Image: Variations of the ice sheet in the northern hemisphere. Source - Brigitte Van Vliet-Lanoë
Glacial and interglacial periods
The study of the Earth's climate, before the influence of human activities for 150 years, makes it possible to predict its future evolution. Although there have been climatic variations over short periods (in a few decades), it is the Milankovitch cycles that make it possible to explain the variations over long periods (in tens of thousands of years).
There is a strong correlation between astronomical movements and terrestrial climatic variations because astronomical movements modify the sunshine received. This correlation has been established for the last 25 million years.
If the solar system were composed of only the Sun and the Earth without any other planets, the rotation of the Earth around the Sun would remain unchanged. There would be no variations in sunshine over time.
In paleoclimate theory, the three astronomical phenomena involved are orbital eccentricity, axial obliquity, and precession of the equinoxes (explained below).
These astronomical phenomena are partly responsible for natural climatic changes, the main consequence of which is the succession of glacial and interglacial periods.
The ice caps expand and then shrink with the rhythm of glacial and interglacial cycles. For example 21 000 years ago at the last glacial maximum, the ice caps were more important and covered the north of Europe and America (image opposite).
Ice caps have always been present for the past 25 million years. Indeed, the Earth without an ice cap dates from the Eocene (56 to 34 million years). 50 million years ago, there was almost no ice cap, palm trees grew in Antarctica where the temperature fluctuated between 10 and 25 °C.
We are currently in a warm interglacial period called the Holocene. The Holocene is a Quaternary period that began 11 700 years ago.
The three parameters of Milankovitch
Image: Variations in the eccentricity of the Earth's orbit.
Milankovitch cycles can explain climatic variations over long periods. They correspond to variations in the volume of polar ice and therefore to variations in sea level. During the last 2.6 million years (Quaternary era) we observe large amplitudes, with a very marked period of 100 000 years, linked to the parameters of Milutin Milanković (1879-1958).
The 3 cycles cover periods of ≈25 000, ≈41 000 and ≈100 000 years.
- The eccentricity defines the shape of the Earth's orbit around the Sun. It characterizes the degree of flattening of the ellipse with respect to a circle. The currently very low eccentricity is equal to 0.017. It varies between 0 (circle) and 0.06 under the influence of two cycles of 100 000 and 400 000 years depending on the position of the other planets in the solar system.
- The obliquity is the inclination of the axis of rotation of the Earth with respect to the ecliptic. It is the angle between the axis of rotation and the perpendicular of the orbital plane (currently 23.5°).
Due to planetary disturbances, the inclination of the Earth's orbital plane changes and fluctuates between 24.5° and 22° over a period of approximately 41,000 years. Moreover, this oscillation of the axis describes the surface of a cone like the axis of a spinning top turning around the vertical. The Earth's axis describes a complete cone approximately every 26,000 years.
- The precession of the equinoxes is linked to the oscillation of the Earth's axis. Currently the summer solstice in the northern hemisphere takes place near aphelion, so the summers are temperate and the winters less cold. About 12,000 years ago, the summer solstice took place near the perihelion, the summers were very hot and the winters very cold (the southern hemisphere is in the opposite situation). Cold summers allow the persistence in high latitudes of winter snow whose high albedo promotes cooling and the annual accumulation of ice. The offset of the axis of rotation is measured relative to the stars. From one century to another (on the same date) the stars are not seen in the same direction. The North Star changes over time. 8000 years ago, Alpha Cygni was the pole star, 4800 years ago it was the star Alpha Draconis, in 12000 years it will be Alpha Lyrae...
Linked to obliquity, this cycle takes place over a period of about 26,000 years.
The combination of Milankovitch parameters will influence the volume of ice stored in the ice caps of the northern hemisphere.
Ocean sediments have recorded the three Milankovitch cycles over the last two million years (≈25,000, ≈41,000 and ≈100,000 years). The cycle of ≈100,000 years is the main wave in which there are two harmonics, one of ≈25,000 years and one of ≈41,000 years which gradually shift.
Concerning the climate there are two periods which are repeated at more or less regular intervals:
- an ice age that lasts about 100,000 years.
- an interglacial period that lasts about 20,000 years.
The last ice age began 115,000 years ago and ended 11,700 years ago. The last glacial maximum was reached around 21,000 years ago. Cooling is slow and lasts 80,000 years while warming is brutal in 20,000 years.
Currently we are at the end of an interglacial period and we should begin a slow cooling of 80,000 years.
NB: The search for a correlation between climatic variations and orbital forcing requires sufficiently detailed climatic indicators on the Milankovitch time scales.
Currently, the indicators used are marine and lacustrine sediments, corals, tree rings, ice cores, pollens, rock cyclostratigraphy, marine sequences of carbonates, etc...
A commonly used indicator for estimating the global volume of continental ice is the isotopic ratio between the isotope 18 of oxygen and the oxygen 16 contained in the carbonate skeletons of benthic foraminifera. These are usually mined from marine sediments drilled from the ocean floor.