Over time, the tilt of the Earth, its orbit, and its wobble change a bit. These very minor and regular adjustments in the angle of the Earth relative to the sun affects the amount of solar radiation, or insolation, that reaches Earth. And of course, less energy from the sun means colder temperatures. During the colder winters, snow falls on the land. If the summers are cool enough, the snow lasts until the next winter. Eventually there will be more and more snow building up, and that will pack down into a glacier.
Ocean currents and carbon dioxide, or CO2, play a role, too. Carbon dioxide levels change more or less in step with changes in ice volumes. This whole process takes thousands and thousands of years. Warming up, however, happens much faster. The bottom layer turns to ice, which becomes a glacier as the weight of accumulated snow causes it to slowly move forward. As temperatures drop in areas adjacent to these ice cliffs, cold-weather plant life is driven to southern latitudes.
Meanwhile, the dramatic drop in sea levels enables rivers to carve out deeper valleys and produce enormous inland lakes, with previously submerged land bridges appearing between continents. Upon retreating during warmer periods, the glaciers leave behind scattered ridges of sediment and fill basins with melted water to create new lakes.
Approximately a dozen major glaciations have occurred over the past 1 million years, the largest of which peaked , years ago and lasted for 50, years. At the height of the recent glaciation, the ice grew to more than 12, feet thick as sheets spread across Canada, Scandinavia, Russia and South America. Corresponding sea levels plunged more than feet, while global temperatures dipped around 10 degrees Fahrenheit on average and up to 40 degrees in some areas.
In North America, the region of the Gulf Coast states was dotted with the pine forests and prairie grasses that are today associated with the northern states and Canada. The origins of ice age theory began hundreds of years ago, when Europeans noted that glaciers in the Alps had shrunk, but its popularization is credited to 19th century Swiss geologist Louis Agassiz.
Contradicting the belief that a wide-ranging flood killed off such megafauna as the wooly mammoth, Agassiz pointed to rock striations and sediment piles as evidence of glacier activity from a destructive global winter. Geologists soon found evidence of plant life between glacial sediment, and by the close of the century the theory of multiple global winters had been established. A second important figure in the development of these studies was Serbian mathematician Milutin Milankovitch.
Along with solar radiation levels, it is believed that global warming and cooling is connected to plate tectonic activity. One significant outcome of the recent ice age was the development of Homo sapiens. The bands in the lighter rock result from the coastline rising after the glaciers that had weighed down the coast retreated.
Image from Mike Beauregard, Wikimedia Commons. When volcanoes release sulfur dioxide, the gas undergoes chemical reactions in the atmosphere to form highly reflective sulfates—particles that block out sunlight, like billions of tiny mirrors.
Likewise, when volcanoes extrude large volumes of basalt, the rock weathering that follows can cool the planet. Over time, rain, wind, and chemical changes all eat away at volcanic rocks. Rainwater and groundwater percolating through rock can dissolve carbon dioxide, stripping it from the atmosphere and ultimately trapping it as carbonate minerals such as limestone. Geologists have identified two glaciations during the Neoproterozoic: the Sturtian about to million years ago and the Marinoan about to million years ago.
Rock layers from these times show the most extensive evidence of extreme glaciations so far found in the geologic record. In between these deep freezes, Earth appears to have endured an equally remarkable hothouse. This climate extreme, too, might be down to volcanic activity. Over the long term, volcanic emissions of carbon dioxide and the depletion of carbon dioxide by weathering of rocks can keep each other in check.
But as ice enrobed most of the planet hundreds of millions of years ago, weathering probably slowed as conditions turned too cold for heavy precipitation. If the most extreme ice ages in Earth history were true Snowball Earth events—with no open ocean—our planet may have looked like a supersized version of Enceladus.
Volcanoes, however, kept cranking out carbon dioxide. With little rock-weathering or photosynthetic activity left to draw from the atmosphere, the greenhouse gas would have accumulated, leading to a gradual increase in global temperatures.
Once conditions warmed enough to melt tropical ice, the temperature increase would have accelerated. The subsequent big melt might have caused such dramatic, rapid weathering that it led to the second glaciation.
As in the Huronian, glaciations of the Cryogenian Period reached sea level at the equator. But just how complete the Neoproterozoic ice coverage was—whether it was a Snowball Earth or a Slushball Earth —remains an area of active research.
The rock record indicates that nothing as extensive as the Huronian and Cryogenian glaciations has happened in the last million years, even though geologists have found evidence of several more ice ages. Although it has some competition from cold conditions occurring between and million years ago, the most significant ice age in the last half a billion years may be the most recent. Striking during the time period known as the Pleistocene Epoch, this ice age started about 2.
Like all the others, the most recent ice age brought a series of glacial advances and retreats. In fact, we are technically still in an ice age. All of human civilization—everything from the earliest scripts such as cuneiform to smartphones and tweets—has occurred within an interglacial.
About 50 million years ago, the planet was too warm for polar ice caps, but Earth has mostly been cooling ever since. Starting about 34 million years ago, the Antarctic Ice Sheet began to form. Besides nauseating generations of ocean travelers, the Drake Passage opening created the Antarctic Circumpolar Current.
Circling the now-frozen continent, the current may have reduced the amount of ocean heat reaching Antarctica, enabling Antarctic ice to form and grow. Wind and waves make trips through the Drake Passage memorable. Its appearance due to plate tectonics maybe have contributed to the development of the Antarctic Ice Sheet.
CC license by Flickr user Christopher Michel. Another land movement likely plunged the planet into its most recent ice age. Prior to its formation, the Atlantic and Pacific Oceans freely exchanged tropical waters.
By cutting off that exchange and sending warm, salty ocean water northward, the isthmus increased precipitation at high latitudes in the Northern Hemisphere. Snow accumulated into glaciers and eventually into ice sheets. Once Earth was cold enough for ice sheets to form, they waxed and waned over timescales of about 20, to , years, due partly to Milankovitch Cycles.
High-resolution without annotations available. Image by Climate. A second part of an ice age definition is the end result of protracted cooling. Ice ages lead to the development of continental ice sheets in the northern and southern hemispheres, and the growth of glaciers in mountainous parts of the world, such as the Himalayas, Alps, Southern Alps and Andes. A third part of the definition involves time. For an ice age to be recorded as significant, it must last for a geologically significant amount of time.
If we bring all these factors together, then an ice age occurs during times of protracted lower temperatures, resulting in significant areas covered in ice for millions to tens or even hundreds of millions of years. Read more: Humans inhabited North America in the depths of the last Ice Age, but didn't thrive until the climate warmed.
Ice ages are not uniformly cold. There can be colder and warmer periods during the overall ice-age period. Colder periods lead to more extensive areas of continental ice sheets, valley glaciers and sea ice, while warmer periods lead to reduced areas of ice. An ice age ends when the Earth warms enough for the ice cover to recede, or disappear completely.
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