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After The Ice Age

The Return of Life to Glaciated North America

The University of Chicago Press

The Physical Setting

Every story has a context. The context of the story in this book is the land, the fresh water, the surrounding sea, the ice, and the climate of northern North America during the past 20,000 years. This is the period since the ice sheets of the most recent ice age were at their greatest extent. It is the period during which the ice in North America has almost completely melted. Almost, but not quite. Some still remains, in the form of glaciers and ice fields in the western mountains and some ice caps in the Arctic islands, as shown in fig. 1.1.

So much for the immediate context, but the context itself has a context. Twenty thousand years is only a short period in geological terms; it is worth considering how climatic change during the period covered in this book fits into the pattern of change over a sizable fraction of the lifetime of the earth, specifically over the past one billion (1,000,000,000) years, which is probably rather more than one-fifth of the earth's lifetime to date.

Research into the climates of the past shows that climatic change is, not surprisingly, never ending. It happens on several scales: there are large variations, taking many millions of years; lesser variations within the large ones, taking tens or hundreds of thousands of years; and so on, with a succession of variations of decreasing magnitude and duration forming a nested series. In this section we consider only the two largest scales.

In the past billion years there have been a number of cold periods, popularly called ice ages, but the term ice age is an imprecise, general term. By itself it can mean a long cool period, of ten million years' duration, or an extra cold period, of a mere 100,000 years or so, within one of the long cool periods. To keep the distinction clear, it helps to use separate names for ice ages of different rank. Those of highest rank are called glacial ages, whereas those of second rank are called glaciations. As an introduction to all that follows, here is a very-brief outline of what is now known or surmised about the timing, and the causes, of glacial ages and glaciations.

To make the relative lengths of enormous stretches of time easy to visualize, let us use as a model one decade to represent the past billion years. During this time there have been two complete glacial ages and the beginnings of a third. On the scale of the model, a glacial age lasts a month or two; one occurred eight or nine years ago; a second, between two and three years ago; the third (which we live in) began only six or seven days ago; we are still near the beginning of it. Still on the scale of the model, the climate has been comparatively warm for nine years (at least) when there was not a glacial age in progress.

A glacial age is not continuously cold. This is known with certainty for the current glacial age and is probably true of the earlier ones. During the six or seven days (on the scale of the model) of the current glacial age that have elapsed so far, there have been about nineteen or twenty intensely cold periods (glaciations), each of about six hours' duration, separated by short, comparatively mild respites (interglacials) of about one and a half hours duration. We are at this moment living in an interglacial and are within minutes of the end of it.

Now for causes. Many theories purporting to account for climatic change have been proposed since the scientific study of earth history first began; by far the most persuasive of the modern theories is that glacial ages come and go because of the constantly shifting pattern of the earth's tectonic plates. The argument is as follows. For most of the earth's history the continents and oceans have been so arranged that warm ocean currents from the tropics were able to flow easily into the north and south polar regions; there were no barriers to the flow of currents from low to high latitudes. That is not how matters stand now. At present, a continent, Antarctica, covers a large area centered on the South Pole; the Arctic Ocean, centered on the North Pole, is almost cut off from surrounding oceans because of the way the northern continents are arrayed around it. This is why we are now experiencing a glacial age; warm ocean currents are prevented, by land barriers, from circulating the sun's heat away from tropical and temperate latitudes into the polar regions. As a result, summers are cool at high latitudes; they are not warm enough to melt all of the ice and snow that forms each winter. Therefore, the ice and snow accumulate, year after year, and we have a glacial age.

Presumably the same mechanism accounts for earlier glacial ages. The last complete glacial age before our present one (which is still in progress) happened about 250 million years ago (2.5 years on the model scale in which one billion years is represented by ten years). It does not follow that the pattern of continents and oceans was the same then as now, only that the continents were so placed as to block the free exchange of tropical and polar ocean waters.

Because a far greater proportion of the earth's surface is covered by sea than by land, patterns that produce a glacial age are comparatively uncommon, and the earth has experienced the conditions of a glacial age for only a small proportion of its total history. The duration of a glacial age, when one happens, is governed by the speed at which the tectonic plates drift; probably each one lasts for ten or more million years. We are only about two million years into the current one.

Now we come to glaciations. Why should the climate vary within a glacial age, so that for parts of the time enormous ice sheets cover lands in temperate latitudes (that is, there are "ice ages"), whereas at other times, during the interglacials, the big ice sheets shrink markedly? They do not necessarily disappear entirely; the ice sheets of Antarctica and Greenland have survived thus far in our present interglacial (which is probably reaching its end) and will no doubt persist into the next glaciation.

