Since Agassiz applied the scientific method to the study of glaciers in 1840, much has been discovered. Although he felt that the earth had been covered once, modern scientific consensus is that much of the earth has been covered by ice many times. By analyzing layers of glacial drift separated by peat and other organic material, geologists have determined that much of the earth has been covered by huge ice sheets between 4 and 10 times in the last million years, the Pleistocene geological epoch,. Each time the expansion and contraction occurred about the same time throughout the world, covering the same routes, and the same area, roughly 17 million square miles. Right now glaciers cover 10% of the EarthÕs land. In an Ice Age glaciers could cover as much as 30% of the EarthÕs land.
Furthermore because ice sucks up the water from the oceans onto the land, the ocean levels fell by about 350 feet. This reduced ocean level allowed many of the early migrations. Both the Bering Straits from Asia to North America, and the English Channel from Europe to the British Isles, were land bridges during the last major Ice Age.
The glaciers did not start at the North Pole and spread south. Instead they started growing locally, moved into the valleys, and then joined forces with other glaciers to cover most of the northern half of the globe with ice - sometimes as much as two miles thick.
There were two ice sheets in North America, east and west; there was a large one covering Scandinavia and Great Britain, a smaller one emanating from the Alps, a Himalayan ice sheet, a Siberian ice sheet, not to mention smaller ice sheets in the Andes, Australia and New Zealand. This is mentioned only to exhibit that the earth was not entirely covered in ice. Furthermore there is evidence of glaciation as far back as 700 million years ago. But while there have been many glaciations in the earthÕs history, we only see the results of glaciation in the Pleistocene.
The last Ice age, called the Wisconsin Stage in the US, began 70,000 years ago and peaked 20,000 years ago. As recently as 8,000 years ago the glaciers had retreated to their present position. It seems like just yesterday.
Glaciologists have determined that there have been three mini-Ice Ages since the last big one ended 8000 years ago. After the Great Thaw, the end of the last Ice Age, the world temperatures were 2 degrees Fahrenheit hotter than they are today - warm enough to melt all the enormous ice sheets which had covered the earth. The world then began cooling again. The first mini-Ice Age peaked 5000 years ago, but the glaciation was only slightly greater than it is today.
The second mini-Ice Age peaked 2500 years ago and was more extreme. It even gave birth to some new glaciers. Many of todayÕs glaciers are remnants of this mini-Ice Age, not the Great Ice Age, which had many of its glaciers melted by the Great Thaw.
The last mini-Ice Age started 600 years ago in the 1400s and continued for 450 years, until about 1850. The European and Scandinavian glaciers grew so much that they covered houses and farms that had been built upon their edges. Then came a global warming for about 100 years, where all the glaciers worldwide began shrinking. Then in about 1940 the thaw ended and the glaciers have been expanding somewhat since that time.
By analyzing the glacial ice sheets, glaciologists have been able to sketch some cycles. It seems that there have been seven Ice Eras since the earth began 4.6 billion years ago. We are in the midst of the last of these Ice Eras. It started 65 million years ago. ÒIn the middle of an Ice Era?Ó you say. ÒHow could it be when we have 100 degree summers?Ó There are glaciers which never thaw still on the Earth, which didnÕt exist 100 million years ago.
Additionally there have been 6 Ice Epochs in this last Ice Era. We are in the midst of the last one, which started 2.4 million years ago. How again, because there are glaciers that didnÕt exist 3 million years ago, which still exist today. In this Ice Epoch we have had no less than 23 major Ice Ages. In the last 125,000-year Ice Age cycle, we are in an interglacial cycle which has lasted 10,000 years. The Ice Age that preceded it was nearly 100,000 years long. The land in the North hasnÕt even thawed yet.
As an indication of our relatively frigid state, the frozen tundra of the North is ground that hasnÕt fully melted yet even though there is no more ice sheet covering the area. The summer heat is not great enough to melt the ground that was frozen over the 100,000 year Ice Epoch. Half of Canada, 85% of Alaska and almost all of Greenland in North America are covered by permafrost. It wasnÕt always that way. It takes thousands of years to freeze the land so deep, which is what the last Ice Age provided. Human settlement is extremely difficult because of the instability of frozen land.
