Ice Ages and Milankovitch Cycles (Leaving Cert Geography): Revision Notes
Ice ages and Milankovitch cycles
What are ice ages?
An ice age represents an extended period in Earth's history when global temperatures drop significantly, causing massive ice sheets and glaciers to cover large portions of the planet's surface. During these periods, ice coverage expands far beyond the polar regions, affecting both northern and southern hemispheres.
Several factors work together to trigger ice ages, including changes in Earth's orbital patterns, variations in solar energy output, shifts in atmospheric composition (particularly greenhouse gas levels), and movements of tectonic plates. When these elements combine, they reduce Earth's average temperature enough to allow ice sheets to grow and persist.
The most recent major ice age was the Pleistocene Ice Age, which lasted from approximately 2.6 million years ago until about 11,700 years ago. During this time, enormous ice sheets covered vast areas of North America, Europe and Asia.
Between ice ages, Earth experiences interglacial periods characterised by warmer temperatures, retreating ice sheets, and rising sea levels. We currently live in an interglacial period called the Holocene, which began when the Pleistocene ended.
Understanding Milankovitch cycles
Milankovitch cycles describe long-term variations in Earth's orbit and orientation that influence our planet's climate over thousands of years. These cycles are named after Serbian astrophysicist Milutin Milanković, who developed this theory in the early twentieth century.
His work became crucial for understanding when and why ice ages occur, providing the mathematical framework that explains the timing of glacial and interglacial periods.
The three components of Milankovitch cycles
The Three Orbital Cycles:
1. Eccentricity (100,000-year cycle) Eccentricity refers to how the shape of Earth's orbit around the sun changes over time. The orbit shifts from being more circular to more elliptical in a cycle lasting approximately 100,000 years. When the orbit becomes more elliptical, it creates greater variation in the distance between Earth and the sun, which affects how much solar energy our planet receives throughout the year.
2. Axial tilt or obliquity (41,000-year cycle) Earth's axis tilts at different angles relative to its orbit around the sun, changing over roughly 41,000 years. Currently, this tilt measures 23.5°, but it varies between 22° and 24.5°. When the tilt increases, it creates more extreme seasonal contrasts - summers become warmer and winters become colder. Conversely, a smaller tilt results in milder seasonal differences.
3. Precession (26,000-year cycle) Precession describes the wobble in Earth's rotation on its axis, similar to how a spinning top wobbles as it slows down. This cycle takes about 26,000 years to complete and affects the timing of seasons in relation to Earth's position in its orbit around the sun.
How Milankovitch cycles influence ice ages
These three cycles work together to determine how much solar energy reaches different parts of Earth at various times. The key to ice age formation lies in what happens to summer solar radiation in polar and high-latitude regions.
When the cycles align to reduce summer solar radiation in these areas, snow and ice accumulate because temperatures remain too low to completely melt winter snow. Over time, this accumulation builds into massive ice sheets, marking the beginning of an ice age.
Conversely, when the cycles combine to increase summer solar radiation in polar regions, it can cause significant melting of existing ice sheets, leading to the end of an ice age and the start of an interglacial period.
Evidence supporting Milankovitch theory
The Pleistocene Ice Age provides strong evidence for Milankovitch cycles. This ice age included multiple glacial and interglacial periods spanning 2.6 million years.
Scientists have studied geological records, including ice cores and ocean sediments, which show a clear correlation between changes in Earth's orbital patterns and the timing of these glacial cycles.
Ice cores preserve atmospheric conditions from thousands of years ago, while ocean sediments contain microscopic fossils that indicate past ocean temperatures. Both types of evidence demonstrate how orbital variations have influenced Earth's climate throughout history.
Key Points to Remember:
- Ice ages are prolonged cold periods when ice sheets cover large parts of Earth's surface, triggered by multiple factors including orbital changes
- The three Milankovitch cycles are eccentricity (100,000 years), axial tilt (41,000 years), and precession (26,000 years)
- These cycles control ice age timing by affecting how much summer solar radiation reaches polar regions
- When summer radiation decreases in high latitudes, ice accumulates and ice ages begin; when it increases, ice melts and ice ages end
- The Pleistocene Ice Age (2.6 million to 11,700 years ago) provides strong evidence supporting Milankovitch theory through geological records