The astronomical theory of the Ice Age

Over long periods of time the temperature of the Earth alternates between a cold ice age phase and a warm interglacial phase. Very small changes in the Earth's orbit result in only a 4 degree Celsius in the global mean temperature which can have very dramatic impacts on the Earth system. One such change is shown above. The tilt (obliquity) of the Earth with respect to is orbit around the Sun varies between 22.2 and 24.2 degrees. When the tilt is low (between 22.2 and 23.4 degrees) ice sheets grow and when the tilt is relatively high (between 23.4 and 24.2 degrees) the ice sheets melt away. The ice age reached its peak 18,000 years ago when the tilt had risen to a value of 23.4 degrees from a low of 22.2 degrees 32,000 years ago. Since then the tilt has reached a maximum of 24.2 degrees (10,000 years ago). Today, the Earth's tilt measures 23.5 degrees, and the large ice sheets have disappeared. The three orbital parameters that are affected are the eccentricity (how circular the orbit is) which varies with periods between 400,000 and 100,000 years; obliquity (how tilted the Earth is with respect to its orbit around the Sun) which varies with a period of about 40,000 and affects the solar radiation most strongly at the poles; and precession (changes in the distance between the Earth and Sun in a given season) which varies with a period of about 23,000 years and affects the solar radiation most strongly at the equator. These changes in the orbit of the Earth produce changes in the solar radiation received by the Earth that vary over periods of about 40,000 years, strongest at the poles, and 20,000 years, strongest at the equator. These changes can be matched with the cycle of the ice ages as revealed by the geologic record; however, the largest variation of the ice ages over the past million years has a period of 100,000 years. The 100,000 year cycle is thought to be the result of the interactions of the 40,000 and 20,000 year variations in solar radiation and various geophysical processes that occur on the Earth over long periods; however, this is a subject of continuing research.

Credit: Data credit: J. Kutzbach, R. Gallimore, S. Harrison, P. Behling, R. Selin, and F. Laarif (1998) Climate and Biome Simulations for the Past 21,000 Years. Quaternary Science Reviews, 17, 473-506; Peltier, W., 1993, Time Dependent Topography Through Glacial Cycle. IGBP PAGES/World Data Center-A for Paleoclimatology Data Contribution Series # 93-015. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

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