Sea-level rise threatens not just the territory but the legal personality of Small Island Developing States. Examine with reference to the Falepili Union.

The Montevideo criteria say a state needs four things: territory, a permanent population, a government and the capacity to conduct foreign relations. For atoll SIDS like Tuvalu, home to about 10,600 people [1], rising seas wear away the first two. That puts the state's international legal personality at risk, not just its land. The Falepili Union gives the first binding answer to this problem, but only a partial one.

Threat to territory

• A 2024 marine heatwave killed about 99.9% of the reef-building coral, which is the system that creates and replenishes the islands [1].
• About 60% of Tuvaluans already live crowded on Funafuti [2].

Threat to legal personality

• Statehood criteria: losing land and steady out-migration weaken the territory and population tests.
• Maritime zones: if coastlines retreat, baselines could shift. That would shrink the EEZ under UNCLOS and, with it, sovereignty over resources [4].
• Status in the world: the UN seat, treaty rights and citizens' nationality would all become uncertain.

Falepili Union as a response

• It was signed on 9 Nov 2023 and came into force on 28 Aug 2024. It is the first legally binding instrument to recognise that Tuvalu's statehood and sovereignty will continue despite sea-level rise [3].
• "Mobility with dignity": up to 280 Tuvaluans a year can live, work and study in Australia [2][3].
• Help to stay: Australia committed AUD 38 million to the Coastal Adaptation Project for 22 hectares of raised land [3].
• It works alongside other measures: Tuvalu's Constitution keeps the state "in perpetuity" with fixed baselines, and the PIF 2021 and 2023 Declarations set the regional position [4]. The ICJ Advisory Opinion (2025) said that losing one element of statehood does not necessarily end the state [2][4].

Limitations

• Bilateral only: the treaty binds Australia alone. Other states still need to recognise Tuvalu's continuity.
• Sovereignty trade-off: under Article 4(4), Tuvalu must agree with Australia before making any security or defence partnership with another state [5].
• Risk of slow depopulation: a fixed migration quota may gradually empty the islands. It also raises concerns about losing culture and identity [4].

Falepili shows that a state's legal identity can be separated from its physical territory. To be secure, though, that idea needs wide acceptance by other states. The UNGA high-level declaration on sea-level rise [4], deeper emissions cuts and adaptation finance can turn it into a general principle. That would help protect SIDS statehood in line with SDG 13 and climate justice.