The evidence for past Ice Ages lies everywhere in the Northern Hemisphere: in lands scraped and gouged by glaciers, in pothole lakes, in piles of gravel, sand, or loess, in high shorelines surrounding arid basins.  These artifacts can be found far from active glaciers or ice caps; aditional evidence is found in the remaining glaciers themselves, and in the great ice caps of Greenland and Antartica.  The evidence is so widespread and obvious, that it seems strange that it wasn't until 1840 when Agassiz first made public his claims for ancient Ice Ages.  It is stranger yet that his ideas were not generally accepted until 30 or more years after he put them forth.

Amid the present concern about Global Warming, study of the Ice Ages is important because it shows how the climate has gone awry many times in the past.  It gives us good reason for concern that the systems that regulate the climate are much more delicate than has been assumed in the past.

Scientists first became aware in the 19th century that the Earth had undergone long periods of intense cold, cold enough that huge ice caps covered much of the continents.  The widespread glaciation of the past was first pointed out in 1840 by Louis Agassiz.  He noted geological features in Wales and Scotland, far from present day glaciers, similar to those near active glaciers in the Alps.  He also demonstrated that glacial till must be produced by glaciers.  Ancient tillites (cemented till) are found almost everywhere.  The records of ancient glaciations goes back many millions of years, to the Pre-Cambrian era.

Till is a jumble of rock fragments and ground rock powder.  The rock fragments often exhibit abrasion marks which indicate that they were dragged, rather than tumbled, over long distances.  The jagged forms of the rock indicated that the till did not originate in ancient river gravels or rock slides.  After thousands of year under heat and pressure, buried till may eventually turn to rock, but its basic characteristics remain readily identifiable.

Evidence of climate change is especially evident in the landforms of the Earth's surface.  Some of the evidence is very subtle, but once you know what to look for, you will see evidence of Pleistocene glaciation everywhere in the landscape of North America and northern Europe:

• Alterations of rivers and drainage systems, caused by diversion of rivers or the erosion of surface topography by glaciers.
• Channels not occupied by significant streams, left behind after the glaciers diverted the rivers that originally excavated them.
• Evidence of catastrophic floods as huge glacial lakes breached ice dams.
• Broad gouges across the landscape that show evidence of glacial surges, indicating that the glacial ages may have been punctuated by periods of warming and melting; many of these are so large they are only recognizable on aerial photographs and topographic maps.
• Moraines and other deposits of gravels and glacial till transported from distant places; some of these deposits are found at the feet of valleys in which the glaciers have retreated; others are found far from any present day glaciers.
• Striated rocks abraded by glaciers; these are found all over northern Europe and North America
• Glacial eratics: huge boulders left far from their original sources, so large that they couldn't have been moved by water or wind.
• Pothole lakes scoured from the land and left by melting glaciers.
• Loess (fine windblown soil) deposits and sand carried by the winds that blew down from the continental glaciers.
• Widespread deposits of finely ground rock left by glacial melting; these are found in mountain lakes, in the oceans, and in dried up lake beds far from the mountains.

Moreover, glaciers and ice caps themselves give evidence of the climate when the snow was originally laid down.  Cores drilled vertially through ancient ice show bands, rather similar to tree rings, indicating cycles of temperature and moisture for thousands of years.

Because till is found in so many places, and in a wide variety of textures, it is difficult to form any precise conclusions about the nature of the glaciers that deposited the till.  Of course the location of the till and the amount give some clues about the former intensity and extent of glaciation.  Sometimes till is carried by icebergs, which drop it on the ocean floors; this provides some clues as to the events that released the ice bergs.

LOESS DEPOSITS
AND SAND DUNES

The climate was generally dry during the Pleistocene glaciation.  Intense katabatic winds blew downward from northern ice caps; deflected in an anticyclonic (counter-clockwise) direction before becoming part of global circulation.  Those winds left huge deposits of loess (fine wind-blown soil, pronounced "less") and sand in the central plains of North America and Europe: rich loess soils in Iowa; poor sandy soils in western Nebraska.  The first map shows the distribution of sand dunes left by the climatic conditions during the last ice age.

Many sand dunes, particularly at sea coasts and in arid intermountain basins of western North America, are relatively recent.  These have seldom had sufficient time to acquire a thin covering of vegetation.  Some of the best examples of ancient dunes can be found in the Plains states: the Dakotas and Nebraska.  Much of northwestern Nebraska is covered by dunes, whose alignment and regular spacing are evident in photographs taken from space.  The dunes on the Plains are mostly covered with grass and small shrubs, that became established in a relatively wet period thousands of years ago— after the end of the Ice Ages.

