New global map shows populations are growing faster in flood-prone areas
And while the world population grew by 18.6% from 2000 to 2015, the population of these regions exceeded that growth, increasing by 34.1% during the same period. This means that between 58 and 86 million more people have been exposed to flooding in these places over the past 15 years.
“It’s not particularly surprising that flooding is increasing,” says Beth Tellman, co-founder of flood mapping startup Cloud to Street and lead author of the study. “But what struck me was that people were heading to places where we have observed flooding in the past.”
Researchers looked at more than 3,000 events in the Dartmouth Flood Observatory database, which records floods reported in media coverage. They matched events containing location data with satellite images from MODIS, an instrument mounted on two NASA satellites that have each captured daily images of the Earth since 2000.
The researchers used an algorithm to map where the flooding had occurred by sorting out which pixels were covered in water and which were not. Then they added population data to see how the trends in the flooded areas changed over time.
Low- and middle-income countries have experienced the fastest population growth in flood-prone areas over the past two decades, with the highest growth rates in sub-Saharan Africa and South Asia.
Socio-economic factors could explain part of the movement, Tellman says. Vulnerable groups may have no choice but to settle in flood-prone areas, where land may be cheaper and more available.
Using satellite images, the researchers were able to describe the impacts of actual flooding more accurately than traditional models. Models can capture certain types of flooding, such as those that occur around rivers and on coasts. But for others caused by heavy rainfall or random events, such as dam failure or a storm surge aligned with high tide, satellite images provide a clearer picture.
The 913 mapped floods still represent only a fraction of the tens of thousands that occur worldwide each year. “This is just the tip of the iceberg,” Tellman says.
MODIS takes images with a resolution of 250 meters, approximately the length of two football fields. This means that the researchers could not map the smaller floods or those in most cities. Clouds also interfered with the image processing algorithm, and since the satellites only passed over a specific location on Earth once or twice a day, they also missed the floods. short term.
Newer instruments have a much higher resolution and can see through clouds, says Bessie Schwarz, co-founder and CEO of Cloud to Street. These tools, combined with artificial intelligence, can map floods even more precisely today. But to consistently map flooding over time, researchers had to stick to images from a single source, using technology that has been around longer.
The effort gives scientists a clearer picture than any other resource of the scale and human impact of recent flooding. And the results will be especially useful for modelers trying to predict risk, says Philip Ward, who studies flood risk assessment at Vrije Universiteit Amsterdam and was not involved in the study.