WEATHER IS WHAT HAPPENS TODAY; climate is the average of what happens over decades and centuries. Earth has gone through past episodes, called ice ages, when the worldwide climate was cooler and dryer and thick layers of ice covered northern latitudes. The earliest known ice age occurred about 570 million years ago, and the next about 280 million years ago. The most recent ice age began only about 3 million years ago and is still going on. You are living in one of the periodic episodes during an ice age when the glaciers melt back and Earth grows slightly warmer. The current warm period began about 12,000 years ago.
Ice ages seem to occur with a period of roughly 250 million years, and cycles of glaciation within ice ages occur with a period of about 40,000 years. Evidence shows that these cyclic changes have an astronomical origin.The Hypothesis
Sometimes a theory or hypothesis is proposed long before scientists can find the critical evidence to test it. That happened in 1920 when Yugoslavian meteorologist Milutin Milankovitch proposed what became known as the Milankovitch hypothesis—that small changes in Earth’s orbit, in precession, and in inclination affect Earth’s climate and can trigger ice ages. You should examine each of these motions separately.
First, astronomers know that the shape of Earth’s orbit varies slightly over a period of about 100,000 years. At present, Earth’s orbit carries it 1.7 percent closer than average to the sun during northern hemisphere winters and 1.7 percent farther away in northern hemisphere summers. This makes the northern climate very slightly warmer, and that is critical—most of the landmass where ice can accumulate is in the northern hemisphere. If Earth’s orbit became more elliptical, for example, northern summers might be too cool to melt all of the snow and ice from the previous winter. That would make glaciers grow larger.
A second factor is also at work. Precession causes Earth’s axis to sweep around a cone with a period of about 26,000 years, and that changes the location of the seasons around Earth’s orbit. Northern summers now occur when Earth is 1.7 percent farther from the sun, but in 13,000 years northern summers will occur on the other side of Earth’s orbit where Earth is 1.7 percent closer to the sun. Northern summers will be warmer, which could melt all of the previous winter’s snow and ice and prevent the growth of glaciers.
The third factor is the inclination of Earth’s equator to its orbit. Currently at 23.5°, this angle varies from 22° to 24°, with a period of roughly 41,000 years. When the inclination is greater, seasons are more severe.
In 1920, Milankovitch proposed that these three factors cycle against each other to produce complex periodic variations in Earth’s climate and the advance and retreat of glaciers (Figure 3-13a). But no evidence was available to test the theory in 1920, and scientists treated it with skepticism. Many thought it was laughable.The Evidence
By the middle 1970s, Earth scientists could collect the data that Milankovitch had lacked. Oceanographers could drill deep into the seafloor and collect long cores of sediment. In the laboratory, geologists could take samples from different depths in the cores and determine the age of the samples and the temperature of the oceans when they were deposited on the sea floor. From all this, scientists constructed a history of ocean temperatures that convincingly matched the predictions of the Milankovitch hypothesis (Figure 3-13b).
The evidence seemed very strong, and by the 1980s, the Milankovitch hypothesis was widely discussed as the leading hypothesis. But science follows a mostly unstated set of rules that holds that a hypothesis must be tested over and over against all available evidence (Window on Science 3-2). In 1988, scientists discovered contradictory evidence.
For 500,000 years rainwater has collected in a deep crack in Nevada called Devil’s Hole. That water has deposited the mineral calcite in layer upon layer on the walls of the crack. It isn’t easy to get to, and scientists had to dive with scuba gear to drill out samples of the calcite, but it was worth the effort. Back in the laboratory, they could determine the age of each layer in their core samples and the temperature of the rainwater that had formed the calcite in each layer. That gave them a history of temperatures at Devil’s Hole that spanned many thousands of years, and the results were a surprise. The evidence seemed to show that the previous ice age ended thousands of years too early to have been caused by Earth’s motions.
These contradictory findings are irritating because we naturally prefer certainty, but such circumstances are common in science. The disagreement between ocean floor samples and Devil’s Hole samples triggered a scramble to understand the problem. Were the ages of one or the other set of samples wrong? Were the ancient temperatures wrong? Or were scientists misunderstanding the significance of the evidence?
In 1997, a new study of the ages of the samples confirmed that those from the ocean floor are correctly dated. This seems to give scientists renewed confidence in the Milankovitch hypothesis. But the same study found that the ages of the Devil’s Hole samples are also correct. Evidently the temperatures at Devil’s Hole record local climate changes in the region that became the southwestern United States. The ocean floor samples record global climate changes, and they fit well with the Milankovitch hypothesis. In this way, the Milankovitch hypothesis, though widely accepted today, is still being tested as scientists try to understand the world we live on.
How do precession and the shape of Earth’s orbit interact to affect Earth’s climate?
Now you can see the importance of precession. As Earth’s axis precesses, its long axis points gradually in different directions, and the seasons occur at different places in Earth’s orbit. In 13,000 years, northern winter will occur at aphelion, and, if Earth’s orbit were highly elliptical, northern winter would be terribly cold. Similarly, summer would occur at perihelion, and the heat would be awful. Such extremes might deposit large amounts of ice in the winter but then melt it away in the hot summer, thus preventing the accumulation of glaciers.
Continue this analysis by modifying your scientific argument further. What effect would precession have if Earth’s orbit were more circular?
We are Earth creatures. We live on the exposed surface of a world spinning on its axis and revolving around the sun. Those motions produce the cycles of day and night and winter and summer, and we have evolved to live within those cycles. One theory holds that we sleep at night because dozing in the back of a cave (or in a comfortable bed) is safer than wandering around in the dark. The night is filled with predators, so sleeping may keep us safe. People who live and work in the Arctic or Antarctic where the cycle of day and night does not occur can suffer psychological problems from the lack of the daily cycle.
The cycle of the seasons controls the migration of game and the growth of crops, so cultures throughout history have followed the motions of the sun along the ecliptic with special reverence. The people who built Stonehenge were marking the summer solstice sunrise because it was a moment of power, order, and promise in the cycle of their lives.
The moon’s cycles mark the passing days and divide our lives into weeks and months. In a Native American story, the mythical character Coyote gambles with the sun to see if the sun will continue to warm Earth, and the moon keeps score. The moon is a symbol of regularity, reliability, and dependability. When you notice the moon in the sky, remember that it is the scorekeeper counting out your days and months.
Like the ticking of a cosmic clock, the passing weeks, months, and seasons mark the passage of time on Earth, but, as you have seen, the cycle of the seasons is also affected by longer period changes in the motion of Earth. Ice ages come and go, and Earth’s climate cycles in ways we do not entirely understand. If you don’t feel quite as secure as you did when you started this chapter, then you are beginning to catch on. Astronomy tells us that Earth is a beautiful world, but it is also a complicated planet, spinning in a complicated universe, and there are probably a lot more surprises for the humans who walk its surface.
Figures and Extras
Window on Science 3-2