Frozen Earth: The Once and Future Story of Ice Ages (14 page)

Croll attacked the ice age problem in the same way he approached all of his work in science: through first seeking to understand the governing principles.
He described this method in the introduction to his book
Climate and Time,
which was published more than a decade after he first became involved in the ice age debate.
Writing about what he termed “The Fundamental Problem of Geology,” Croll notes:

We may describe, arrange, and classify the effects as we may, but without a knowledge of the laws of the agent we can have no rational unity. We have not got the higher conception by which they can be
comprehended.
It is this relationship between the effects and the laws of the agent, a knowledge of which really constitutes a science. We might examine, arrange, and describe for a thousand years the effects produced by heat, and still we should have no science of heat unless we had a knowledge of the laws of that agent. The effects would never be seen to be necessarily connected with anything known to us; we could not connect them with any rational principle from which they could be deduced
à priori [sic].
The same remarks hold, of course, equally true of all sciences, in which the things to be considered stand in the relationship of cause and effect. Geology is no exception.

Croll’s words could profitably be taken to heart by some scientists today.
They show quite clearly why he was so successful as a scientist: he was not content to tabulate observations, or even, as Agassiz did, to synthesize them into a theory, without first trying to understand the underlying principles.
In the case of glaciation and ice ages, he recognized that a major determinant of climate is the amount of heat energy the Earth receives from the sun—that had to be, in his terminology, one of the “agents” of climate.
He also knew that the heat received from the sun varies because of the constantly changing orbit the Earth follows in its travels around the sun, and he made those variations the centerpiece of his investigation.

Croll was not the first to attempt to relate climate to the effects of the Earth’s variable orbit, but he was the first to put the theory on a firm scientific footing.
The idea had actually been discussed on and off for almost a century before Croll entered the debate.
By the 1850s, a section on “Astronomical Causes of Fluctuations in Climate” was even included in Charles Lyell’s widely read
Principles of Geology,
generally considered to be the first true textbook of geology, then in its ninth edition.
In his book, Lyell summarized the evidence that had been accumulated up to that time, but he remained lukewarm about the possibility that astronomical influences were important.
However, he did suggest that someone should carry out the laborious calculations that would be necessary to fully investigate this possibility.

Prior to Croll’s work, a few scientists had looked into the mathematics of the Earth’s orbital variations, but those who had thought about the implications for climate had concluded that the changes
would not be of much consequence.
They had considered the fact that the Earth’s orbit around the sun is not circular, but is actually an ellipse, with the sun located at one of its two foci, slightly offset from its center (see figure 9).
This means that as the Earth makes its yearly journey around the sun, it is sometimes closer (and thus receives more solar energy) and sometimes farther away.
It was also known that the shape of the elliptical orbit changes over time, from being almost circular at some times to much more elliptical at others.
Such changes are formalized in the concept of eccentricity—the more elliptical the orbit, the greater the eccentricity.
Changes in eccentricity affect the amount of solar energy received by the Earth at various points in its orbit—when the orbit is almost circular, the energy received is almost constant throughout the year; when the orbit is more elliptical, the variation
between times of closest and farthest approach is much greater.
But calculations showed that when averaged over a year—one complete revolution of the Earth around the sun—the differences resulting from these variations are negligible.
Few believed they could be responsible for ice ages.
There was an exception, however.
In 1842, Joseph Adhémar, a French scientist, wrote a book titled
Révolutions de la mer,
in which he proposed that the combined effects of eccentricity and the tilt of the Earth’s axis of rotation play an important role in glaciation.

Figure 9.
Plan (above) and side (below) views of the Earth’s orbit around the sun.
The eccentricity of the orbit is much exaggerated, and the sizes of Earth and sun are not to scale.
As the eccentricity changes, the orbit becomes either more or less elliptical.
The tilt of the Earth’s axis of rotation is shown in the side view.
At present, the Northern Hemisphere is tilted toward the sun in June, when the Earth is distant from the sun.

Because the details of the Earth’s orbit are important for understanding the arguments made by Adhémar, Croll, and others about glaciation, it is worth pausing briefly to consider them more carefully.
Without perturbing influences, the orbit of the Earth around the sun would be unchanging.
But due to gravity, each planet has an effect on the orbits of the others.
And because the planets orbit the sun on different timescales, their relative positions, and therefore their influences on one another, are constantly changing.
Thus the shape of the Earth’s elliptical orbit—its eccentricity—is also continuously changing.
These changes are slow and regular, and they can be calculated.
Over long periods of time, the Earth cycles through the same orbital conditions over and over again.
With computers and accurate knowledge of the masses of the planets, it’s possible to plot out the Earth’s orbit, and those of the other planets, very accurately far into the past or the future.
These kinds of calculations are routine for NASA engineers—when they launch an exploratory spacecraft to another planet, they have to know precisely where the Earth is in relation to other bodies in the solar system and where the target planet will be several years later when the spacecraft reaches it.
The very first calculations of the planetary orbits, a truly monumental achievement in applying Newton’s gravitational theory to the solar system, were made by the French mathematician and astronomer Pierre-Simon Laplace in 1773.
By the time Croll tackled the problem, the elliptical shape of the Earth’s orbit was well known, and it was also known that the eccentricity gradually changes.
However, no one had yet done the systematic calculations to
determine exactly what these changes were over very long periods of time.

