what is an ice age and how often do they happen?
- Written by Michael Petterson, Professor of Geology, Auckland University of Technology
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.
These environments are called “periglacial” and occupy areas between relatively warmer ice-free regions and permanent ice fields.
Ice ages and the Earth’s climate
Ice ages change the Earth’s climatic belts. 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? Earth’s average global temperature today is around 16℃.
Analysis of proxy temperature data (e.g. from the modelling of deep-ocean sediment isotope compositions) over the past 500 million years of Earth’s history indicates that average global temperatures have varied between around -10℃ and +30℃. During the most recent glacial maximum (stadial, 23,000 to 11,000 years ago), the average global temperature was about 8℃, with polar regions experiencing average temperatures of -2℃. Ice-free periods over the past 500 million years correspond with average global temperatures of over 20℃.
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. Rivers regularly froze over in winter in western Europe. The stunning artwork of the Dutch painter Hendrick Avercamp (1585–1634 CE) documents aspects of this period.
How many ice ages has the Earth experienced?
Geologists agree Earth has gone through six major global ice ages. The oldest ice age occurred some 2,900-2,780 million years ago. The most recent ice age is the one we are currently experiencing, the late Cainozoic-Quaternary Ice Age, which began around 34 million years ago with the glaciation of Antarctica.
Between these two ice-age periods, other ice ages occurred at 2,400-2,100, 715-550, 450-420 and 360-260 million years ago. These six major ice ages lasted between 300 and 30 million years respectively.
Ice ages vary in length of time, extent, and extremes of temperature. The most extensive ice age was the period referred to as “Snowball Earth” when geologists think ice reached all the way to the equator, some 700 million years ago.
Read more: The last ice age tells us why we need to care about a 2℃ change in temperature
At other times within ice ages (such as today), ice is mainly restricted to polar regions and higher mountain chains. But at its greatest extent, the present glacial period produced ice sheets as far south as the southern Great Lakes in the US and the river Thames in the UK. Mountain glaciers also extended much further and sea levels were some 120 metres lower than today.
There are many factors that cause ice ages. The main ones include variations in Earth’s orbit, known as Milankovitch cycles, reductions in solar energy emissions, lower atmospheric greenhouse gas concentrations, variations in ocean currents, tectonic activity, continental configurations, mountain building periods and global volcanism.
Authors: Michael Petterson, Professor of Geology, Auckland University of Technology
Read more https://theconversation.com/climate-explained-what-is-an-ice-age-and-how-often-do-they-happen-166652
