How many ice ages have we had

Lasting roughly 2. During the Stone The Iron Age was a period in human history that started between B. During the Iron Age, people across much of Europe, Asia and parts of Africa began making tools and weapons from iron and The Bronze Age marked the first time humans started to work with metal.

Bronze tools and weapons soon replaced earlier stone versions. Humans made many technological advances during the The most recent ice age peaked between 24, and 21, years ago, when vast ice sheets covered North America and northern Europe, and mountain ranges like Africa's Mt.

Kilimanjaro and South America's Andes were encased in glaciers. At that point our Homo sapien ancestors had Led by Mark Pagel, an evolutionary biologist, the study attempts to sketch out a linguistic family tree that stretches back some 15, years to southern Europe. During this era, America became The Cretaceous-Tertiary extinction event, or the K-T event, is the name given to the die-off of the dinosaurs and other species that took place some For many years, paleontologists believed this event was caused by climate and geological changes that The Neolithic Revolution, also called the Agricultural Revolution, marked the transition in human history from small, nomadic bands of hunter-gatherers to larger, agricultural settlements and early civilization.

The Neolithic Revolution started around 10, B. Live TV. This Day In History. History Vault. 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. The regions on the fringes of extensive ice sheets and glaciers experience a cooling to the point that a consistently cold environment forms.

Usually, the ground is frozen for much of the year, growing seasons are short, and only the hardiest of flora and fauna survive. The Russian tundra is an example of this landscape. Temperate and tropical zones become restricted to the lower equatorial latitudes. A question that follows on from the definition of an ice age is: how cold does Earth have to become to produce one? Analysis of proxy temperature data e.

There is no official minimum period of time for an ice age. Some colder periods in historical times are termed little ice ages, including between the 13th and 18th centuries. This period was characterised by longer and colder winters, and shorter, cooler summers. In all, scientists have identified upwards of a dozen ice ages in the geologic record, several of them in the last half a billion years.

Among the earliest ice ages so far found in the geological record are the Huronian ice ages. Interspersed with non-glacial periods, the ice ages occurred between 2. Paleontologists surmise that when microbial life arose on Earth over 3. Although nitrogen levels may have been similar, other gases were much more—or much less—abundant. Carbon dioxide was anywhere from 10 to 2, times present levels, and methane may have been as much as 10, times higher than present levels.

Atmospheric oxygen was virtually nonexistent. Scientists debate when exactly microbes capable of carrying out photosynthesis and making oxygen as a byproduct first evolved. Estimates range from about 3. The earliest oxygen makers were probably ancestors of modern cyanobacteria, or blue-green algae.

At first, the oxygen produced by these early photosynthesizers probably reacted with iron in the ocean, settling into layers of rusty sediment on the seafloor before starting to accumulate in the atmosphere. Some oxygen reacted with methane, converting it to carbon dioxide and water. Meanwhile, photosynthesizing microbe populations kept growing, consuming more carbon dioxide. Ancestors of modern cyanobacteria blue-green algae may have been the first oxygen producers on planet Earth, and ushered in significant changes in climate.

CC license by Flickr user Richard Droker. Carbon dioxide is a greenhouse gas, and methane is an even more potent greenhouse gas. As atmospheric concentrations of these greenhouse gases dropped, global temperatures plummeted, plunging the planet into a series of ice ages. The Huronian ice ages and non-glacial periods separating them likely lasted a total of million years. Evidence suggests these glaciations reached equatorial regions at sea level. Ice occurs in equatorial regions today, but only at high elevations.

Geologic evidence of these ice ages was first discovered in , in glacial deposits near Lake Huron. Since then, geologists have discovered more evidence elsewhere in North America, as well as in South Africa, Western Australia, and northeastern Europe. Found near Whitefish Falls, Ontario, along the northern shore of Lake Huron, this dropstone landed in seafloor sediments under a floating glacier some 2. Lindsey, USGS.

The rise of oxygen did more than freeze the planet. At least twice between and million years ago, Earth fell into a deep freeze. Because the Cryogenian Period events occurred during a longer geologic era known as the Neoproterozoic Era, the deep freezes are sometimes referred to as the Neoproterozoic Snowball Earths.

Scientists continue to debate the causes of Neoproterozoic freezes and the subsequent thaws. Volcanoes may be the force that both pushed the planet into the glaciations and also pulled it out. About million years ago, most continents were clustered around the equator. Within this continental mix, geologists have identified evidence of what they call a large igneous province. Eruptions in this province could have cooled the planet in two ways.

Evidence of the once-equatorial large igneous province that may have kickstarted the Cryogenian is preserved in Nunavut, Canada. Sills—intrusions of volcanic material into older rock layers—cut across older, sand-colored rock.

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.