The German meteorologist Wladimir Köppen was one of the scientists who took note of Milankovitch's theoretical approach to climate. Köppen's daughter was married to geophysicist Alfred Wegener. Wegener is the scientist who first put forth a serious theory of continental drift, which we investigated in Chapter . Milankovitch began a collaboration with Köppen and Wegener that was to last their lifetimes. When Köppen and Wegener published Die Klimate die Geologischen Vorzeit (Climates of the Geological Past), in 1924, it helped to bolster Milankovitch's work.
Among the key contributions of Milankovitch's work were the ideas that different latitudes experienced the impact of orbital variation differently, and that the key to the onset of glaciation is cool summer weather, not colder winters as had previously been assumed. Understanding that there were a number of factors involved in regulating climate, Milankovitch didn't try to give absolute temperature values. Instead, he used “equivalent latitude.” If a location's equivalent latitude decreased, effectively moving the climate south, temperatures were warmer. Similarly, an increase in equivalent latitude meant temperatures were colder.
Once again, Europe was preparing for war when Milankovitch decided to publish his definitive book on climate, Cannon of Insolation and the Ice Age Problem, printed in German in 1941. For a time, this resurgent theory of climate change found wide acceptance, but it was not to last.
New dating techniques and geological data began to uncover discrepancies in Milankovitch's predictions. Around the time of his death, in 1957, Milankovitch's theory was out of favor for much the same reasons as Croll's. History was repeating itself.
Milankovitch had managed to out-live the Nazis and the second World War, only to find himself trapped in the dismal reality of Communist Yugoslavia. Fortunately, his international reputation as a scientist garnered him some respect from the new regime, allowing him to live out his days in relative peace. But the story doesn't end here.
In the 1960s, several advances were made in geophysics that improved the collection of data about Earth's past climate. Analysis of radioactive isotopes of hydrogen and oxygen, along with the ability of scientists to collect sediment cores from deep ocean beds, greatly improved knowledge of ice age climate changes. In 1970, Wally Broecker and J. van Donk published a paper that detailed temperature changes going back 400,000 years.
In this paper, a number of the apparent discrepancies in Milankovitch's theory were resolved. Though he didn't live to see his theory vindicated, Milankovitch's astronomical theory of climate change is now recognized as the best explanation of the cycles of glacial-interglacial change. In his honor, these periodic changes in Earth's orbital orientation are called the Milankovitch Cycles.
The Croll-Milankovitch Cycles
Though scientists had long considered the variation in insolation warming from the Sun too weak to cause the waxing and waning of ice ages, the cycles found by Croll, and expanded on by Milankovitch, fit the climate data so well some form of link had to exist. Unlike atmospheric carbon dioxide, where several Earth-bound explanations exist for changing CO2 levels, there is no way that terrestrial forces can cause the changes in Earth's orbit.
Illustration 81: Variation in Axial Obliquity, Orbital Eccentricity, and Polar Precession. Images from NOAA.
Today, we know variations in the intensity and timing of heat from the Sun are the most likely cause of glacial/interglacial cycles. This solar variability is partially driven by changes in the Sun's output, but is affected more strongly by variations in Earth's orbit.
There are three major components of Earth's orbit about the Sun that contribute to changes in our climate. These are, in order of longest to shortest cycle, Orbital Eccentricity, Axial Obliquity, and Precession of the Equinoxes. These three variations are shown in Illustration 81.
Earth's orbit goes from measurably elliptical to nearly circular in a cycle that takes around 100,000 years. Presently, Earth is in a period of low eccentricity, about 3%. This causes a seasonal change in solar energy of 7%. The difference between summer and winter is a 7% difference in the energy a hemisphere receives from the Sun.
When Earth's orbital eccentricity is at its peak (~9%), seasonal variation reaches 20-30%. Additionally, a more eccentric orbit will change the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes.
