More often than not, there have been no polar icecaps on Earth.
The temperature on Earth has been much hotter than it is now and has been for long stretches of time (more than 100 million years in the Mesozoic).
CO2 levels in Earth's atmosphere have been much higher than they are now and were higher for most of the history of life.
There have been a number of times when Earth has been very much colder than it is right now. There have been several Ice Ages.
After each ice age, Earth's climate has warmed back up to modern temperature levels or higher.
There have been a number of times when Earth has been very much colder than it is right now. There have been several Ice Ages.
After each ice age, Earth's climate has warmed back up to modern temperature levels or higher.
All of these variations in climate, the warm periods and the ice ages, have come and gone without the presence of mankind. Earth has experienced climate shifts far more dramatic than the recent global warming. Conclusive fact? Man could not have caused them.
As we have seen, the claim that Earth's climate is undergoing “unprecedented” change is totally inaccurate. In fact, when viewed from a long-term historical perspective, the recent warming trend isn't even noteworthy. If anything, our current cool climate conditions are abnormal. We have also seen that elevated CO2 levels have neither ended nor prevented ice ages in the past, implying that CO2 is not the primary driver of climate.
What, then, does cause Earth's climate to change? We will examine the mechanisms of climate change that science has discovered in the next several chapters. We will start with a closer look at the causes of radical climate change in the past—ice ages.
Ice Ages
“The snow doesn't give a soft white damn whom it touches.”
— E. E. Cumming
People are fascinated by the thought of an Ice Age—a frozen world populated by club wielding cavemen and animals like mammoths and saber-toothed cats. We know that our ancestors actually lived during such an ice age because they left evidence of their presence behind.
Some of the earliest examples of human art date from the last glacial period, stone age cave paintings. Paintings dating back as far as 30,000 years ago have been found in France, China, Tanzania and Australia. This indicates that humans were widely spread across the world by that time.
Illustration 31 Paleolithic cave painting of a horse, Lascaux France.
Other than a few scattered cave paintings, our ancient ancestors left no record of the ice age behind. How did we find out about ice ages? When were they first discovered by Science? How many have there been, how long ago and how often? In this chapter, we will examine the geologic evidence for ancient ice ages predating our ice age, the Pleistocene. Ice ages that may have frozen the planet from pole to pole. But first we will start with the story of a scientific theory and the clues that led scientists to believe that our planet was once covered in ice.
The Father of Glaciology
Louis Agassiz, a nineteenth century naturalist, is considered the “Father of Glaciology.” The son of a minister, Agassiz was born in the village of Montier, in the French-speaking part of Switzerland, on May 28, 1807. A physician by training he was also one of the leading naturalists of his day, establishing his reputation with his meticulous studies of fish fossils.
An inquisitive man, with a keen eye for observation, it is not surprising that he would have noticed the signs of glacial activity in his native Switzerland. After all, he had grown up in the shadow of the Alps surrounded by mountain glaciers. For many years, early geologists had been debating the source of a number of unexplained natural phenomenon; scoured rocks, strange mounds of debris called moraines, and erratic boulders.
Erratic boulders, large rocks found in the oddest places, having nothing in common with the surrounding rock on which they rested had, for years, puzzled the young science of geology. In the Alps, large boulders had been found perched high up on valley walls, well out of the path of any river or stream. Many, at that time, thought that these boulders had been carried to their strange locations by icebergs during the biblical flood. Agassiz examined the evidence he saw all around him, and thought the answer clear—at one time, glaciers must have covered most of Europe.
Illustration 32 Jean Louis Agassiz, 1807-1873.
He first publicly voiced his opinions in 1837 and later, in 1840, published his theory in a book, Étude sur les glaciers. It was not well-received by the scientists of the day. One reason for this chilly reception was that geologists and other scientists were still highly influenced by the history of Earth as told in the Bible. Not only did the Bible fail to mention a time when Earth was covered in ice, the time frames needed for Agassiz's glacial theory to work were far longer than what was theologically acceptable.
Add to this theological opposition, the bruised egos and trampled reputations of the geological establishment, and the new theory of an age of ice was roundly met with ridicule. The debate over Agassiz's theory raged for decades. As is often the case in science, great advances can require a fresh point of view, the point of view of an outsider.146 Agassiz, a physician who had made his reputation as a zoologist, was certainly that.
He was also a scientific genius and in the end, his ideas won out. But, it is not surprising that the geological community regarded the ideas of this outsider with hostility, or that it took decades to displace the older, inaccurate, but accepted ideas. As we will see in the later chapter on how science works, this is an all too frequent story in the history of science.
Agassiz came to the United States and, in 1848, he accepted a professorship at Harvard. He went on to help found the Museum of Comparative Zoology, the National Academy of Sciences and served as a regent of the Smithsonian Institution.147 Today, he is rightly regarded as a great scientist and visionary.