ÒAbout 20 per cent of the worldÕs land area remains permanently frozen—in some cases to depths of almost a mile.Ó (Ice Age p30)
The interglacial warmings in the last million years lasted from 10 to 40 thousand years and were normally followed by Ice Ages, which were longer in duration – from 10 to 200 thousand years. The four previous interglacials lasted from 8 to 12 thousand years. It has been 10 thousand years since the last Ice Age. Doing the math we could have another Ice Age any time now. One glaciologist predicted another Ice Age coming between 3,000 to 7,000 years.
Let us point out that earthquakes, floods, fires, or drought are localized phenomenon, no matter how intense. A hurricane might devastate Florida; an earthquake might level San Francisco; a fire might burn down hundreds of houses in Santa Barbara; a flood might wreak havoc up and down the long Mississippi. But each of these events is localized in its effect. There might be a ripple effect in the surrounding areas affected by immigration or the sharing of precious resources. But the impact is for the most part local. Japan does not care about AmericaÕs disasters except from a compassionate stance. Additionally the abovementioned environmental disasters are also localized temporally. An earthquake and the after shocks last for maybe weeks; a bad fire might take weeks to contain; a flood may take a season to subside; a drought might last for years.
Alternately an Ice Age is global in effect and in time. The whole planet will have to relocate. The environment of Arctic Canada would move down to the northern temperate zones where most of the worldÕs population is located. This is not a temporary scenario either. The duration of Ice Ages is recorded in millennia, not weeks, not years, nor even centuries. The last Ice Age was almost 100,000 years. That is almost as long as modern man has been around, 25 times as long as historical records, 10 times as long as the last few interglacial periods. Even if it was a short Ice Age of 10,000 years, this is 5 times longer than the Common Era and, 100 times longer than a long human life.
We can build walls to contain floods. We can build our houses to withstand earthquakes; we can clear brush around our homes to prevent fire; we can build basements to wait out a hurricane. There is nothing we can do to prevent an Ice Age. And they are inevitable.
Most scientists agree that the general mechanism of the global warming and cooling has to do with the earthÕs ability to absorb solar radiation. Ice reflects most solar radiation into space. Hence the more ice, the less solar radiation absorbed; the more ice, the less solar energy absorbed - a feedback effect. The more ice, the less precipitation because all the water is sucked up into the ice packs. At a critical point there is not enough precipitation to maintain the ice sheets and they begin to shrink. As they shrink the earth begins absorbing more solar radiation, further reducing the ice sheets, allowing the earth to accumulate more heat. Hence the earth is in dynamic cycle of Ice Ages and interglacials.
Although not universally accepted, many scientists believe that the variations in the distance between the Earth and Sun is that little difference that precipitates an Ice Age or an interglacial period. When the Earth is relatively closer we get an interglacial, when it is a little further away we get an Ice Age. There are three cycles that affect the distance between Earth and Sun: 1) the EarthÕs tilt 41,000 years, 2) the EarthÕs wobble 22,000 years, and 3) the EarthÕs orbit 100,000 years.
Because of temperatureÕs feedback effects, i.e. cold accumulating cold and warm accumulating warm, there are a few events that could trigger another Ice Age; 1) a large meteor colliding with the earth raising lots of dust, 2) several years of excessive volcanic activity, or 3) many years of exceptionally cold winters. Further there is some evidence that that an Ice age can occur within 20 years because of the aforementioned feedback effect.
Ironically the global warming caused by the burning of fossil fuels could deflect another Ice Age by artificially elevating the worldÕs temperature and thereby preventing the global freezing that would be so devastating to the human species. Maybe the humans have accidentally done something right. Global warning an antidote for an Ice Age?
It is evident from this discussion that glaciers have regularly covered the Earth throughout her history. This explains why we see so much evidence of glacier action in the Sierras. Before we begin to analyze the glacial landforms that we see in the Sierras let us discover how glaciers are formed and how they move.