The phrase “Ice Age” is poorly defined and often abused, so let’s first define the climate state during most ice ages. It is called “Icehouse Earth.” The earth is in an icehouse state when either or both poles are covered in a thick, permanent icecap (Scotese 2015). Today, both poles are covered in ice year-round, so you may be surprised to learn this is very rare in Earth’s history. In fact, out of the last 550 million years, the earth has had permanent ice caps on one or both poles only nine percent of the time.
An “Ice Age” is best defined as a geologically (or millions of years long) long period of low temperatures. This usually results in the presence of continental and polar ice sheets and alpine glaciers. We are currently living in the Quaternary Ice Age, this is only the fifth significant and severe ice age in Earth’s known history, and, so far it has lasted about 2.6 million years. It is the most severe ice age in the Phanerozoic, the geological name for the past 550 million years. Ice Ages are rare, but humans evolved during one, so it seems normal to us.
Not by Fire but by Ice...
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As Figure 1 makes clear, the “normal” or “optimum” global average temperature of the Earth is 19.5 degrees C. or 67 degrees F. This is over 5 degrees C. (9 degrees F.) warmer than today. Over the past 550 million years, the Earth has normally been in the green area of Figure 1, the “Greenhouse” or optimal temperature regime. There are five periods when the Earth became very warm with average surface temperatures of more than 24 degrees C. or 75 degrees F. This area is called “hothouse” and is shaded in red in Figure 1. The blue area in Figure 1 is called the “icehouse” and we are living in the fourth or fifth icehouse period. Normally the Karoo Ice Age is considered one icehouse period, but it briefly returns to greenhouse conditions in the middle. The sharp cooling period labeled “KT Impact Winter” occurred 66 million years ago and was caused by a large asteroid striking the Earth near the Yucatan Peninsula in southern Mexico. The ejecta, which included a lot of SO2, from the crater caused a sudden cooling of the Earth and the extinction of all large animals, including the dinosaurs. This impact also marks the end of the Cretaceous Period and the beginning of the Tertiary. It is not considered an Ice Age, as it is too brief. In this article, the Jurassic-early Cretaceous Cool period will be considered an ice age, although the temperatures were not low enough to enter the icehouse state for any significant period.
The maximum swing in temperature in Figure 1 is from 13 degrees to 28 degrees C. or an increase of 15 degrees C. (27 degrees F.) between 280 million years ago and 250 million years ago. This is from the depths of the Permo-Carboniferous icehouse to the peak of the Triassic hothouse. Tropical temperatures change more slowly than polar temperatures, compare Figure 1 to Figure 2.
As shown in Figure 2, the global temperature of the tropics (roughly 23.5 degrees north latitude to 23.5 degrees south latitude) varies less than the average global temperature of the entire Earth’s surface (see Figure 1). This means that the temperatures in the polar regions vary a lot from warmer periods to cooler periods. In the cooler periods, like our current ice age, the polar temperatures are low enough for ice to survive the summer months and thus, form permanent ice caps. Ice reflects more solar radiant heat than the soil or ocean under it, which amplifies the cooling.
