The Paleogene Period
The Paleogene Period consists of the Paleocene, Eocene and Oligocene Epochs. During the Paleogene, which lasted about 42 million years, mammals evolved from relatively small, simple animals into many diverse, larger forms. These large animals would come to dominate the land, taking the place of the extinct dinosaurs. Mammals also returned to the sea, adapting and evolving into whales, seals and other present day marine mammals.
Earth's climate cooled throughout the Paleogene, eventually leading to the great Pleistocene Ice Age in the mid-Neogene Period. But before the world cooled down, something unexpected happened; an event that still confounds scientists—the Paleocene-Eocene Thermal Maximum (PETM).
This dramatic event occurred around 55 mya, at the start of the Eocene, and was one of the most rapid and extreme periods of global warming in geologic history. Sea surface temperatures rose between 10°F and 15°F (5°C and 8°C) over a period of a few thousand years. In the Arctic regions, sea surface temperatures rose to a sub-tropical 73°F (23°C).123 Not only did the surface of the Arctic ocean heat up about 10 degrees, but the ocean depths warmed as well.124 The temperature of shallow waters off the North American coast rose to a balmy 92°F (33°C).125
Rapid warming in the higher latitudes led to significant changes in animal life both on land and in the oceans. Oxygen levels in the oceans dropped sharply and 40% of marine life perished. Around the world, water flowing in ocean currents reversed directions. Scientists now think as much as 4.5 trillion tons of CO2 were added to the atmosphere,126 raising the level of CO2 to ten times current levels. This was real, uncontrollable global warming.
What caused the PETM? As with many events in science, there are a number of competing theories. Some think it was a volcanic outbreak, others a massive release of methane from the deep sea floor.127 No one knows for sure. Better known is the effect the PETM had on Earth's wildlife.
It is no surprise that such a dramatic shift in climate would have an equally dramatic effect on the living creatures of the time. Still, the effect was rather surprising—animals became significantly smaller. Paleontologists discovered this downsizing from the size of teeth found in the fossil record. During the period of hottest temperatures, mammals were half the size of those that came before them.128 This warm interval lasted 150,000 years and marked fundamentally different climatic conditions than were present at any other time in the Cenozoic.
Aside from the anomalous PETM, the Eocene Epoch's climate was the most homogeneous of the Cenozoic, with an average temperature difference between the equator and the poles only half what it is today. The polar regions were significantly warmer than today, with temperate forests extending north and south, as far as land allowed. The deep ocean waters remained exceptionally warm, moderating temperatures worldwide. Rainy tropical climates extended as far north as 45 degrees latitude. Only in the tropics was the climate similar to today. Earth's climate during the Eocene was as close to a Garden of Eden as could be imagined.129 ,130 Though the ancestors of modern animals were emerging during the Eocene, they were vastly different than animals of today. There were strange creatures around, like Mesonyx, a carnivorous wolf-like animal that had hooves at the ends of its stubby toes. Mesonyx belonged to an extinct order of animals called mesonychids, who were the only known group of ungulates (hoofed animals) to become carnivorous.
Illustration 25 Mesonyx, a wolf-like mammal from the Eocene Epoch.
After the PETM, larger animals began to reappear. Another member of the mesonychids, Andrewsarchus, was found in Mongolia and parts of Asia. “Andrews' Beast,” named after paleontologist Roy Chapman Andrews, is the largest carnivorous mammal ever to walk on Earth. Standing more than 12 feet (3.5 meters) at the shoulder and weighing more than 3000 lbs, Andrewsarchus was twice the size of a modern grizzly bear.131
At the end of the Eocene, yet another extinction event occurred called the Grande Coupure (“Great Break”). It severely affected European and Asian mammals.132 The cause of the extinction could have been cooling climate, large swings in sea level, or the impact of large asteroids in what is now Russia133 and the Chesapeake Bay.134 Whatever the cause, it brought the Eocene to a close.
