World Weather Attribution said climate change is the most likely explanation for extreme downpours in southern Spain, as a warmer atmosphere can hold more moisture, leading to heavier downpours. Human-caused climate change made Spain’s rainfall about 12% heavier and doubled the likelihood of a storm as intense as this week’s deluge of Valencia, according to a rapid but partial analysis Thursday by World Weather Attribution, a group of international scientists who study global warming’s role in extreme weather.