Climate change in cities and towns exposed to flood risks: The 1985-2015 global footprint of human settlements in the East Asia, Pacific and sub-Saharan Africa
“In a time when human settlements should be adapting to climate change, many countries are actually rapidly increasing their exposure to floods,” says Jun Rentschler, an economist at the World Bank in Washington DC who led the study. “This is a concerning trend, especially as climate change is intensifying flood disasters worldwide.”
Between 1985 and 2015, the extent of human settlements around the world grew by 85.4% according to Rentschler and his co-authors. However, development in areas exposed to the biggest flood hazards — those where the worst floods can see water rise higher than 150 centimetres — increased by 122%, far outpacing the growth in flood-safe areas. The team referred to the floods caused by rising rivers and torrential rain, as well as coastal flooding caused by storm surge and sea-level changes.
There are many reasons construction takes place despite the danger, and Dr. Rentschler and his colleagues did not attempt to analyze which ones were important in specific places. In some countries, the safest land could already be occupied, meaning new development will have to take place in areas that had previously been avoided. Governments want to collect property taxes. People want second homes. And waterfront living has long had a certain appeal.
In the study, which was published on Wednesday, the researchers analyzed decades of satellite data to see how the physical footprints of cities and towns changed worldwide between 1985 and 2015. They then compared this expansion with high-resolution maps of present-day flood exposure.
The East Asia and Pacific region was the area most exposed to flooding. Almost a quarter of all settlements were prone to flooding in 2015, the highest portion in the world. North America and sub-Saharan Africa had the lowest exposure, at 4.5% and 4.6% of settlements, respectively.
About 6 percent of land was in the very high flood- hazard category. A further 76,400 square kilometres was in the ‘high’ flood-hazard bracket — between 50 and 150 centimetres.
The study didn’t take into account flood defense systems such as levees. There is not a good global inventory of these structures, said Dr. Rentschler. Figuring out what to do in the event of a flood can be done by understanding the vulnerability, even in places with artificial flood protections.
We work with countries to make sure they know not to assume the job is finished because the risk is gone.
Dr. Rentschler said he hoped that this kind of data could someday help inform government policies by tracking economic development that exposes people and structures to greater flood risk.
Rentschler and his colleagues plan to monitor trends in climate risk and urban growth in the future. They hoped to identify high-risk developments early on and announce success stories, thus helping to build people’s resilience to climate change.
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The figures were striking for China. The authors wrote that the extent of settlements in the highest Flood-hazard category more than tripled over three decades.
Rentschler says development on flood plains is driven by land scarcity. He says that in many areas, much of the safe land suitable for development is already occupied, forcing new development into areas that were previously avoided because of flood risk.
“Countries face trade-offs between economic opportunities and disaster risk,” he notes, highlighting that this is a factor in the expansion of major port cities, beach-front communities and tourism hubs in flood-prone coastal areas.
Development in flood zones can be a poor choice if the flood risk assessment is poor.