David A. Wharton’s book, Life at the Limits: Organisms in Extreme Environments (Cambridge University Press, 2002), contains an interesting observation about temperatures on our planet. It shows that cold temperatures are the norm, not the exception, for the planet as a whole:
What are normal temperatures? The Earth is, on average, a cold place. More than two-thirds of the Earth’s surface is covered by ocean and the temperature of most of the ocean stays close to 2 °C. Including the ocean depths, the polar ice caps and the land, four-fifths of the planet is below 5 °C all the time. What we might think of as “normal” temperatures, say 10 °C to 30 °C, are really not normal at all but occur only in restricted parts of the world. Abundant life is associated with the warmer parts of the Earth (but not too warm!).