During the Oligocene Epoch, roughly 34 – 23 mya, Earth continued its slow transformation into the world we know today. The first elephants with trunks appeared, along with early horses. Modern grasses made their debut, plants that would produce vast grasslands during the following Miocene Epoch. The climate still supported vast tracts of tropical greenery, but there was a distinct hint of drier, cooler times to come for much of the world.
The world's climate stabilized. Fragments of the supercontinent Gondwana had fully broken off, with Australia and India moving northwards while Antarctica remained isolated in the extreme south. Once Australia moved far enough north to allow ocean waters to flow unhindered around Antarctica, the circumpolar current formed. This isolated the south polar region and accelerated that continent's descent into frigidity.
Illustration 26 Mesohippus bairdi, a three-toed Oligocene horse. Painting by Heinrich Harder.
Temperatures cooled throughout the Oligocene, with an icecap forming for the first time over Antarctica. Global temperature dropped as much as 15°F (8.2°C).135 There is evidence of glacial episodes occurring at 29.16, 27.91, and 26.76 mya, causing sea level fluctuations of 160–210 ft. (50–65 m).136 More dramatic changes were in store as the Neogene Period began.
The Neogene Period
The Neogene started about 23 mya and continues on to the present day; it is our final geologic Period. As we approach modern times, our knowledge of Earth's past climate grows more detailed because there has been less time for age to destroy the evidence. During the Neogene, the warm and relatively stable climatic conditions, present for so much of the Mesozoic and Cenozoic eras, came to an end. This stable period lasted 200 million years―since the world recovered from the Permian-Triassic Extinction.
Through the Paleogene Period, the world was still a warm, humid, and heavily forested place, unlike the drier, colder and much harsher conditions of today. During the Neogene, continental forests were replaced by grasslands and broad savannas. In North Africa and Central Asia, large deserts formed.
The Neogene world looked much like our own. Continents began to take on the familiar shapes recognized today. The continents were changing, as they still are today, creating mountains by slow motion collisions. Throughout the Neogene, new mountain ranges grew. India collided with the underside of Asia and formed the Himalayas, Spain pushed into southern France raising the Pyrenees, Italy moved north into Europe and created the Alps. In what are now the Americas, the Rockies and Andes formed.
Also during the Neogene, northward-moving South America finally caught up with North America. Their collision created the Isthmus of Panama, one of several land bridges formed between continents during this period. The Isthmus of Panama allowed North and South America to exchange animals. Dogs, cats, bears, and horses migrated south while North America gained armadillos, porcupines, and opossums.
The Miocene Epoch, 23–5 mya, makes up most of the Neogene. During this time, the transition from forests to grasslands forced existing animals to adapt or die. Many forest dwellers became extinct and new animals developed that could live on grass. Horses, bison, sheep, giraffes and camels appeared on the plains.
With clear vistas, good eyesight and long legs, these new grazing animals proved harder for predators to catch, causing an escalation in the age-old arms race between predator and prey. Wolves and big cats emerged as the new apex predators. Able to run fast to catch prey, and equipped with powerful jaws and teeth to take them down, these new animals became the dominant predators on the Miocene grasslands.
Around six million years ago, colliding plates in Europe and Africa blocked off the Mediterranean basin, causing the Mediterranean Sea to dry up and disappear. In his book, The Mediterranean Was a Desert, geologist Ken Hsu paints a dramatic picture of a “deep, dry, hot he
llhole,” some 1.8 miles (3 km) below sea level. Geological evidence indicates that this happened not just once, but ten times or more over a million-year span.137
The evaporation of the Mediterranean Sea brought about the Messinian Salinity Crisis.138 Aside from having rather drastic impact on the creatures trapped in the Mediterranean, water from the sea was redistributed to the world's oceans. This caused sea levels to rise and ocean salinity to drop, affecting life worldwide. Eventually, the barrier at the Strait of Gibraltar broke permanently, re-flooding the basin and temporarily creating a spectacular set of water falls bigger than Niagara and higher than today's Angel Falls,139 over 3,200 ft (979 m).
