Heat gets a bad rap for fueling human hostility. But what's the deal when the mercury drops? The cold effect has been somewhat less studied, although there are hints that being uncomfortably chilly can contribute to conflict in some situations and quell it in others.

A Swiss-led group using tree-ring data to look at Central European summer climate patterns during roughly 2,500 years saw that periods of prolonged warming and of colder than usual spells coincided with social upheavals. As they reported online in January 2011 in Science, the researchers uncovered cold periods that overlapped with raucous historical events ranging from a Celtic expansion around 350 b.c. to modern migrations from Europe to the Americas in the 1800s.

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