An ice age is a long period of time where the earth is covered in large ice sheets and glaciers. Although we are currently in an interglacial period, the last major ice age ended about 11,700 years ago. Scientists are still working to understand exactly what causes ice ages to begin and end, but there are a few key factors that are believed to play a role. The triggers for an ice age are extremely complex and involve interactions between the atmosphere, oceans, ice sheets, and Earth’s orbit around the sun. Some quick answers to basic questions about ice ages include:
How often do ice ages occur? On a scale of millions of years, ice ages are actually the norm for Earth. Major ice ages have occurred at least 5 times in the past 1 million years.
How long do ice ages last? Ice ages can persist for millions of years and end very abruptly. The past 4 major ice ages have lasted between 2-3 million years.
What’s the difference between an ice age and a glacial period? An ice age is a long interval of time where ice sheets cover large areas of land. Glacial periods are colder periods during an ice age where glaciers expand. The ice age contains both warmer interglacial and colder glacial periods.
Are we heading into another ice age? No, due to the current pattern of Milankovitch cycles, the next expected ice age is at least 15,000 years away. Smaller glacial periods may still occur.
Milankovitch Cycles
One of the most well-supported triggers of an ice age involves cyclical changes in Earth’s orbit and tilt called Milankovitch cycles. Milankovitch cycles influence how much solar radiation reaches different parts of Earth across long timescales. The 3 different changes in Earth’s orbit and tilt are:
Eccentricity – The shape of Earth’s orbit around the sun varies from more circular to more elliptical in a cycle that takes around 100,000 years. A more elliptical orbit means Earth receives more uneven solar radiation, with more extreme seasons.
Obliquity – The tilt of Earth’s axis varies between 22.1 and 24.5 degrees over 41,000 years. Changes in obliquity affect how solar radiation is distributed across the seasons.
Precession – Earth’s axis wobbles and shifts direction over a 19,000 to 23,000 year cycle. Precession alters which hemisphere is tilted closest to the Sun during each season.
Together these Milankovitch cycles impact the seasonal and latitudinal distribution of solar energy on Earth. When cycles align to concentrate summer solar radiation in the northern hemisphere, warming ensues which promotes growth of northern hemisphere ice sheets over many thousands of years. When ice sheets reach a critical mass, the ice-albedo feedback loop can take hold, reflecting more radiation and allowing the descent into a major glacial period.
Role of CO2 Levels
Carbon dioxide levels in the atmosphere also play a crucial role in ice age cycles. Lower CO2 levels allow more cooling because CO2 is a greenhouse gas that normally traps heat. Orbital cycles initiate cooling, but the cooling is intensified by corresponding drops in CO2 that further reduce the greenhouse effect.
During the onset of an ice age, chemical reactions caused by cold ocean water allow more CO2 to be absorbed into the deep ocean. And colder conditions slow plant and animal respiration rates, reducing CO2 released back to the atmosphere. This acts as a positive feedback loop, allowing strong cooling sustained over thousands of years under the right orbital cycle conditions.
Rising CO2 can help end an ice age by increasing the greenhouse effect. As orbital cycles shift, any corresponding increase in CO2 helps accelerate deglaciation.
Role of Other Greenhouse Gases
While CO2 plays the largest role, fluctuations in other greenhouse gases like methane (CH4) may also contribute to the onset and end of ice ages. Methane is released by natural wetlands, permafrost, oceans, and animal/plant decay. More methane in the atmosphere during interglacial warm periods enhances warming through the greenhouse effect.
Ice-Albedo Feedback
The ice-albedo feedback loop is another critical factor in driving glacial periods. As ice sheets grow under the right conditions, their white surface is highly reflective. This increased reflectivity (albedo) results in more incoming solar radiation being reflected back into space unable to warm the planet.
With more ice cover and higher albedo, global cooling intensifies. This causes more ice growth and reflection in a positive feedback loop. As cooling continues, the ice-albedo effect can sustain and deepen a glacial period independent of initial orbital triggers.
Ocean Circulation Changes
Shifts in major ocean current circulations, especially the Atlantic Meridional Overturning Circulation (AMOC) have been linked to past ice age cycles. The AMOC carries warm water from the tropics into the North Atlantic. During colder glacial periods, meltwater from ice sheets can cause freshening of surface water.
This lighter fresh water layer reduces sinking of surface water required to drive the AMOC. A slowed AMOC would further cool the Northern Hemisphere and stimulate additional ice growth. Ocean circulation changes likely amplify glacial periods rather than directly cause their onset.
Volcanic Eruptions
Increased volcanic activity during colder periods may enhance cooling into an ice age by emitting sulfate aerosols. These aerosols reflect incoming solar radiation and produce overall cooling over years to decades if eruption frequency is high. Though not a direct cause of ice ages, spikes in volcanism likely accelerate cooling in conjunction with shifts in orbital cycles and greenhouse gases.
Plate Tectonics and Continental Position
The positions and connections of continents can impact ocean currents, global albedo, and the ability for ice sheets to develop. One controversial hypothesis suggests that the current position of the continents clustered around the poles makes the development of large ice sheets much more feasible. Continental drift plays out over extremely long timescales, facilitating ice growth at times of high continental concentration around the poles.