In addition to having an elliptical orbit around the sun, the Earth exhibits a peculiar feature—it rotates around an axis (an imaginary line drawn through the north and south poles) that is tilted relative to the plane of its orbit around the sun (this is also shown in figures 9 and 10).
The tilt today is 23 1/2°, but just why the axis is tilted is still a mystery.
Some scientists believe that it is residual from a gigantic collision early in the Earth’s history, when a small planet, about the size of Mars, crashed into the Earth and knocked it into its tilted position.
Regardless of its origin, however, we’re fortunate to live on a tilted planet—it’s the reason we have seasons.
You can work this out from the diagrams showing the Earth’s orbit, or by experimenting with a flashlight and a round object to represent the tilted Earth.
If the axis were perpendicular to the plane of the orbit around the sun, the length of day would be twelve hours everywhere, throughout the year.
With a tilted axis, it changes.

As the Earth makes its yearly journey around the sun, the direction of tilt remains constant, so that at one point along the orbit, the North Pole tilts directly toward the sun (the Northern Hemisphere summer solstice) and at another it tilts directly away (the winter solstice).
This simple picture is complicated, however, by the fact that the Earth not only rotates, it also wobbles, exactly like a spinning top (see figure 10).
The wobble, caused by the combined force of gravity from the moon and the sun, is very slow on a human timescale, so that we don’t notice it at all.
But over time, the orientation of the Earth’s axis of rotation changes, tracing out a circle that takes approximately twenty-six thousand years to complete.
Today, the north end of the rotation axis points toward the North Star.
Thousands of years from now, because of the Earth’s wobble, it will point at a different part of the heavens, and some other star will have to be identified as the “pole star.”

One consequence of the Earth’s wobble is the phenomenon called the precession of the equinoxes.
Because of the 26,000-year-long wobble
cycle, the points along the Earth’s orbit around the sun where the equinoxes occur—the fall and spring days when daylight and darkness have equal lengths—gradually change.
But because the shape of the elliptical orbit also changes over time, the precession of the equinoxes follows a slightly different timetable than the wobble itself.
One full cycle of this phenomenon is about 23,000 years, which means that every 23,000 years, the equinoxes occur at exactly the same point in the Earth’s orbit.
It’s easier to think about this—and of more importance for glaciation—in terms of the winter and summer solstices.
Today, the Northern Hemisphere has its longest day on June 21.
That’s the point along the Earth’s orbit when the North Pole is tilted most directly toward the sun.
But because of our wobbling axis, halfway through the precession cycle—11,500 years from now—when the Earth is at exactly the same point in its orbit, the tilt will be in the opposite direction (see figure 10).
If there is anyone around in the Northern Hemisphere then, it will be December 21, the shortest day of the year and the beginning of winter in 13500 A.D.
After another half cycle, 23,000 years from now, the tilt will be back to today’s orientation.

Figure 10.
The Earth’s axis of rotation wobbles relative to the plane of the Earth’s orbit around the sun, just as a spinning top wobbles.
The result is that for any particular point along the orbit, the direction of tilt gradually changes from year to year.
One complete cycle takes approximately 23,000 years, so that half a cycle from now (in 11,500 years) the tilt will be opposite that of today.

But to return to Adhémar.
He proposed that the wobble of the Earth’s axis of rotation, combined with the eccentricity of its orbit around the sun, would cause the Northern and Southern Hemispheres to be alternately glaciated.
His reasoning was that when the North Pole pointed away from the sun at the same time as the Earth was at its greatest orbital distance from the sun, the Northern Hemisphere would accumulate less heat and become covered with ice.
Halfway through the cycle of the Earth’s wobble, the same conditions would occur for the Southern Hemisphere.
Adhémar did his computations assuming no change in the present-day elliptical shape of the orbit; the effects he predicted were entirely due to the wobble.
He buttressed his arguments by pointing out that his theory predicted that there should be no current ice age for the Northern Hemisphere, because when the Earth is farthest away from the sun in its orbit, the North Pole points toward the sun, and it is Northern Hemisphere summer.
Adhémar suggested that
glaciation would occur again in the Northern Hemisphere only when the reverse is true, when the Earth is most distant from the sun during winter.
As it turned out, there were errors in his calculations of the amounts of heat that would be accumulated in each hemisphere.
But what really doomed Adhémar’s theory were the wildly imaginative consequences he predicted.
He claimed that at the cold pole, there would be a buildup of ice so massive that the Earth’s center of gravity would shift toward it, catastrophically attracting the waters of the ocean in a kind of huge tidal wave.
Every half cycle of the Earth’s precession—every 11,500 years—as the ice built up at the opposite pole, the center of gravity would change again, with similar results.
Adhémar envisioned a gigantic mushroomlike structure forming during the transition as the warming ocean water ate away at the base of the ice at the glaciated pole, leaving a huge cap supported on a thin column, which would eventually collapse, triggering the shift in the Earth’s center of gravity and tidal waves crammed with icebergs.
The land everywhere would be devastated.