Astronomical cycles
Main articles: Milankovitch cycles and orbital forcing
See also: 100,000-year problem
Relationship of Earth's orbit to periods of glaciation
The role of Earth's orbital changes in controlling climate was first advanced by James Croll in the late 19th century.[5] Later, Milutin Milanković, a Serbian geophysicist, elaborated on the theory and calculated these irregularities in Earth's orbit could cause the climatic cycles known as Milankovitch cycles.[6] They are the result of the additive behavior of several types of cyclical changes in Earth's orbital properties.
Changes in the orbital eccentricity of Earth occur on a cycle of about 100,000 years.[7] The inclination, or tilt, of Earth's axis varies periodically between 22° and 24.5°.[7] (The tilt of Earth's axis is responsible for the seasons; the greater the tilt, the greater the contrast between summer and winter temperatures.) Changes in the tilt occur in a cycle 41,000 years long.[7]Precession of the equinoxes, or wobbles on Earth's spin axis, complete every 21,700 years. According to the Milankovitch theory, these factors cause a periodic cooling of Earth, with the coldest part in the cycle occurring about every 40,000 years. The main effect of the Milankovitch cycles is to change the contrast between the seasons, not the amount of solar heat Earth receives. These cycles within cycles predict that during maximum glacial advances, winter and summer temperatures are lower. The result is less ice melting than accumulating, and glaciers build up.
Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that sufficiently long and detailed chronology of the Quaternary temperature changes was worked out to test the theory adequately.[8] Studies of deep-sea cores, and the fossils contained in them indicate that the fluctuation of climate during the last few hundred thousand years is remarkably close to that predicted by Milankovitch.
A problem with the theory is that the astronomical cycles have been in existence for billions of years, but glaciation is a rare occurrence. Astronomical cycles correlate perfectly with glacial and interglacial periods, and their transitions, inside an ice age. Other factors such as the position of continents and the effects this has on the earth's oceanic currents, or long term fluctuations inside the core of the sun must also be involved that caused Earth's temperature to drop below a critical threshold and thus initiate the ice age in the first place. Once that occurs, Milankovitch cycles will act to force the planet in and out of glacial periods.[citation needed]
Atmospheric composition
Glacial and interglacial cycles of the late Pleistocene epoch, as represented by atmospheric CO2, measured from ice core samples going back 650,000 years
One theory holds that decreases in atmospheric CO2, an important greenhouse gas, started the long-term cooling trend that eventually led to glaciation. Recent studies of the CO2 content of gas bubbles preserved in the Greenland ice cores lend support to this idea. The geochemical cycle of carbon indicates more than a 10-fold decrease in atmospheric CO2 since the middle of the Mesozoic Era.[9] However, it is unclear what caused the decline in CO2 levels, and whether this decline is the cause of global cooling or if it is the result.[citation needed]
CO2 levels also play an important role in the transitions between interglacials and glacials. High CO2 contents correspond to warm interglacial periods, and low CO2 to glacial periods. However, studies indicate that CO2 may not be the primary cause of the interglacial-glacial transitions, but instead acts as a feedback.[10] The explanation for this observed CO2 variation "remains a difficult attribution problem."[10]
Plate tectonics and ocean currents
Further information: Plate tectonics and ocean current
An important component in the long-term temperature drop may be related to the positions of the continents, relative to the poles (but it cannot explain the rapid retreat and advances of glaciers).[11] This relation can control the circulation of the oceans and the atmosphere, affecting how ocean currents carry heat to high latitude. Throughout most of the geologic time, the North Pole appears to have been in a broad, open ocean that allowed major ocean currents to move unabated. Equatorial waters flowed into the polar regions, warming them with water from the more temperate latitudes. This unrestricted circulation produced mild, uniform climates that persisted throughout most of geologic time.
