The Universe Within (26 page)

Driving this exploration of the atom was the development of new devices that could measure particles in parts per billion. With this resolution, new kinds of answers to old questions were now possible.

Libby set up two junior scientists in his lab with five thousand dollars to carry out a research program on
carbon. Like most atoms, carbon exists in a number of different forms in the natural world. All carbon atoms have the same number of protons inside their nuclei; the different
versions are distinguished from each other by the number of neutrons inside. Libby’s insight was that all living things will have the same amount of
carbon 14 in their bodies as the atmosphere in which they live. Living creatures breathe, eat, and drink carbon atoms in their daily lives and thus share the same balance of carbon with the atmosphere. Once organisms die, this balance with the atmosphere is disrupted, and no new carbon enters as food or nutrients. Whatever carbon atoms remain in the body begin to
decay into other forms. As we’ve seen with other atoms, this reaction happens at a constant rate set by the laws of physics and chemistry. Knowing this, Libby ventured that if you can measure the amount of carbon 14 in a sample of old bones, you can, with some assumptions, calculate how long ago the animal died. This was a huge
advance: it was like finding clocks inside ancient bones, teeth, shells, and wood.

To
Harold Urey, who worked in a lab just steps away, atoms were imagined to be clues to the history of the planet, solar system, and universe. One of his main objects of fascination was an atom familiar to us all—
oxygen. An abundant player in our air,
water, and skeletons, oxygen has some distinctive properties that make this infinitesimal atom a window into our past and a much larger world.

Urey knew that oxygen, like carbon, exists as heavy atoms with extra
neutrons and light atoms with fewer. On purely theoretical grounds, he guessed that the balance of these forms in any substance depends on temperature. The timing of his guess could not have been better, because accurate machinery could test his ideas.

And it worked: the ratio of heavy and light oxygen atoms in a material was dependent on temperature. To Urey and his team, this success meant that if you could measure the infinitesimal amounts of the different forms of oxygen in any substance—water or bone, for example—you might be able to guess the temperature of the environment in which it formed. The trick was to find the right kind of record that could reveal the details of Earth’s climate with precision. Only then could the tool kit derived from the work of Libby, Urey, and their colleagues pull together cause and effect.

Seashells are durable and hard because they contain a crystal,
calcium carbonate. This molecule, so vital to their hardness, also fortunately contains oxygen. Urey and others saw that as seashells develop during the life of the animal, the molecules that make the shell are ultimately derived from the water in which they lived. The relative amounts of the different forms of oxygen in the shell could, then, reflect the temperature of the waters that the creatures grew in. And since shells preserve well, they could contain an excellent record of ancient events.

With oxygen atoms as the thermometer, carbon atoms as the timekeeper, and the regularity of the layers as a guide, the teams set off to see how climate changed over the ice ages. One group looked at the most continuous record of seashells they could find, to map the temperature changes over time. The bottom of the sea is ideal: it contains layer after layer of sediment that drifts down the water column. By looking at the oxygen composition of the seashells inside these layers, the researchers could get an approximation of how climate changed over time. The team found that the planet’s temperatures waxed and waned with peaks of high temperature and valleys of low temperature. What’s more, the temperature seemed not to change randomly
over time: if you squinted really hard at the graphs they made, you could see that the peaks and valleys seemed to rise and fall every 100,000 years. This was not some random number but one of those proposed by
Milutin
Milankovitch years before. One-hundred-thousand-year pulses started cropping up in other people’s data as well. Maybe astronomical events were influencing things after all?

The problem was that the data were messy; the plots of temperature versus time have lots of wiggles, not just the 100,000-year one. Then three scientists, one British and two American, took a new look and applied a method developed by one of
Napoléon’s regional governors after his conquest of Egypt. The bureaucrat, bored on the job, set off to understand
heat and its transfer among different materials. It wasn’t heat that was to help geologists over a century later; it was a new mathematical approach he devised. If you have a graph with lots of different wiggles in it, perhaps that mess is made by several different rhythms superimposed on one another. The mathematical technique, known as
Fourier transform analysis, is a way of revealing how a complex pattern can be made by a number of regular and more simple ones.

With that simple analytic tool, the data revealed not chaos but a deeply buried signal. The pattern emerges from a number of rhythms superimposed on one another: 100,000-year cycles onto cycles of 40,000 and 19,000 years. Milankovitch and
Croll were right: ice ages are correlated in a broad way to the changing orbit, tilt, and gyration of Earth.

Graphs of climate, with peaks and valleys reflecting the rise and fall of temperature over the millions of years of
geological time, look something like an EKG of a human heart. The heartbeat of our planet has drummed on for countless eons, beating to rhythms in Earth’s orbit and the workings of air and water. Before the global
cooling 45 million years ago that so fascinated scientists such as Maureen
Raymo, these orbital changes did not
often lead to
ice ages. With a newly cool Earth, orbital wiggles became written in the waxing and waning of sheets of polar ice. And it is the ice itself that reveals the biggest surprises.

