How do Milankovitch cycles affect climate?
JAMES EDWARDS
Earth has a long history of episodic alternation between glacial and interglacial periods. And so far, these alternations are best explained by periodic deviations in Earth’s orbit around the Sun, called Milankovitch cycles. These cycles cause regular variations in the amount of heat we receive from the Sun.
Changes in the shape of Earth’s orbit around the Sun occur cyclically; the pull of gravity from other bodies in the Solar System causes our orbit to vary from nearly circular to more elliptical. When Earth’s orbit is at its most elliptical, around 23 per cent more solar radiation reaches Earth during our closest approach to the Sun. Currently, Earth is near its most circular orbit (least elliptic). This cycle is known as eccentricity and occurs over a period of around 100,000 years.
Obliquity, also known as axial tilt, involves changes in the angle of tilt of Earth’s rotation axis, and it’s the reason why we have seasons. Over the last million years or so, it’s varied between 22.1 and 24.5°, and as obliquity decreases, our seasons become milder.
In addition, Earth wobbles a little on its axis of rotation, like a toy spinning slightly off-centre. This gives rise to precession and is caused by the gravitational influence of the Sun and the Moon on the Earth’s equator. Precession dictates when the perihelion (the closest approach to the Sun) will be, which currently occurs on 3-4 January when it’s winter in the northern hemisphere, and summer in the southern hemisphere. Precession creates more extreme seasonal contrasts in one hemisphere, and less extreme in the other. So, in around 13,000 years, precession will cause our current conditions to switch, and the northern hemisphere will experience more extremes in solar radiation.
Milankovitch cycles do not, however, account for the current period of rapid warming, as they operate on much longer time scales than we are currently seeing. They range from tens of thousands, to hundreds of thousands of years, and without the influence of humans, our current orbital positions suggest that our planet should be cooling, rather than warming.
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