What are Milankovitch Cycles?
The age-old question, which I’m sure everyone uses medium to find the answer. The Milankovitch conundrum. It keeps you awake at night, doesn’t it? Gnaws away at the back of your mind day in, day out. Is it really as simple as some guy called Milankovitch on a bike? No. Is it relevant to the woes of everyday life? Not especially. Will this article change your life and lift you to a higher plain of understanding? Definitely not, but is it some pretty interesting orbital mechanics that influence global climate on a geological scale? Yes. So if that sounds even remotely interesting, stay awhile and listen.
Way back when in the ancient times of 1930, a Serbian called Milutin Milankovic (anglicised to Milankovitch), building on the work of British scientist James Croll, hypothesised that regular, predictable changes in the Earths position relative to the sun are the key to understanding the Earths long-term climate. In particular, he theorised that orbital variations trigger ice ages every 41 000 years. As finding evidence for his theory became possible thanks to advances in palaeoclimatology, the ice age intervals he predicted were found recorded in the geological record, and his ideas became accepted.
There are three primary cycles; eccentricity, obliquity and precession—each with different cycle lengths. The first and most extended cycle is eccentricity. Eccentricity is a 100 000 year change in the Earths orbit with the sun from perfectly circular to slightly elliptical. Currently, the Earths eccentricity is at its most circular; as it becomes more elliptical over the next 100 000 years, the difference between the Earths perihelion (closest approach to the sun) and aphelion (furthest point from the sun) will cause a more significant difference in solar radiation received by the Earth at different times of the year. This pattern is of prime importance in impacting periods of glaciation and regulates the effects of another Milankovitch cycle: precession. Currently, the radiation difference between perihelion and aphelion is 6%; when eccentricity is at its most elliptical, the difference will increase to between 20–30%.
Obliquity (also known as axial tilt) is the tilting of the Earth’s axis of rotation, which controls how extreme our seasons are. As obliquity decreases, seasons become milder. Present-day obliquity is currently at 23.4° and decreasing; it varies between 22.1° and 24.5° across the 41 000 year cycle. The effect of obliquity is not equally distributed globally, with higher latitudes greater impacted by the changes in axial tilt. When obliquity increases, it results in colder winters, warmer summers, and periods of deglaciation; the increased angle means each hemisphere receives greater solar radiation levels during summer.
The final Milankovitch cycle is that of precession. Precession is the 26 000 year circular movement of the Earths axis. This “spinning top” movement of the axis can create huge seasonal differences between hemispheres when combined with eccentricity. Precession causes different hemispheres to point towards the sun at perihelion (the shortest distance between the Earth and Sun) and away from the sun at aphelion (greatest distance between the Earth and Sun). Currently, summer in the southern hemisphere occurs close to perihelion, and winter occurs close to aphelion; this will switch to the northern hemisphere in 11 000 years. If eccentricity were at its most elliptical, precession would cause the south to have extreme seasons compared to the north. As eccentricity (the first Milankovitch cycle discussed) increases (increasing the radiation difference between perihelion and aphelion), the effect precession has on the distribution of solar radiation between the two hemispheres also increases. When eccentricity is at 0 (circular orbit), the effect of precession is minimal; however, when eccentricity is at its maximum (slightly elliptical orbit), precession has a dramatic impact on global radiation distribution.
Milankovitch’s prediction that ice ages occur every 41 000 years (correlating to changes in obliquity) was correct; however, 800 000 years ago, the interval between ice ages increased to 100 000 years, matching eccentricity; the reason for this is not yet fully understood. Understanding Milankovitch cycles and how they operate both individually and together to change the Earth climate is crucial when constructing models of the Earths climate timeline both in the past and future. This is especially important in a world dominated by the effects of anthropogenic climate change. While Milankovitch’s theory is accepted, there is still much to understand.
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