Historic U.S. weather catastrophes took an unprecedented toll in human lives and livelihood in 2011. At least 12 natural disasters wreaked more than $1 billion in damage apiece, according to the U.S. National Climate Data Center. When all the lost crops have been tallied, insurance claims filed and ravaged assets accounted for, the number is likely to be 14, with a total cost of more than $50 billion. The previous high was eight, in 2008.The frequency of mass weather calamities has been increasing since 1970, based on records going back to 1910. It's a trend that's "virtually certain" to continue, according to a recent United Nations report by a panel of dozens of scientists. Rising global temperatures put more water vapor in the air, intensifying storms. Climate change also exacerbates the impact of drought, heat waves and wildfires.