The variation in eccentricity doesn't change regularly over time, like a sine wave. This is because Earth's orbit is affected by the gravitational attraction of the other planets in the solar system. There are two major cycles; one every 100,000 years and a weaker one every 413,000 years.262
Illustration 82: Precession of Earth's axis of rotation.
The second Milankovitch cycle involves changes in obliquity, or tilt, of Earth's axis. Presently Earth's tilt is 23.5°, but the 41,000 year cycle varies from 22.1° to 24.5°. This tilt is depicted in the upper-left panel of Illustration 81. The smaller the tilt, the less seasonal variation there is between summer and winter at middle and high latitudes.
For small tilt angles, the winters tend to be milder and the summers cooler. Cool summer temperatures are thought more important than cold winters, for the growth of continental ice sheets. This implies that smaller tilt angles lead to more glaciation.
The third cycle is due to precession of the spin axis. As a result of a wobble in Earth's spin, the orientation of Earth in relation to its orbital position changes. This occurs because Earth, as it spins, bulges slightly at its equator. The equator is not in the same plane as the orbits of Earth and other objects in the solar system. This is shown in Illustration 82.
The gravitational attraction of the Sun and the Moon on Earth's equatorial bulge tries to pull Earth's spin axis into perpendicular alignment with Earth's orbital plane. Earth's rotation is counter-clockwise; gravitational forces make Earth's spin axis move clockwise in a circle around its orbit axis. This phenomenon is called precession of the equinoxes because, over time, this backward rotation causes the seasons to shift.
The full cycle of equinox precession takes 25,800 years to complete. Presently, Earth is closest to the Sun in January and farther away in July. Due to precession, the reverse will be true 12,900 years from now. The Northern Hemisphere will experience summer in December and winter in June. The North Star will no longer be Polaris because the axis of Earth's rotation will be pointing at the star Vega instead (see Illustration 83).
A consequence related to this phenomenon is that the Moon is slowly becoming more distant from Earth. The Moon is departing from us at the rate of 1.5 inches (3.8 cm) each year. The idea of the Moon retreating proposed over a century ago by English mathematician and geophysicist George Howard Darwin, Charles Darwin’s son, has been confirmed by measuring the distance to the Moon with lasers.263 ,264
Illustration 83: Path of the north celestial pole among the stars due to precession. Original image by Tau'olunga.
The tidal drag caused by the Moon's gravity slows Earth's rotation and accelerates the Moon. One of the counter intuitive things about physics is that, if you speed up an object in orbit, it takes longer to complete a full orbital revolution. In orbit, you slow down if you speed up, at least when viewed from Earth's surface. This means that both days and months are getting longer.
This mechanism has been working for 4.5 billion years, since Earth and Moon first formed. There is evidence in the geologic record that Earth rotated faster and that the Moon was closer to Earth in the distant past. We know from silt deposits that, 620 million years ago, a day was 21.9 hours long, there were 13 months/year and 400 solar days/year.265
Tidal forces are also why the Moon always keeps the same side facing Earth. One day, Earth will reach a similar state—a day will be as long as a month. A month will be longer than it is today because the Moon will be farther away, and Earth will show only one face to the Moon. But this shouldn't be a major concern: There is good reason to believe that the Sun will expire, taking Earth and its Moon with it, long befo
re this happens.
Cycles Summarized
Individually, each of the three cycles affect insolation patterns. When taken together, they can partially cancel or reinforce each other in complicated ways. Illustration 84 shows how the three cycles combine to affect solar forcing over the past 200,000 years. It is the complex pattern of insolation change created by the interaction of all three factors that caused so much confusion verifying Croll and Milankovitch's predictions.
Adhémar based his predictions on the mathematics of d'Alembert, who calculated precession in 1754. Alexander von Humboldt discredited this theory by pointing out that, though the seasonal insolation varied, the total energy received remained constant. Precession alone was not enough.
Illustration 84: Relationship of cycles with insolation. Source NOAA.
