Urban areas in the low-elevation coastal zone in Japan, 2015. Source: CUNY Institute for Demographic Research (CIDR), Institute for Development Studies (IDS), and the Center for International Earth Science Information Network (CIESIN). In Climate Emergency, Urban Opportunity (2019). Credit: Hasim Engin, CIDR

Study: Rising seas threaten low-lying coastal cities, 10% of world population

The recent Typhoon Hagibis—the most powerful storm to hit Japan since 1958—caused massive destruction. The reported death toll as of October 22 has climbed to 80, with another 398 injured and 11 people still missing. Tens of thousands of homes were flooded, damaged, or without power after torrential rain and powerful winds resulted in tornadoes, widespread mudslides, and overflowing rivers. In addition, an earthquake in the northeastern area of Japan (Chiba-Tokyo) compounded landslides and flooding. Insured losses throughout the country are estimated at more than US$10 billion.

Part of the reason the damage was so intense was because of the concentration of settlements in Japan's low-lying coastal cities. Approximately a fifth of the population lives in low-lying coastal urban centers—comprised of densely clustered residents, buildings, and infrastructure—less than 10 meters above sea level. These areas are especially vulnerable to flooding and storm surge impacts.

A new report from the Coalition for Urban Transitions finds that, because sea level rise exacerbates flooding and storm surge, it is a critical threat to urban coastal areas. More than 10 percent of the world's population now resides in urban centers or quasi-urban clusters situated at less than 10 meters above sea level. Research and analysis by Columbia University's CIESIN contributed to the new findings.

When the Coalition for Urban Transitions decided to investigate the global scale of urban vulnerability to sea level rise, it was quickly apparent that new estimates of exposed populations would be needed. Fortunately, population estimation in an area in which CIESIN has particular expertise. A 2007 paper by a team from CIESIN and the International Institute for Environment and Development provided the first global estimates of impacts to urban populations from sea level rise. The 2007 paper relied on a data set called the Low Elevation Coastal Zone Urban Rural Population Estimates. These estimates were based on population count data from the Gridded Rural Urban Mapping Project (GRUMP), developed by the NASA Socioeconomic Data and Applications Center (SEDAC) managed by CIESIN, and elevation data from the NASA Shuttle Radar Topography Mission. To update these estimates, the Coalition engaged the authors of the original research: Gordon McGranahan of the Institute of Development Studies; Deborah Balk, formerly of CIESIN, now at the CUNY Institute for Demographic Research; and CIESIN senior systems analyst/GIS developer Kytt MacManus, who had processed some of the original estimates and led SEDAC's development of version two of the Low-Elevation Coastal Zone dataset in 2013.

Read the full study here.