This report builds closely on a paper first published online in October 2015 in the Proceedings of the National Academy of Sciences of the United States of America by the same authors. That research used relationships between cumulative carbon emissions, warming, and the future global sea level rise they lock in to assess implications for the United States and its cities. What distinguishes the current report is its application of sea level projections to global elevation, tidal, population, and administrative boundary data, instead of U.S. data only.

Among all nations, this report finds that China has the most to lose from business as usual, with 145 million citizens today on implicated land. China also has the most to gain from limiting warming to 2 °C, which would cut the total to 64 million. Twelve other nations have more than 10 million people living on implicated land under 4 °C warming — India, Bangladesh, Viet Nam, Indonesia, Japan, the United States, Philippines, Egypt, Brazil, Thailand, Myanmar, and the Netherlands, in descending order of total threats. A carbon path that limits warming to 2 °C would reduce exposure by more than 10 million in each listed nation except the last two, and by half or more in all listed nations but Viet Nam (still achieving 44% reduction), Brazil (45%) and the Netherlands (13%). Global megacities with the top ten populations in the balance include Shanghai, Hong Kong, Calcutta, Mumbai, Dhaka, Jakarta, and Hanoi. 4 °C warming could lead to submergence of land inhabited by more than half the population of Shanghai, Mumbai and Hanoi, among these.

These results are based on median sea level rise projections. They are also based on global elevation data with a tendency to understate exposure.

Carbon emissions this century can lock in these projected threats, but the associated sea level rise is expected to play out over a longer period, likely centuries.

This report assesses and lists global nations and urban agglomerations at risk by projected total population exposure, percent exposure, and differences in exposure under warming scenarios of 1.5, 2, 3 and 4 °C (2.7, 3.6, 5.4 and 7.2 °F). Results do not account for present or future shoreline defenses, such as levees, that might be built, nor for future population growth, decline or relocation.

In conjunction with this report, Climate Central has extended its interactive and embeddable Mapping Choices platform globally (choices.climatecentral.org). Users can now type in any coastal city name or postal code worldwide, and visually compare the potential consequences of different warming or emissions scenarios on a local map. Climate Central is also serving Google Earth layers for visualizing sea levels associated with 2 °C or 4 °C warming in areas with 3-D building data, available here; and offering spreadsheets for download with analytic results for comprehensive lists of global nations and coastal urban agglomerations, available here.