Coastal Flooding
Roughly 1 billion people live within 10 meters of sea level. About 300 million are exposed to flooding from 100-year coastal events under current conditions. Climate-change-driven sea-level rise will roughly triple this exposure by 2100 under medium emission scenarios. Bangladesh, Vietnam, China, Indonesia, India, Egypt, Netherlands have the largest populations at risk.
Key insights
Bangladesh tops absolute exposure rankings
Bangladesh has ~50 million people living in cyclone-vulnerable coastal zones. The Ganges-Brahmaputra delta is sinking due to sediment compaction and groundwater extraction, even before factoring in sea-level rise. Cyclone shelters and warning systems have reduced cyclone mortality from hundreds of thousands (1970 Bhola: ~300,000 deaths) to thousands or hundreds per major event. But asset and economic damage continues to rise.
Small island states face existential threat
Tuvalu, Kiribati, Maldives, Marshall Islands, the Bahamas: each has substantial populated land at or near sea level. Some islands could become uninhabitable mid-century. Migration agreements emerging — Australia-Tuvalu Falepili Union (2023) provides Tuvaluan citizens special access to Australia. The legal status of climate refugees remains underdeveloped — current refugee conventions don't cover environmental migration.
Wealthy countries adapt; poor countries can't afford to
The Netherlands has the largest population at risk relative to land area but has invested 300+ years in coastal engineering. ~$3B/year in dike maintenance; major upgrades scheduled. New York post-Sandy invested $20B+ in coastal protection. Adaptation cost can be paid by wealthy economies. Bangladesh, Vietnam, Indonesia face similar exposure with 1/20th the fiscal capacity. The 'adaptation gap' is the real climate-justice issue at coastal margins.
Population in low-elevation coastal zones — top countries
Millions of people within 10m of sea level
Key Finding: China, Bangladesh, Vietnam top exposure rankings.
Frequency increase of 1-in-100-year floods by 2100
Years between events that were 100-year frequency in 2020
Key Finding: What was a 100-year event will be much more frequent under climate change.
Methodology & caveats
Low-elevation coastal zone
LECZ = land area within 10m elevation and within 100km of coast. ~2% of global land area, ~12% of global population. The 10m threshold captures most cyclone-flood and sea-level-rise exposure. Some studies use 5m (more conservative); some use coastal floodplains directly mapped.
Storm surge vs sea level rise
Storm surge: temporary water rise from cyclones/storms, can reach 6-9m above normal sea level. Sea-level rise: permanent gradual elevation. The two interact: each cm of SLR roughly doubles the frequency of storm-surge floods at a given height. Future flood exposure combines SLR with storm-surge climatology. Both are projected to worsen with warming.
Adaptation options
Hard adaptation: dikes, seawalls, flood barriers, raised buildings. Soft adaptation: managed retreat, building codes, early warning, ecosystem-based (mangroves, dunes). Most coastal cities use a mix. Costs scale with engineering complexity — Manila estimating $200B for protection; Jakarta planning capital relocation (Nusantara) partly due to coastal flooding.