In recent years astrophysicists have proposed links to other extraterrestrial phenomena, including nearby supernovae194 and the solar system's course in and out of the plane of the galaxy.195 Here is a list of causes that have been proposed for one or more mass extinction:
Extraterrestrial Impacts — asteroids or comets striking Earth.
Massive Volcanoes — in particular the effect on climate.
Moving Continents — destruction of habitat due to continental drift.
Ice Ages — glaciation, global cooling, lowered sea levels.
Disappearing Oxygen — deep water overturn or methane ice.
Cosmic Peril — impact of cosmic rays and supernovas.
Coincident Causes — the “murder on the orient express” model.
Our planet's past is filled with extinctions, some large, some small, some solitary. All the ages in the fossil record chronicle the departure of species from this Earth. The sweep of geologic time, comprising more than 90 recognized time periods, is partitioned by changes in the fossil record. The most complete records come from the seas and oceans where the departed settle to the bottom in a silent rain of death. The vast ocean sediment beds accept all comers.
Though life on land has been similarly affected by changing climate, drifting continents, and the occasional rogue asteroid, the fossil record there is much poorer. This is because it is much less likely for a land animal or plant to die under circumstances conducive to preservation. Sometimes, a tree toppled into a peat swamp where the oxygen-poor water prevented decay until rising oceans buried the swamp with sediments. Hundreds of millions of years later, we find the imprint of a leaf or twig embedded in coal. On other occasions, animals were buried by volcanic ash or blundered into a tar pit. But, despite the astonishing number of extinct creatures that have been found, scientists believe many times more have vanished than we have uncovered. Of these creatures, we will never know. For us, the history of life is written in rock by the remains of the extinct.
Just as climate experts have been unable to find a model that would explain the sudden onset of ice ages, scientists are now looking to more sudden events for the cause of mass extinctions. Just as ice ages seem to need a “jump start” to begin or end, extinction events seem to need a trigger. These sudden catastrophes underscore our planet's often violent past.
Invasive Species
As we have seen, ice ages sometimes contribute to extinctions. They are suspected as the major cause of the Ordovician and Devonian extinctions and may have participated in many other minor extinction events. But sometimes the occurrence of an ice age in juxtaposition with a species' demise is just coincidence.
We know from evidence that both mammoths and mastodons survived the worst of the Pleistocene Ice Age in North America. What made them go extinct is not certain but many think it was related to the ice age. During the last glacial period of the Pleistocene, ocean levels were as much as 400 ft lower than today. This would have created numerous bridges between land masses, including one connecting Asia to North America. In the waning days of the ice age, a new, super-predator arrived in the Americas—Homo sapiens.
Illustration 47: Early Paleoindian Clovis points. Drawings by Richard McReynolds, from Chandler and Kumpe (1997).
The Clovis people, migrants from Asia who entered North America approximately 12,000 years ago, are thought to have hunted mastodons, mammoths, and many other large mammal species to extinction. The Clovis people were primitive hunter-gatherers that subsisted on wild animals and plants. The height of their technology is embodied in the well-formed flint spear points they left behind (Illustration 47).
From 12,000 to 8,000 years ago, the Great Plains of North America were populated by small bands of hunters. These groups developed the weaponry and the expertise necessary to successfully hunt large animals. For a band of primitive hunters, wandering from place to place, living off what they could kill, a mammoth would have been a treasure on legs. They would have valued not just the meat, but the woolly hide, the sinew and bone, and the ivory in the animal's tusks. Not that this treasure was easily taken. Attacking a creature several times the size of a modern elephant, with only stone-tipped spears, would have been as hazardous to the hunters as the hunted.
Over time, these ancient people developed more effective hunting methods. In a number of locations in North America, scientists have found evidence that ancient hunters lured or herded their prey into dead falls. Mammoths became extinct on the Plains not long after the arrival of humans and, although climate conditions were worsening, many researchers think hunting by Paleoindians hastened the mammoth's demise.196 With the disappearance of the mammoth, the main target of the hunters shifted to other large plains animals, Bison antiquus and Bison occidentalis, sometimes (incorrectly) called buffalo.
