As a species, we've enjoyed a run of luck in the Holocene, the geological period covering the last ten thousand years. Migration as a response to adversity has become progressively less viable, yet warming and cooling trends and attendant sea level fluctuations have remained small. Even the Little Ice Age, from approximately 1300 to 1850, amounted to an average cooling (in Europe, where the data are best) of just about 0.5°C (0.9°F). It made harvest failures more frequent in northern Europe and probably contributed to the demise of the tiny Greenland Norse settlement in the early fifteenth century. In lower latitudes, the Little Ice Age probably brought reduced rainfall and more frequent droughts—a much more disruptive experience than mild cooling or warming. But as nature's surprises go, the climate change of the Little Ice Age was modest.2
In the past, nature's shocks and stresses challenged all societies. In recent millennia, the most dangerous of these included epidemics, droughts, floods, earthquakes, and volcanic eruptions. Warming, cooling, and sea level changes were far down the list. Broadly speaking, these challenges came in two varieties: short, sharp shocks with durations of days, weeks, or a year or two; and long, slow stresses that played out over decades or centuries, and were often invisible to people at the time. In terms of demographic losses, epidemics were by far the most serious.3
Box 2-1 ranks the demographic seriousness of nature's shocks in very rough terms. The mortality figures, given only as an order of magnitude, represent the maximum, meaning 95 to 99 percent of such incidents would kill fewer people. So, for example, although there may have been a flood or even ten floods that killed more than 1 million people, this represents the worst that floods have ever done to humankind.
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