Ice House World
For reasons that are still not fully understood, 14 million years ago the Antarctic ice sheets stabilized and became permanent. Around seven million years ago, a small temporary ice sheet formed on Greenland for the first time. Still, the early Pliocene was about 4°F (2°C) warmer than today. Some scientists think that the world was undergoing perennial El Niño conditions.140
El Niño initially referred to a weak, warm current appearing annually at the end of December along the coast of Ecuador and Peru. It usually lasted only a few weeks to a month. Every three to seven years, an El Niño event would last for many months, having significant economic and atmospheric consequences worldwide. During the past forty years, ten of these major El Niño events have been recorded, the worst of which occurred in 1997-1998.
Midway through the Pliocene Epoch, the great Pleistocene Ice Age began in earnest. This period of extreme glaciation was originally thought to have started during the next epoch, the Pleistocene, hence the name of the ice age. The boundary layer that marks the transition from the Pliocene to the Pleistocene has been dated between 1.8 and 1.6 mya, and shows a minor extinction event that affected populations of shallow water clams. Even though we now know this ice age started in the Pliocene, the name remains unchanged―it is called the Pleistocene Ice Age.
One possible explanation for the onset of global cooling is that ocean surface temperatures began dropping when winds started bringing cold, deep ocean waters to the surface in low latitudes. Other theories range from long-term cyclical variation of solar energy to massive volcanic eruptions. Whatever the cause, Earth's last, great, warm period was over.
About three million years ago, large ice sheets began forming over North America and Europe. These ice sheets were to become bigger than the Antarctic ice sheet of the present day. In North America, ice came as far south as Ohio and New York, while glaciers covered most of northern Europe. Ocean sediments from the northern Pacific show ice-rafted debris abruptly began appearing ~2.7 mya,141 evidence of drifting icebergs that came from glaciers, not pack ice. Only ice that scrapes across land picks up embedded dirt and rocks. Earth had become an ice house world.
The ice age lasted the remainder of the Pliocene and throughout the Pleistocene, with alternating episodes of glacial advance and retreat. During periods of heavy glaciation, sea levels dropped by as much as 400 ft (120 m), establishing land bridges between continents that allowed animals, including man, to migrate to new areas.
The Holocene Epoch
Ice core records indicate that Earth started warming up about 18,000 years ago, though it proceeded in a number of fits and starts. The Pleistocene Epoch ended with the termination of the last glacial period of the Pleistocene Ice Age, about 12,000 years ago. The Holocene Epoch (“new whole”) begins with the retreating ice at the beginning of the current interglacial.
Illustration 27 Neanderthal man, an extinct cousin.
Scientists do not know if the Pleistocene Ice Age is truly over, or if we will return to another glacial period, but most seem to think we are going back into the deep freeze.142 There seems to be a cycle of 20,000 year interglacials separating 100,000 year glacial periods. This is possibly caused by changes in Earth's orbit and the wobbling of its axis. If this is true, then the start of the next glacial period could be right around the corner or 10,000 years in the future.
We are creatures of the Ice Age. Many scientists believe that, without the extreme challenges brought on by the advancing ice sheets, mankind would never have developed the survival skills that have brought us to planetary dominance. Great intelligence, ability to collaborate using language, and facile use of tools, has enabled Homo sapiens sapiens (“wise wise man”) to spread from nearly pole to pole.
Most of our closest relatives were not as adaptable. They were also not as fortunate. Earth is littered with the bones of other failed members of the genus Homo; Homo habilis, Homo ergaster, Homo erectus and Homo neanderthalensis. Anatomically, modern humans generally have a lighter physical build compared to our earlier cousins. Physically, we were weaker, yet our species was the one that survived.
But we shouldn't be cocky, mankind hasn't been around very long. The oldest fossil evidence for anatomically modern humans is about 130,000 years old143 ― a mere instant in geologic terms. The entire recorded history of our species is contained within the Holocene, the rest of Earth's long past belongs to pre-history and to other species.
