Milankovich cycles and climate change The Milankovich theory of climate change was first advocated by James Croll in the late nineteenth century and later elaborated by Milutin Milankovich (1879–1958), a Serb mathematician. The mathematical basis of this astronomical theory of the ice ages was substantially refined by André Berger. More recently, Berger has provided highly detailed information on past insolation variations.
The Milankovich theory of climate change is based on the premise that the alternating cold and warm periods of the Quaternary were principally due to changes in the nature of the Earth's orbit around the Sun. One important assumption of the Milankovich theory of orbital changes in that there has been no absolute annual change in the amount of incoming solar radiation. Long-term changes in the Earth's orbit are believed to cause a redistribution of insolation across both hemispheres, and these changes, in turn, lead to changes in climate.
Milankovich orbital processes
At present, the Earth has an elliptical orbit around the Sun. The Sun is not located at the centre of the ellipse but at one of the foci (Fig. 1). During the winter solstice in the northern hemisphere, the Earth is at one end of its elliptical orbit when it is nearer the Sun (in perihelion) and thus receives greater heat. In contrast, the northern hemisphere summer solstice is characterized by a position in the elliptical orbit that is more distant from the Sun (in aphelion). The principal cause of the seasons is the fact that the Earth's axis is tilted at an angle that is not perpendicular to the plane of its orbit. During the course of its annual orbit, the tilting of the Earth results in the seasonal heating of each hemisphere. The timing of perihelion at present coincides with the period when the southern hemisphere is tilted towards the Sun. Conversely, aphelion coincides for the northern hemisphere in summer, when the Earth as a whole receives approximately 3.5 per cent less solar radiation than the annual mean (as measured at the outer edge of the Earth's atmosphere).
Throughout time, there has been a continual change in the eccentricity of the elliptical orbit. Marked changes in the axial tilt (obliquity) of the Earth have also taken place. In addition, there also have been changes in the timing of perihelion and aphelion with respect to seasonal changes on Earth (the precession of the equinoxes).
Changes in orbital eccentricity
Changes in orbital eccentricity have varied with time from a circular orbit (when perihelion and aphelion are identical) to maximum eccentricity, when the values of incoming solar radiation may have varied by as much as 30 per cent between perihelion and aphelion. The periodicity of this cycle is 95 800 years, during which time the Earth alternates from a circular orbit to a highly eccentric orbit and back again to a circular orbit. It should be stressed that changes in orbital eccentricity do not cause any change in the amount of solar radiation reaching the Earth during summer or winter, nor any change in the total annual heat received by either hemisphere. Instead, the effect is to increase the contrast in seasonality in one hemisphere and reduce it in the other. Croll believed that when this contrast was at its maximum it would cause increased snowfall in the northern hemisphere during winter. The increased global albedo (the fraction of the incident radiation that is reflected from the surface) resulting from a widespread snow cover might then modify the climate of the succeeding seasons and, in this way, initiate ice ages.
Changes in inclination (obliquity)
Changes in the inclination of the Earth's axis have varied between extreme values of 21.39° and 24.36° (the present value is 23.44°) with a periodicity of 41 000 years. Increases in axial tilt result in a lengthening of the period of winter darkness in polar regions. They also result in changes in the seasonal range of latitude in which the Sun occurs overhead. Changes in obliquity therefore cause significant changes in the amount of solar radiation received at high latitudes but do not greatly affect the amount of incoming solar radiation at low latitudes. Because changes in axial tilt are equal in both hemispheres, the resulting changes in incoming solar radiation are the same in both hemispheres.
Precession of the equinoxes
Variations in the timing of perihelion and aphelion are caused by a ‘wobbling’ in the Earth's axis of rotation as it rotates around the Sun. Over a period of 21 700 years, the axis slowly swings in a conical manner around a line perpendicular to the orbital plane (Fig. 1). During this period of time, the northern hemisphere is tilted towards the Sun at successively different points in the Earth's orbit. At present, this takes place during summer when the Earth is in aphelion. However, approximately 11 000 years ago during the Younger Dryas time period, the Earth was tilted towards the Sun in midsummer during perihelion. In theory, therefore, northern hemisphere winters during the Younger Dryas were much colder and longer than at present, and summers were shorter and warmer.
