Warming U.S. could see a large increase in extreme rains


precipitation_nca_1991-2012_lrg
Precipitation change in the U.S. from 1991 to 2012. (NASA)
Jake Slobe | January 4, 2017

According to a new study by scientists at the National Center for Atmospheric Research (NCAR), the number of summertime storms that produce extreme downpours could increase by more than 400 percent across parts of the United States  including sections of the Gulf Coast, Atlantic Coast, and the Southwest by the end of the century.

The study, published in the journal Nature Climate Change, also found that the intensity of individual extreme rainfall events could increase by as much as 70 percent in some areas. That would mean that a storm that drops 2 inches of rainfall today would be likely to drop nearly 3.5 inches in the future.

“I think this is one of the most severe consequences of climate change, at least in the US,” said Andreas Prein, study co-author.

An increase in extreme precipitation is one of the expected impacts of climate change because scientists know that as the atmosphere warms, it can hold more water, and a wetter atmosphere can produce heavier rain. In fact, an increase in precipitation intensity has already been measured across all regions of the U.S. However, climate models are generally not able to simulate these downpours because of their coarse resolution, which has made it difficult for researchers to assess future changes in storm frequency and intensity.

Prein and his co-authors used the new dataset to investigate changes in downpours over North America in detail. The researchers looked at how storms that occurred between 2000 and 2013 might change if they occurred instead in a climate that was 9 degrees Fahrenheit warmer (the temperature increase expected by the end of the century if greenhouse gas emissions continue to rise unabated).

The study also found that the intensity of extreme rainfall events in the summer could increase across nearly the entire country, with some regions, including the Northeast and parts of the Southwest, seeing particularly large increases, in some cases of more than 70 percent.

A surprising result of the study is that extreme downpours will also increase in areas that are getting drier on average, especially in the Midwest. This is because moderate rainfall events that are the major source of moisture in this region during the summertime are expected to decrease significantly while extreme events increase in frequency and intensity. This shift from moderate to intense rainfall increases the potential for flash floods and mudslides, and can have negative impacts on agriculture.

The study also investigated how the environmental conditions that produce the most severe downpours might change in the future. In today’s climate, the storms with the highest hourly rainfall intensities form when the daily average temperature is somewhere between 68 to 77 degrees and with high atmospheric moisture. When the temperature gets too hot, rainstorms become weaker or don’t occur at all because the increase in atmospheric moisture cannot keep pace with the increase in temperature. The new study found that storms may continue to intensify up to temperatures of 86 degrees  because of a more humid atmosphere. The result would be much more intense storms.

“Understanding how climate change may affect the environments that produce the most intense storms is essential because of the significant impacts that these kinds of storms have on society,” Prein said.

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