Ice Ages and Glaciations

Glaciers are layers of ice that form on Earth’s lithosphere where the temperature is sufficiently low to support year-round ice and snow. Extended periods when temperatures drop sufficiently low to support large-scale increases in glaciation are called glacial epochs or ice ages. Earth is now in an ice age that began 2.4 million years ago and has involved twenty or more fluctuations between glacial and interglacial periods. Estimates indicate that Earth is undergoing cycles of glaciation that occur every eleven thousand years.

Glaciers of Modern Earth

The part of the Earth where the temperature is permanently below the freezing point of water (0 degrees Celsius, or 32 degrees Fahrenheit) is known as the cryosphere. Glaciers are large bodies of ice that form within the cryosphere and are considered permanent by the standard of the human life cycle.

Glaciers are formed primarily from layers of snow that have become compacted by the weight of overlying snow and by the pull of gravity to recrystallize into ice. In the twenty-first century, glaciers exist primarily in the far Northern and Southern Hemispheres past the snow line, the latitudinal mark beyond which the ambient temperature remains below the freezing point of water.

The portion of the Earth covered in permanent ice changes over time according to cycles that affect Earth’s global temperature. Multimillion-year periods of prolonged reduced temperature, during which glaciation spreads, are sometimes called glacial epochs or ice ages.

The presence or absence of glaciers has a domino effect on Earth’s geography and climate. Ocean depth is one factor highly dependent on the amount of water frozen in glacial zones. Estimates show that the world’s glaciers in Antarctica and Greenland hold enough frozen water to raise the Earth’s ocean levels by more than sixty meters (two hundred feet), drastically altering the amount of land available for habitation.

Glaciers can also influence sea levels through their effect on isostasy, the gravitational equilibrium between the Earth’s crust and the mantle below. The crust of the Earth floats along the mantle because it is less dense, and the mantle is more fluid (caused by heat from the Earth’s core). As a portion of the crust becomes heavier, that portion sinks into the mantle, causing a depression in the Earth’s surface and raising water levels in the surrounding area.

During the last ice age between thirty thousand and one hundred thousand years ago, portions of North America and Eurasia were covered in thick glacial ice, creating a deep depression in the mantle. As most of this ice melted, the mantle and crust began to return to equilibrium, a phenomenon known as isostatic rebound. As the crust rose, sea levels receded in many parts of the Northern Hemisphere.

Global climate change is a trend caused by human activity, a reduction in vegetation worldwide, and an increase in greenhouse gases. This warming phenomenon is noted by the level of glacial retreat, or the melting of glacial ice, measured since the nineteenth century. One focus of the study of glaciers and glaciation is to develop a more complete understanding of climate cycles on Earth and the future of Earth’s environmental evolution.

Short-Term Glacial Cycles

The Cenozoic period, the current geologic age, began approximately seventy million years ago. Evidence from geologic sources indicates that climate change and glaciation have occurred on a relatively regular cycle in this period of Earth’s history.

Earth is in an ice age called the Pleistocene-Quaternary glaciation, which began more than 2.5 million years ago. During an ice age, short-term fluctuations in climatic variables lead to periods known as glacial stages, or simply glacials, characterized by relatively low temperatures and increased glacial buildup. These glacial periods alternate with interglacial periods, in which temperatures increase and glacial ice retreats as the climate becomes warmer. Geologic evidence indicates that in the Pleistocene-Quaternary glaciation, the Earth has experienced twenty or more alternating glacial and interglacial periods. The Earth is now in an interglacial period.

Geologists believe that glacial and interglacial periods are partially related to changes in Earth’s solar orbit. In the current geologic period, the Holocene, the Earth orbits the Sun in a circular pattern. However, the shape of the Earth’s solar orbit gradually shifts over thousands of years, alternating between circular and elliptical patterns. The extent to which an object's orbit deviates from a circular path is known as its eccentricity. Changes in Earth’s eccentricity can majorly impact the amount of solar radiation that reaches the planet's surface at various times of the year.