Though modern day bison are large animals, the bison of the late stone age were even larger. From skulls found at various sites, it has been estimated that these creatures were twice the size of their modern cousins. They were alert, powerful wild animals who were not easily killed. Noted anthropologist George Frison, of the University of Wyoming, described Paleoindian hunting this way:
“Large mammals (mammoths, mastodons, and bison) usually are portrayed mired in bogs with the hunters throwing spears and rocks, dogs barking, and crippled and dead hunters being dragged from the scene. Hunters wave blankets at a small group of bison and they jump off a cliff to their deaths. In reality, bison are extremely agile and, to force them over precipices, a large herd must be stampeded in the proper direction so that the ones in the rear can push the leaders over the edge.”197
While stampeding a herd of bison off a cliff may not seem very sporting, these hunters were worried about survival, not ethical questions. This is primitive Man in his natural state as alpha predator. No different than crocodiles schooling at the one river crossing where all wildebeest must transit during their annual migration, or a pack of hungry wolves harassing and driving a caribou herd, killing the old, the young and the weak. This is why humans survived the ice age, humans are the most fearsome predators on the planet.
Contributions of human hunting to mammoth and mastodon extinction are still debated.198 Sudden climate change, a nearby supernova,199 and even trees have been blamed for their demise.200 Still, many researchers believe that human over-hunting directly caused the extinction of the mastodon and mammoth. People like to think that early Man existed in harmony with nature, all creatures living in idyllic balance. This is a static and naive view. Often, as Earth and climate change, species previously separated come into contact for the first time. Then, nature arrives at a new balance. Unfortunately for some species, this means their time on Earth is over.
Species invasions have occurred innumerable times in the past from natural causes, but the actions of humans have also been responsible. Regardless of the source, the effect of introducing foreign species to new habitats can be devastating.
Examples of these unintended invaders are legion: Formosan termites imported in the wood of shipping pallets, tropical fish species flushed down toilets, zebra mussels carried in ships' bilge water, the introduction of kudzu and water hyacinths, and on and on.
Australia has been suffering a plague of rabbits for over 100 years. Introduced by the first European settlers in 1788, rabbits were released into the wild. By 1890 they had reached plague-like proportions, causing widespread eco-logical damage. They compete with native animals for habitat and food, such as the now-endangered Greater Bilby. Also known as the Rabbit-eared Bandicoot, the Bilby is an iconic marsupial once found across wide areas of Australia.
By consuming seeds and seedlings, rabbits are driving native Australian plants to extinction. They also cut away strips of bark around a tree's circumference, called ring-barking, which kills the tree. Rabbits cost Australian agriculture $600 million a year in lost production.201
Sometimes, invasive species are introduced unintentionally. Around the time of World War II, the brown
tree snake was accidentally transported from its native range in the South Pacific to Guam, probably as a stowaway in a cargo ship. With no natural predators, it has wreaked havoc on the native ecology and threatened the island's human inhabitants.202 Good climbers, that hunt primarily at night, these aggressive, venomous pests grow up to 8 ft (2.5 m) long. There are reports of these snakes entering houses during the night, attacking pets and small children.
In one incident, a brown tree snake bit a human baby, and attempted to eat the child, starting with its hand and arm. Discovered in time by the child's parents, the snake was dispatched and the baby recovered after a stay in hospital.203 The brown tree snake has since migrated to other Pacific islands and now threatens Hawaii.
Illustration 48: A dodo bird, now extinct.
