What is an ice age?

Geologists define an ice age as a period when more of the Earth's surface was covered by larger ice sheets than in modern times, or when cool temperatures endured for extended periods of time, allowing the polar ice to advance into lower, more temperate latitudes. The last ice age event, which ended about 12,000 years ago, is referred to as the "Ice Age" or the "Great Ice Age"—both usually capitalized—a time when ice covered nearly 32 percent of the land and 30 percent of the oceans.

Who first proposed the idea of ice ages?

Several scientists over the centuries came close to proposing the idea of ice ages. Scottish naturalist James Hutton (1726-1797) observed strangely shaped glacial boulders (erratics) near Geneva, Switzerland. Based on this, he published his theory in 1795 that alpine glaciers were more extensive in the past. In 1824, Jens Esmark (1763-1839) proposed that past glaciation had occurred on a much larger continental scale.

But the most persuasive argument for ice ages came in 1837, when Swiss-American geologist Louis Agassiz (1807-1873) gave his now-famous speech on past widespread ice age conditions. He proposed that nearly all of northern Europe and Britain had once been covered by ice, and he subsequently found evidence for his theory in New England. Others eventually uncovered additional evidence. In 1839, United States geologist Timothy Conrad (1803-1877) discovered evidence of polished rocks, striations, and erratic boulders in western New York, supporting Agassiz's theory that Ice Age glaciation was worldwide. In 1842, the first attempt to explain the ice ages using an astronomical connection was made by

Geologist Louis Agassiz was among the first to form a theory about past ice ages. (NOAA)

Geologist Louis Agassiz was among the first to form a theory about past ice ages. (NOAA)

French scientist Joseph Adhemar (1797-1862). He proposed that the ice ages were the result of the 22,000-year precession of the equinox, a natural movement of the Earth's axis that causes the seasons to switch over thousands of years. In other words, the current summer months would become winter months and vice versa.

What causes ice ages?

No one knows why ice ages occur, but there are several theories. One possibility is that the Sun's energy varies in intensity over time. Each time there is a decrease in activity, an ice age may occur as the Earth cools. Another possibility is an increase in dust in the atmosphere, either from volcanoes or a large meteorite impact. The debris from either event would reflect more of the Sun's light into space (albedo), cooling down the atmosphere and causing more snow and ice to form. This would also further increase the world's albedo, as even more sunlight would reflect off the ice and snow. However, this theory has a problem, as many of the other theories do, in that it doesn't explain what causes the ice sheets to retreat.

What were the major glacial ice ages over time?

Evidence from the geologic record shows that ice ages have occurred relatively few times in Earth's history, with the first known large-scale ice age taking place approximately 2.3 billion years ago. (In the last about 670 million years, ice ages have occurred less than 1 percent of the time.) Geologists believe there have been five major ice ages over geologic time. The following lists the occurrences:

• 1.7 to 2.3 billion years ago (Huronian Era, Precambrian)

• 850 million years ago (Cryogenian Period)

• 670 million years ago (Proterozoic Era, Precambrian)

• 420 million years ago (Paleozoic Era, between the Ordovician and Silurian Periods)

• 290 million years ago (Paleozoic Era, between late Carboniferous and early Permian Periods)

• 1.7 million years ago (Cenozoic Era, Quaternary Period, Pleistocene epoch) What was Snowball Earth?

Scientists now believe that at between two to four times between 850 and 580 million years ago, the planet was covered completely in ice and early life forms were nearly wiped out: a "Snowball Earth." There are a couple of theories as to why this occurred: one being that the Sun was about six percent cooler than it is today; the other is that the continents had all traveled—due to plate tectonics—mostly south; ocean currents traveled easily around the planet without interruption, and volcanic activity dwindled to a minimum. The result was that significantly less carbon dioxide was being generated and expelled into the atmosphere. In addition, the Earth's axis was significantly more tilted at the time (by 54 degrees versus today's 23.5 degrees), causing more drastic seasonal extremes.

Once a period of cooling began and ice shelves formed, more of the Sun's light was reflected, thus causing the cooling cycle to accelerate more and more until everything was frozen. Early theories about this, proposed by such scientists as George Williams at the University of Adelaide and Joe Kirschvink at the California Institute of Technology, include the effects of plate tectonics as causing massive volcanic eruptions worldwide. This would lead to an extended winter of unprecedented scale that turned Earth into one big frozen lump in space that would last for millions of years. Nearly all life was wiped out. In fact, one matter of debate concerning this theory was that critics felt such a Snowball Earth would have extinguished all life. This notion was laid to rest in the 1990s when life was found to thrive near geothermal vents deep under the ocean.

The Snowball Earth cycle was only broken because volcanic action and plate tectonics would cause carbon dioxide levels to build under the ice, especially since there would be no liquid water to dissolve minerals or aid in the dissipation of CO2 levels. The result would be an inevitable, sudden massive release of CO2 that would lead to a comparatively brief period of extreme heat with temperatures averaging 120°F (50°C). However, the thaw retreated to another snowball period a couple more times over millions of years before the continents moved into positions that created a more stable geophysical state of the planet. Volcanic activity moderated, as did carbon dioxide levels. Another Snowball Earth could happen again, however, as the continents, hundreds of millions of years from now, drift back together to form a new supercontinent.

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