In 1964, during the heyday of
Camp Century, a Danish geologist,
Willi Dansgaard, visited the major air base in the region,
Thule Air Base—the supply station for the camp—to look at local snow. Dansgaard spent some time in Chicago, even working in Urey’s lab. Students then remember his fondness for the cold, leaving windows open during the long Chicago winters.

While on base, he heard buzz of the military
project going on a hundred miles to the east. Asking permission to visit Camp Century, he was rejected on the grounds that it was a top secret operation. With some luck, in the form of a visionary senior administrator in the U.S. Army’s
Cold Regions Research and Engineering Laboratory, he was given access to the pristine cores of ice that the air force dug up to make the city under the
glacier. Perhaps within these chunks of ice were keys to understanding the planet’s climate?

Dansgaard had yearned to see a huge uninterrupted column of ice for much of his professional life, and now the most complete
ice cores yet known were within his grasp. Two features of ice cores are immediately apparent. They are colorful, varying from iridescent green to blue. And they are layered, with thick layers, thin ones, and everything in between. Almost anything in the atmosphere or in the water can get caught in ice. Debris of all sizes and kinds can get trapped: not only seeds, plants, and ash, but vintage
World War II planes. Air from the atmosphere can get caught as bubbles. The layers of ice themselves can reveal the extent of the
seasons. Arctic winters are dark and cold, whereas the summers are bright and less cold. With the sun come melt, flowing water, and the detritus water brings. Summer bands in the layers are darker and messier than the ones made in winter.
Dust blown by the winds can make some layers darker than others. With so much trapped in the ice, it becomes a very precise and informative record of ancient climates.

Dansgaard’s breakthrough came from applying the tools developed by
Harold Urey to the
Greenland ice core. Since his focus wasn’t shells but ice, the work required a few modifications, but he nevertheless was able to see a climate record. He measured oxygen along an ice core over half a mile deep, representing more than
100,000 years. Dansgaard saw the remarkable chilling taking place 17,000 years ago, during the
ice ages first seen by Agassiz. He also encountered a
warming period 500 years ago, corresponding to when humans first settled Greenland. And he found a cooling period extending from 1700 to 1850, when much of Europe was cold and Hans Brinker was ice-skating in the canals of Amsterdam.

Dansgaard’s was a rough first effort because his core, having been dug for missiles and churches, didn’t allow for great scientific resolution. A scientifically useful core is drilled, sectioned, and kept in conditions that allow long stretches of unbroken ice to be analyzed. Needed were new, more precise cores. And if these data were to have meaning, he’d need to see ice from different places on the planet: from both poles and from mountaintops of different continents.

Drilling scientifically accurate cores requires collaboration among engineers, scientists, and governments working on the planet’s largest ice sheets. This is expensive science: rigs need to be set up, and teams housed, in some of the most remote places on Earth. Since the 1970s a number of cores have been drilled, and to date the most complete of these are several drilled into the Greenland ice, the glaciers in
Antarctica, and several mountain glaciers from around the world.

The fine-grained view of climate and ice reveals surprises. Earth’s climate during the past 100,000 years has swung wildly on occasion. The ice ages weren’t just long invariant cold periods:
glacial periods have witnessed warm intervals, and warm intervals have seen glacial conditions. The emerging picture is that Earth’s climate depends on the
heat balance of the planet—the amount of heat coming in from the
sun minus the heat that escapes into space—and the ways that this heat is transferred among the oceans, land, air, and ice. Music is an analogy for what drives climate: a composition can be heard as one entity but be decomposed into rhythms, backbeats, and harmonies of different instruments acting on their own cycles.
Orbital motions of the kind revealed by
Milankovitch define the main cadence. The movement of heat through ocean currents, winds, and ice floes form other beats. The result of the interacting effects of these components is a system that has a long-term rhythm and
short-term riffs.

Climate at the end of the last glacial period, about 12,500 years ago, exemplifies one of the riffs. At this time, when by all accounts things should have continued to warm, there was a dramatic shift to a sharp cold spell that happened in the blink of an eye in geological terms—over decades. The record from pollen, oxygen atoms, and other markers implies a climate that converted from warm to cold on a dime. Global mean temperatures changed 15 degrees in as little as a decade. If wiggles of the climate curves are like an EKG, fluctuations like this are the equivalent of planetary heart attacks. When you think of the extent to which coastlines, arable land, and deserts can be transformed by changes in global temperature of just 2 or 3 degrees, the prospect of a 15-degree shift is staggering. Yet that is the kind of change that has taken place during the history of our species.

Orbits, climates, and ice define the way living things spread across the globe and through time. Changes in global climate
fragment some populations into isolated groups separated by ice. Others are offered new migration routes, enabling them to reach portions of the globe inaccessible under previous climatic conditions. DNA of
Native Americans reveals that they are derived from a single male who likely crossed the Bering Strait when an ice bridge formed during the last
ice age.
European populations, too, carry the signal of ice in their family trees. The DNA of many Europeans derives from populations that formerly lived in Ukraine and spread out during the last recession of ice. Ice is carried deep inside our human family tree, in the DNA we share with our diverse human cousins.