Sea Level Rise

Since 1992, the TOPX/Poseidon satellite has been recording ocean surface height across the world, using pulses of radio waves that reflect off the ocean surface back toward the satellite. Five missions with similar altimeters have repeated the same orbit every 10 days and together, this data has facilitated a unified standardised ocean topography record that is long and sensitive enough to detect global and regional sea level changes beyond the natural seasonal, yearly and decadal cycles.

Sea levels have already risen about 20–23 cm since 1880, and the rates of sea level rise are now accelerating, climbing 4.8 mm per year in the last decade due to ocean warming and melting ice, with regional variations driven by ocean circulation. Even at 1.5 °C of warming, substantial further rise is locked in, amplifying risks such as saltwater intrusion, habitat loss, infrastructure damage, disease, displacement and threats to the 900 million people living near coasts, Rising sea levels also create feedback loops like collapsing mangrove systems worsen climate change, making emissions cuts, adaptation measures, early‑warning systems and managed retreat increasingly urgent.

Global mean sea level is rising faster due to human‑driven warming of oceans and melting ice, with future rise projected to reach 0.43–0.84 m by 2100 and continue for millennia. Local risks are intensified by non‑climatic human pressures like land subsidence, altered sediment flows and coastal development, making it hard to attribute impacts cleanly but ensuring that extreme sea‑level events become far more frequent, threatening ecosystems, infrastructure, public health and low‑lying states. Without adaptation, flood damages could increase by orders of magnitude, while effective responses will require locally tailored mixes of land‑use planning, governance, public participation and long‑term decision‑making as technical and social limits to protection are reached.

Coastal residents are experiencing far faster sea level rise than global averages because many live in places where the land itself is sinking. Human-driven subsidence (from groundwater extraction, oil and gas drilling, sand mining, etc.) means cities like Jakarta, New Orleans, Shanghai and Bangkok have sunk metres over the past century. Without intervention, tens of millions more will be exposed by 2050, making locally tailored adaptation (such as regulating groundwater use, restoring sediment delivery and sharing successful mitigation strategies) essential.

Freshwater flows are declining in many coastal regions because of land‑use change, over‑extraction, drought and rising temperatures. As these flows weaken, seawater can move inland and salinise aquifers, estuaries and drinking‑water supplies. Sea‑level rise, reef degradation, altered sediment delivery and wave overwash all intensify this process. Salinisation poses serious health risks, especially where chloride levels rise in drinking water, and it threatens vulnerable freshwater lenses in places like Pacific Island states. Not all communities are exposed, but where water supplies are at risk, adaptation requires layered strategies such as conservation, relocating water intakes, desalination, diversifying water sources and watershed‑based management.

Sea‑level rise is already transforming coastal ecosystems, pushing saltwater into freshwater habitats and drowning nesting and breeding areas, leaving species like freshwater turtles, Hawaiian monk seals, green sea turtles and Florida’s Key deer increasingly vulnerable. As habitats shrink, salinity rises, storms reshape coastlines and invasive species spread, many animals face displacement or decline, with survival hinging on whether suitable habitat exists nearby or whether species can adapt quickly enough to rapidly changing conditions.

Although atolls are often portrayed as the first islands destined to vanish under rising seas, decades of aerial and satellite imagery show a more complex picture. While some islands have eroded or disappeared, many have remained stable or even grown as waves naturally redistribute sediment across reefs. However, the long‑term outlook depends on how quickly reefs degrade under warming and bleaching, how engineered structures disrupt natural sediment dynamics, and whether communities choose to adapt by coexisting with changing shorelines rather than relying solely on hard defences.

Low‑lying atoll islands are highly vulnerable to sea‑level rise, but those with intact coral reefs, native forests and healthy seabird populations are far more resilient; reefs generate the sediment that builds atolls, seabirds fertilise reefs, and native vegetation stabilises shorelines, while introduced species like rats and coconut palms undermine these systems. Although some islands have already disappeared and others are growing or stable, maintaining strong marine and terrestrial ecosystems gives atolls the best chance of withstanding rising seas through this century.

There are several steps a city can take to adapt to sea level rise and coastal flooding. The first is to understand the city’s vulnerability. Additionally, coordinating with other cities and regional authorities can increase effectiveness of action and help to pool resources, especially as sea level rise is not a localised issue. Cities can also improve coastal flood defences to reduce the likelihood and severity of flooding, either through restoring natural coastal ecosystems and/or building physical engineered structures. Alongside these measures, cities can adapt the built environment to reduce the impact of flooding when it does occur (e.g. policy changes related to zoning, or raising buildings). Finally, managed retreat is a contentious issue but will become increasingly necessary as sea levels continue to rise.

The International Court of Justice, by way of an Advisory Opinion concerning the obligations of States with respect to climate change amidst the climate crisis, recognises sea‑level rise as an urgent and existential threat and confirms that states retain their legal continuity even if they lose territory to rising seas. It also clarifies that countries are not required to redraw maritime boundaries or update geographic coordinates solely because of physical shoreline changes, thereby protecting the permanence of their maritime rights under international law.

Tuvalu, a low‑lying Pacific nation already experiencing regular king‑tide flooding and projected widespread inundation by 2050, is pursuing digital statehood as a way to preserve its sovereignty, identity and governance as its physical territory becomes increasingly threatened. Through 3D LiDAR mapping, virtual modelling, upgraded communications infrastructure and plans for blockchain‑based civic systems, Tuvalu is creating a “digital twin” to safeguard its maritime rights, cultural heritage and constitutional continuity, asserting that the Tuvaluan state will endure even if its land is lost to rising seas.