A new study finds that possible global mean sea level rise of up to 4 feet (1.23 meters) by the year 2100 may lead to the flooding of up to 4.6 percent of the global population, in absence of adaptation measures. The findings are detailed in the Proceedings of the National Academy of Sciences. The researchers added that the cost of coastal strategies to adapt to sea level rise, such as dikes, is much smaller than the potential costs of flooding damage.
Coastal flood damage is expected to increase significantly in the 21st century as sea levels rise. However, climate adaptation researcher Jochen Hinkel at the Global Climate Forum in Berlin and his colleagues noted few studies have analyzed the global impacts of this damage. Although those studies did explore factors such as a range of sea-level scenarios, protection strategies, population statistics and socioeconomic changes, none explored all these dimensions together, they added.
To address this gap, the researchers analyzed a variety of datasets. They relied on the Dynamic Interactive Vulnerability Assessment (DIVA) model for a global-scale exploration of coastal zones. To model rising sea levels, they analyzed four different climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), each combined with three scenarios of how melting ice sheets and glaciers may contribute to rising sea levels. These helped generate sea-level rise projections based on oceanic thermal expansion (how the oceans will expand as they get warmer) as well as water coming in from glaciers and the Greenland and Antarctic ice sheets.
In addition, for continental topography data, they used the Global Land One-kilometer Base Elevation (GLOBE) dataset and the Shuttle Radar Topography Mission (SRTM). For population data, they used the Global Rural-Urban Mapping Project (GRUMP) and the LandScan high-resolution global population dataset. For socioeconomic data, they used five population and gross domestic product growth scenarios. When it came to how people might adapt to rising sea levels, they considered two strategies involving dikes — one where they are maintained but not raised in height, and another were they are raised as the demand for safety increases with growing affluence and increasing population density.
If dikes are not raised, the scientists estimated 0.2 to 4.6 percent of the world’s population is expected to be flooded annually in 2100 under 0.8 to 4 feet (25 to 123 cm) of global average sea-level rise, with expected annual losses of 0.3 to 9.3 percent of global gross domestic product. Although the global costs of protecting coasts by raising dikes are substantial, with annual investment and maintenance costs of $12 billion to $71 billion in 2100, these are much smaller than the global costs of flooding damage.
“This research highlights the need for substantial investments in coastal protection,” Hinkel says. “The cost of not doing anything are much higher than the costs of doing something — if we don’t do anything, the effects are traumatic, with trillions of dollars worth of assets damaged.”
The researchers noted many uncertainties remain in such analysis. For instance, they noticed that in many places, coastal population and the amount of assets exposed to flood damage are growing faster than the trends they assumed, since factors such as migration and urbanization are complex and variable. In addition to sea-level rise, possible changes in storminess and potential increases in hurricane intensity may alter flood damage.
Although the researchers explored the effects of dikes, “we don’t want to suggest that we should protect the whole world via dikes,” Hinkel says. “More and different options, such as dune or wetland restoration as well as establishing set-back zones, are also available.”
In the future, the researchers would like to explore which sea-level-rise adaptation strategies are best for any given location. They also want to focus on socioeconomic and institutional aspects of these strategies, such as city planning and public-private partnerships, Hinkel says.
“At a global scale, protection is very efficient in reducing impacts for densely populated areas,” Hinkel says. “Sea level rise will not simply wipe out low-lying cities like Miami or New York — humans will adapt.”