Milankovitch Cycles - "Natural Climate Change"
In this activity, we will explore the different Milankovitch Cycles that lead to natural cycles in the Earth's Climate.
About one hundred years ago Serbian scientist Milutin Milankovitch hypothesized that changes in the Earth’s position relative to the Sun are a strong driver of Earth’s long-term climate, and are responsible for triggering the beginning and end of glaciation periods (Ice Ages). He proposed three cycles.
1. The shape of the Earth's orbit, is known as eccentricity.
2. The angle at that which the Earth's axis is tilted with respect to the Earth's orbital plane, known as obliquity.
3. The direction in which the Earth's axis of rotation is pointed, is known as precession.
The diagram below shows how these three cycles correlate with the Earth's temperature and Ice Ages. These cycles are in the order of tens of thousands of years.
This section is adapted from an excellent article by NASA's Science Editorial Team.
Eccentricity (shape of the Earth's orbit)
The Earth's orbit varies over a cycle of about 100,000 years between being elliptical and being circular. This is due to the pull of gravity from the giant planets, Jupiter and Saturn. When the Earth's orbit is at its most elliptic, about 23% more incoming solar radiation reaches the Earth at our closest approach to the Sun than does at its farthest departure.
Obliquity (change in tilt)
This cycle is about 41,000 years. The angle of the Earth’s axis of rotation is tilted as it travels around the Sun is known as obliquity. Obliquity is why Earth has seasons. Over the last million years, it has varied between 22.1 and 24.5 degrees with respect to Earth’s orbital plane. Larger tilt angles favour periods of deglaciation (the melting and retreat of glaciers and ice sheets). The Earth’s axis is currently tilted 23.4 degrees, or about halfway between its extremes. It will reach its minimum tilt about 10,000 years from now. This change allows snow and ice at high latitudes to build up into large ice sheets, reflecting more sun (higher albedo) and promoting more cooling.
Axial Precession (Wobble)
As the Earth rotates, it wobbles slightly upon its rotational axis, like a slightly off-centre spinning toy top. This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon that cause Earth to bulge at the equator, affecting its rotation. This cycle is about 25,771.5 years.
In 1976, a study in the journal Science by Hays et al. using deep-sea sediment cores found that Milankovitch cycles correspond with periods of major climate change over the past 450,000 years, with Ice Ages occurring when Earth was undergoing different stages of orbital variation. Cooling acts in a positive (amplifying loop) feedback cycle, It is well accepted that Milankovitch cycles drive the timing of glacial-interglacial cycles but more research is needed to explain how they affect climate more generally but they do not explain the current warming.
Changes in orbital cycles do not immediately cause rises or falls in atmospheric CO2. Rather, initial increases in ice cover in high-latitude areas trigger feedbacks that cause atmospheric CO2 to fall at the start of ice ages. This is a complex story, with changes in sea levels, ocean currents, amounts of carbon dioxide dissolved in the oceans (due to temperature and salinity changes) and thus photosynthetic rates in the oceans and on land with differences between the north and south hemispheres. You can read more about this in this article from Zeke Hausfather for Carbon Brief.