It is believed that glaciations and interglacials alternate regularly. The climate has a 100,000-year cycle, in which glaciations lasting 60,000 to 90,000 years alternate with interglacials lasting between 40,000 and 10,000 years, respectively. (These values are rounded for convenience; now that we are considering the geologically recent past, durations and dates are given in "real" time; the model time scale used previously will be dropped.)

What must be explained is the reason for the 100,000-year climatic cycle. In light of recent research, it seems almost certain that the climatic cycle is forced by a 100,000-year cycle in the behavior of the earth in its annual orbit around the sun. This astronomical cycle is known as the Milankovitch cycle after its discoverer. It is, itself, the resultant of three other cycles: a 105,000-year cycle in the shape of the earth's orbit, from a more elongated to a less elongated ellipse, and back; a 41,000-year cycle in the tilt of the earth's axis relative to an axis perpendicular to the orbital plane (the tilt is known as the obliquity of the ecliptic); and a 21,000-year cycle, called the precession of the equinoxes, during which the moment in the year at which the earth is closest to the sun, as it traverses its elliptical orbit, shifts forward from January through February, then March, and so on, around to January again.

The climatic effect of these cycles is a variation in the degree of contrast between summer and winter temperatures. There is no change in the total amount of solar radiation falling in one year on the whole earth; what is affected is the partitioning of this radiation between the northern and southern hemispheres and between high latitudes and low. At one extreme of a Milankovitch cycle there is a combination of comparatively warm summers and cold winters at high northern latitudes. At the other extreme there is a combination of cool summers and mild winters. The same sequence of events takes place at high southern latitudes as well. It is not yet known whether, and by how much, the glaciations in the two hemispheres are out of step with each other; this depends on the different climatic effects of the three cycles that together make up the Milankovitch cycle. In all that follows we concentrate on the climate of our own hemisphere, the northern.

The degree of contrast between the seasons is the climatic factor that, more than any other, accounts for the formation and disappearance of ice sheets over the land during a glacial age. This contrast depends, at all latitudes, on the eccentricity of the earth's orbit (which varies in the 105,000-year cycle) and on the season when the earth is closest to the sun (which varies in the 21,000-year precession cycle). The 41,000-year cycle in the obliquity (or tilt) of the earth's axis has most effect at high latitudes; the contrast between the seasons at high latitudes is much greater when the tilt is large (24.4 degrees) than when it is small (21.8 degrees), but the angle of tilt makes little difference to seasonal contrasts at low latitudes.

Throughout most of a glacial age (that is, during the glaciations), when the contrast between seasons is comparatively slight, summer temperatures are not high enough for the previous winter's snow and ice to melt. They accumulate year after year, inexorably building up huge continental ice sheets in temperate latitudes, even though the winters are relatively mild. But during one short segment of each Milankovitch cycle, the contrast between the seasons is great enough, that is to say the summers are hot enough, for each winter's snow and ice to melt before the onset of the succeeding winter (except in polar land masses like Antarctica and Greenland). It is the high summer temperatures that count; the winters are extra cold as well during this phase of the cycle, but that does not affect the completeness of the thaw each summer. The result is an interglacial such as the one now coming to an end.

The change from a glaciation to an interglacial is not abrupt, of course. As summers become imperceptibly warmer year after year, near one extreme of a Milankovitch cycle, the ice begins to melt. The current Milankovitch cycle reached this stage about 18,000 years ago and the results, for North America, have been dramatic. The series of maps in figures 1.2 through 1.5 and finally fig. 1.1 show how the two huge ice sheets of the most recent glaciation, the Laurentide and the Cordilleran, melted away.

As the ice sheets vanished, they left a growing expanse of bare ground where, at first, nothing lived. Over the millennia, the area has been colonized by an enormous number of plants and animals whose permanent home is in the low-latitude regions that are ice free during the glaciations. The invaders managed to occupy the newly exposed land and water as it became available, and now they form the biosphere of most of northern North America.

Where they came from and how they got there are the topics of this book.

The Changing Climate of the Last 20,000 Years

To recapitulate: we are now living in an unusual interval in an unusual age. The age is unusual in being a glacial age; glacial ages have probably occupied only a small fraction of the past history of the earth since the oceans and continents first formed. The interval is unusual in being an interglacial; interglacials are short gaps between the glaciations that make up most of a glacial age.

