Earth and its ice ages

Earth's climate has fluctuated for as long as the globe and its atmosphere have existed. Many factors contribute to these changes: variations in the Earth’s orbit, but also more local influences, like carbon dioxide levels in the atmosphere and the amount of volcanic eruptions. As a result of climate fluctuations, occasionally the Earth enters into very cold periods. During these times, called ice ages, large sheets of ice form and spread over the Earth and global temperatures drop. Scientists believe that the Earth has experienced at least five major ice ages.

The oldest and longest ice age that scientists know about is the Huronian ice age. It lasted from about 2.4 to 2.1 billion years ago, at a time when there were still only unicellular organisms on Earth. The next ice age was the Cryogenian, which occurred from 850 to 635 million years ago. Scientists have discovered evidence of glaciers near the equator at this time, leading some to believe that during this ice age, the Earth may have been almost entirely covered in ice - resembling a snowball. The Andean-Saharan ice age occurred between 460 to 430 million years ago at a time when most of the life on Earth lived in the ocean.

This period saw the second largest mass extinction of species in Earth’s history — it is believed that approximately 85% of all species died. However, as the ecosystem recovered, it also expanded, leading to further evolution of land plants. As plants spread, they absorbed carbon dioxide from the atmosphere and released oxygen. Carbon dioxide levels fell, triggering another ice age: the late Paleozoic ice age. This ice age lasted around 100 million years, from 360 to 260 million years ago.

The most recent ice age is the Quaternary ice age. This period began approximately 2.58 million years ago when ice sheets expanded over both Greenland and Antarctica. We are actually still in this ice age today. How is that possible? Temperatures on Earth don't stay cold the entire time an ice age is happening.

Instead, the climate goes between what scientists call "glacial periods" and "interglacial periods." During glacial periods, the temperature is low and ice extends far away from the poles. During interglacial periods, the temperature is somewhat warmer and the ice retreats. We are currently in an interglacial period of the Quaternary ice age. During the last glacial period of this ice age, which lasted from about 115,000 to 11,700 years ago, the ice sheets grew over three kilometres thick as they spread across Canada, Scandinavia, Russia and South America. In many areas, sea levels dropped by nearly 125 metres.

Meanwhile, the average global temperature was only around 8 degrees celsius. If ice ages happened so long ago, how can scientists know about them? As masses of ice move and spread, they transport material like rocks and soil far from their original location. Their movement and weight also erodes the land, creating unique landforms. By studying certain rocks and geological features, scientists gain clues as to which regions of the world may have been covered in ice in the past.

Scientists also study ice samples called cores. Every layer of ice in an ice sheet has a unique chemical composition, meaning a large ice core can provide hundreds of thousands of years of climate records. Despite this, there is still so much scientists do not know about ice ages. They continue to study these important periods, both to learn more about Earth’s history and to help predict how the climate may change in the future.