Rising Seas Threaten Low-Lying Coastal Cities, 10 percent of World Population

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 (CIDR); and CIESIN senior systems analyst/GIS developer Kytt MacManus, who had processed some of the original estimates and led SEDAC’s development of version 2 of the Low-Elevation Coastal Zone data set in 2013. Hasim Engin, GIS Analyst at CIDR, also contributed.

This time, the research team selected the MERIT DEM: Multi-Error-Removed Improved-Terrain Digital Elevation Model as the basis for its update, chosen because it offered improved vertical accuracy, especially in low-lying delta regions. For population data, the team selected a newly released data collection by the European Commission’s Joint Research Center, the Global Human Settlement Layer 2019 release (GHSLr2019), because of its state-of-the-art methods for identifying built-up areas and their populations, as well as its urban dichotomy. The GHSLr2019 data set included population estimates for the years 1990, 2000, and 2015, and differentiated categories of settlement types such as high density urban clusters, low density urban clusters, and rural areas. This let researchers estimate the type of settlements and population vulnerable to flood impacts worsened by or even caused by or climate change-driven sea level rise.

The report finds that urban areas, like urban residents, are disproportionately located in low-elevation coastal areas. Although just 2 percent of the world’s total land is urban, almost 10 percent of coastal land lower than 10 meters above sea level is already urbanized or quasi-urbanized. To this end, 13 percent of the world’s total urban land mass is located in low-elevation coastal zones.

Moreover, coastal cities are growing faster than average. Between 2000 and 2015, urban centers at an elevation lower than 10 meters above sea level had an average annual growth population rate of 1.98 percent, compared to 1.62 percent for all urban centers. Mega-cities such as Lagos, Nigeria; Tianjin, China; and Manila, Philippines, are largely responsible for this difference in rate, but the trend is also observable elsewhere.

According to the report, some countries’ urban populations are especially concentrated in low-elevation coastal zones. For example, in Guyana, Maldives, Belize, and Suriname, 100 percent of the urban population lives at an elevation lower than 10 meters above sea level, and 81 percent of the urban populations of Thailand and Bahrain live at this low elevation. Because most countries’ economies depend heavily on the activity of their cities, the increased vulnerability caused by the low elevation poses a risk to future national economic development and growth.

The report further finds that in terms of the total number of people living in the low-elevation coastal urban centers, China has the most (129.5 million people), followed by India (55.2 million people), and Bangladesh (40.9 million people). Notably, some of the largest numbers of people are in countries with the least capacity to prepare for or recover from climate-related shocks.

To protect urban areas and residents of low elevation coastal areas, the report finds, it is essential to build resilience in coastal cities, and to guide future urban growth towards less vulnerable areas. Urban adaptation and resilience is especially important in lower-income countries with more complex and widespread vulnerability to climate events, lacking the technological prowess, advance planning, and robust social services that helped Japan withstand Typhoon Hagibis. The recommendations of the Coalition for Urban Transitions’ report are meant to support decision makers navigating difficult choices. The report findings, along with up-to-date, high-resolution data such as the latest version of the Low Elevation Coastal Zone data set, are critical resources in preparing for and adapting to the localized impacts of climate change.

This blog post was written by Leah Lazer, research analyst for Coalition for Urban Transitions (CUT), which is hosted and managed by WRI Ross Center for Sustainable Cities and the C40 Cities Climate Leadership Group.This story is published courtesy of Earth Institute, Columbia University.