Migration will shape this century as much as it shaped the last. Two forces stand out: a changing climate and ageing populations. Behind both lies a question older than either: what does a person keep when they lose a home, or a country?

Climate and movement

Weather has always moved people. In the 1930s the Dust Bowl drove hundreds of thousands of families off the American plains. Today, sudden disasters such as floods and storms displace tens of millions of people in a typical year, though most return home within weeks or months. Slower changes, such as rising seas, failing rains and saltwater in wells, push people more gradually, usually mixed with economic and political causes. Here the human pattern matters more than the physical science. Most climate-related movement happens within the affected country, often from countryside to city.

The phrase "climate refugee" is common in the press, but it is not a legal category under the 1951 Convention, because a drought or flood is not persecution on one of the five grounds. That gap was tested by Ioane Teitiota, a man from the low-lying Pacific nation of Kiribati who sought protection in New Zealand. In 2020 the UN Human Rights Committee found that New Zealand had not violated his rights in his case, but that climate harm could someday bar returns, if conditions in a home country came to threaten life itself.

Not everyone affected can leave. Researchers warn of trapped populations: people too poor to move away from growing environmental danger. This echoes the migration hump in Level 1: moving takes resources, and the most vulnerable often lack them.

Forecasts vary widely. A World Bank study in 2021 projected that up to 216 million people could move within their own countries because of climate change by 2050 under a pessimistic scenario. Such numbers are models, not predictions, and some researchers warn that alarming headlines about climate migrants can be used to justify harsher borders rather than help.

Ageing societies

Birth rates in Europe, East Asia and much of the Americas have fallen below the level needed to replace each generation, while lifespans have grown. Fewer workers support more retirees. Migration is often proposed as a remedy, and it does help. But a 2000 United Nations study of "replacement migration" showed that very large inflows would be needed to keep the ratio of workers to retirees steady, far beyond what any country has accepted. Migration can ease ageing, but it cannot by itself undo it; later retirement, higher productivity and family policy would all play a part.

Cooperation and its limits

In 2018 most UN members endorsed two global compacts, one on refugees and one for safe, orderly and regular migration. The migration compact is not legally binding on the states that endorsed it; it sets shared goals such as reducing deaths in transit and improving data. Several states, including the United States and Hungary, refused to support it, arguing that it could erode national control over borders. Supporters reply that cooperation is the only way to manage a movement no single country controls.

Regional agreements go further. Citizens of European Union countries may live and work in any other member state, and West African states have a protocol on free movement among their members. These zones show that open borders between countries can work when they share rules, institutions and comparable incomes, though Britain's 2016 vote to leave the EU showed they can also provoke backlash.

The right to belong somewhere

Millions of people have no citizenship anywhere. Myanmar's 1982 citizenship law left most Rohingya stateless, and many were later driven into Bangladesh. Writing after the Second World War, the philosopher Hannah Arendt, herself once stateless, argued that losing citizenship meant losing the protection on which all other rights depend. Human rights were supposed to belong to everyone, yet in practice the stateless found that no government would enforce them. Conventions on statelessness in 1954 and 1961 try to close that gap, with limited success.

Here the course's underlying theme comes into view. People disagree, often in good faith, about how many migrants a country should admit and on what terms. Yet the law built after 1945 rests on a shared premise: a border can decide where a person may live, but not whether that person has rights.

Worked example: Tuvalu

Tuvalu is a nation of about 11,000 people on coral atolls whose highest points rise only a few metres above the sea. Its government has pursued adaptation, reclaiming land and building sea defences, while preparing for the possibility that some islands may become unlivable. In 2023 it signed the Falepili Union with Australia, which created a visa pathway for a limited number of Tuvaluans to move to Australia each year, along with a promise of help after disasters. Tuvalu has also declared that it will remain a state even if its land is lost. The case gathers this course's threads: climate pressure, a planned and legal route instead of desperate flight, a bargain between sovereign states, and a people determined to keep their membership in the world.