The Earth has had at least five major ice ages, with the most recent one ongoing. Ice ages are caused by various factors and are cyclical, with glacial ice expanding and retreating rhythmically. During an ice age, positive feedback effects kick in, causing further temperature drops and increased glaciation. Most of the time, the Earth is not in an ice age, and the average global temperature is around 22°C. The Sturtian/Marinoic glaciation was severe, with evidence of continental glaciers found around the equator. The most recent ice age covered continents in glaciers as far away as Chicago and Paris, making it difficult for humans to colonize certain areas.
The Earth has experienced at least five major ice ages in its 4.57 billion year history: the Huron glaciation (2.4 to 2.1 billion years ago), the Sturtian/Marinoan glaciation (710 to 640 million years ago), the Andean-Saharan glaciation (460 to 430 million years ago). mya), the Karoo Ice Age (350 to 260 mya), and the most recent, currently ongoing Ice Age (40 to 0 mya). The definition of an ice age is a long-term decline in global temperatures from the historical norm accompanied by an extension of continental ice sheets. Each ice age is cyclical, usually on time scales of 44,000 and 110,000 years, during which glacial ice expands and retreats rhythmically.
The precise causes of historical ice ages are unknown, but likely arose due to a variety of factors, including: location of continents, atmospheric composition (greenhouse gases), volcanic activity, Earth albedo (reflectivity), variations in Earth’s distance from the Sun (Milankovitch cycles), variations in solar output and asteroid impacts. When the right variables are in place, an ice age sets in, and once it does, positive feedback effects kick in. The strongest is simply that ice is more reflective than land or forest, so large areas covered in ice sheets reflect the sun’s rays, causing further temperature drops and increased glaciation.
Most of the time, the Earth is not in an ice age and the average global temperature is around 22°C (71°F). Ice caps are almost completely absent, present only at higher altitudes (alpine glaciers). The poles are cold, but not covered in ice, and forests stretch from pole to pole. Dinosaur fossils have been found less than 10° latitude from the ancient South Pole. Only during about 15% of the Earth’s history was there an ice age.
The two most famous ice ages are probably the Sturtian/Marinoic glaciation and the more recent ice age. The Sturtian/Marinoic glaciation was so severe that evidence of continental glaciers has been found around the equator from this period. The average global temperature may have fallen below -30°C (-22°F), colder than present-day Antarctica. Some scientists even believe that the oceans froze from top to bottom during this period, resulting in a “Snowball Earth” scenario. Life would have survived in shelters such as deep-sea hydrothermal vents.
The most recent ice age is well known because we humans have had all of our history in it. We think the ice sheets covering Greenland and Antarctica are typical, even if they’re not. More than 10,000 years ago, there was a severe glacial period that covered the continents in glaciers as far away as Chicago and Paris. During this time period, humans mostly had to avoid colonizing Europe or northern Asia, as these areas were frozen. For this reason, human fossils from before the last glacial period are found only in Africa, the Middle East, China, Southeast Asia, Australia and only in small parts of Europe such as Spain and southern France.
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