For the first time in history, we are an “urbanized” society. More people live in urban spaces than rural spaces, and this trend is only projected to continue. As a former NASA scientist and current professor at the University of Georgia, I have spent a significant portion of my research career exploring ways urbanization affects weather or climate. Devastating California wildfires and recent hurricanes prompt important questions about climate change attribution. However, I see another human influence, often ignored, that makes weather related events worse – and that’s urbanization itself.
2018 California fires from space. NASA/JPL/CALTECH
Have you heard of WUI (“woo-we”)? I bet the majority of you answered “no.” WUI is an acronym that stands for wildland-urban interface. The U.S. Forest Service defines the wildland-urban interface as “the area where structures and other human development meet or intermingle with undeveloped wildland, and it is where wildfires have their greatest impacts on people.”
The WUI as mapped by the U.S. Forest Service (graphic below) is very important for wildfire management. The wildland-urban interface has a greater chance of being impacted by fire because it typically has ample fuel sources, plants within the landscape, and structures that can burn. However, the interface also contributes to the fires themselves. The most obvious way? Humans are in the WUI and about 95% of wildfires are started by humans according to CalFire. However, there is also a less obvious reason the WUI is problematic.
The wildland urban interface of the United States. USFS
Wildfires are a part of the natural ecosystem. A 2013 U.S. Forest Service Report noted that “fire plays a vital role in the maintenance of the health of many ecosystems, in part by promoting a mosaic of vegetation and by stimulating the establishment and growth of particular trees and other plants. However, the irony is that as more people live and matriculate in the wildland-urban interface (see graphic below), they want fires suppressed. This allows fuel to build up and increases the chances of fire ignition and rapid spreading. This process is called “fuel loading.” Dr. Marcus Williams, a U.S. Forest Service scientist in Athens, Georgia, adds that “the WUI is also a problem for smoke management…Harder to do controlled burns which could reduce some fuels.” Kimiko Barrett, a policy and research analyst at Headwaters Economics, a Montana-based nonprofit research, recently told U.S. News and World Report reporter Gabrielle Levy:
You have this dual problem of fuel loading through decades and decades of fire suppression, in addition climate change, in addition to people building in high-risk areas…It’s a three-pronged sword that current federal management is trying to have to respond to.
The fingerprint of urbanization can also be detected in hurricanes. As I did with fires, I will start with the most obvious way. Impervious surfaces like parking lots and roadways modify the natural water cycle leading to lack of infiltration into soils and increased surface runoff. When a hurricane like Florence or Harvey sits over an area with intense or sustained rainfall rates that is a recipe for flooding. According to the U.S. Geological Survey (USGS), Hurricane Florence set 28 flood records.
A new study out of Princeton University also revealed that the urban sprawl of Houston made the flooding worse in Hurricane Harvey. In a press release, the University noted:
The researchers found that as Houston’s urban center grew upward and outward, flood risk changed, including a greater risk of higher flood peaks and more serious and dangerous flooding overall. As the city’s miles of impervious sidewalks and streets increased, heavy rainfall could not be absorbed into the ground.
However, the Princeton researchers also describe another way Houston affected Hurricane Harvey. The actual impervious surfaces and buildings may have affected the hurricane itself. The study, published in the journal Nature, simulated the meteorology of Hurricane Harvey with and without the urban landscape coupled to the weather model. They found that with the Houston landscape removed, the rainfall amounts and patterns changed.
This is very similar to results my research group at the University of Georgia published in the journal Environment and Planning B in 2010. In that study, we modeled the impact of Houston landscape on summertime rainfall and projected what rainfall patterns might look like with future landscape growth. The Princeton researchers suggested that surface roughness and turbulence associated with buildings can alter the wind flow and perhaps amplify the meteorological processes (primarily lift) that cause precipitation clouds.
Gabriele Villarini is an associate professor of civil and environmental engineering and director of Iowa’s IIHR—Hydroscience and Engineering research center. He said “friction caused by high winds buffeting tall buildings created a drag effect that influenced air and heat movement and resulted in optimal conditions for precipitation.” I saw some media reports mischaracterize this process as “slowing the hurricane down.”
This caused criticism in some circles. As someone that has studied these very processes for almost two decades, I understood what the Princeton researchers were trying to convey, but it is perhaps counter-intuitive to some. In another study of Houston published in the Journal of Atmospheric and Oceanic Technology, our University of Georgia group found that wind effects of buildings and the urban heat island also altered the formation and movement of the sea breeze front.
By the way, if you look at the NASA satellite based summary of rainfall in Hurricane Harvey, I cannot help but noticed something that is very supportive of the Princeton study. The values of rainfall are maximized over the Houston metropolitan area. While it is important to note that this satellite based estimate from the Global Precipitation Measurement (GPM) IMERG product is a more coarse and averaged value, it is compelling enough to continue this line of research inquiry.