Lessons from the Past
So what lessons can we learn from Earth's long history? For one thing, we can say that life is tenacious. Everywhere you look on Earth you will find life, from the Arctic to the Tropics, from the highest mountains to the bottom of the deepest abyssal ocean trench. Living organisms can be found in pitch dark caves, boiling hot volcanic ocean vents, and underneath mile thick glaciers. There are microorganisms, related to ancient archaea, that live in underground oil deposits and have been found in oil wells.144 There are even “radiation eating,” single-cell fungi that have been found thriving inside the collapsed nuclear reactor at Chernobyl, Ukraine145 —when humans make a mess, life finds a new habitat.
Illustration 28 Biodiversity in terms of number of species types (genera) during the Phanerozoic Eon. After R. Rohde, Global Warming Art.
We are constantly warned that “Mother Earth is in trouble,” that we are losing species right and left, that diversity is vanishing. The notion that nature is fragile is puzzling. As we have seen, after every catastrophe, every mass extinction, life springs back. Illustration 28 shows a graph of species diversity throughout the Phanerozoic Eon. Note how after each extinction event diversity increases. Life rises to a challenge.
Illustration 29: Temperature over the Phanerozoic from oxygen-18 proxy data. After R. Rohde, Global Warming Art.
Far from being the fragile, breakable thing described by some ecologists, life is tougher than ice ages, tougher than asteroids, tougher than anything the Universe has been able to throw at it. A life is fragile, Life is tougher than rock.
Another trend to observe is temperature. Illustration 29 shows estimated average temperature during the Phanerozoic derived from oxygen isotope proxy data. How these temperatures and other environmental factors from the past are measured will be discussed in Chapter 13, Experimental Data and Error. The main observation that can be drawn from these data is that Earth's modern day climate is the anomaly. For most of Earth's past, temperatures have been significantly higher than they are today. Only during previous ice ages are temperatures similar to today found. Comparing modern CO2 and temperature levels with the past, it becomes clear why scientists say we are still in an active ice age.
The final important observation is the variation of carbon dioxide levels during the Phanerozoic Eon, shown in Illustration 30. CO2 levels at the beginning of the Cambrian were 25 times the average level during the Holocene. They steadily declined through the coal bed forming Carboniferous, but began to rise again during the Permian. Throughout the Age of the Dinosaurs, CO2 levels were 5 to 10 times modern levels.
Illustration 30: Carbon dioxide levels during the Phanerozoic Eon. After R. Rohde, Global Warming Art.
The general trend of CO2 reduction is not surprising, given that there are numerous biological mechanisms that work to store carbon within Earth's strata. The number of ways biolo
gy has conspired with geology, both on land and in the oceans, to remove CO2 from the atmosphere, is astounding. These natural processes dwarf humanity's release of greenhouse gases. The sequestration of carbon will be discussed in greater detail in Chapter 7, Changing Atmospheric Gases. It will be shown that even if all the fossil fuels buried in Earth's crust were burned, it would not make an appreciable bump in the overall carbon cycle. The CO2 graph would show hardly a wiggle, at least on the geologic timescale.
This is not to say that releasing that much CO2 would have no effect on Earth's climate. It undoubtedly would. The real question is, would this be a catastrophe? Earth's ecosystem would change and adjust, like it always has, and humanity might not like all of the changes. But nature is always changing, as shown in the graphs. Species change, temperatures change and CO2 levels change. One of the persistent mysteries of the global warming debate is how those who claim to love nature most, seem to understand it least.
Atmospheric carbon dioxide did not reach modern levels until the onset of the Pleistocene Ice Age. When placed in historical context, the current level of CO2 in the atmosphere is near a historic low point. Clearly, life has thrived during periods when CO2 levels were significantly higher than today. This makes the IPCC's obsession with CO2 levels all the more puzzling. To summarize observations from this review of Earth's past:
Earth's climate is constantly changing. There is no one “normal” climate pattern.
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