Milankovitch (orbital) cycles and their role in Earth's climate - climate change: vital signs of the planet (2023)

information| February 27, 2020

by Alan Buies,
NASA Jet Propulsion Laboratory

Our lives do revolve around loops: regular repetitions of a series of events in the same order. There are hundreds of different types of loops in our world and universe. Some are natural, like the changing seasons, the annual migration of animals, or the circadian rhythms that control our sleep patterns. Others are artificial, such as farming and harvesting, musical rhythms, or economic cycles.

Cycles also play a key role in Earth's short-term and long-term climate. A century ago, Serbian scientist Milutin Milankovitch hypothesized that the long-term collective effect of changes in the Earth's position relative to the Sun was a powerful driver of Earth's evolution.longclimate, and is responsible for triggering the beginning and end of ice ages (ice ages).

Specifically, he studied how changes in three types of Earth's orbital motion affect the amount of solar radiation (called insolation) reaching the top of Earth's atmosphere, and where the insolation reaches. These periodic orbital movements, known as Milankovitch cycles, cause variations in the amount of sunlight entering Earth's mid-latitudes (the region of our planet that lies between about 30 and 60 degrees north of Earth) and regions south of it by as much as 25 percent. equatorial). ).

Milankovitch cycles include:

  1. The shape of Earth's orbit, known aseccentricities;
  2. The angle at which the Earth's axis is tilted relative to the plane of Earth's orbit, calledinclination;yes
  3. The direction in which the Earth's axis of rotation points, known asPrecession.

Let's take a look at each(Further read Why Milankovitch Cycles Can't Explain Current Earth Warminghere).

Eccentricity-Earth's annual pilgrimage around the sun isn't perfectly circular, but it's pretty close. Over time, the gravitational pull of Jupiter and Saturn, the two largest gas giants in our solar system, caused the shape of Earth's orbit to change from nearly circular to slightly elliptical. Eccentricity measures how far the shape of Earth's orbit deviates from a perfect circle. These changes affect the distance between the Earth and the sun.

A quirk is the reason our seasons have slightly different lengths, currently summer in the northern hemisphere is about 4.5 days longer than winter, and spring is about 3 days longer than autumn. As eccentricity decreases, our season lengths gradually flatten out.

The distance difference between the position where the earth is closest to the sun (called perihelion) around January 3 and the position farthest from the sun (called aphelion) around July 4 every year is currently about 5.1 million kilometers (about 3.2 million miles), a difference of 3.4%. This means that 6.8% more incident solar radiation reaches Earth in January than in July.

When the Earth's orbit is at its most elliptical, about 23% more incident solar radiation reaches the Earth's point closest to the sun each year than its furthest point from the sun.Currently, the Earth's eccentricity is close to its point of minimum ellipse.(more circular)It is slowly declining in a cycle of about 100,000 years.

The total change in global annual insolation due to eccentric cycles is very small. Because changes in Earth's eccentricity are so small, they are a relatively minor factor in annual seasonal climate variability.

inclination– The angle at which the Earth's axis of rotation is tilted as it orbits the Sun is called the inclination. The tilt is why the Earth has seasons. Over the past million years, it has varied between 22.1 and 24.5 degrees relative to the plane of Earth's orbit. The greater the Earth's axial tilt, the more extreme our seasons are because each hemisphere receives more solar radiation during summer (when the hemisphere is tilted toward the sun) and in winter (when the hemisphere is tilted the other way) ) receive less solar radiation. Larger tilt angles favor periods of glaciation (the melting and retreat of glaciers and ice sheets). These effects are not uniform across the globe: Total solar radiation varies more at high latitudes than near the equator.

The Earth's axis is currently tilted at 23.4 degrees, or about halfway between its ends, and this angle is decreasing very slowly over a period spanning about 41,000 years.It was at its last maximum dip about 10,700 years ago and will reach its minimum dip in about 9,800 years. As the obliquity decreases, it gradually helps our seasons become milder, leading to warmer winters and colder summers, gradually allowing snow and ice to accumulate in the high latitudes over time cover. As the ice cover grows, it reflects more of the sun's energy back into space, promoting more cooling.

Precession– As the Earth spins, it wobbles slightly on its axis, like a spinning top slightly off center. This wobble is caused by tidal forces caused by the gravitational influence of the Sun and Moon causing the Earth to bulge at the equator, affecting its rotation. This tendency to wobble in direction relative to the fixed position of the star is known asaxial precession. The axial precession period spans about 25,771.5 years.

Axial precession makes seasonal contrast more extreme in one hemisphere and less extreme in the other. Currently, perihelion occurs during winter in the northern hemisphere and summer in the southern hemisphere. This makes summers hotter in the Southern Hemisphere and moderates seasonal variations in the Northern Hemisphere. But over about 13,000 years, axial precession will cause those conditions to change, with more extreme solar radiation in the northern hemisphere and milder seasonal variations in the southern hemisphere.

Precession affects the timing of the seasons relative to the Earth's closest/farthest point around the Sun. However, the modern calendar system is tied to the seasons, so that, for example, winter in the northern hemisphere never occurs in July. Earth's current pole stars are Polaris and Polaris, but thousands of years ago they were Kochab and Pherkad.

besidesapex precession. Not only does the Earth's axis wobble, but Earth's entire elliptical orbit wobbles irregularly, mainly due to its interactions with Jupiter and Saturn. The apical precession cycle spans about 112,000 years. Aptic precession changes the orientation of Earth's orbit relative to the plane of the ellipse.

The combined effects of axial and apex precession cause the entire precession cycle to span on average about 23,000 years.

weather machine

Small changes triggered by Milankovitch cycles act individually or together to affect Earth's climate over long periods of time, leading to larger changes in our climate over tens to hundreds of thousands of years. Milankovitch combined these cycles to create a comprehensive mathematical model to calculate the difference in solar radiation at different latitudes on Earth and the corresponding surface temperature. The model is something likeweather machine: Can run forward and backward to check past and future weather conditions.

Milankovitch hypothesized that radiative changes at certain latitudes and during certain seasons are more important for the growth and retreat of ice sheets than others. In addition, he argues that obliquity is the most important of the three climatic cycles, as it affects the amount of sunlight in the Earth's northern high latitudes in summer (the relative role of precession versus obliquity is still a matter of scientific study). study).

He calculated that ice ages occur approximately every 41,000 years. Later research confirmed that they occurred at a 41,000-year interval between 1 and 3 million years ago. But about 800,000 years ago, the Ice Age cycle extended to 100,000 years, coinciding with Earth's eccentric cycle. While various theories have been proposed to explain this transition, scientists still don't have a definitive answer.

Milankovich's work was supported by other researchers of his time, and he authored numerous publications on his hypothesis. But it wasn't until about a decade after his death in 1958 that the world's scientific community began to pay attention to his theories. In 1976, a study by Hays et al. was published in the journal Science. Using deep-sea sediment cores, he found that Milankovitch cycles corresponded to periods of major climate change over the past 450,000 years, when the Earth went through different phases of orbital changes, called ice ages.

Several other projects and studies have confirmed the validity of Milankovitch's work, including studies using ice core data from Greenland and Antarctica, which provided strong evidence that the Milankovitch cycle dates back hundreds of thousands of years. Additionally, his work has been accepted by the National Research Council of the National Academy of Sciences.

Scientific research is underway to better understand the mechanisms that cause changes in Earth's rotation, and how Milankovitch cycles specifically combine to affect climate. But the theory that they drive cycles of glacial and interglacial periods is widely accepted.

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