Agassiz's original concept of an ice age was a period in the distant past when major parts of the northern hemisphere were covered by thick glacial ice. Today, we know that there have been many such periods in the past, and that these periods are not single episodes of glacial conditions. Ice ages consist of many alternating periods of glacial advance and retreat, called glacials and interglacials. When modern scientists refer to an ice age, they mean the entire climatic episode, including both the warm and the cold periods.
Thanks to Louis Agassiz, and those who followed him, we now know that there have been many dramatic temperature swings in our planet's past. Geologic evidence suggests that Earth has experienced several intervals of intense, global glaciation during Precambrian times148 and many times since.
Ancient Ice Ages
The true history of ice ages goes back much further than the stone age. In fact, scientists now think that there have been ice ages dating back all the way to the middle of the Archean Eon, around 2.8 billion years ago. We have evidence of this from layers of sediment found in rock formations known to belong to that period.
Glaciers are massive, weighing billions of tons, but they are not static. They move, flowing over the underlying rock of the lands they rest on. The constant grinding of ice against rock, under tremendous pressure, creates a type of sediment, called glacial drift. This finely ground material is carried away in melt water, often causing the water in glacial streams and rivers to be milky white.
Geologists have been able to find evidence of glacial drift lying on top of the grooved bedrock of ancient periods. Drift (also called till), turned to rock is called tillite and is recognized by geologists as proof of glacial activity. As early as 1891, glacial sediments dating to the Precambrian were found in Scotland and Norway.
Over time, many more sediment layers have been found and identified; the Gowganda tillite in North America and the Bigganjarga tillite in Finland are among the oldest. Presently, there is evidence for an ice age in the late Archean, and five or six in the Proterozoic. Within the Phanerozoic there is evidence for glaciations in the Cambrian, the Ordovician/Silurian, the P
ermian/Carboniferous, the Devonian, and the late Cenozoic.149
Snowball Earth
Eight hundred million years ago, during the Neoproterozoic Era, Earth underwent a monstrous ice age. There is evidence of glacial ice in tropical latitudes, only 15° to 30° north of the equator. In our world, this would mean glaciers as far south as Miami, Florida. Earth would have looked like a different planet, with almost no open ocean and few areas of exposed rock. Only ice and snow, a world of almost pure white.
At that time, most of the land belonged to the super-continent of Rodinia, which formed around 1,100 mya. Rodinia, contained the land that makes up the modern continents today, but not in a configuration we would recognize. North America was in the middle. South America, Australia and Antarctica were packed around North America. Rodinia straddled the tropics, leaving a single vast ocean sweeping across the other side of the globe. There was no land at either pole.
In 1992, Joseph L. Kirschvink, of the California Institute of Technology in Pasadena, put forward a theory that our planet had almost completely frozen from pole to pole, with the only open ocean choked with pack ice. He named this condition “Snowball Earth.” Other researchers have calculated that some of the glacial periods during this time had lasted as long as 10 million years. During these periods, the ocean may have frozen over completely, blocking all sunlight and killing most ocean life.150
When Rodinia began to break up, 760 mya, it created a new ocean near the equator that was not covered in ice. This allowed a cyanobacterial bloom which depleted atmospheric CO2. This, in turn, reduced the greenhouse effect, a change that, when combined with the Sun's lower output at the time, created conditions for a permanent deep freeze.
Scientists think that over time volcanoes replenished the atmosphere's CO2. In fact, they think that the CO2 levels in the atmosphere rose to 350 times current levels, leading to intense greenhouse warming. The ice sheets rapidly melted, and another giant algae bloom occurred, with similar results to the previous time. CO2 dropped and the planet reentered its frozen state. This cycle repeated at least four times.151 Even with the occasional thaw, this time was so pervasively cold, it was named the Cryogenian Period.
Eventually, the growing strength of the Sun, perhaps with a bit of help from emerging forms of life, managed to break the manic cycle of freezing and thawing. When the ice retreated for the last time, life exploded across the planet and the Cambrian Era began.
Researchers now believe that there have been ice ages dating back as far as 2.8 billion years ago. On occasion, these episodes lasted several hundred million years, and may have rivaled the ice age during the Neoproterozoic in intensity. There may have been several Snowball Earth periods in our planet's past.152
It is thought that a period of global glaciation ending 2.4 billion years ago may have been the cause of the Oxygen Crisis. Melting of the oceanic ice could have induced a cyanobacterial bloom, leading to an oxygen spike and radically changing Earth's atmosphere. It seems that ice ages, and the development of life on Earth, have been tightly linked from the earliest times.
Phanerozoic Ice Ages
Since the Precambrian, ~550 mya, ice ages have occurred at widely spaced intervals of geologic time, approximately every 200 million years. Varying in intensity and lasting for millions, or even tens of millions of years, these ice ages have had significant impact on the development of life on Earth.
Evidence has been found in South America, South Africa and Morocco for an ice age spanning the late-Ordovician / early-Silurian.153 ,154 This ice age occurred from 460 to 430 mya and has been linked to the Ordovician Extinction that occurred 450 to 440 mya.155 This extinction was the second most devastating extinction for marine life in Earth's history. At this time, much of the world's dry land was concentrated in the super continent Gondwana. One possible cause for this ice age was Gondwana's passing over the North Pole, which resulted in global climatic cooling and widespread glaciation.