Solar Variability
Changes in solar output may also play a supporting role in ice age cycles. Lower solar output would induce cooling, though solar changes are considered relatively minor compared to the effects of greenhouse gases and orbital cycles over key timescales. Occasional drops in solar activity due to sunspots or other factors may work together with other climate forcings to enhance cooling into an ice age.
Conclusion
In summary, ice ages are prompted by a complex interplay of many factors rather than a single cause. Orbital cycles initiate the cooling, while greenhouse gases amplify these cycles along with other positive feedback loops related to ocean circulation and the ice-albedo effect. A reduction in atmospheric greenhouse gases, especially CO2, remains the most well-supported primary driver for allowing orbital cycles to progress into a major glacial period. But many accelerating feedback mechanisms are then required to sustain an ice age for millions of years once it begins. Currently, the farthest we appear to be from the next glacial inception is at least 15,000 years unless mitigated by human greenhouse gas emissions.
Frequently Asked Questions
What causes an ice age to end?
The main triggers ending an ice age are shifts in the Milankovitch orbital cycles along with corresponding increases in atmospheric greenhouse gases like CO2 and methane. As orbital cycles change to boost summer solar radiation in the northern hemisphere, warming commences which releases more greenhouse gases and accelerates melt of large continental ice sheets over thousands of years.
How quickly do ice ages start and end?
The onset and ending of full glacial conditions occurs gradually over thousands of years. However, recent evidence indicates the final transitions into and out of an interglacial period can occur more abruptly over centuries to decades.
How many ice ages have there been?
Major glacial periods have recurred multiple times over the past million+ years. The 4 most recent ice ages occurred about 300,000, 450,000, 750,000, 2.6 million years ago.
How much has Earth’s temperature changed between ice ages and warmer interglacial periods?
Global temperature changes of approximately 5°C (9°F) between peak ice age and warm interglacial conditions have been typical based on paleoclimate data. More extreme regional changes likely occurred as well.
How much of Earth was covered in ice during past ice ages?
At maximum extent during the last ice age about 20,000 years ago, ice sheets covered nearly 1/3 of Earth’s land and reached thicknesses up to 4 km (2.5 miles) in places.
What is a “Snowball Earth” and have any happened?
A “Snowball Earth” refers to the controversial hypothesis that Earth was entirely or nearly entirely frozen over around 700-800 million years ago. While Earth has experienced periods of major glaciation throughout its history, the extent of past “Snowball Earth” events is still debated.
Key Facts and Data
| Last Major Ice Age Start and End Dates | Began ~110,000 years ago, ended ~11,700 years ago |
| Estimated global temperature change from Last Glacial Maximum to interglacial | Approximately 5°C (9°F) |
| Approximate maximum ice extent during Last Glacial Maximum | Ice sheets covered ~30% of Earth’s land area |
| CO2 levels at Last Glacial Maximum | Around 180-200 ppm, vs 280 ppm during pre-industrial Holocene |
| Timing of Milankovitch orbital cycles | Eccentricity – 100,000 years; Obliquity – 41,000 years; Precession – 19,000 to 23,000 years |
| Length of recent major ice ages | Past 4 ice ages lasted ~2-3 million years |
Key Scientists and Research
Milutin Milankovitch – Formulated theory connecting ice ages to variations in Earth’s orbit and tilt in the 1920s/1930s.
Willie Dansgaard – Studied oxygen isotope ratios in Greenland ice cores in 1960s/1970s to quantify past climate changes including ice age cycles.
Wallace Broecker – Demonstrated role of ocean circulation in ice ages starting in 1980s based on sediment cores.
J.D. Hays, John Imbrie, Nicholas Shackleton – Developed orbital theory of the ice ages using deep sea sediment cores in 1970s.
EPICA Antarctic Ice Core – Over 3km long core drilled in 1990s/2000s provided climate record back 800,000 years.
Petit et al. 1999 – Antarctic ice core analysis showing correlation between temperature, greenhouse gases, and ice volume over past 420,000 years.
James Croll and Milutin Milankovitch – Nineteenth century researchers who first proposed orbital cycles as an ice age trigger.
Impacts on Human Civilization
While impacts were minimal during the most recent major ice age which peaked 20,000 years ago, earlier ice ages had dramatic effects on early human civilization:
– Habitat ranges and food sources were disrupted as environments changed across glacial/interglacial transitions.
– Sea levels dropped up to 130 meters (400 feet) during glacial periods due to water trapped in enormous ice sheets. Extended coastal areas were exposed that could be inhabited or traveled.
– Some migration may have been northward or southward following warmer or colder climates. Isolated groups were likely displaced.
– Tools, clothing, and shelter needed to be adapted to survive in the bitter cold temperatures of ice age environments.
– Development of social networks and trading would have been vital to support survival during harsh ice age conditions and climate changes.
Potential Future Ice Ages
Based on the current status of Milankovitch cycles, Earth’s next gradual shift into an ice age would typically begin in around 15,000 years. However, the high amount of human-generated greenhouse gases beginning in the Industrial Age may delay the next ice age. Models show anthropogenic global warming may disrupt the ongoing pattern of ice age cycles for the next 50,000 years unless greenhouse gas levels are reduced. Geoengineering schemes have also been proposed to artificially regulate global temperatures to prevent future ice age inception.
