Understanding Milankovitch cycles

| 5th June 2019
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What are orbital theories of climate and how can they help us understand climactic shifts in the geologic past, and the severity of human-induced climate change in the present?

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The orbital theory of climate is the prevailing explanation for glacial-interglacial climate change over thousands to hundreds of thousands of years. 

But news reports written by climate deniers often blame contemporary climate change on these solar cycles – despite solar irradiance decreasing as temperatures continue to soar. In some cases, those deniers blame the Earth’s orbit in order to shirk the blame from human impacts. 

Understanding Milankovitch cycles shows that solar activity and orbital cycles are not responsible for climatic change over the last few decades. Instead, these cycles help us to understand climate change in the geological past. 

Feedback loops

In its most basic form, Milankovitch theory postulates that variations in the shape of the Earth’s orbit and tilt changes the seasonal and geographic distribution of approaching solar radiation entering the planets upper atmosphere.

Cyclicity was the catalyst for the growth and decline of ice sheets during the Ice Ages - to the point that they are nicknamed the “pacemaker of the Ice Ages” - and other fluctuations in the Earth’s climate system, resulting in a cyclical environmental change in the geological record. 

The theory that orbital cycles could influence climate was first put forward by Serbian mathematician Milutin Milanković in the early twentieth century. He calculated that variations in solar radiation during the summer in the northern hemisphere was critical in the formation of Arctic ice sheets.

Milanković argued that because of a reduction in solar radiation during the summer, snow from the previous year would not have melted. This enhanced albedo, reflecting radiation into space leading to further cooling creating a positive feedback loop. 

There are three Milankovitch cycles: eccentricity, tilt and procession.

Eccentricity

Firstly, orbital eccentricity is the longest of the orbital cycles taking roughly 100,000 years. It refers to the change in the shape of the Earth’s orbit around the sun from a more circular shape to a more elliptical one.

The more elliptical orbits lead to greater seasonal changes, changing the distance the Sun's short-wave radiation must travel to reach Earth, subsequently reducing or increasing the amount of radiation received at the Earth's surface in different seasons.

The variation in the shape of the orbit results from the gravitational attraction of the other plants in the solar system. 

Today, there is only about a three percent difference between the farthest point (aphelion) and the closest point (eperihelion) between the Earth and the Sun. This 3 percent difference in distance means that Earth experiences a 6 percent increase in received solar energy in the middle of Winter than in the Summer.

Thus, seasonal variations are less extreme now than compared to the last glacial maximum, and are currently near the minimum of the cycle. When the Earth's orbit is most elliptical, the amount of solar energy received varied between 20 to 30 percent more in the summer than in the winter. 

Obliquity

Second, we have the tilt, or obliquity, of the Earth’s axis. We experience seasons because Earth’s axis is not perpendicular to the plane of the planet’s orbit. Over time, the tilt angle varies between 22.5 and 24.5 over a 41,000-year cycle.  

An increase in obliquity results in less solar radiation in the northern hemisphere and more in the southern hemisphere. A lesser axial tilt leads to a more even distribution in solar radiation in the summer and winter resulting in less seasonal variation, but increases the disparity between higher and lower latitudes.

A smaller axial tilt promotes the growth of glacial ice sheets. This is because warmer winter temperatures lead to more snowfall, due to warmer air holding more moisture, and cooler summers discourage melting in the polar regions, so ice and snow build up strengthening the albedo, further encouraging cooling.

The Earth's axial tilt is current in the middle of its range. 

Precession

Thirdly we have precession of the equinoxes. This wobbling of the Earth’s axis and turning ellipse occurs over a 19,000 to 23,000-year cycle, changing 1° every 72 years and effects both poles equally.  

Precession can be visualised as a spinning top running down and starting to wobble back and forth on its axis. When the axis points toward Vega, the brightest star in the constellation of Lyra, the northern hemisphere will experience winter - when the Earth is furthest from the Sun - and summer - when the Sun is closest to the Earth, resulting in massive seasonal contrasts.

Currently, winter in the northern hemisphere occurs when the Earth is closest to the Sun resulting in less seasonal variation. 

Geological evidence 

Milankovitch cycles are identifiable in the geological record by examining sedimentological and isotopic records.

Cyclic sediments are sequences of sedimentary rocks, with repeating patterns of facies within a sequence. A clear example of cyclic sediments are the alternating layers of mudstone and limestone’s present at Lyme Regis on southern England’s Jurassic Coast. It is worth noting that the origin of this apparent cyclicity at Lyme Regis is still problematic.

Stable isotope records can be used to identify rhythmic patterns in Earth’s climate history. The isotope most commonly used to identify potential cyclicity is oxygen-18 from fossil calcite, ocean sediments and ice cores, which is used to track temperature changes. 

There is a clear relationship between the shape of Earth’s orbit and natural climate change. However, Earth’s current warming trend is happening during a relatively cool orbital phase.

The fact that current human activity, the prime cause for current climate changes as accepted by the vast majority of scientists, is strong enough to counteract a cool orbital phase goes to show the profound effect we are having on the climate. 

This Author

Jack Wilkin is a graduate research student at the Camborne School of Mines (University of Exeter) in the United Kingdom. His research focuses on the isotopic geochemistry of fossils from the Jurassic of Germany for paleoclimate studies.   

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