Posted in | News | Climate Change | Pollution

Urban Development, Greenhouses Gases can Trigger Extreme Floods

When the rain started to pour down in northern Georgia on September 15th, 2009, the residents of Atlanta were completely unaware that they would be witnessing epic flooding across the city.

Image Credit: Discovod/

Localities, including Peachtree Hills, were inundated; the busiest expressway in Georgia was submerged, and so were the bridges and roads; the untreated sewage combined with the growing floodwaters; and people and cars were swept away. Sonny Perdue, who was the presiding governor of Georgia at that time, had announced a state of emergency.

Moisture from the Gulf of Mexico drove the 2009 flood. Now, after 10 years, scientists from Arizona State University (ASU) are asking whether a mix of climate change driven by greenhouse gases and urban development could lead to similar situations in U.S. cities. And based on a recently published analysis, the answer is yes.

When we account for these twin forcing agents of environmental change, the effect of the built environment and the effect of greenhouse gases, we note a strong tendency toward increased extreme precipitation over future US metropolitan regions.

Matei Georgescu, Study Lead Author and Associate Professor, School of Geographical Sciences and Urban Planning, Arizona State University

Earlier analyses have demonstrated that precipitation is modified by urban development because of the so-called urban heat-island effect—that is, the variation between the surrounding rural area and the temperature in a city.

When a city develops, it becomes warmer. The extra warmth adds energy to the atmosphere, which, in turn, pushes it to rise more quickly, condense and create precipitation, and finally rain down across the city or downwind of the city.

Hence, the amount of precipitation received by a city either decreases or increases in reaction to the urban heat-island effect.

But when both urban development and greenhouse gases are taken into consideration, the regional climate modeling targeting the continental U.S. demonstrated the compensating effects between the impact of greenhouse gas emissions and urban development on extreme precipitation.

The study was published online in the Environmental Research Letters journal.

Earlier, scientists did not explore these two variables together. Analyses on upcoming precipitation over urban surroundings usually investigate the effects for a restricted number of events, and they do not consider the twin forcing agents of greenhouse gas and urban development that caused climate change.

This new study is unique. 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 gases, across dozens of cities across the continental United States.

Matei Georgescu, Study Lead Author and Associate Professor, School of Geographical Sciences and Urban Planning, Arizona State University

The latest study essentially demonstrated that if greenhouse gases are incorporated into a regional climate model, the occasional declining effect of urban development on extreme precipitation can be compensated, added 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,” explained Georgescu.

Similar to Atlanta, cities across the United States, such as Houston, Phoenix and Denver, seem to be susceptible to high precipitation and its corresponding flooding.

According to Georgescu, the study results demonstrate the urgent need for cities to design new policies to deal with the flooding events that threaten the special resilience and planned infrastructure investments of every city.

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 gases, separately and then together, and trust what the model will tell us.

Matei Georgescu, Study Lead Author and Associate Professor, School of Geographical Sciences and Urban Planning, Arizona State University

However, Georgescu also noted that it is not just about decreasing the emission of greenhouse gases.

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,” he added.

As a matter of fact, the new study holds significant implications for the adaptation and planning of climate change.

The analysis also emphasizes the regionally-specific and complex ways through which the competing forces of urban development and greenhouse gases can affect rainfall across the metropolitan areas in the United States, elaborated Ashley Broadbent, an assistant research professor in the School of Geographical Sciences and Urban Planning at ASU.

This complexity reinforces that future adaptation efforts must be informed by simulations that account for these interacting agents of environmental change,” Broadbent concluded.

Other authors of the study included M. Wang and M. Moustaoui from ASU; and E. Scott Krayenhoff from ASU, and also the University of Guelph in Ontario, Canada.

The study was financially supported by the National Science Foundation via the Urban Water Innovation Network.

Journal Reference:

Georgescu, M., et al. (2021) Precipitation response to climate change and urban development over the continental United States. Environmental Research Letters.


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