Climate Through Human History

The Pleistocene - The Age of Ice

Throughout much of Earth's history, long before humanity came onto the scene, the global climate was much warmer than now, with the global mean temperature perhaps between 8°C and 15°C warmer than it is today. During most of this time, the polar regions were free of ice.

These comparative warm conditions, however, were interrupted by several periods of glaciation. Geologic evidence suggests that one glacial period occurred about 700 million years ago (or million years before present, MyBP) and another 300 MyBP.

The most recent one — the Pleistocene epoch or, simply the Ice Age — began about 2 MyBP.

Figure 1. Ice-core oxygen isotopic measurements from Greenland (right hand side) and from Antarctica (left hand side). The isotope measurements can be interpreted to yield the global sea surface temperatures to ~160,000 years ago (colder temperatures to the left). The two traces are consistent with each other and depict the most recent glacial period, ending ~15,000 years ago.

The last geologic era, the Cenozoic, consists of two periods. The first, the Tertiary, lasted 63 million years. The second, the Quaternary, includes the last 1.8 million years of Earth history. The Quaternary Period contains two Epoch, the Pleistocene and the Holocene. During this time of rapid and extreme environmental change, the human species evolved into a position of dominance in the biological world.

About 65 MyBP, the earth was warmer than it is now; polar ice caps did not exist. Beginning about 55 MyBP, the earth entered a long cooling trend. After millions of years, polar ice appeared. As average temperatures continued to lower, the ice grew thicker, and by about 10 MyBP a deep blanket of ice covered the Antarctic. Meanwhile, snow and ice began to accumulate in high mountain valleys of the Northern Hemisphere, and alpine, or valley, glaciers soon appeared.

About 2 MyBP, continental glaciers appeared in the Northern Hemisphere, marking the beginning of the Pleistocene epoch. The Pleistocene, however, was not a period of continuous glaciation but a time when glaciers alternately advanced and retreated (melted back) over large portions of North America and Europe.

The Pleistocene experienced the most important single environmental event since the human species has been on Earth: The oscillation between glaciation and interglacial during the Pleistocene Epoch. This period is remarkable not just for the alternation of warm and cold phases but for the scale and rapidity of the changes. There were at least 20 glacial/interglacial cycles during the Pleistocene.

Figure 2. Alaska and Siberia and the Bering strait

Coastal environments during the Pleistocene were controlled in large part by the fluctuating level of the sea. As a result of the many glaciations on land and the subsequent release of melt-water during interglacial times, sea level has fluctuated almost continuously between interglacial levels, like those of today, and levels during times of maximum glaciation, such as 18,000 years ago when sea level was more than 100 meters lower.

At that time all the continental land areas were larger. The Bering Shelf was exposed at this time and Siberia was connected to Alaska by a land bridge, thus allowing intercontinental migration of animals, including early humans.

More water locked into ice -> sea-level fell by over 100 meters -> a dry plain formed between Asia and North America (Bering Sea) -> this allowed the ancestors of American Indians to walk across the Bering Strait linking Alaska and Siberia.

In areas not covered by ice -> climate differences during the ice ages:

• inland areas had much wetter climate
• evidence of large inland lakes in present basin areas - Great Basin, Africa
• shifts in rainfall belts; reduced evaporation and increased cloudiness

The last Pleistocene ice age reached a maximum 22,000-14,000 yBP. During this period two large ice sheets have been identified in the Northern Hemisphere: the Laurentide Ice Sheet covering parts of Eastern North America, and the Scandinavian Ice Sheet covering parts of northern Europe.

During this last ice age, the maximum area of the Northern Hemisphere ice sheets was equal to approximately 90% of the maximum achieved during the last million years of the Pleistocene. At the glacial maximum the sea level dropped by approximately 85 meters and sea-surface temperature fell by as much as 10°C in mid-latitudes of the North Atlantic and 3°C in the Caribbean.

This image compares modern day glacier and ice coverage of the Northern Hemisphere with the same region 18,000 years before present. Note that the modern sea ice coverage is during the summer months; winter coverage is more extensive.