Climate Cycles and Climate Transitions as a Response to
Astronomical and CO 2 Forcings
A. Paul and W. H. Berger
1
Introduction
The natural variability of climate occurs on a wide range of time scales, ranging from decades to millions of years. The time scales of importance to the ice
ages of the late Cenozoic are of the order of 10 to 1000 ka. On these time scales,
the so-called astronomical theory of paleoclimates and in particular its Milankovitch version have become the almost universally accepted basis of research
(Berger 1992). In its traditional form, the astronomical theory states that
changes in high-latitude summer insolation cause the waxing and waning of
the continental ice sheets. This was first quantitatively formulated by Milankovitch (Berger 1988; Milankovitch 1995), who calculated how the incoming radiation at the top of the atmosphere varies as a function of latitude and the
orbital parameters e, E and e sin ill, where e denotes the eccentricity of the
Earth's orbit around the Sun, E is the obliquity and e sin ill is the precessional
effect, with ill being the longitude of the perihelion as measured from the moving equinox. Each of the orbital parameters can be expressed as a quasi-periodic function of time. According to the astronomical theory, low summer insolation could prevent the winter snow from melting. The high albedo of
snow- and ice-covered areas would initiate a further cooling of the Earth.
Eventually, this positive feedback would result in the build-up of the continental ice sheets.
The success of the astronomical theory is mainly due to two reasons. (1)
Climatic time series as recorded in deep-sea sediments exhibit power spectra
that show peaks corresponding to climate cycles with frequencies near 1/19,
1/23 and 1/41 ka- 1 • The first two frequencies correspond to the precessional
effect, with the frequency of the precessional angle ill 1/22 ka- 1 split into two
by the eccentricity frequency, whose mean is about 1/100ka- 1 (the most important terms in the series expansion of eccentricity have frequencies of about
11404, 1195, 11124, 1/99 and 1/131 ka- 1 , in decreasing order of amplitude -
(Berger and Loutre 1991). The third frequency corresponds to obliquity. (2)
Climate models forced at the astronomical frequencies produce changes in global ice volume in fair agreement with the deep-sea records.
Astronomical and CO 2 Forcings
A. Paul and W. H. Berger
1
Introduction
The natural variability of climate occurs on a wide range of time scales, ranging from decades to millions of years. The time scales of importance to the ice
ages of the late Cenozoic are of the order of 10 to 1000 ka. On these time scales,
the so-called astronomical theory of paleoclimates and in particular its Milankovitch version have become the almost universally accepted basis of research
(Berger 1992). In its traditional form, the astronomical theory states that
changes in high-latitude summer insolation cause the waxing and waning of
the continental ice sheets. This was first quantitatively formulated by Milankovitch (Berger 1988; Milankovitch 1995), who calculated how the incoming radiation at the top of the atmosphere varies as a function of latitude and the
orbital parameters e, E and e sin ill, where e denotes the eccentricity of the
Earth's orbit around the Sun, E is the obliquity and e sin ill is the precessional
effect, with ill being the longitude of the perihelion as measured from the moving equinox. Each of the orbital parameters can be expressed as a quasi-periodic function of time. According to the astronomical theory, low summer insolation could prevent the winter snow from melting. The high albedo of
snow- and ice-covered areas would initiate a further cooling of the Earth.
Eventually, this positive feedback would result in the build-up of the continental ice sheets.
The success of the astronomical theory is mainly due to two reasons. (1)
Climatic time series as recorded in deep-sea sediments exhibit power spectra
that show peaks corresponding to climate cycles with frequencies near 1/19,
1/23 and 1/41 ka- 1 • The first two frequencies correspond to the precessional
effect, with the frequency of the precessional angle ill 1/22 ka- 1 split into two
by the eccentricity frequency, whose mean is about 1/100ka- 1 (the most important terms in the series expansion of eccentricity have frequencies of about
11404, 1195, 11124, 1/99 and 1/131 ka- 1 , in decreasing order of amplitude -
(Berger and Loutre 1991). The third frequency corresponds to obliquity. (2)
Climate models forced at the astronomical frequencies produce changes in global ice volume in fair agreement with the deep-sea records.