience; and I think that experience is against the supposition.”178
Illustration 37: American Mammoth, source U.S. National Parks Service.
Years of observation had convinced Jefferson that animals were actually larger in America than in Europe. When an expedition to the Ohio river valley returned with bones from what appeared to be a gigantic elephant, he viewed the remains as conclusive proof that Buffon was in error. Judging from the size of bones and teeth, this animal was five or six times larger than an elephant. Though nobody had ever seen such a beast roaming the American countryside, Jefferson was sure it existed somewhere in the vast continental interior.
“Our quadrupeds have been mostly described by Linnaeus and Mons. de Buffon. Of these the Mammoth, or big buffalo, as called by the Indians, must certainly have been the largest. Their tradition is, that he was carnivorous, and still exists in the northern parts of America.”179
Whether any of these “Mammoths” were found alive or not, here was proof that America was home to Earth's largest animal. In 1797, Jefferson arranged for some specimens to be shipped to Europe to bolster his assertions.
Georges-Louis Leclerc, Comte de Buffon, was the most influential naturalist of his day. He originated Buffon's Law, which states that despite similar environments, different regions have distinct plants and animals. These observations led him to conclude that species must have both “improved” and “degenerated” after dispersing away from a center of creation, a foreshadowing of Darwin's theory of evolution.
Illustration 38: Georges-Louis Leclerc, Comte de Buffon.
One hundred years before Darwin, in his Histoire Naturelle, Buffon noted the similarities of humans and apes and even talked about a common ancestry of apes and Man. When Buffon published Les Epoques de la Nature, in 1788, he openly suggested that Earth was much older than the 6,000 years commonly accepted and proclaimed by the church. Based on the cooling rate of iron, he calculated that the age of the planet was 75,000 years. This proclamation was condemned by the Catholic Church in France, who burned Buffon's books. A man ahead of his time, Les Epoques discussed concepts very similar to Charles Lyell's “uniformitarianism,” which will enter our tale 40 years later.
A prolific and flamboyant writer, Buffon was criticized by many of his contemporaries. Voltaire did not approve of his writing style, and d'Alembert180 called him “the great phrasemonger.” Unfazed, Buffon said of his critics' attacks, “I shall keep absolute silence ... and let their attacks fall upon themselves.”
Though Buffon set the stage for the discovery of extinction he was not the naturalist who first offered proof of the transient nature of terrestrial species. But, it was through the spat between Jefferson and Buffon that another French naturalist, Georges Cuvier, would come to know of the “Ohio Animal.” It was to Cuvier that Jefferson shipped his samples of Mammoth bones in 1797.181
George Cuvier, was born on August 23, 1769, at Montbéliard, a French-speaking community in the Jura Mountains that was not under French jurisdiction at the time. This area was ruled by a German nobleman, the Duke of Württemberg.182 The son of a retired army officer, his family had emigrated to avoid religious persecution. After a strict Lutheran upbringing, the young Cuvier studied at the Carolinian Academy in Stuttgart, a school which the Duke had founded. During his studies he became familiar with the works of Linnaeus183 and Buffon, sparking an interest in nature and science.
Illustration 39: George Cuvier, painted by F.A. Vincent.
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Eventually Cuvier moved to Paris, where he became a professor of animal anatomy at the newly reformed Musée National d'Histoire Naturelle. In his first important paper, read publicly in 1796, Mémoires sur les espèces d'éléphants vivants et fossiles, he analyzed skeletal remains of Indian and African elephants as well as mammoth fossils found in Siberia. Also included in his study was the fossil remains of the animal de l'Ohio, the “Ohio animal.”
Cuvier's analysis proved that African and Indian elephants were, in fact, different species, and that mammoths were not members of either species. He further stated that the “Ohio animal” represented another distinct species that displayed even greater differences from living elephants than mammoths. He went on to hypothesize that both mammoths and the “Ohio animal” represented species that must be extinct.
Those first fossils Cuvier examined turned out to not be the same species as the later bones sent to him by Thomas Jefferson. Jefferson's creature was identified as an American Mammoth, one of three species that lived in what is now the mainland United States. In honor of Jefferson, that particular species is now known as Mammuthus Jeffersonii, Jefferson's Mammoth. Years later, in 1806, Cuvier would return to the original “animal de l'Ohio” in another paper and give it the name Mastodon.
Ever since fossils had been discovered, scientists assumed that they were remains of living species. This belief was reinforced by religious teachings of the time, which said that no species had ever gone extinct. This was based on the assumption that anything God had designed was perfect and couldn't die out. By the start of the 19th century, this ideology began to come under increasing scrutiny from scientists, most notably Cuvier.
In his second paper, Cuvier analyzed the fossil remains of a giant ground sloth, named Megatherium, that had been found in South America. Megatherium was one of the largest mammals to walk on Earth. Weighing as much as an African bull elephant and, when standing on its hind legs, about twice as tall, it was unlike anything ever seen alive. Comparing the fossil bones with the skeletons of modern sloths, he found that Megatherium represented a distinct species, and again the evidence indicated that this species was extinct.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 10