Box 1.1 Analogy of Abrupt Climate Change

FIGURE 1.1

Abrupt climate change is not unusual, and in fact many simple physical systems exhibit abrupt changes. Here, we illustrate a few basic points using a mechanical analogy.

Imagine a balance consisting of a curved track poised on a fulcrum, as shown above. The track is curved so that there are two “cups” where a ball may rest. A ball is placed on the track and is free to roll until it reaches its point of rest. This system has three equilibria denoted (a), (b) and (c) in the top row of the figure. The middle equilibrium (b) is unstable: if the ball is displaced ever so slightly to one side or another, the displacement will



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