What is an ice age and how often do they occur?
By Michael Petterson, Professor of Geology, Auckland University of Technology
Auckland (Australia), Sep 30 (The Conversation) What is an Ice Age? Do they have to last a while to count, how have they varied, and how many ice ages has Earth experienced?
As with many definitions of natural phenomena, a precise definition of an ice age is not straightforward.
Ice ages form during extended periods of a relatively cooler Earth. A definition must include the condition that the Earth is cold enough for permanent ice formation.
A second part of an ice age definition is the end result of prolonged cooling. Ice ages lead to the development of continental ice caps in the northern and southern hemispheres, and the growth of glaciers in mountainous regions of the world, such as the Himalayas, the Alps, the Southern Alps and the Andes.
A third part of the definition concerns time. For an ice age to be recorded as significant, it must last for a geologically significant time frame.
If we put all of these factors together, then an Ice Age occurs during periods of prolonged low temperatures, resulting in significant areas covered in ice for millions to tens, if not hundreds of millions of years.
Variations within an Ice Age
Ice ages are not uniformly cold. There can be colder and warmer periods throughout the Ice Age. Colder periods lead to larger areas of continental ice caps, valley glaciers and sea ice, while warmer periods lead to reduced areas of ice.
Cold ice ages on Earth are called “stages”, while the warmer parts of an ice age are called “interstades”. An ice age ends when the Earth warms enough for the ice cover to recede or disappear completely.
Regions bordering large ice caps and glaciers are cooled to the point where a constantly cold environment is formed.
Usually the ground is frozen for much of the year, the growing seasons are short, and only the hardiest flora and fauna survive. The Russian tundra is an example of this landscape.
These environments are called “periglacial” and occupy areas between relatively warmer ice-free regions and permanent ice fields.
Ice ages and Earth’s climate
Ice ages change the Earth’s climatic belts. Temperate and tropical zones are limited to low equatorial latitudes.
One question that stems from the definition of an Ice Age is, how cold does the Earth have to get to produce one? The average global temperature of the Earth today is around 16 degrees Celsius.
Analysis of indirect temperature data (e.g. from modeling the isotopic compositions of deep-sea sediments) over the last 500 million years of Earth’s history indicates that mean global temperatures have varied between about -10 degrees Celsius and +30 degrees Celsius.
During the most recent glacial maximum (stadial, 23,000-11,000 years ago), the average global temperature was around 8 degrees Celsius, with the polar regions experiencing average temperatures of -2 degrees Celsius. Ice-free periods over the past 500 million years correspond to average global temperatures of over 20 degrees Celsius.
There is no official minimum period for an Ice Age. Some colder periods in history are called small ice ages, including between the 13th and 18th centuries. This period was characterized by longer, colder winters and shorter, cooler summers. Rivers froze regularly in winter in Western Europe. The breathtaking work of Dutch painter Hendrick Avercamp (CE 1585-1634) documents some aspects of this period.
How many ice ages has the Earth known?
Geologists agree that the Earth has passed through six major global ice ages. The oldest ice age occurred around 2,900 to 2,780 million years ago. The most recent ice age is what we are experiencing now, the Late Cainozoic-Quaternary Ice Age, which began around 34 million years ago with the glaciation of Antarctica.
Between these two ice ages, other ice ages occurred 2,400-2,100, 715-550, 450-420, and 360-260 million years ago. These six great ice ages lasted between 300 and 30 million years, respectively.
Ice ages vary in duration, extent and temperature extremes. The most extensive ice age was the period known as “Snowball Earth,” when geologists believe the ice reached the equator around 700 million years ago.
At other times during Ice Ages (such as today), ice is mostly confined to polar regions and higher mountain ranges. But at its peak, the current Ice Age produced ice caps as far south as the southern Great Lakes in the United States and the Thames in the United Kingdom. Mountain glaciers also extended much further and the sea level was about 120 meters lower than today.
Many factors are at the origin of the ice ages. The main ones include variations in Earth’s orbit, known as Milankovitch cycles, reductions in solar energy emissions, decreasing atmospheric concentrations of greenhouse gases, variations in ocean currents, activity tectonics, continental configurations, periods of mountain formation and global volcanism. (The Conversation) NSA
Disclaimer: – This story has not been edited by Outlook staff and is auto-generated from news agency feeds. Source: PTI