The Milankovitch Theory
The Milankoitch theory refers to the changes in the Earth’s movement relative to the Sun and its knock-on effect on its climate. This was first referenced by Milutin Milankovitch in the 1920s. He claimed that changes to the rotation and orbit of the Earth alter the amount of radiation it receives from the Sun. (Smith, 1994)
Three main factors have been identified (Goudie, 1992), which I will discuss below:
The Eccentricity of the Earth’s Orbit (96,000 year cycle)
Figure 1
The eccentrcity of the Earth’s orbit is simply how epliptical the Earth’s orbit around the Sun is at any given point in time. (Montana.edu, Date unstated) The further away we are from the Sun, the cooler our climate. We generally are in an glacial period at this furthest distance of 94.3 million miles and an interglacial (a period between glacials considered to be relatively warmer that lasts thousands of years) when we are closest to the Sun, at 91.4 million miles. The eccentricity varies due to the gravitation pull of Jupiter and Saturn upon the Earth. (National Geographic Magazine, 2007) A perihelion is the point when the Earth is closest to the Sun regardlss of its stage in this cycle. Similarly, the point furthest away from the Sun is an aphelion. Our summer takes place at an perihelion and winter at an aphelion. The therefore, the greater the eccentiricty, the more severe the winters. At current the difference in distance between an aphelion and a perihelion is just 3%, however when the orbit eccentricity is greatest the difference becomes more in the range of 20-30%. (Montana.edu)
The Precession of the Equinoxes (21,000 year cycle)
Figure 2
As the Earth spins on its axis, it wobbles slightly, this is known as ‘precession’. It is often given the metaphor of a spinning top beginning to wobble before it falls. (Montana.edu) Precession slowly changes th time of year when the solstices (when the Sun reaches the furthest distance away from the equator, known as an aphelion) and the equinoxes (when the sun crosses the earth’s equator and day and night are equal length) occur. This therefore shifts the seasons, in 12,000 years time we’ll experience summer in our current winter. (National Geographic, 2007)
Changes in Obliquity (40,000 year cycle)
Figure 3
It is the tilt of the earth that causes seasons. As the earth orbits the Sun, the hemisphere that is tilted towards the tilt is in summer and that that is tilted away from it is in winter. This ‘axial tilt’ varies, which is known as a chang in obilquity. The change in obliquity is the varying angle of the earth away from the plane perpendicular to which it orbits. (Montana.edu) The greater the tilt the more intense the seasons and equally the lesser the tilt the lesser the intensity. This angle varies between 22° and 24.5° and it takes 40,000 years to complete this cycle. (National Geographic, 2007) The cool summers caused by low levels of obliquity are thought to allow snow and ice to last at high latitudes from year to year, which in the long term can create massive ice sheets and glaciers. These massive ice sheets reflect further light in to the atmosphere, known as the albedo effect. This is a positive feedback system which increases the effect of cooling, but similarly warming when the obliquity begins to increase again. (NOAA, 2009)
Figure 4
Collectively, the tilt fundamentally defines the seasons and the amount of such impacts the severity. These can further be accentuated by the precession effect and the eccentricity of orbit. Throughout the “quaternary era” which dates back 2.5 million years until the present, the most recent geological era, specifically in the last ice age, during the “Pleistocene epoch” which ended 11,000 years ago. When all of the three factors line up we have historically been in an interglacial period, meaning it caused a warm period. The last time that this happened was as the Pleistocene epoch ended 11,000 years ago and we entered the modern-day Holocene epoch. The last glacial occurred during the end of the Pleistocene epoch and we are therefore currently in an interglacial. It is possible therefore that we will continue warming, but more likely that we will eventually go back into a glacial period as predicted by previous cycles. (Goudie)
These factors do explain climate in the long term, but climate change is often observed over 30 years (Met Office, date unstated), rather than the thousands of years that these cycles take. Therefore change takes place very slowly and cannot account for the rapid rise in temperature that I showed in the introduction.