THE GAIA HYPOTHESIS* (Lovelock & Margulis)

"...the physical and chemical condition of the surface of the Earth, of the atmosphere, and of the oceans has been and is actively made fit and comfortable by the presence of life itself."

This is an interesting notion and may be at play at some level. There are certain things we know about the Sun which concern us when looking at the Terresrtrial climate. As we noted in earlier in the lecture, Venus apparently is much different from the Earth today because it was closer to the Earth (Venus is 0.7 A.U. while the Earth is 1 A.U. from the Sun). This corresponds to a difference in Solar heating (ignoring clouds) of a factor of 2. According to some modelers, even smaller increases in Solar heating are important. This is disturbing because the Sun gets brighter as it ages. At birth the Sun was around 70 % of its current brightness and by the time the Sun dies in 5.4 billion years, it will be more than twice as bright as it is currently.

Recall that the equilibrium temperature for the Earth is currently below the freezing point of water. In the past it should have been much colder. It appears, however, the Earth was warmer in the past (The Faint Young Sun Paradox)! Huh? What is a possible explanation?

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^*The Gaia Hypothesis, named after Gaea, Mother Earth, a goddess of the early Greeks.