From The Environmental Defense Fund: https://www.edf.org/blog/2016/09/01/we-just-had-five-1000-year-floods-less-year-whats-going
September 1, 2016
A 1,000-year flood is supposed to be extremely rare. Its chance of occurring in a given year: 0.1 percent.
So how do we explain that in the span of just five months, the United States logged no fewer than four deadly 1,000-year floods in states as widespread as Texas, West Virginia, Maryland and Louisiana – following a 1,000-year-flood that ravaged South Carolina last October.
It appears that the calculation of a 1,000-year event may no longer be the most accurate statistic. It was based, as are our increasingly common 100-year natural disaster events, on data from the past. We may, in other words, already have shifted so far into a new climate regime that probabilities have been turned on their head.
Climate change “supercharges” normal weather
Like any climate scientist will tell you, there is more to the story than what you see on the surface.
All climate and weather events are influenced to some degree by both natural climate variations and human-made climate change. The amount that each of these influences can exert on a particular event can theoretically range from 0 to 100 percent.
Rigorous scientific analysis has found that the extreme rainfall that caused a Texas flooding in May of 2015, for example, was caused by a fairly typical rainfall pattern associated with that year’s El Niño, a naturally occurring climate cycle, which had been supercharged by human-made climate change.
Working in tandem, these two phenomena together produced one of the largest multi-day flooding events Texas has ever experienced.