It's Raining a Little Bit More

 "Observed Changes in Daily Precipitation Intensity in theUnited States" is a new article published this month in the Geophysical Research Letters. This is a study by two Northwestern University researchers, Daniel Horton and Ryan Harp on how the intensity of rainfall has changed over time. The below is from the Northwester University press release:

“When people study how climate change has affected weather, they often look at extreme weather events like floods, heatwaves and droughts,” said Northwestern’s Daniel Horton, the study’s senior author. “For this particular study, we wanted to look at the non-extreme events, which are, by definition, much more common. What we found is pretty simple: When it rains now, it rains more.” 

Horton is an assistant professor of Earth and planetary sciences in Northwestern’s Weinberg College of Arts and Sciences, where he also leads the Climate Change Research Group. Ryan Harp, an Ubben Postdoctoral Research Fellow at the Institute for Sustainability and Energy at Northwestern, is the paper’s first author.

To conduct the study, Harp and Horton compared two climatologically distinct time periods: 1951-1980 and 1991-2020. For each time period, they used historical precipitation data from the Global Historical Climatology Network, a publicly available database maintained by the National Oceanic and Atmospheric Administration. For the past 10 years I have been a rain gauge reader for the Community Collaborative Rain, Hail and Snow Network, the source of the data.

The researchers broke the observations into 17 distinct climate regions in the United States. These regions reflect differences in temperature, precipitation, vegetation and ecosystem dynamics. After analyzing data from two time periods across regions, Drs. Harp and Horton discovered that precipitation intensity (including rain and snow) had increased across much of the United States, particularly in the East, South and Midwest. Changes in the western United States were not detected.

In this study, Harp and Horton narrowed their focus to examine how much precipitation falls during each rain or snowfall event. For their next study, they plan to investigate if annual precipitation is becoming more variable and if precipitation events are becoming more or less frequent. Although this study does not attribute the changes in precipitation rates to climate change, Harp said the findings are consistent with human-caused global warming and climate model predictions.

“Warmer air holds more moisture,” he explained. “For every one degree Celsius the atmosphere warms, it holds 7% more water vapor. So, these observations are consistent with the predicted effects of human-caused global warming.”

If this is an indication of what we can expect in the future as the climate, weather and rainfall respond to the ever increasing CO2 equivalent emissions in the atmosphere, we need to take action to prevent increased damage from more intense rain. Increased precipitation intensities affect many sectors, including agriculture and infrastructure, as well as lead to increased risks of landslides and flooding. We need to design infrastructure that can accommodate more intense rainfall in our plans for future growth and development. We need infrastructure that is more resilient to changing weather patterns because they are changing.

“You don’t need an extreme weather event to produce flooding,” Horton said. “Sometimes you just need an intense rainstorm. And, if every time it rains, it rains a little bit more, then the risk of flooding goes up.”