Extreme downpours — like those that flooded Louisiana, Houston and West Virginia earlier this year — will happen nearly three times as often in the United States by the end of the century, and six times more frequently in parts of the Mississippi Delta, according to a new study. Scientists have long pointed out that warmer air holds more moisture, so man-made climate change will increase the frequency of extreme downpours. That increase has already started , they say, but new work with much stronger computer simulations shows just how bad it will get, and where.