A major ice age, from the end of the Carboniferous well into the Permian (350 to 260 mya), is known from South African strata. There is strong evidence for extensive polar ice caps during this period, which is known as the Karoo Ice Age.156 The name derives from the Karoo region of South Africa where the first clear evidence of this ice age was uncovered. From the sediments found, there appears to have been major ice advance–retreat cycles lasting between 9 and 11 million years. Also found were shorter term ice fluctuations that can be linked to Earth's orbital variations.
There was a dip in temperature towards the end of the Triassic Period. Evidence has been found indicating there was limited glaciation on some continents at the time,157 but temperatures did not sink to the level of an ice age.
During the late-Jurassic / early-Cretaceous, there is evidence of a rise in global sea level and a climatic shift from warm humid conditions to arid cold climates in the higher latitudes. Studies imply the existence of stable climatic belts during this time with significant temperature differences between the tropics and temperate zones.158 While not a full ice age, the global climate experienced moderate “ice house” conditions.159 Finally, an ice age started in the middle of the Cenozoic Era and has continued right up to the present day.
Our Ice Age, the Pleistocene
Technically, we are still in the midst of the most recent ice age, the Pleistocene. During the peak of the Pleistocene Ice Age, vast glaciers, thousands of feet thick, covered most of the northern hemisphere. At their greatest extent the ice sheets covered a third of all land on Earth. Though the ice has retreated significantly during the current warm period, the glaciers on Antarctica and Greenland still cover a tenth of Earth's surface.160
As ice ages go, the Pleistocene has not been the longest or coldest Earth has ever seen. Still, a return to glacial conditions would be very disruptive for human civilization. Many entire countries would disappear under the advancing ice. Twenty thousand years ago, the regions now occupied by Chicago, St. Paul, Toronto, London, Copenhagen, and Moscow were all covered by sheets of ice more than a half mile (~1 kilometer) thick.
Illustration 33: South East Asia as it looked in the Pleistocene. After Fairbanks.
During an ice age, some areas lose land and others gain. Sea levels drop significantly changing the familiar shapes of coastlines. An example of this can be seen in the shallow seas of Southeast Asia, where there are prominent submerged river valleys, much like the Hudson Canyon off New York.161 ,162 As shown in Illustration 33, Borneo, Java, and Sumatra were once joined with the Asian continent, forming a great peninsula. In places where there are shallow seas today, there used to be broad, fertile plains crossed by mighty rivers.
During the last glacial period, shifting coastlines and merging landmasses caused some plant and animal species to go extinct due to loss of habitat. Others found new areas open to them as land bridges between islands and continents appeared. Make no mistake, the impact of a renewed ice age would be much more dramatic and disruptive than that predicted for global warming.
Will the ice sheets come again? That depends on a number of factors that help drive Earth's climate. As with the causes of global warming, there are many competing theories that attempt to explain why our planet should suddenly ice over. We will cover the major ones in the next section.
But before we talk about why ice ages occur, we will examine what the northern hemisphere looked like at the peak of the last glacial period. Illustration 34 shows the view looking down on the North Pole during the last glacial peak, 18,000 years ago, and how it looks today.
Quoting Doug Macdougall, Emeritus Professor of Earth Sciences at the Scripps Institution of Oceanography:
“It is worth reiterating ... the Earth is still in an ice age. We are in a warm period, one of the many interglacial intervals that have occurred throughout the Pleistocene Ice Age, but even so, there are significant amounts of permanent ice in the polar regions. It is easy to forget that this may be just a short respite before another glacial interval begins.”163
Illustration 34 Earth's North Polar region at the peak of the last glacial period and today. If the glaciers return Earth may look like this again. Source NOAA.
As strange as it seems, the globe in the future could again look like it did 18,000 years ago.
The Causes of Ice Ages
There are a number of factors that could cause Earth to slip into an ice age. Among these are changes in the energy output of the Sun, variations in Earth's orbit, depletion of greenhouse gases in the atmosphere, rising particulates in the atmosphere, and the influence of cosmic rays on low level cloud formation. Some factors are tied to natural cycles; changes in Earth's orbit, the sunspot cycle, the solar system's journey around the galaxy. Others factors are more random in their timing.
Even more interesting might be what caused the various ice ages to come to an end. Covering large portions of Earth's surface with ice and snow would make it reflect more sunlight. Scientists would say that glaciers raise Earth's albedo, the measure of how reflective the atmosphere and surface are. A small change in albedo can have a big impact on Earth's climate. A drop of as little as 1% in Earth’s albedo would have a major warming influence on climate—roughly equal to the effect of doubling the amount of carbon dioxide in the atmosphere.164
Today, Earth's average albedo is approximately 30% meaning it reflects one third of the sunlight hitting it. If Earth was covered in ice, like a giant snowball, its albedo would be about 84%, causing it to reflect most of the Sun's rays. Without some external influence, a frozen Earth would tend to remain frozen.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 9