These stories have been repeated all over the world, sometimes by design, and sometimes by accident. The introduction of domestic animals, dogs and pigs, to the islands of Mauritius is thought to have caused the extinction of one of the most famous recently extinct animals, the Dodo. The Dodo bird is famous for two things—being dumb and being dead. The phrases “dead as a Dodo” and “gone the way of the Dodo” are synonymous with extinction. For the most part, the dodo was described as a lazy, rather dumb animal, with foul-tasting flesh.204 As American humorist Will Cuppy said, “The Dodo never had a chance. He seems to have been invented for the sole purpose of becoming extinct and that was all he was good for.”
The Sixth Extinction
Despite the impressive advances of science we are unable to bring back a single vanished species. When a species becomes extinct all the knowledge we might have gained from studying it, all the pleasure of observing it, is also gone forever. The genetic code of each life-form, its genome, contains a treasure house of information. New drugs, cures for disease, and unique evolutionary insights can all be lost. We should be very careful when dealing with such living treasure.
Some ecologists have cited Man's impact on nature, accelerating the extinction of numerous species, as the sixth major Phanerozoic extinction event.205 ,206 Others have stated that it will take humans another 10,000 years to join the extinction big league. Either way, we are the only species that actually has a choice about which species we drive to extinction. Regardless of the outcome of the global warming debate, we humans need to pay closer attention to the effect our actions have on the environment. Sometimes technological advances can have a positive effect on nature. Man had hunted whales to the brink of extinction when the discovery of oil, in 1859, granted them a reprieve. Of course, oil is part of the problem today.
But even without Man's actions species will go extinct, and not all can or should be saved. Remember, every ecosystem—tropical rain forests teaming with life, magnificent redwood forests towering above the ground, northern tundra that springs to life during the brief Arctic summer, sun baked deserts that wait years for a rain shower to bloom—all these environments will be destroyed over time. Any species that overspecializes, whether plant, animal or bacterium, has signed on for early extinction. All species go extinct—that is nature's way.
So we see that there are numerous causes for extinctions, with climate change a contributing factor, but not the leading cause. Past climate fluctuations much more dramatic than the current, slight warming trend have failed to have a significant impact on Earth's biota (living things). Polar bears, penguins, and coral reefs have all lived through interglacial periods before. The ancestors of most modern species lived through the Paleocene-Eocene Thermal Maximum, a period of global warming that was an order of magnitude more intense that our current “crisis.” In truth, Man is more likely to cause extinctions by the accidental introduction of foreign species than by warming the planet.
Man's recent impact aside, scientists have come to suspect that many of the events that have shaped life and climate on Earth have extraterrestrial sources. These events are not limited to the odd, random asteroid impact. Recent research has identified cyclic changes in Earth's climate on scales as short as tens of years to as long as hundreds of millions. But these suggestions come from outside the climatological community, and have been resisted or summarily dismissed. Others point to long-term terrestrial processes as the root cause of climate change. Next, we will examine these theories, starting with earthbound processes that can cause climate to change and species to die out. Because greenhouse warming caused by CO2 emissions is central to the IPCC's case, we will start there.
Changing Atmospheric Gases
“A theory's validity depends on whether or not it can be verified, it is constantly tested against the facts; wherever it can no longer explain the latter, it shows its limitations and unsuitability. It must then be rethought.”
— Pope John Paul II
Life requires energy to exist, and the energy, for almost all life on Earth, comes from the Sun. Without the Sun, our planet would be a frozen, dark ball drifting through space, lifeless and alone. Energy is transferred from the Sun to Earth in the form of electromagnetic radiation. Part of this radiation appears to us as visible light, but there are other frequencies that we cannot see: infrared, ultraviolet, and x-rays are also forms of light.
Illustration 49: Scattering and absorption of solar radiation. Source the COMET Project.
When radiation from the Sun strikes Earth, part is reflected by the atmosphere and clouds, part is reflected by the surface, and part is absorbed by the land and oceans. It has long been known that some gases in the atmosphere trap heat radiating from Earth's surface, preventing it from escaping into space. This has an effect similar to a greenhouse, trapping heat and warming the planet. For comparison, look at Earth's sister planets, Venus and Mars.