Our interglacial is now more than half over. It peaked about 10,000 years ago. The way in which the quantity of solar radiation received at 65 degrees north latitude has varied over the past 20,000 years because of the Milankovitch cycle is shown by the dashed curve in fig. 1.6. Presumably the climate of high northern latitudes would have varied to match if the Milankovitch cycle were the only factor controlling the climate. It is almost certainly the most important factor, and when it caused a warming of northern summers sufficient to melt the previous winters' snows, the last glaciation quickly gave way to the current interglacial. But as the solid line in the figure shows, there were other, lesser climatic variations superimposed on the main one, and they caused the alternate warm and cool periods shown in the graph. In particular, there appears to have been a cyclical variation with a period of about 2,500 years throughout the last 10,000 years; it may have been caused by cyclical variations in the sun's output. The figure does not attempt to show finer detail than this, although there are a series of lesser climatic variations believed to be controlled by a series of cycles of progressively shorter periods. For example, a 200-year cycle may have resulted from smaller-scale solar variations. In addition, there is the eleven year sunspot cycle, which amounts to a solar cycle with period an order of magnitude shorter. Noncycling factors may also exist; a steady increase in atmospheric carbon dioxide has been reported in the 7,000-year interval from about 17,000 to 10,000 years ago; it may have been accompanied by a warming caused by the greenhouse effect.

No attempt has been made to show the minor, short-term ups and downs of climate on the graph in fig. 1.6, as they are unlikely to have significantly affected the ecological developments that are the subject of this book. Many organisms, especially plants, lag in their response to climatic change. It obviously takes considerable time for mature vegetation to become established on newly exposed ice-scoured rocks or glacial till (the unsorted material, ranging from clay to pebbles to boulders, deposited by an ice sheet). It also takes considerable time for whole ecosystems to change, with their numerous interdependent plant species, the habitats these create, and the animals that live in the habitats. Therefore, climatically caused fluctuations in ecological communities are a damped, smoothed-out version of the climatic fluctuations that cause them.

Several of the climatic intervals shown in fig. 1.6 have traditional names, which have been shown in the figure. A digression is desirable here, on the naming of geological time intervals. The largest type of interval that need concern us is known as a period. We live in the Quaternary Period, which is merely a name for the current glacial age; it began between one and a half and two million years ago. Examples of earlier periods are the Tertiary Period, which immediately preceded the Quaternary and began about sixty-five million years ago. Before that was the Cretaceous Period, which began about 140 million years ago, and so on back into the more distant past. Periods are divided into geologic epochs. For example, the Tertiary Period is made up of five epochs (Paleocene, Eocene, Oligocene, Miocene, and Pliocene). The Quaternary Period so far has two, the Pleistocene and the Holocene. It would take us too far from our topic to delve into the reasons for these divisions.

The past 20,000 years, the time interval we shall be considering, brackets the end of the Pleistocene Epoch and the beginning of the Holocene Epoch, the epoch in which we live. In the light of modern knowledge, the division of the Quarternary into two epochs now seems unjustified. The transition between them, the Pleistocene/Holocene boundary, marks the end of the last glaciation, known as the Wisconsin glaciation (sometimes called Wisconsinan), and the beginning of the present interglacial. There seems no reason to doubt, however, that a new glaciation will soon begin, in which case there will be no reason in the future to pick out our present interglacial from the twenty-odd others that have interrupted the Quarternary Period (our glacial age) and treat it as in any way noteworthy. The only thing that distinguishes the Holocene Epoch from earlier interglacials (which are not treated as separate epochs) is that it includes our lifetimes, a thoroughly anthropocentric reason. Some geologists consider that the Holocene should not be recognized; they prefer to consider the Pleistocene as still going on. Among those who do favor the recognition of both Pleistocene and Holocene (the majority), there is a certain amount of disagreement over where the boundary between them should be put, but the "official" date is 10,000 years ago.

As figure 1.6 shows, comparatively warm intervals alternated with comparatively cool ones throughout the last 20,000 years. A list of names exists for those in the first 10,000 years (the final years of the Pleistocene), but they are not in common use among paleoecologists and so are not given here. However, the intervals into which the second 10,000 years (the Holocene so far) have been divided have commonly used names that are shown in the figure. Precise boundary marks have been deliberately avoided for two reasons: first, beginnings and endings would be arbitrary, since climatic change was gradual; second, as we shall see in later chapters, these intervals were not simultaneous from one region to another. The correspondence between climatic changes in northern North America and factors affecting the whole earth need not have been close. Global temperatures would naturally have influenced the ice sheets, but their responses would have been delayed, since huge volumes of ice take a very long time to melt. Local climates would have been strongly affected by the extent of the ice sheets, which would have governed the temperatures of overlying air masses and consequently wind strengths and directions.

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Excerpted from After The Ice Age by E. C. Pielou. Copyright © 1991 The University of Chicago. Excerpted by permission of The University of Chicago Press.
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