This is a recurring theme of an Earth with constantly changing temperature. A review of this chart showing four billion years of temperature data reveals that the Earth usually has been either very warm or very cold, oscillating between very warm “hothouse” conditions and much colder “icehouse” or “snowball Earth” conditions. During hot-house periods, high temperatures prevail and ice is rare. During icehouse periods such as the present, the Earth cycles between long periods of glaciation and shorter interglacial periods of somewhat warmer temperatures, but with extensive ice at both poles. The current “icehouse” phase has lasted 3.5 million years.

Fortunately for mankind, we are currently in a blessedly warm interglacial period. For that, we should be thankful.