New research published this week in Nature used direct satellite observations of floods, rather than the standard modeled estimates, to reveal that the proportion of the population exposed to floods has grown by 24% globally since the turn of the century. That's 10 times more than scientists previously thought, and it's due to both increased flooding and population migration. This marks an increase of the population inside flood-prone regions to as many as 86 million people.

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The research was led by University of Arizona assistant professor Elizabeth Tellman and postdoctoral researcher Jonathan A. Sullivan, who both joined the School of Geography, Development and Environment in the College of Social and Behavioral Sciences this month.

Tellman, who is joining UArizona from Columbia University, researches the causes and consequences of global environment change on vulnerable populations and is also the chief science officer and co-founder of Cloud to Street, a global flood tracking and risk analytics platform for disaster managers and insurers.

She has three NASA grants underway related to flooding and said she is eager to work with UArizona students at the Tellman lab.

"Using satellite observation data of floods with improved spatial-temporal resolution will help policymakers understand where flood impacts are changing and how best to adapt," she said.

Global Flood Database Shows True Impact of Flooding

Cloud to Street hosts the Global Flood Database, the database behind the Nature paper and the largest and most accurate dataset of observed floods ever produced, providing a new view into the true scope of flooding. Powered by twice-daily, global satellite imaging, the Global Flood Database has mapped and analyzed 913 flood events in 169 countries since 2000.

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Most flood maps, including those used by the Federal Emergency Management Agency, rely on modeling that simulates floods based on available ground data, such as elevation, rainfall and ground sensors. These models are time intensive and can have substantial limitations, entirely missing flooding incidents in regions not historically prone to flooding, Tellman said.

The Global Flood Database instead relies on satellite observations of actual flooding over the past two decades. This allows for additional analyses of the scope, impact and trends of recent flooding.

"The Global Flood Database can improve the accuracy of global and local flood models and vulnerability assessments, increase the efficacy of adaptation measures, and deepen our understanding of how climate, land cover change, and floods interact," Tellman said.

Increased Exposure to Floods

Most flood events in the database were caused by heavy rainfall. The other top causes were tropical storms or surges, snow or ice melt, and dam breaks, illustrating the increasingly deadly cost of failing infrastructure. Other key findings of the Nature paper include:

• Between 58 million and 86 million people moved into observed flood regions between 2000 and 2015, marking a 20 to 24% increase in the proportion of the population exposed to floods.
• Worldwide, 2.23 million square kilometers, or about 861,000 square miles, were flooded at some point between 2000 and 2018, affecting between 255 million and 290 million people. 
• By 2030, climate and demographic change will add 25 new countries to the 32 already experiencing increasing floods, the researchers estimate. 
• Despite causing less than 2% of floods, dam breaks had the highest increased incidence (177%) in proportion of population exposed. 
• Population growth in flooded areas is driven by people moving into flood-prone areas, and economic development in those regions.
• Nearly 90% of flood events occurred in South and Southeast Asia.
• The satellite data uncovered previously unidentified increases in flood exposure in Southern Asia, Southern Latin America and the Middle East.

"We found that economic development and people moving into flood-prone areas is significantly increasing the number of people exposed to floods in those regions," Sullivan said. "Furthermore, increasing flood exposure is rooted in underlying conditions that give vulnerable populations no choice but to settle in flood zones."

Bessie Schwarz, CEO and co-founder of Cloud to Street, said the data available in the Global Flood Database can lead to actionable lessons for policy makers and improve the velocity of payments to the survivors of floods.

"In the aftermath of a major flooding event, the greatest determinant of a community's recovery time and future resilience is how soon they access capital," Schwarz said. "More people and more assets are impacted by flooding than any other climate-fueled disaster, which in turn keeps poor countries poor and drives up the price of food and housing everywhere. We are proud to enable governments and insurers to protect people and billions of assets they have never been able to before."

Additional co-authors of the Nature paper were from NASA, Google Earth Outreach, Columbia University, University of Michigan, University of Colorado, University of Texas at Austin and the University of Washington.

Funding for the research was provided by Google as a Google Earth Engine research award and NASA award 80NSSC18K0426, 16-GEO16-0068, Integrating Global Remote Sensing and Modeling Systems for Local Flood Prediction and Impact Assessment.