Venus, named after the Roman goddess of love, is almost Earth's twin in terms of size. Unfortunately, Venus is so close to the Sun that its temperatures are far too hot for life to exist. The Venusian atmosphere is a thick blanket composed chiefly of carbon dioxide with swirling clouds of sulfuric acid. This massive blanket of carbon dioxide generates a surface pressure 90 times greater than that on Earth. This causes a runaway greenhouse effect that heats the planet's surface to an average temperature of 872°F (467°C )—hot enough to melt lead.
Illustration 50 Venus, Earth, and Mars. Source NASA.
Mars, on the other hand, is farther away from the warming Sun. As a result, it is a cold, forbidding world. The Martian atmosphere, like Venus's, is made mostly of CO2. Unlike Venus, the surface pressure on Mars is only about 0.7% of the average sea level pressure on Earth, with an average surface temperature around -63°F (-53°C). The polar icecap, visible in Illustration 50, is frozen CO2, not water. Mar's atmosphere is frozen, thin and unbreathable; perhaps befitting a planet named after the Roman god of war.
When compared to its sisters, Earth is often called the “Goldilocks” planet: Venus is too hot and Mars is too cold, but Earth is just right. Earth is, indeed, the “Miracle Planet” for we currently know of no other world where life exists. It is still possible that one of the moons of the outer planets—Jupiter, Saturn, Uranus, or Neptune—will harbor life. If they don't we will have to look to other star systems for company.
Earth isn't quite close enough to the Sun for temperatures to rise above freezing without help from the atmosphere. If it were not for the greenhouse effect, our planet would be about 60°F (33°C) colder than it is—far too cold for life. Earth's climate is the result of a balance mediated by gases in the atmosphere. Therefore, since life regulates atmospheric gases through a number of feedback mechanisms, it is fair to say that life itself creates livable conditions on our planet.
Discovering the Greenhouse Effect
Sir Frederick William Herschel (1738-1822) was a well-known English musician and astronomer. Born in Hanover, Germany, he moved to England in 1757 with his sister Caroline. Together, they constructed telescopes and surveyed the night sky. Their work resulted in several catalogs of double stars and nebulae. As an astronomer, Herschel is most famous for his discovery of
the planet Uranus in 1781, the first new planet found since antiquity.207
In 1800, Herschel made a dramatic discovery. While observing sunlight through colored filters, he noted that filters of different colors seemed to pass different amounts of heat. To determine if the colors themselves were of varying temperatures, he devised a clever experiment to investigate his hypothesis.
Herschel directed sunlight through a glass prism to create a rainbow spectrum and then measured the temperature of each color. To measure the temperatures, he used three thermometers, their bulbs blackened to better absorb the heat. He placed one bulb in the visible light of a single color, while the other two were placed out of the light for comparison. As he measured the individual temperatures, he noticed that all the colors had temperatures higher than the unilluminated thermometers. Moreover, he found that the color's temperatures increased from the violet to the red part of the spectrum.
After noticing this pattern, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region where no sunlight was visible. To his surprise, he found that this region had the highest temperature of all. Herschel had discovered a form of light beyond red, invisible to human eyes. This is a classic example of how the unexpected often leads to important scientific discoveries. Herschel had not been looking for a new form of light, he was just trying to measure the heat from the visible spectrum.
Illustration 51: Jean Baptiste Joseph Fourier (1768-1830).
In 1816, Herschel was made a Knight of the Royal Guelphic Order by the Prince Regent, granting him the title “Sir.” Sir William helped found the Astronomical Society of London in 1820, which later became the Royal Astronomical Society. On 25 August 1822, Herschel died at Observatory House, Slough, and was buried at nearby St Laurence's Church.208 It would fall to one of his contemporaries, Joseph Fourier, to make the connection between the heat carried by invisible light and the warming of Earth's atmosphere.
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