During the past approximately 2.6 million years (the Quaternary), glacial and interglacial periods have alternated. The modern standard theory explains this periodicity through changes in Earth's orbit, known as the Milankovitch cycles.
Basic Knowledge
The Milankovitch cycles (Milankovitch theory) state that changes in Earth's orbit and rotational axis alter the seasonal and latitudinal distribution of solar radiation, causing glacial–interglacial cycles. Three orbital variations are combined.
① Eccentricity (eccentricity variation): changes in the degree to which Earth's orbit is elliptical. Periods of about 100,000 years and 400,000 years. ② Axial tilt (obliquity): currently about 23.4°, varying between 21.5° and 24.5°. Period of about 41,000 years. A greater tilt produces a larger seasonal difference. ③ Precession (wobbling of the rotational axis): changes in the direction of the axis, like the wobbling of a spinning top. Periods of about 23,000 years and 19,000 years.
When the three variations overlap, summer insolation in the Northern Hemisphere changes greatly. During periods of weak summer insolation, snow is more likely to remain unmelted. The resulting increase in albedo (snow and ice reflect sunlight) acts as a positive feedback and leads to an ice age. The last ice age reached its coldest point about 20,000 years ago, and the present is an interglacial period (the Holocene).
Name the three elements of the Milankovitch cycles and state the variation with the longest period and its period.
Answer: The three elements are ① eccentricity variation (about 100,000 years and 400,000 years), ② obliquity variation (about 41,000 years), and ③ precession (about 23,000 years and 19,000 years). The longest period is about 400,000 years for eccentricity variation (or about 100,000 years as the dominant major cycle).
- Milankovitch cycles: changes in insolation caused by changes in Earth's orbit and rotational axis
- Eccentricity variation: periods of about 100,000 years and 400,000 years
- Obliquity variation: period of about 41,000 years; greater tilt = greater seasonal difference
- Precession: periods of about 23,000 years and 19,000 years
- Decreased Northern Hemisphere summer insolation → snow remains → albedo increases → ice age
- Coldest point of the last ice age: about 20,000 years ago
- The present is the Holocene (the interglacial period after the last ice age)
Points to Note
① Milankovitch cycles only “change insolation”; actual temperature changes are amplified by CO₂ and albedo feedbacks. ② The approximately 100,000-year glacial cycle is most prominent, but eccentricity variation alone produces only a small change in insolation, an issue known as the “100,000-year problem” that remains under study. ③ Precession is a change in the direction of the rotational axis, not in the angle of axial tilt (the wobbling of a spinning top).
Practice
- State the three orbital variations that make up the Milankovitch cycles.
- Which seasonal change in Northern Hemisphere insolation is important as a trigger for entering an ice age?
- Approximately how many ten-thousands of years ago was the coldest point of the last ice age?