There have been five or six major ice ages in Earth's history over the past 3 billion years. The Late Cenozoic Ice Age began 34 million years ago, and its most recent phase, called the Quaternary glaciation, has been ongoing since 2.58 million years ago.
During ice ages, Earth experiences times of colder conditions, called glacial periods, and times of warmer conditions, called interglacial periods. The Earth is now in an interglacial period of the Quaternary glaciation. The last glacial period of this time ended about 11,700 years ago. This current warm period is known as the Holocene epoch. Scientists use climate proxies to study past climate changes caused by ice ages.
Descriptions
The third ice age, which may have been the most extreme, is believed to have happened between 720 and 635 million years ago during the Neoproterozoic Era. Scientists think this period may have caused a second "Snowball Earth," a time when Earth was entirely covered in ice. Some researchers suggest that the end of this icy period led to the Cambrian explosion, a time when many new types of complex life forms appeared quickly during the Cambrian Period. This idea is still being debated by scientists, though more evidence supports it.
A smaller series of ice ages occurred between 460 and 430 million years ago. Larger ice ages happened between 350 and 289 million years ago.
The Late Cenozoic Ice Age included large ice sheets in Antarctica for the past 34 million years. In the last 3 million years, ice sheets also formed in the northern hemisphere. This phase, called the Quaternary glaciation, involved repeated cycles of widespread ice coverage. These cycles first repeated every 41,000 years, but later changed to longer cycles averaging 100,000 years after a major shift known as the Mid-Pleistocene Transition.
Nomenclature of Quaternary glacial cycles
During the first 30 million years of the Late Cenozoic Ice Age, Antarctica was mainly affected by ice. However, during the Quaternary period, large ice sheets spread over parts of Europe and North America that are now inhabited and easy to reach. Early geologists gave names to repeated cycles of cold and warm periods in the Quaternary Ice Age based on unique geological features, and these names differed by region. Evidence from ocean sediments shows all past ice ages, while land-based evidence is less complete because later ice ages can erase signs of earlier ones. Ice cores from large ice sheets also provide full records, but they do not cover as much time as ocean data. Pollen found in lakes, bogs, and layers of wind-blown dust (loess) helped scientists match land-based events with ocean records. Today, most scientists no longer use these traditional names and instead refer to periods by specific numbers from marine isotopic stages. For example, ocean sediments show five cycles of ice and warmth during the last 500,000 years, but only three major ice periods (Mindel, Riss, and Würm) were originally identified on land during that time.
Land-based evidence is useful for studying events as far back as Marine Isotope Stage 6, but it has been hard to match earlier stages using only land data. This means the traditional naming system is incomplete, and earlier ice ages identified on land are not fully certain. However, land-based data remains helpful for understanding landforms and matching them with marine isotopic stages.
It has been challenging to match traditional regional names for ice ages with global ocean and ice core records. Marine isotopic stages often show multiple ice periods that overlap in time with a single traditional regional name. Some scientists still use traditional names to describe these overlapping events, while others use the word "complex" to describe long periods that include both cold and warm phases. As shown in the table below, traditional names only clearly match marine isotopic stages for the last 200,000 to 300,000 years. There has been much debate about how Marine Isotope Stages 10 and 12 relate to older European ice periods called Elster and Mindel.