And while the global population grew by 18.6% from 2000 to 2015, the population in these areas outpaced that growth, increasing by 34.1% over the same period. This means that between 58 and 86 million people were more exposed to floods in those places over 15 years.
“It’s not particularly surprising that the floods will increase,” he says Beth Tellman, co-founder of the flood mapping initiator Cloud to Street and lead author of the study. “But what struck me was that people were moving into places where we had seen floods in the past.”
The researchers looked at over 3,000 events in Dartmouth Flood Observatory a database, which records floods reported by the media. They reconciled events that had location data with satellite imagery FASHION, an instrument mounted on two NASA satellites, each capturing daily images of the Earth since 2000.
The researchers used an algorithm to map the location of the flood by sorting which pixels were covered with water and which were not. They then added population data to see how trends in flooded areas changed over time.
Low- and middle-income countries have recorded the fastest population growth in flood-prone areas in the last two decades, with the highest growth rates in sub-Saharan Africa and South Asia.
Socioeconomic factors may explain some of the movements, Tellman says. Vulnerable groups may have no choice but to settle in floodplains, where land could be cheaper and more accessible.
Using satellite imagery, researchers were able to more accurately describe the impact of actual floods than traditional models. Models can include some types of flooding, such as those that occur around rivers and on banks. But for others caused by heavy rainfall or accidental events – such as the bursting of dams or storm surges that follow the tide – satellite images provide a clearer picture.
The 913 mapped floods are still only a fraction of the tens of thousands that occur globally each year. “It’s just the tip of the iceberg,” Tellman says.
MODIS captures images with a resolution of 250 meters, the length of two football fields. This means that researchers could not map minor floods or those in most cities. Clouds also interfered with the image processing algorithm, and because the satellites crossed a particular spot on Earth only once or twice a day, they also missed short-lived floods.
Newer instruments have a much higher resolution and can see through the clouds, he says Bessie Black, co-founder and CEO of Cloud to Street. These tools, along with artificial intelligence, can map floods even more accurately today. But in order to systematically map floods over time, researchers had to stick to images from a single source, using technology that has been around for a long time.
The effort gives scientists a clearer picture than any other source of the scale and human impact of the recent floods. And the results will be especially useful for modelers trying to predict risk, he says Philip Ward, which studies the flood risk assessment at Vrije Universiteit Amsterdam and was not included in the study.