Atmospheric

Part 14 - Ice Ages

A mere 20,000 years ago, the site of Canada's national capital city, Ottawa, Ontario, was buried under ice two kilometres (6,500 feet) thick and much of northern Europe was also buried under thick ice.So much water was locked up in glaciers (on land) that the sea level was lower, exposing land bridges in many parts of the world which allowed Asians to cross to Alaska and begin colonising the American continent. 

 This was the most recent glacial maximum of the last ice age which began around 115,000 years ago and ended about 11,700 years ago.

Several major ice ages and many less severe glaciations have occurred throughout Earth's history, the earliest known occurred between 2.4 and 2.1 billion years ago.

The Cryogenian series of ice ages occurred between 720 and 635 million years ago and the Andean-Saharan was between 440 and 420 million years ago.

The Earth may have been almost completely covered in ice and snow, as many as three times in history when much of the Sun's radiation was reflected back into space from the snow cover.

The most probable Snowball Earth occurred between 717 and 660 million years ago during the Cryogenian series of ice ages. Another occurred about 650 to 635 million years ago. This last period ended when tectonic activity (driven by the heat from decaying radio-active isotopes in the Earth's interior) released enormous amounts of dust, methane, carbon dioxide and other gases from volcanoes. When this reached the atmosphere it prevented much of the Sun's radiation from being reflected and the planet began to warm up.

Many multi cellular Eukaryotic organisms survived the extreme cold in warm spots near the equator or where volcanic activity provided a warmer local environment near underwater volcanic vents.

Remnants of the last ice age are still with us in the form of polar ice caps and glaciers on mountainous areas.