http://www.nytimes.com/2016/12/06/science/global-warming-extreme-storms.html?_r=0

 

Decades of research (and perhaps your own recent experiences on hot, humid days) have suggested that climate change will lead to an increase in big storms that cause flash floods, landslides and other natural disasters. Now, a new study shows that such intense precipitation will most likely increase across the continental United States, but with important regional variations.

 

The study by scientists from the National Center for Atmospheric Research in Boulder, Colo., published on Monday in Nature Climate Change, found that across the country, rainstorms may become more frequent and intense if greenhouse gas emissions remain at current levels. The biggest increase would be in the Northeast and the Gulf Coast.

In fact, the study shows that these intense storms could become five times as frequent. And when they do happen, there could be up to 70 percent more rain, potentially turning a heavy but not catastrophic storm into something closer to a biblical flood.

Other regions, like the Pacific Northwest, already quite wet, and the central United States, might become drier on average, but even there extreme rainfall is likely to intensify, the researchers said. That means there could be both an increased risk of droughts and of flash floods. The study found that humidity was a key factor. Extreme precipitation is likely to increase with rising temperatures because of growing atmospheric humidity, leading to a higher risk of flash flooding nationwide.

Climate scientists had anticipated these results and have already seen some of them play out, with serious flooding in New England and Louisiana over the last several years. But this study provides a new layer of detail and analysis, said Kevin E. Trenberth, a senior researcher at the National Center for Atmospheric Research who was not involved in it.

Dr. Trenberth said that while the future may not look exactly as predicted, "It builds confidence in our previous work and adds to the urgency that this is the sort of thing we need to pay attention to."

The researchers divided the continental United States into roughly 2.5-mile square grids and used computer simulations to look at extreme precipitation - on an hourly basis - in each square, with precipitation data from 2000 to 2013.

Andreas Prein, the lead author of the study, said that there was some variation depending on region, and that some areas of the country would not be as affected by these types of storms. For instance, while the Pacific Northwest is likely to see more intense winter storms, summer thunderstorms will probably not increase in intensity, he said.

Dr. Prein said the results of this study showed that many areas of the country would need to build or restore infrastructure, including reservoirs and storm sewers, to handle the intense rainfall.

One area of particular concern is the Midwest, which is already becoming drier on average, especially in the summer, Dr. Prein said. However, he added, "The extreme events there are getting more frequent and more intense."

It might seem as though big rainstorms would help alleviate drought in the Midwest or elsewhere, but the opposite can actually be true, Dr. Prein said. If a heavy rainstorm occurs during a drought, it can lead to soil erosion, washing away the plants that help the soil absorb moisture.

Without regular, gentler rain to restore the soil, he said, conditions will continue to worsen.

"Droughts and extreme rainstorms?" Dr. Prein said. "This could be a very harmful combination."