Careful study of the long-term climate record has shown that even a minor shock to the system can cause an enormous change in outcome, a nonlinear response that has come to be called "abrupt climate change." Less well recognized is that our social and economic systems are also highly sensitive to climate perturbations. Seemingly modest fluctuations in rainfall, temperature and other meteorological factors can create havoc in vulnerable societies.

Recent years have shown that shifts in rainfall can bring down governments and even set off wars. The African Sahel, just south of the Sahara, provides a dramatic and poignant demonstration. The deadly carnage in Darfur, Sudan, for example, which is almost always discussed in political and military terms, has roots in an ecological crisis directly arising from climate shocks. Darfur is an arid zone with overlapping, growing populations of impoverished pastoralists (tending goats, cattle and camels) and sedentary farmers. Both groups depend on rainfall for their livelihoods and lives. The average rainfall has probably declined in the past few decades but is in any case highly variable, leaving Darfur prone to drought. When the rains faltered in the 1980s, violence ensued. Communities fought to survive by raiding others and attempting to seize or protect scarce water and food supplies.

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