The winds during the Ice Ages tended to sort the fine materials, first dropping the sand, and then the finer loess.  There is a range of particle sizes: the the loess of central asia tends to have a texture like extremely fine sand, while the loess further east tends to decompose and form rich soils.  The following map shows the distribution of loess.  Comparison of these two maps demonstrates that the finest loess soils were indeed deposited eastward and southward of the sand.  These have provided the basis for rich agricultural production in central North America and in China

Besides evidence of aridity, the deposits of wind-blown material can tell us something about the nature of the atsmospheric circulation during periods of continental glaciation.  The evidence is difficult to interpret, but could provide data to confirm the computer models of the local and global circulation.

THE DISTRIBUTION
OF CONTINENTAL GLACIERS

The following map shows roughly where the major continental glaciers were distributed.  The estimated temperatures and weather regimes are also noted.  Especially significant are some large Northern areas that remained free of glaciers, particularly in Siberia and Alaska.  These were intensely cold and inhospitable, but they provided corridors for man and other animals, by which they were able to migrate from Asia to North America.

The climate during the Ice Ages was extremely variable, so this picture should not be considered a snapshot of the ice distribution at any time, but rather a map indicating where the ice might accumulate to great depths.  During the Ice Ages various kinds of evidence suggest that there may have been periods lasting hundreds of years when the climate became almost as warm as the present.  Such intervals would have caused much melting and erosion around the edges, especially where the ice was not very well compacted.  In North America these brief warming periods may have opened a corridor between the Rocky Mountains and the Laurentian Ice Cap centered over the location of Hudson's Bay.  It has been speculated that such a corridor might have offered a passageway by which animals (including humans) could have reached the North American interior from their origins in Siberia.

The Earth has spent most of the past million years in the grip of intense cold, with great sheets of ice over the northern continents (See a time-line of the history of the Earth's climate. ).  The Pleistocene Ice Ages were first called to the attention of the world by Louis Agassiz, after he had observed the effects of large scale glaciation in Europe.  Though his ideas initially met with disbelief, other scientists established that the world had experienced at least four very cold periods, with glaciers advancing over much of Europe.  In Europe these Ice Ages became known by the names "Günz," "Mindel," "Riss," and "Wurm," after places in Switzerland where their effects were observed.  In North America they became known as the "Kansan," "Nebraskan," "Illinoian," and "Wisconsin."  The effects of the most recent, the "Wisconsin/Wurm," lie all about, and give evidence of monstrous accumulations of ice and thousands of years of intense cold.  Even so, it may not have been the most intense, most extensive, nor most long-lived; that honor may go to the "Nebraskan/Mindel" Ice Age.

We now know more about the timing, duration, and extent of the Ice Ages.  Generally they have lasted longer than the Interglacials, which typical have run for 20,000 to 50,000 years.  Measurements of isotope ratios in cores of ice from Greenland and Antarctica, and isotope ratios of microscopic shells dredged up from the deep oceans reveal that there may actually have been seven or more Ice Ages in the Pleistocene, recurring at intervals of about 100,000 years.  It appears that the Earth began cooling rapidly more than 15 million years ago, and eventually reached a point 1.8 million years ago where repetitive Ice Ages could become established through the Milankovich mechanism.  Each Ice Age began with a gradual cooling over several thousand years; lasted many tens of thousands of years; and then, shortly after reaching its maximum extent, warmed quite rapidly.  The warming often took less than 1000 years to reach atmospheric temperatures comparable to the present.  The tremendous thermal inertia of the ice meant that large remnants remained for several thousand years after the warming had begun.  Likewise, the sea level dropped by hundreds of meters during Ice Ages, and took several thousand years to recover.

Though the evidence of many periods of glacial advance can be found in long-lived ice; many of them were not intense enough to leave evidence on the ground—evidence that could not be erased by successive events.  The 19th century geologists could not construct a complete picture of the many glacial advances because each episode tends to erase the evidence of the previous episode.  A glance at a map of North America indicates why the Kansan, Nebraskan, Illinoian, and Wisconsin Ice Ages were recognized as distinct: each was first revealed by geological evidence that could be dated in each of those states.  The distribution of the evidence could be interpreted as suggesting that each of those episodes was less severe than the one that preceded it.

The nomenclature of Ice Ages is not always clear or useful; and it is not entirely certain whether events labelled "Nebraskan" in North America, "Mindel" in Europe and other names in other places are actually synchronous.  Moreover there are no such names for several of the previous episodes.  So it has become standard practice in recent years to denote the episodes by their Oxygen Isotope Stages (OIS), or by the ages derived from the OIS combined with other data.

Articles in the popular press have occasionally raised alarms that we are due soon for a renewed Ice Age.  They reason that the typical length of a warm Interglacial may be as short as 10,000 years, and it has been about 10,000 years since the last episode of Ice Age temperatures.  Such fears have largely been discredited by the results of climate modelling, which suggest that there are at least several thousand years before the beginning of the next cooling episode.  Moreover, predictions of Global Warming suggest that the Earth could soon warm sufficiently that another Ice Age is unlikely.  So there is some good news associated with Global Warming.

