Ice Ages - Natural History Notebooks
Over the Earth's long history, there have been a number of times when much of the northern hemisphere was covered by vast sheets of ice and snow. Such periods are known as ice ages. During ice ages, huge masses of slowly moving glacial ice—up to two kilometres (one mile) thick—scoured the land like cosmic bulldozers. At the peak of the last glaciation, about 20 000 years ago, approximately 97% of Canada was covered by ice.
It may seem hard to believe, but an ice age can occur if the average daily temperature drops by only a few degrees Celsius for an extensive period. Ice ages include colder and warmer fluctuations. During colder intervals, called glacial periods, glaciers and ice sheets grow and advance. (As the snow gets deeper and deeper, the lower portion turns to ice and its incredible weight makes the ice sheet flow across the land). In warmer intervals, known as interglacial periods, glaciers and ice sheets shrink and retreat.
The Earth is in an ice age now. It started about 2 million years ago and is known as the Quaternary Period. Despite the many warm periods since then, we identify the entire time as one ice age because of the continuous existence of at least one large ice sheet—the one over Antarctica. (The glaciers and the Greenland ice sheet are also of long standing, but they are more recent). We are currently enjoying a warm interval: our climate represents an interglacial period that began about 10 000 years ago. The preceding glacial period lasted about 80 000 years.
At least seven ice ages have been recognized. At least four of them are considered significant because of the extent of their glaciation or because they lasted for an extremely long time:
- about 2 million years ago to the present—the Quaternary Ice Age
- 350 to 250 million years ago—the Karoo Ice Age
- 800 to 600 million years ago—the Cryogenian (or Sturtian-Varangian) Ice Age
- 2400 to 2100 million years ago—the Huronian Ice Age.
Some regions escaped glaciation during the Pleistocene Epoch of the Quaternary Ice Age because they were too dry for enough snow to fall to form glaciers. Other regions were too high in elevation for the ice to cover them, or they were farther south than the glaciers advanced. Glacier-free zones are called refugia, and the plants and animals that survived there repopulated the land once the glaciers melted. During the Pleistocene, the distribution and kinds of plants and animals were greatly affected.
In Canada, the richest stores of Pleistocene bones are in Yukon's Old Crow Basin—part of the Eastern Beringian refugium. There, thousands of fossils have been collected that demonstrate the existence of a remarkable fauna, including woolly mammoths, bison, mastodons, giant beavers, small horses, camels, cow-sized ground sloths, American scimitar cats and lions. These species became extinct toward the close of the Pleistocene—perhaps due to a combination of rapidly changing climate and human hunting. However, wolves, caribou, muskoxen, moose and other animals survived.
[Illustration: Charles Douglas].