Frozen Earth: The Once and Future Story of Ice Ages (22 page)

With the inclusion of snow reflectivity in his calculations, Milankovitch had completed work on all aspects of the problem that he thought were important.
But he had published the results of his investigations in bits and pieces, in different languages and at different times.
His most famous graph was known by most people because of its inclusion in Köppen and Wegener’s book.
He was now corresponding with scientists from around the world, many of whom were working on the ice age problem, and most of them requested copies of his papers.
There were no Xerox machines or electronic versions of his work, and he was quickly running out of the few copies his publishers had given him.
He resolved to put all of his investigations of climate together in a book that would encompass everything, from the details of the equations necessary to calculate the motions of the planets around the sun to a discussion of ice ages.
He called this massive work
Canon of Insolation and the Ice Age Problem.
It was written in German and published in 1941 (an English translation did not appear until 1969).
Like Harlan Bretz, Milankovitch was very organized and systematic; his book begins with the classical laws of mechanics and then proceeds, step by step, through all of the calculations necessary to address the problem of climate variations.

Canon of Insolation
was written during the early part of World War II, and it took Milankovitch, he says, 539 days (ever the mathematician, Milankovitch is precise; he doesn’t say “about a year and a half,” but gives us the time to the day!).
The last pages were printed in Belgrade in April 1941; a few days later, Germany invaded Yugoslavia.
Belgrade was bombed and the printing shop where his book was being produced was reduced to rubble.
Fortunately, however, most of the printed pages
survived intact.
Even under occupation, some things continued to work in Belgrade—Milankovitch’s rescued manuscript was bound and distributed, a brief happy occurrence in an otherwise unhappy time.
His memoirs make it clear that his experience of life under German occupation was not pleasant.
“Our civilized existence,” he wrote, “had disintegrated into a life of hard grind.”

Figure 15.
Milutin Milankovitch working at his desk in 1954.
Photograph courtesy of his son, Vasko Milankovitch.

Milankovitch was sixty-two and isolated from the world of science because of the war.
The university was closed; his friend and colleague Köppen had died almost two years before, Wegener a decade earlier.
His work on climate had been completed with the publication of
Cannon of Insolation,
and his scientific career was effectively over.
Living conditions were grim, and the wait for the war’s end seemed interminable.
Not content to be idle, Milankovitch decided to write a history of science—an activity, he said later, that kept him sane.
It was published in Serbian at the end of the war.
After the Germans had left, life in Yugoslavia under the communists was only marginally better.
But Milankovitch had an international scientific reputation, and he was for the most part left in peace.
He worked on his memoirs and some manuals for university courses (figure 15), and he was able to travel abroad occasionally.
Late in 1958, at the age of seventy-nine, he died of complications from a stroke.

By then, the astronomical theory of glaciation was not in good health, either.
This idea, which had seemed so promising when James Croll first proposed it, was, for the second time, losing influence among scientists who had initially embraced it.
Milankovitch’s work had revived the theory at a time when it had been all but forgotten, and his comprehensive calculations put it on a much firmer footing than Croll had been able to do.
But once again issues of timing began to cast doubt on the theory.
First, the gravel terraces of the Alps that had been thought to record glacial periods turned out to contain fossils incompatible with a cold climate.
Later, it was discovered that the terraces had not been formed by glaciation, but had another origin altogether.
The glacial timescale devised by Penck and Brückner, which had been one of the most important pieces of confirming evidence for Milankovitch’s theory, was completely meaningless.
As if that were not enough, there was a second blow to the theory when a new dating method showed that glacial deposits in North America did not correspond well in age to predictions of the theory.

The new technique was carbon-14 dating, invented in the late 1940s by Willard Libby and his students at the University of Chicago.
It was an elegant method, making use of the fact that radioactive carbon-14 is continually produced in the Earth’s atmosphere by cosmic rays, but then begins to decay away when it is incorporated into organic material.
It had potential applications in a wide range of subjects, and Libby was later awarded the Nobel Prize in chemistry for his work.
For the geologists working on ice ages, the new technique was a godsend.
Here, finally, was a method that could provide “absolute” dates for the deposits of the glacial periods.
Unlike other methods commonly used for measuring ages in geology, the carbon-14 technique cannot be
used to date rocks; it works only for organic material that was once alive and exchanging carbon with the atmosphere.
This meant that a fossil in glacial drift, or a piece of wood preserved in a peat bog were perfect samples for dating.
There was one problem, however: radioactive carbon-14 has a half-life that, in geological terms, is quite short.
Especially in the early days of its application, it could only be used to date materials that were not more than a few tens of thousands of years old.
In older deposits, so much of the radioactive carbon-14 had decayed away that the few remaining atoms could not be detected.

Soon after Libby had demonstrated the feasibility of the carbon-14 method, others began to set up the necessary equipment, and before long, especially in the United States, there were a number of laboratories capable of making the analyses.
Geologists studying ice ages had more than enough samples to keep them busy.
Very quickly they established a timescale for the movement of the North American ice sheet by dating the moraines and drift that marked the margins of the ice at different times in its history.
Even though they could not extend the analyses very far into the past, the carbon-14 data presented a far more complex story of glacial advances and retreats than Milankovitch’s graph, which showed single cold-summer spikes at 25,000 and 72,000 years ago.
History was repeating itself.
Just as Croll’s attempt to prove an astronomical cause for ice ages had foundered because the timing didn’t seem to be right, so too the carbon-14 analyses in North America, especially in light of the complete abandonment of the Penck and Brückner timescale for European glaciation, seemed to sound the death knell for Milankovitch’s revised and updated version of the theory.
Furthermore, some meteorologists had looked again at the solar radiation balances calculated by Milankovitch and declared that the variations were just too small to produce drastic changes in climate, even when the effects of increased snow cover were incorporated.
The pendulum was again swinging away from the astronomical theory.
But the story has yet one more twist, one that brought Milankovitch’s calculations back to center stage.