In addition, Earth's axis of rotation is tilted with respect to the Sun at an angle of about 23.45 degrees; this tilt is called its axial tilt or obliquity. Earth's obliquity oscillates between 21 and 24.5 degrees over a forty-two-thousand-year cycle. Finally, the Earth’s axis of rotation itself rotates on a twenty-six-thousand-year cycle, a phenomenon known as precession. While the Earth’s North Pole is currently pointed toward the star known as Polaris, or the North Star, the Earth’s axis will gradually rotate toward the star known as Vega.

Serbian geophysicist and engineer Milutin Milanković used the Earth’s precession, obliquity, and eccentricity to calculate the cyclic pattern of the planet’s orbital relationship with the Sun; these cycles came to be called Milankovitch cycles. He then correlated these data with information on climate change to develop the Milankovitch hypothesis, which posits that repeating patterns of heating and cooling on Earth are related to the planet’s angular orientation and relative distance from the Sun.

The Milankovitch hypothesis predicts that Earth will experience significant changes in climate on three separate cycles—every one hundred thousand years, forty-one thousand years, and twenty-one thousand years—corresponding closely to changes in the pattern of Earth’s orbit. Geologic and climatological evidence indicates that Milankovitch cycles may largely be responsible for the alternating glacials and interglacials during the ice age.

Long-Term Glacial Cycles

While Milankovitch cycles help explain short-term variations in climate, geologic evidence indicates that the Earth has experienced periods of long-term glaciation, or glacial epochs, that cannot be explained by orbital variation. Some glacial periods last hundreds of millions of years and involve glaciation far more extensive than the current Quaternary ice age.

One of the first glacial epochs to be studied occurred between 2.2 and 2.4 billion years ago, in the Paleoproterozoic Era, and is thought to have lasted 200 million years or longer. Though this ancient ice age, called the Makganyene (or Huronian) glaciation, is poorly understood, global changes in tectonic movement and volcanism have been identified as potential causal factors. Geologists believe that the Makganyene glaciation might have marked the first time that most or all of the Earth was covered in glacial ice, a hypothesis now called the snowball Earth theory. Global glaciation results when contributing factors such as tectonic movement and atmospheric composition converge to create a positive feedback loop that allows glaciers to spread until they cover vast portions of the planet’s surface.

The period from 850 to 630 million years ago, constituting a major portion of the Neoproterozoic period (1 billion to 540 million years ago), is sometimes called the Cryogenian period because of the two major glacial epochs that occurred during this span, which included the most extensive glaciations in the known history of the Earth. The Marinoan (635 million years ago) and Sturtian (710 million years ago) glaciations left widespread geologic evidence around the world, indicating that glacial ice was present at all latitudes and covered the Earth in thick layers.

During the Paleozoic Era (540–340 million years ago), two glacial epochs existed, corresponding to major biosphere changes. The Andean-Saharan glaciation occurred between 460 and 430 million years ago, spanning the Ordovician and the Silurian periods, and corresponds with a mass extinction of life on Earth, after which terrestrial plants began to spread across the surface. The Karoo ice age (360–260 million years ago) spanned part of the Mississippian and Pennsylvanian periods and is partially explained by the spread of terrestrial plants, which removed greenhouse gases from the atmosphere and caused global cooling.

Geologists believe that glacial epochs are largely the result of tectonic shifting on the Earth’s surface. Evidence suggests that glacial epochs occur when larger continents break into smaller continents, a process known as rifting. Increases in tectonic rifting might affect climate by altering levels of silicate weathering. This is the process by which atmospheric carbon dioxide is removed from the atmosphere through interaction with minerals dissolved from the Earth’s crust into the oceans.