An area where the adverse effects of the Little Ice Age have been well documented was the Arve Valley in the French Alps.  The valley lies just below Mont Blanc (off the map at the bottom), and several huge glaciers descend toward the valley in several locations on the north side of the mountain.  These glaciers threatened the town of Chamonix (just slightly off the map to the lower left), particularly the Mer de Glace Glacier, which advanced to the very edge of the neighboring village of Les Bois.  The map shows a reconstruction of the margins of the glacier,  The earlier outlines were taken from the work of LaDurie, based on narrative accounts and sketches; later outlines are from more recent observations; all superimposed on a recent topographic map.  The glacier has shrunk to a fragment of its former self, and, from observation points erected in the 19th century ("Hotel" near the lower center, site of the photograph above), appears as a narrow stream of rocks and dirty ice at the bottom of its trough.  None of the Mont Blanc glaciers comes anywhere near the floor of valley today.

In an earlier age, prior to the 19th century, when paintings were devoted to religious subjects and human dramas, we look for evidence of climate shifts in contemporary chronicles and stories.  A rich source of information, is the long record of harvests in Europe; the celebrated historian LaDurie was among the first to delve into those chronicles to reconstruct past climates. 

The past thousand years have seen many climate changes.  One thousand years ago the Earth experienced a warm period, when the Vikings established flourishing settlements in Greenland and Iceland.  The name "Greenland" has frequently been interpreted as a real-estate promoter's ploy, but the land truly was green during the brief summers in the 10th and 11th centuries.  The first cooling event we can make out in the historical record occurred in the 12th and 13th centuries, when travel to Greenland became difficult.  There were several warm respites in later years, but the Little Ice Age finally cut off Greenland from contact with Europe, and even made voyages to Iceland difficult.  The Greenland colonies disappeared; Iceland suffered famines and a precipitous decline in population.

It is worthwhile to reflect on the potential consequences of Global Warming, which could result in a temperature rise of several degrees within less than 100 years.  The global temperature during the depths of the Little Ice Age fell by no more than 1 to 2 C; but there was misery and suffering everywhere.  In continental Europe the chronicles are full of famines, and periods when travel became nearly impossible because rivers froze over.  We also find stories that in some places travel by land in the winter was eased where large rivers could be crossed on solid ice.  The years 1816 and 1817 were probably the worst.  To add to the calamity, the volcano Tamobora in southeast Asia exploded, filling the atmosphere with dust which caused the temperatures to drop even further.  1816 has been called "the year without a summer;" crops failed everywhere.  Fortunately Europe had plundered enough wealth and exploited enough tropical colonies that it recovered quickly and came through the 18th and 19th centuries with little difficulty, and fewer wars than had been the custom in the past.

The scarcity of documentary evidence in North America makes the effects of the Little Ice Age somewhat difficult to interepret.  Most of the population was concentrated in the relatively fertile East and near the great rivers of the interior.  The population density and and distribution prior to 1500 is a controversial topic, but there is much evidence of precipitous population declines in years following the first incursions by Europeans.  If the population densities were not high, hard times might habe been partially averted by exporting people to new, uncultivated lands.  But west of the Great Lakes, there must have been considerable environmental stress.  The recorded history of the western prairies and mountains is complicated because of the great displacements of Native Americans.  The Indians were first subjected to European diseases, then to pressure from settlers along the east coast.  The Indians on the Plains propered briefly, after they obtained horses from the Southwest settlements and guns from eastern traders.  All these factors collided on the Great Plains in the 19th century.  At the same time the bison, on which the Indians had come to depend, were under stress from prolonged droughts.  It is probably time for North American history to be reinterpreted, and the Little Ice Age given its due.  What was the role of environmental and climate deterioration in the population crashes of Native Americans?

Because of thermal inertia, many mountain glaciers did not reach their greatest extent until long after the coldest years.  Their melting was also delayed by many years.  The growth of mountain glaciers during the Little Ice Age is well documented for the northern Rocky Mountains of the United States and Canada.  In many places, particularly the National Parks of the Canadian Rockies, the valleys scoured by glaciers remain barren of vegetation, with prominent terminal moraines.  Those glaciers were probably at their peak when first discovered by Europeans; they have since retreated far back into their high mountain sources.

The rapid shrinkage of the glaciers of Glacier Park in the northern Rocky Mountains of Montana has been cited as one of the consequences of global warming.  Examination of moraines, trim lines, and vegetation suggests that those glaciers during the Little Ice age covered a maximum extent of nearly 90 square km (35 square miles).  Their total extent in the early 21st century was about 30 square km (12 square miles).  But those glaciers are at relatively low altitudes, several hundred miles south of the limit of semi-permanent ice sheets.  If the annual temperatures and precipitation had remained at the levels they attained in 1860, the glaciers would have been certain to disappear by the end of the twenty-first century.  The glaciers of Glacier Park owe their existence to the Little Ice Age, and are bound to melt away in the absence of another cooling event.


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