The evidence that revived the Croll-Milankovitch theory was discovered more than a decade after Milankovitch’s death, and it came from the depths of the ocean.
Many geologists had recognized that sediments on the sea floor might provide a long-term record of climate and other environmental conditions on the Earth’s surface.
Unlike moraines or loess or other glacial features on land, ocean sediments were presumed to accumulate slowly and continuously, century by century and millennium by millennium, without disturbance.
If it were possible to core into these sediments, it might be possible to retrieve a record of events far into the past.
Indeed, James Croll had presciently suggested that ocean sediments might provide the best clues about glacial cycles.

By the 1960s and 1970s, the technology for sampling the sea floor had improved to the point where it was possible to retrieve long cores of sediment reaching back millions of years into the past.
Geochemists studying such cores noted that there are regular variations in the chemical composition of the sediments, and paleontologists examining fossils reported that there are similar alternations in the abundance of species that lived in warm and cold conditions.
Oceanographers began to link these changes to glacial-interglacial cycles.
But there was still the problem of assigning a timescale—accurate ages were needed to compare the dates of the sediments with the ages of glacial deposits on land and the timing of astronomical variations.
Carbon-14 dating turned out to be more complicated for ocean sediments than for land deposits, and in addition there was the problem that it is limited to the past few tens of thousands of years.
To probe farther into the past and investigate the cyclic changes that characterized the deep-sea sediment cores would require a new approach.

The breakthrough that came to the rescue occurred in another, unrelated, area of earth science.
Geophysicists studying the Earth’s magnetic field discovered that it has reversed periodically in the past—the north and south magnetic poles have switched positions.
When rocks form on the Earth’s surface—when lava erupts from a Hawaiian volcano, for example—the magnetic minerals they contain line up their own small
magnetic fields in the same direction as the Earth’s.
By dating such rocks and measuring their magnetic orientation, a record has been built up of how the Earth’s magnetic field has varied in the past.
Particularly important is the timing of the magnetic reversals, the geologically short intervals when the field switches from “normal” (today’s situation) to reversed, because these serve as markers or time lines that can then be used to date other events.
Through the work of many different laboratories, the reversals have been dated quite accurately.

Ocean sediments, like the igneous rocks of Hawaii, contain magnetic minerals.
As they slowly settle to the sea floor, these minerals too line up with the Earth’s field.
The long cores from the oceans thus contain a continuous record of changes in the magnetic field, a built-in timescale for dating variations in sediment properties that might be related to glacial climate cycles.
Many of these properties seemed to vary in a regular way, but the question was, Which would be most useful for understanding ice age climate?
The answer was not immediately obvious, but one property in particular turned out to be crucial for confirming, once and for all, the link between climate and variations in the Earth’s orbit.
It was the oxygen isotopic composition of fossil shells in the sediments.
What in the world, you may wonder, do oxygen isotopes have to do with glaciation or the astronomical theory of climate?
The connection was made by Harold Urey, a chemist who, like Willard Libby, worked at the University of Chicago, and who, again like Libby, was awarded the Nobel Prize, his for the discovery of deuterium, one of the isotopes of hydrogen.
Urey was especially interested in how different isotopes of the same element behave when they take part in chemical reactions or processes such as evaporation and precipitation.
Most of the elements in the periodic table have multiple isotopes; oxygen, for example, has three.
All three have the same chemical properties—they are all oxygen—but they exhibit minute differences because of their different masses.
In a tank of oxygen gas all of the molecules have the same energy.
They whiz around, bumping into the walls of the tank and each other, but those containing oxygen-16
travel slightly faster than those containing oxygen-18, because oxygen-16 is lighter.

From theoretical considerations, Urey discovered that during chemical reactions, the oxygen isotopes are fractionated from one another because of the slight differences in their masses.
One isotope is preferred over another in the products of the reaction.
Furthermore, he found that the amount of fractionation depends on the temperature.
In a flash of insight, he realized that oxygen isotopes could act as a natural thermometer.
Many organisms that live in the oceans make their shells of calcium carbonate, an oxygen-containing compound.
The oxygen comes from seawater, and when the shells are being precipitated, one oxygen isotope is preferred over another.
Thus the shells end up with different proportions of the three oxygen isotopes compared to seawater, and the amount of that difference depends on the water temperature.
By measuring the ratio of oxygen-16 to oxygen-18 in an ancient shell, Urey realized, he could determine the temperature of seawater in the distant past—a stunning concept.

Like most such ideas, bringing this possibility to fruition took some time.
Urey and his students had to perfect the measurement techniques so that they could measure oxygen isotope proportions accurately in small amounts of calcium carbonate shell.
On the basis of their calculations, the variations would probably only amount to a few tenths of 1 percent.
They also faced the perennial problem of geochemists: Which samples would be most representative and provide the most important information?
Their method, too, they soon learned, had its own complications.
For example, they realized that both evaporation and precipitation would change the proportion of the various oxygen isotopes in seawater.
How could they distinguish these variations from those caused by temperature changes?