Combined effects
The solar radiation received in low-latitude areas is principally affected by variations in eccentricity and precession of the equinoxes. By contrast, higher latitudes are mainly affected by changes in axial tilt (obliquity). The combined influence of changes in eccentricity, obliquity, and precession of the equinoxes produces a complex pattern of variations in insolation (Fig. 2). These are indicative of increased seasonal contrasts in one hemisphere and diminished contrasts in the other. It is not, however, known whether, during certain time periods in the Quaternary, insolation variations in the high latitudes of the northern hemisphere may have induced similar environmental changes in the southern hemisphere. Similarly, it is not known whether, on certain occasions, insolation changes in the high latitudes of the southern hemisphere (particularly in Antarctica) may have led to a climatic response in the northern hemisphere. Some scientists have even argued that climate changes in both hemispheres have taken place in an approximately synchronous manner.
A critical factor that affected rates at which ice sheets built up and decayed in the middle latitudes during the Quaternary was the seasonal temperature gradient between low and high latitudes—known as the insolation gradient. During periods when the insolation gradient from the Equator to the pole was high, meridional (north–south) circulation would have been increased, thus increasing the rate at which snow-bearing precipitation was delivered to high latitudes.
It has often been argued that the observed patterns of Quaternary environmental changes reflect well-defined cycles of Milankovich insolation. For example, it is generally agreed that the timing of the principal glacial–interglacial cycles during the Quaternary reflects the influence of the 96 000-year eccentricity cycle and that the last glacial–interglacial cycle, approximately corresponding to the Late Quaternary, was principally due to this process. For shorter timescales, scientists have proposed that climate changes during the last 100 000 years are indicative of the occurrence of the 41 000-year obliquity cycle. Similarly, attention has been drawn to the likelihood that some of these changes may have been related also to a 21 000-year precessional cycle.
Milankovich cycles and the ocean sediment record
Milankovich cyclicity and its relationship to climate change has been given increased credence in recent years by the similarity between the oxygen isotope curve derived from sediments on the ocean floor and the Milankovich curve. For a long time the oxygen isotope curve was considered to represent long-term changes in temperature, and, because the Milankovich curve was similar to the oxygen isotope curve, it was argued that these temperature changes were related to Milankovich cycles. However, in recent decades, persuasive arguments have been made proposing that the oxygen isotope curve derived from the ocean sediment record is a record of changes in the isotopic composition of sea water over time, and is therefore a proxy record of palaoglaciation. Scientists have accordingly sought to explain past global changes in ice volume as a result of Milankovich cyclicity. Thus, the present view is that Milankovich cycles have been the prime driving force behind the many complex ice ages that have taken place during the Quaternary.
Notwithstanding this recognition, it should be noted that Milankovich cyclicity cannot be used as an explanation for the pattern of glaciation that has taken in any particular region of the world. Milankovich cycles can be related only to aggregate changes in global ice volume. Furthermore, there are many complex climate changes that have taken place in the past that have occurred at timescales shorter than 10 000 years. For example a well-known dramatic period of ice accumulation appears to have taken place during the Younger Dryas, approximately 11 000 to 10 000 radiocarbon years ago. Dramatic periods of climatic deterioration such as the Younger Dryas cannot be related in any way to Milankovich cyclicity because of the relatively short time interval over which such events took place. For these, and for many other periods of short-term climate change, scientists will have to seek other explanations that do not include Milankovich cyclicity as part of the set of causal mechanisms.
Alastair G. Dawson
Bibliography
Imbrie, J. and and Imbrie, K. P. (1979) Ice ages: solving the mystery. Macmillan, London.
Dawson, A. G. (1992) Ice Age Earth: Late Quaternary geology and climate. Routledge, London.
