Heavy Rain & Flooding
More heavy rain is one of the hallmark signs of climate change. As the atmosphere warms, more water evaporates from soils, plants, lakes, and oceans. For every additional 1°F of warming, the atmosphere is capable of holding an additional 4 percent of water vapor. So when this additional water vapor condenses into precipitation, it leads to heavier rain — or when cold enough, heavier snow. The amount of precipitation falling in the heaviest one percent of all events is increasing across the U.S. — up 55 percent in the Northeast and 42 percent in the Midwest since 1958.
While there are other human factors associated with flooding, such as engineering of waterways and land development, the fact is that more downpours increase the risk of flooding. This expands the risk zone into areas that were previously safe from floods. Also, heavier rain increases erosion and runoff, removing agricultural topsoils and increasing the flow of pollutants into our waterways.