Croll used both eccentricity and precession, as well as the effects of glaciation and changing air currents. In 1875, he added tilt to his calculations. Even then, his interpretation of the factors was not totally correct. Not until Milankovitch produced integrated curves combining all the orbital elements were the cycles on firm footing. But the apparent disagreement with experimental data still caused the theory to fall into disrepute.
Only after work by Erickson, Broecker, et al,266 in the 1950s, was the theory revisited. In 1978, Berger corrected the theory with more accurate formulae for calculating insolation variations.267 Today, we use numerical integration to calculate the effects of all the bodies of the Solar System. This is difficult, requiring minimization of the error in present observational data and running long calculations on supercomputers.268 Croll and Milankovitch had to do their calculations by hand. Science is often a long and tortuous process.
Today, it is widely accepted that Milankovitch cycles are the forcing that decides the timing of glacial/interglacial periods. Data from the glaciation record are in strong agreement with this theory. In particular, during the last 800,000 years, the dominant period of glacial-interglacial oscillation has been 100,000 years, which corresponds to changes in Earth's eccentricity and orbital inclination.
Glacial periods can be triggered when tilt is small, eccentricity is large, and perihelion, when Earth is closest to Sun, occurs during the Northern Hemisphere's winter. Perihelion during the Northern Hemisphere winter results in milder winters but cooler summers, conditions that keep snow from melting over the summer.
Deglaciation is triggered when perihelion occurs in Northern Hemisphere summer and Earth's tilt is near its maximum. There are other factors which act to enhance the forcing effects of the cycles. These include various feedback mechanisms such as snow and ice increasing Earth's albedo, changes in ocean circulation and enhanced greenhouse heating due to increased CO2 and water vapor concentrations. Earth's current place in the three cycles are as follows:
Eccentricity. Earth's current orbital eccentricity is 0.0167, which is relatively circular. Presently, Earth's distance from the Sun at perihelion, on January 3rd, is 95 million miles (153 million km). Earth's distance from the Sun at aphelion, on July 4th, is 98 million miles (158 million km). This difference between the aphelion and perihelion causes Earth to receive 7% more solar radiation in January than in July. Currently, Earth's orbital eccentricity is close to the minimum of its cycle.
Obliquity. Currently, axial tilt is approximately 23.45 degrees, reduced from 24.50 degrees just a thousand years ago. Even so, Earth's current tilt is almost at its maximum. This explains the contrast in Earth's seasons. A lower degree tilt would result in cooler summers and warmer winters, thus, moderating global temperatures. Some argue that this would cause growth in ice sheets in the high latitudes. Snow would accumulate over the winter and would be less prone to melting and recession during the summer.
Precession. The variation in the direction of Earth's axial tilt is thought to be the most important influence of climate. Today, Earth is closest to the Sun during Northern Hemisphere winter and farthest away during Northern Hemisphere summer.
So currently, Earth's orbit meets only one of the three conditions which lead to the onset of glaciation, Perihelion during Northern Hemisphere winter. As stated above, glacial maximums have occurred roughly every 100,000 years for the past 800,000 years with the last glacial maximum occurring 18,000 years ago. After each glacial period there has been a period of rapid warming. These warming periods are followed by a relatively slow cooling trend leading to the next glacial maximum. Today, we are poised to enter another glacial period but, there are still uncertainties.
The European Project for Ice Coring in Antarctica (EPICA) team has noticed the interglacial period of 400,000 years ago closely matches our own because the shape of Earth's orbit was the same then as it is now. That warm spell lasted 28,000 years so we might not be as close to the next glacial episode as often thought.
