information| February 27, 2020
by Alan Buies,
NASA Jet Propulsion Laboratory
Our lives do revolve around cycles: a series of events that repeat regularly 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 planting and harvesting, musical rhythms, or economic cycles.
Cycles also play an important role in Earth's short-term and long-term climate. A century ago, Serbian scientist Milutin Milankovitch hypothesized that the collective long-term effects of changes in the Earth's position relative to the Sun were 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 reaching the top of Earth's atmosphere, known as insolation, and where insolation lands. These periodic orbital movements, known as Milankovitch cycles, cause variations in the amount of sunlight of up to 25% in Earth's mid-latitudes (the region of Earth between about 30 and 60 degrees north and south of the equator).
Milankovitch cycles include:
- The shape of Earth's orbit, known aseccentricities;
- The angle at which the Earth's axis is tilted relative to the plane of Earth's orbit, calledinclination;electronic
- The direction in which the Earth's axis of rotation points is calledPrecession.
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 three days longer than autumn. As eccentricity decreases, our season lengths gradually flatten out.
The difference in distance between the Earth's closest point to the sun around January 3 (called perihelion) and its closest point to the sun around July 4 (called aphelion) is currently about 5.1 million kilometers (about 32,000 miles), a change of 3.4 percent. This means that in January, 6.8% more solar radiation reaches the Earth than in July.
When the Earth's orbit is more elliptical, 23% more solar radiation reaches the Earth closest to the sun each year than that farthest from the sun.Currently, the Earth's eccentricity is close to its point of minimum ellipse(more circular)And it is continuing to slowly decrease in cycles 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 relatively minor contributors to 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 few 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 will be, as each hemisphere receives more solar radiation in summer, when the hemisphere is tilted toward the sun, and in winter, when it tilts toward the sun Less solar radiation. Larger tilt angles favor melt periods (melting and retreat of glaciers and ice sheets). These effects were not uniform across the globe—total solar radiation varied more at high latitudes than near the equator.
The Earth's axis is currently tilted at 23.4 degrees, or about halfway between its extremes, and that angle is decreasing very slowly over a cycle spanning about 41,000 years.It reached its maximum dip about 10,700 years ago and will reach its minimum dip about 9,800 years later. As the obliquity decreases, it gradually helps to make our seasons milder, leading to warmer winters and colder summers, and snow and ice build up in high latitudes over time. Large sheets of ice. As the ice sheet grows, it reflects more of the sun's energy back into space, cooling it even further.
Precession– As the Earth spins, it wobbles slightly on its axis, like a toy 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 warmer 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, modern calendar systems are tied to seasons, for example, winter in the northern hemisphere never occurs in July. Today, the North Stars on Earth are Polaris and Polaris, but thousands of years ago they were Kochab and Pherkad.
besidesapex precession.Not only the Earth's axis wobbles, but the entire elliptical orbit of the Earth also wobbles irregularly, mainly due to its interaction with Jupiter and Saturn. The apical precession cycle spans about 112,000 years. Apical precession changes the direction of Earth's orbit relative to the ecliptic plane.
The combined effect of axial and apex precession results in a mean precession cycle lasting about 23,000 years.
climate time machine
Small changes triggered by Milankovitch cycles occur individually and together over long periods of time to affect Earth's climate, 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 that calculates differences in solar radiation and corresponding surface temperatures at different latitudes on Earth. The model is aclimate time machine: Can be run back and forth to check past and future weather conditions.
Milankovitch hypothesized that radiation changes at certain latitudes and during certain seasons were 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 and obliquity is still debated). Knowledge). study).
He calculated that ice ages occur approximately every 41,000 years. Subsequent research confirmed that they occurred every 41,000 years between 1 and 3 million years ago. But about 800,000 years ago, the cycle of ice ages increased to 100,000 years, matching Earth's eccentric cycle. Although several theories have been proposed to explain this shift, scientists still don't have a clear answer.
Milankovitch'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 global scientific community began to take his theories seriously. In 1976, a study by Hays et al. was published in the journal Science. Using seafloor sediment cores, they found that Milankovitch cycles corresponded to periods of major climate change over the past 450,000 years, with ice ages occurring when the Earth went through different phases of orbital change.
Several other projects and studies have also confirmed the validity of Milankovitch's work, including studies using ice core data from Greenland and Antarctica, which provide strong evidence for Milankovitch cycles dating back hundreds of thousands of years. evidence of. Additionally, their work has been adopted by the National Research Council of the National Academy of Sciences.
Scientific research aimed at better understanding the mechanisms responsible for changes in Earth's rotation and how Milankovitch cycles specifically affect climate is ongoing. But the theory that they determine the timing of glacial-interglacial cycles is widely accepted.