Orbital effects on climate, the equinox, and seasons
Did you know that long term warming and cooling of the Earth, enough warming and cooling to cause ice ages to come and go seem to be caused by slow changes in Earth’s orbit? It has long been known, through both observation and calculation, that Earth’s orbit, and indeed the orbits of all the planets, are not fixed. All the bodies of the solar system tug on each other as they go around causing little changes over time.
We all learned in school that the Earth is tilted in space — the equator does not line up with the imaginary plane of the ellipse that the Earth traces out around the Sun. This is the cause of the seasons. When one hemisphere of the Earth is tilted away from the Sun (winter) that hemisphere gets less Sunlight and, therefore, less heat; half a year later that hemisphere is tilted towards the Sun (summer) causing it to receive more Sunlight and, therefore, more heat. Observations and calculations show that the tilt of the Earth is not stable at 23 1/2 degrees, but can rock back and forth a couple of degrees as the Moon and Sun tug on it. This is known as obliquity changes.
From ancient times observers have noticed that the date of the equinoxes and solstices gradually shift, becoming earlier and earlier. We eventually learned that this is because the Earth is wobbling in space like a top, with the rotational axis gradually changing the direction it is pointing in space. The Earth’s rotational axis points at the pole star, Polaris, but gradually it is moving away from that point. Calculations show that it will eventually be pointing at Vega. This is known as precessional changes.
We might imagine that Earth’s orbit is a circle, but in fact it is an ellipse. Each year, the Earth moves a little closer to the Sun (perihelion) in January, and a little further from the Sun (aphelion) in July. Using Newtonian physics, it is possible to calculate the effects of all the bodies in the solar system on the orbits of all the other bodies. It turns out that the interactions of all the bodies in the solar system cause Earth’s orbit to shift a little over time in predictable ways. This is known as eccentricity changes.
Who was Milton Milankovitch?
A brilliant Serbian mathematician named Milutin Milankovitch (1879-1958) who had an interest in Geophysics and Meteorology began to wonder if these changes in Earth’s orbital parameters might affect how much Sunlight is received at the surface of the Earth. He wondered whether these known changes could actually be enough to cause the Earth to cool so much that ice ages could be the result. He decided to make the calculations to determine whether or not the interactions of eccentricity, obliquity, and precession could be the explanation for Ice Ages.
Antarctic Ice Core Confirmation~
Decades after his death, deep sea sedimentary cores and Antarctic ice cores provided a record of the advance and retreat of continental scale glaciers and the heating and cooling of the global climate. When these cores were compared to the global temperature changes predicted by Milankovitch, the brilliant mathematician was vindicated. Predicted changes in Earth’s temperature based on changes in Earth’s orbital parameters line up with the measured temperature changes indicated by the sedimentary and ice core record over the past 400,000 years.
The cycles of warming and cooling are now named Milankovitch Cycles in honour of the man who predicted them.
Putting it all together (it’s cool)~
Above~have a look at this neat little tool that lets you explore the changes in Earth’s orbital parameters, then line them up yourself:
Try out the Vostok Ice Core Applet.
You can view the rest of the presentation on this fascinating man and his theories: Milankovitch-Cycles-by-Matt-West.pdf