"Harvey is a 1,000-year flood event unprecedented in scale," per WashPost weather editor Jason Samenow:"An analysis from the University of Wisconsin's Space Science and Engineering Center has determined that Harvey is a 1-in-1,000-year flood event that has overwhelmed an enormous section of Southeast Texas equivalent in size to New Jersey.""There is nothing in the historical record that rivals this, according to Shane Hubbard, the Wisconsin researcher who made and mapped this calculation.""A 1,000-year flood event ... signifies just a 0.1 percent chance of such an event happening in any given year."But, but, but: "[R]ainfall and flood data generally go back only 100 years or so, so statistical tricks must be applied to determine what 500-year and 1,000-year events actually represent. Furthermore, the climate is changing and precipitation events have become more intense in recent decades, so what constitutes different return frequencies (100-year, 500-year, 1,000-year and so forth) is probably changing."Why it matters: "Climate change studies have found that what's considered a 500-year flood today may become much more frequent in coming decades."