On September 15, 2009, the Atlanta metropolitan area was hit with intense rainfall and catastrophic flooding. Entire neighborhoods were submerged, the state’s busiest expressway was underwater, cars were swept away, and rising flood waters mixed with sewage.
The new research suggests that comparable scenarios will strike other urban areas in the coming years as a result of two coinciding human activities, greenhouse gas emissions and urban development.
“When we account for these twin forcing agents of environmental change, the effect of the built environment and the effect of greenhouse gasses, we note a strong tendency toward increased extreme precipitation over future US metropolitan regions,” said study lead author Professor Matei Georgescu.
Previous research has shown that urban development intensifies precipitation through the urban heat island effect, which makes it hotter in cities compared to surrounding regions. The heat creates additional energy that forces the air to rise, condense, and form precipitation.
The amount of precipitation a city receives either increases or decreases in response to the heat island effect.
The researchers used regional climate modeling to investigate how extreme precipitation in cities throughout the continental United States would be affected by both urban development and climate change caused by greenhouse gas emissions.
“This new study is unique,” said Professor Georgescu. “We used climate-scale simulations with a regional climate model to examine potential changes in future extreme precipitation resulting from both urban expansion and increases in greenhouse gasses, across dozens of cities across the continental United States.”
In essence, the results showed that incorporating greenhouse gasses into a regional climate model offset the sometimes-diminishing effect of urban development on extreme precipitation, said Professor Georgescu.
“These are the effects our cities are likely to experience when accounting for the twin forcing agents of urban expansion and greenhouse gas emissions, simultaneously. What this means for U.S. cities in the future is the need for a consistent response to an increase in extreme precipitation. We’re no longer likely to see a decrease in precipitation as we’ve seen before.”
Denver, Phoenix, and Houston are some of the cities like Atlanta that appear to be vulnerable to extreme precipitation and flooding. Professot Georgescu said the study’s findings show the pressing need for cities to develop policies to address flooding that threatens each city’s unique resilience and planned infrastructure investments.
“If we trust the models’ capability to simulate average and extreme precipitation so well, and our results demonstrate such simulation skill, then we can conduct simulations that include future urbanization, future greenhouse gasses, separately and then together, and trust what the model will tell us.”
Professot Georgescu noted that it’s not just about reducing greenhouse gas emissions. “It’s also about how you build cities. How extensive they are, how vertical they are, how dense they are, how much vegetation there is, how much waste heat you put into the environment through electricity use, through air conditioning, or through transportation. All of these things can impact future precipitation in our cities.”
The study highlights the complex and regionally specific ways in which the competing forces of greenhouse gases and urban development can impact rainfall across U.S. metropolitan regions, explained Professor Ashley Broadbent of ASU’s School of Geographical Sciences and Urban Planning.
“This complexity reinforces that future adaptation efforts must be informed by simulations that account for these interacting agents of environmental change,” said Professor Broadbent.
The study is published in the journal Environmental Research Letters.