Silicate weathering increases with temperature. Thus, as continents move over the equator, where they are exposed to higher levels of solar radiation, silicate weathering increases, thereby leading to a reduction in atmospheric carbon dioxide and the beginning of a cooling cycle. Rifting and tectonic convergence are part of the theoretical supercontinent cycle, which suggests that the continents converge on a cycle of three hundred million to six hundred million years.

Evidence for Glaciation

Ice-core samples help geologists learn about the climate of the distant past. Ice in glaciers is built of successive layers of snow that have become compacted to form ice. By taking vertical samples of glacial ice, geologists can examine the layers in the ice and discern information about climate from the differences between and among layers.

Water vapor contains different molecular varieties of water based on the types of isotopes contained within water molecules. Isotopes are variations of an element, such as hydrogen or oxygen, which have the same number of protons but differ in the number of neutrons. While these isotopes are of the same element, variations in the number of neutrons cause each isotope to behave differently in certain chemical reactions.

The standard variety of water is H¹⁶OH, which contains an isotope of oxygen known as oxygen-16 (¹⁶O), considered the most common oxygen isotope. However, water also occurs in the formula H¹⁸OH, which contains the oxygen-18 (¹⁸O) isotope. In addition, some water molecules contain deuterium, an isotope of hydrogen, and may, therefore, occur in the chemical formula HOD. When water vapor rises before condensing because of heat, it tends to lose deuterium and oxygen-18 more readily than it will lose the oxygen-16 isotope. This means colder temperatures favor ice accumulation with greater proportions of deuterium and oxygen-18. Therefore, geologists can measure changing temperatures in ancient ice samples by measuring the occurrence of various isotopes. Ice cores taken from Antarctica preserve temperature-induced isotope variations from four hundred thousand years ago or earlier.

In addition to isotope chemistry, ice cores preserve various other environmental elements representing the time when the ice was first deposited. Levels of dust and debris captured in glacial ice indicate that dry air and heavy winds dominated during glacial periods. In addition, geologists have found trapped gas inclusions within ice cores containing carbon dioxide and methane from the Quaternary Ice Age. Measurements indicate that these gases were more abundant during warmer periods and declined during glacial periods, providing evidence that greenhouse gas levels are a major determinant in glaciation.

Geologists have found acids related to volcanic activity in glacial ice in some cases. Increased volcanism occasionally preceded periods of glacial activity because gases released by volcanoes can block solar radiation, leading to more pronounced differences between seasons.

The study of Earth’s ice ages and glaciation continues to be critical in mitigating the effects of global climate change. Analysis of data from Earth’s most recent Last Glacial Maximum, which occurred 21,000 years ago, offers insight into Earth's climate sensitivity, providing predictive models of Earth’s temperature rise as more carbon dioxide is released into its atmosphere. Climatologists also continue to study Milankovitch cycles closely. Because oceans interacting with the Earth's atmosphere played a significant role in previous ice ages, scientists look into how contemporary conditions may affect climate change. The study of ice ages continues to inform scientists about Earth’s past, information that can be used to protect its future. These studies are valuable to the Earth's environmental protection and help inform further climate research and public policy. 

Principal Terms

cryosphere: the portion of the Earth’s surface where the year-round temperature remains constant enough to support permanent ice and snow

eccentricity: variation in the shape of the Earth’s orbit around the Sun, ranging from circular to elliptical

glacial epoch: an extended period of global temperature reduction and glaciation that generally lasts for millions of years and includes internal glacial and interglacial periods

glacial stage: short-term period of glaciation, generally lasting for less than one million years and alternating with interglacial periods

glacier: buildup of frozen ice on some portion of Earth’s lithosphere

interglacial: a period of reduced glacial coverage that alternates with glacials within a global glacial epoch

isostacy: equilibrium between the lithosphere of the Earth and the liquid layer of rock in the inner layers of the strata

obliquity: the tilt of an orbiting body's rotational axis relative to its orbital axis

Pleistocene-Quaternary glaciation: the current ice age, beginning approximately 2.4 million years ago

precession: the gradual change of a rotating body's axis of rotation

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