What is an ice age?
An ice age is a period of time—usually millions or tens of millions of years—when vast glaciers, called ice sheets, cover much of the Earth’s land surface. Several ice ages have occurred throughout our planet’s history. The latest ice age began about 2.5 million years ago. During this time, giant ice sheets have advanced and retreated many times in North America and Europe.
Recent cycles of advancing and retreating ice sheets have occurred every 100,000 years. Each cycle consists of a long, generally cold period during which the ice sheets slowly reach their maximum extent over tens of thousands of years, and a relatively short warm period during which the ice sheets rapidly retreat.
We are now in a warm period that has lasted more than 10,000 years, which is longer than many of the previous warm intervals. If the pattern of glacial cycles still holds true, we should be about due for the beginning of the next cold phase. Indeed, several hundred years ago, global temperatures began decreasing during a period known as the Little Ice Age.
Over the last century, however, average global temperatures have instead started to rise. Scientists have recently concluded that much of the recent warming is due to the release of greenhouse gases from human activities such as the burning of fossil fuels. A sobering possibility is that continued human-caused global warming could disrupt or override the natural climate cycle of the ice age.
How do we know that ice ages occurred?
The idea that an ice age had occurred in the past was first proposed by Louis Agassiz in the early 1800s. Agassiz noted that the slow action of mountain glaciers in Switzerland produced certain kinds of features in the surrounding landscape. These include glacial till and erratics, rock striations and loess. He also noted that these features sometimes occurred in areas far from the mountains, where no glaciers existed. Based on these observations, Agassiz came up with the idea that extensive glaciers had existed in the past.
Scientific progress since the time of Agassiz has not only confirmed his idea, but has also begun to reveal a much more detailed picture of what conditions were like during the last ice age. To obtain a more detailed picture of the last ice age, scientists study natural recorders of climate change, such as fossil pollen, ancient coral reefs, ocean sediments and ice cores.
What causes an ice age?
This question can actually be broken down into two separate questions:
1) W hat causes the long-term changes leading to an ice age?
Factors that are thought to play important roles in long-term changes in Earth’s climate over millions or tens of millions of years are
Changes in the positions of the continents
Variations in the energy output of the sun
Changes in atmospheric carbon dioxide concentration
Changes in volcanic activity
Influence of the biosphere
2) What causes the glacial cycles within an ice age?
During an ice age, the shorter-term cycles of advancing and retreating glaciers are thought to be driven by regular, predictable variations in Earth’s orbit and orientation relative to the Sun.