Glaciations
Glaciations refer to significant periods in Earth's history characterized by the advance of continental ice sheets, often resulting in global cooling. Scientists have studied these phenomena for over a century, uncovering the complex interplay of climate drivers that have influenced glaciation cycles throughout geological history. The evidence indicates that while Earth’s average temperatures have remained relatively stable over billions of years, fluctuations due to greenhouse gases and other factors have led to ice ages—periods of extensive glacial coverage. Major glaciation events, particularly during the Pleistocene epoch, demonstrate how ice sheets have expanded and retreated across the northern regions of the globe.
Research suggests that astronomical cycles, known as Milanković cycles, play a critical role in the timing of these glaciations. These cycles are influenced by variations in Earth’s orbit and axial tilt, affecting climate over thousands of years. Understanding glaciations is not only vital for grasping past climate dynamics but also offers insights into current climate change challenges. Human evolution occurred alongside these glacial cycles, with significant migrations and adaptations taking place in response to changing environmental conditions. Overall, the study of glaciations provides essential context for understanding both historical and contemporary climate issues.
Glaciations
Discovering glaciations and trying to understand them have occupied scientists for over a century. In the process, great progress has been made in understanding the drivers of climate change in the past and the likely drivers in the future.
Background
Glaciations presented a challenge to climate science. With ice sheets today only on Greenland and Antarctica, huge continental glaciers were difficult even to imagine. As the idea took root, however, evidence for glaciations throughout geologic history became apparent. What controls the timing of glaciations is now largely understood, but how they grow or retreat—and why—continue to be important subjects of research.
Global Factors
The temperature of the surface of the Earth is a function of how much energy it receives from the Sun and how much of this energy is radiated back into space. The atmosphere plays an important role, as its clouds and aerosols limit how much light energy reaches the surface and its limit how much infrared energy escapes. Geologic evidence suggests that average global surface temperatures have remained within a limited range over the past two billion years, and GHGs, particularly carbon dioxide (CO2), may have been responsible for this consistency.
CO2 is produced by volcanism and removed by weathering. Weathering is temperature dependent, so it represents a negative feedback system: Higher temperature results in more weathering, which removes more CO2, resulting in lower temperatures, or vice versa. Although over periods measured in hundreds of millions of years this feedback loop seems to have maintained a relatively constant temperature, its effects are not instantaneous, and perturbations have occurred. When these perturbations produce colder conditions, ice ages can result.
Evidence of a number of ice ages has been identified in rocks more than one billion years old. Little is known about these ice ages, as data are sparse and difficult to interpret. Between 750 and 550 million years ago, there were several major ice ages, usually lumped together and called Snowball Earth, as there is evidence that glaciers then existed at sea level near the equator. Although they are of academic interest, at the time these ice ages occurred, there were no land plants, and the atmosphere had far less oxygen, so it is not clear that efforts to understand them will help in the study of contemporary climate change.
Other ice ages, including a short one 440 million years ago and the Permo-Carboniferous between 325 and 240 million years ago, provide insight into what conditions are required for ice ages. These occurred when the land on which the continental glaciers formed was over the South Pole. As continents were also over the South Pole in the time period between these two ice ages and Antarctica sat over the South Pole for 90 million years before its current glaciers formed, this location seems to be a necessary but not a sufficient condition for ice age formation.
Pleistocene Glaciations
About 50 million years ago, the Earth began to cool. Deep-ocean temperatures gradually dropped from 13° Celsius to the present 1° Celsius. Study of ocean sediment cores has identified some minor ice ages around 40 million years ago, another ice age formed the East Antarctica 34 million years ago, and a third ice age around 13 million years ago left evidence in Alaska. Orbital factors probably influenced these advances, perhaps by affecting the carbon cycle, but the details remain obscure.
Starting about three million years ago, the climate developed two different states. Since then, it has oscillated between them, causing glaciations—with continental glaciers extending down to the 40° north parallel of latitude—and interglacials such as the current period, some with temperatures even higher than those of the present. Geologists identified glacial deposits, determined that some were on top of others, and gave names to each, but the names were not standardized internationally. The name of the most recent is Valdaian on the Russian Plains, Devensian in Britain, Weichselian in Scandinavia, Würm in the Alps, and Wisconsinan in North America. The deposits of one glacier are easily removed by subsequent glaciers, so there might be some for which no evidence remains on the continents.
Oxygen isotope ratios of marine sediments, which are not removed by later glaciations, show that there have actually been around fifty glaciations. Because these ratios indicate how much water is tied up in ice, this record is now seen as the best way of delineating when an advance ended and a retreat began. Each (MIS) is given a number (odd ones for interglacials, even ones for glaciations) starting with the most recent. Interglacial MIS 103 occurred 2,580,000 years ago.
Once all these additional glaciations were known, analysis showed that astronomical cycles controlled their timing, as had been suggested by James Croll in 1864 and Milutin Milanković in 1941. These results are so robust that geologists now use these cycles to calibrate the more recent part of the geologic time scale.
That Milanković cycles control the timing of glaciations is beyond dispute. How they do so, however, is not well understood. Feedback systems in the oceans, the atmosphere, the biosphere, or perhaps elsewhere are needed to amplify the tiny signal produced astronomically. Identifying and understanding these systems will help scientists understand current climate change.
Context
Skeptics who wonder if the climate is changing should consider glaciations. Doomsayers, who fear the human race is threatened by global warming, should also consider glaciations. Much of geologic history concerns things that happened so long ago that it is easy to dismiss or ignore them, but glaciations are recent history. Homo erectus was on the planet and using fire twenty or so glaciations ago. All human evolution has taken place as glaciers ebbed and flowed. Human migration to North America occurred 14,600 years ago, just after the peak of the last glaciation. Ice sheets then, although they were getting smaller, still covered most of Maine and northern parts of New York, Vermont, and New Hampshire. They are not there now. Climate changes.
Key Concepts
- glaciation: the advance of a continental ice sheet
- ice age: a period of time during which major continental ice sheets advanced and retreated
- interglacial: the warm period between glaciations
- marine isotope stage: half of a glacial cycle, as identified in the oxygen isotope data from ocean cores
- Milanković cycle: period of variation in Earth’s orbital parameters, including axial inclination, climatic precession, and orbital eccentricity
Bibliography
Imbrie, J., and K. P. Imbrie. Ice Ages: Solving the Mystery. Short Hills, N.J.: Enslow, 1979.
Lindsey, Rebecca. "Climate Change: Mountain Glaciers." Climate.gov, 10 May 2024, www.climate.gov/news-features/understanding-climate/climate-change-mountain-glaciers. Accessed 10 Dec. 2024.
Macdougall, J. D. Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press, 2004.
Ruddiman, William F. Earth’s Climate Past and Future. 2d ed. New York: W. H. Freeman, 2008.