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.