Limits of Orbital Forcing
To sum up, scientists believe the Croll-Milankovitch cycles caused the onset of the Holocene interglacial period in the following way. At the beginning of the Holocene, around 15,000 years ago, variation in Earth's orbit and attitude caused a small increase in the solar radiation received from the Sun. Those changes also resulted in a redistribution of solar energy within Earth's atmosphere and ocean, which caused a slight warming, ending the glacial period. Retreating glaciers, melting snow cover, and diminished sea ice exposed larger areas of land and open ocean. The exposed areas absorbed more solar radiation, reinforcing the warming trend. This accelerated warming of the ocean, releasing of large amounts of CO2 and further reinforcing the warming trend. Temperatures increased to modern levels and have stayed there since—though, the climate continues to undergo variations on century and decade long time scales. How long the warm period will last cannot be predicted.
Today, scientists believe that the principal cause of glaciations is the intensification of the hydrologic cycle caused by Earth's orbital cycles. Variation in insolation patterns cause tropical oceans to warm, increasing the equator-to-pole temperature gradient. This leads to the growth of land-based ice in high latitudes. In other words, increased heating of the oceans is needed to start a glacial period.
This argument was first made by John Tyndall, the Irish physicist, naturalist and educator, in the late 1800s.269 Greater solar radiation in winter and spring at the expense of summer and autumn, leads to higher frequency of El Niño anomalies. From studying the start of the previous glacial period, similarities can be seen in current orbital changes.270
Although the current variations are less extreme, researchers have concluded, “association of recent positive seasonal anomalies of global mean temperature with El Niño events suggests that the ongoing global warming may have a significant, orbitally influenced natural component. The warming could continue even without an increase of greenhouse gases.”271
Even if there is a connection to current global warming, the Croll-Milankovitch cycles do not explain short term, decadal variations, or the longer term changes that signal the beginning and end of ice ages. We need to look for other influences—in particular, the Sun.
Varying Solar Radiation
“The sun is an example. What it seems it is and, in such seeming all things are”
— Wallace Stevens
In our modern, technology-driven world, it is easy to take the Sun's light for granted. With electric power and indoor lighting, our civilization is no longer a slave to daylight. We can work and play around the clock, with illumination available at the flick of a switch. But, for most of human history this was not the case.
In earlier times, candles and oil lamps provided a poor substitute for the natural light of the Sun. And, in ancient times, there was often no substitute for sunlight available, aside from the flickering of a fire. Weak stuff compared with the golden radiance of our local star. It is no wonder that people around the world revered and worshiped the Sun as a deity.
To the ancient Greeks, the Sun was the chariot of the god Helios, driven across the heavens by four horses. For the anci
ent peoples of Meso-America, the Sun was a god, and they carefully observed and recorded the changing path that the Sun traveled across the sky throughout the year. The Inca, Maya and Aztecs created detailed calendars and astronomical tables going back thousands of years.
According to astronomer and stellar physicist David Dearborn, there is reason to believe that some Meso-American cultures recognized sunspots. There is no clear evidence that the Inca or the Maya noticed sunspots, but the Aztec myth of creation involves a Sun god with a pock-marked face. This strongly suggests that they had seen dark blemishes on the face of the Sun.272
As early as 28 B.C., astronomers in ancient China recorded systematic observations of what looked like small, changing, dark patches on the surface of the Sun. These observations were recorded in the Hanshu, or Book of Han, an historical text chronicling the history of the early Han dynasty.
Illustration 85: Galileo Galilei, by Giusto Sustermans.
In western cultures, the visage of the Sun tended to be viewed as perfect and without blemish. There are some early references to sunspots in the writings of Greek philosophers, but Aristotle held that the Sun and the heavens were ideal geometric objects. The ancient Greeks, and the Europeans after them, were highly influenced by Aristotle. Even when faced with contradictory observations, it was hard to supplant his teachings. Not until Galileo pointed his telescope at the Sun did the opinions of Aristotle begin to lose their grip on the western imagination.
Galileo Galilei (1564-1642) was an Italian physicist, mathematician, and philosopher, and probably the most famous astronomer of all time. Often called the “Father of Astronomy,” Galileo was an important figure in the development of modern science.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 17
