The slug of slow-moving destruction that is making its way from Iowa down the Mississippi River Valley is extraordinary by any measure.
That the last flood of this magnitude occurred just 15 years ago should be cause for concern.
Floods happen. Big floods happen. Epic floods happen.
But they don't happen all the time. Scientists talk about the 100 Year Flood, and the 500 Year Flood. These are floods of such magnitude they could be expected to occur every 100 years, or every 500 years. Infrequently, in other words.
In Iowa, and on down the Mississippi River, that once-a-century event has happened twice in the life of some teenagers.
In December, a coalition of environmental groups analyzed weather data from around the United States and found that the storms that pack the heaviest rainfall were unleashing their deluges more frequently. The frequency of "extreme rainfall" increased 24% from 1948 to 2006, and that observation is consistent with scientific models that project more intense bouts of rainfall as a consequence of global warming. (Worldwide, natural disasters have increased fourfold in two decades, which the international aide group Oxfam attributes to global warming's affect on extreme weather.)