Timescales of glaciation
The amount of glacier ice on earth is constantly fluctuating, and is presently at a minimum relative to the vast areas of the earth covered by ice in the past.
Over the last 2.6 million years (known as the Quaternary period (Holden 2012: 22), the Earth has experienced numerous glaciations, commonly known as 'ice ages'. These occur at fairly regular intervals of about 100,000 years, largely due to changes in the amount of energy received from the sun (Berger and Loutre 1991). During these glaciations, up to 30% of land on earth was ice-covered, including most of Northern Europe.