Of all the transitions brought about on the Earth's surface by temperature change, the melting of ice into water is the starkest. It is binary. And for the land beneath, the air above and the life around, it changes everything. That is the main reason climatologists are interested in the Earth's north and south poles. The waxing and waning of the ice provides an unambiguous signal of what is going on-and it is a signal which can be read in rocks a billion years old almost as easily as it can be observed today. But the poles are only two examples. Another would be welcome. And there is one.
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