Rising seas are already reshaping economies, livelihoods and cultures for millions of people worldwide, and the pace is accelerating. Global ocean levels rose an average of 20 centimetres between 1901 and 2018, with 10.6cm of that increase occurring since 1993 — a rate faster than at any point in at least the past three millennia. This week, world leaders at the UN General Assembly are set to adopt a declaration stating that sea level rise is “not a distant scenario, but a real and lived experience for many.” Under a moderate-emissions scenario, scientists project a further 56cm of rise by 2100 relative to a 1995–2014 baseline.
The human toll is already significant. Approximately 770 million people — roughly 10% of the global population — live in locations at “acute risk,” defined as less than five metres above the high-tide line, according to a recent UN secretary general report. Residents of major cities including Mumbai, Kolkata, Shenzhen and Guangzhou face mounting dangers, as do the entire populations of many low-lying island nations. Higher seas amplify storm surges, accelerate coastal erosion, raise water tables and drive saltwater intrusion into farmland. Critically, the frequency of once-in-a-century extreme sea level events has already increased twelvefold since 1900, and under moderate emissions these events could become annual occurrences in many places by 2100. Even without further sea level rise, annual flood losses in the world’s 136 largest coastal cities could reach $52bn by 2050, up from $6bn in 2005. Compounding the risk, coastal populations are growing faster than inland ones: between 2000 and 2018, the number of people living within 5km of a coast grew by 28%, outpacing overall global population growth of 23%.
Coastal and island ecosystems face an equally severe threat. A phenomenon known as “coastal squeeze” traps wetlands between rising seas and human-made structures, preventing them from migrating inland. Nearly 28 million hectares of coastal wetlands — an area roughly the size of Ecuador — have disappeared since 1970, according to the 2025 Global Wetland Outlook report. In the continental United States, only 16% of coastal wetlands are migrating inland fast enough to keep pace with local sea level rise; 11% are already submerging. Low-lying islands are especially vulnerable: a study modelling one metre of sea level rise across nearly 4,450 islands found that around 6% of island habitat area would be completely inundated, with serious consequences for biodiversity, since more than 20% of the Earth’s known plant species exist only on islands.
Some communities have moved to adapt. Venice completed a floodgate system in 2020 that has since been deployed dozens of times to protect its lagoon. Elsewhere, cities and governments are restoring mangrove forests, building seawalls and relocating infrastructure. However, researchers caution that even under a low-emissions scenario, existing coastal protections could be breached ten times more frequently over the next three decades than they are today, suggesting that current adaptation efforts may be insufficient to match the scale of the threat.
The compounding pressures of climate-driven sea level rise, land subsidence — in cities like Jakarta, the ground is sinking by up to 15cm per year due to groundwater extraction — and continued coastal development mean that both the human and ecological costs of inaction are set to grow substantially in the decades ahead.