Introduction

Is Earth's climate stable? At some level the answer is almost certainly “yes.” The evidence for this is two-fold. First, the geologic record indicates that liquid water has been present on Earth's surface more or less continuously since about 4 Ga. (“Ga” stands for “giga-aeon,” which means “billions of years ago.”) We say “more or less” because, as discussed below, there appear to have been brief periods in Earth's history when the planet was almost entirely frozen. And, second, life appears to have been present since at least 3.5 Ga (Schopf, 1993) and perhaps 3.9 Ga, if carbon isotopes are admitted as indirect evidence (Mojzsis et al., 1996). This latter requirement overlaps the first one to some extent because all organisms require liquid water during at least part of their life cycle. It is more stringent, however, in that liquid water can exist right up to the critical point (374 °C, 220 bar for pure water), whereas the upper temperature limit for life is ∼ 113 °C. (A common misconception is that liquid water requires temperatures below 100 °C, but this is only the boiling point at one atmosphere pressure. The ocean contains the equivalent of ∼ 270 bar of water vapor and so, like water in a pressure cooker, it would not boil until the temperature exceeded the critical temperature.)

Another way of evaluating Earth's climate stability is to compare Earth to its neighboring planets, Venus and Mars.