The influx of migrants to the U.S. southern border has taken over the news — and climate change, among other factors, ensures it won't be going away.

Why it matters: The migration of tens of millions of people, exacerbated by a changing climate, will be one of the mega-trends of the 21st century. For both humanitarian and political reasons, wealthy countries like the U.S. will need to figure out a way to handle a flow of people that may never sto.

Driving the news: During President Biden's first presidential press conference Thursday, questions about the renewed flow of migrants — including unaccompanied minors — dominated the discussion.

• As my Axios colleague Stef Kight has reported, the administration "is struggling to keep up with a migration surge" as it tries to balance humanitarian and COVID-19 concerns with border security.
• While the current increase in migrants pales in comparison to the peak in 2019 under then-President Donald Trump — and is likely due in part to a backlog of migrants unable to travel during the worst of the pandemic last year — the flow is expected to grow as the weather warms.

The big picture: People make the difficult decision to leave their homes for many reasons, including conflict and crime, political persecution, and the simple desire for a better life.

• But a factor now — and even more so in the future — is the push of extreme weather and climate change, which will disproportionately affect the people living in the poorer, hot countries that are already a major source of migrants to the U.S.
• That means the U.S., as well as the rich nations of Europe and even countries like India, will likely face a permanent and likely growing flow of climate migrants that they and the international refugee system more broadly are ill-equipped to handle.

By the numbers: The World Bank estimates that three regions — Latin America, sub-Saharan Africa, and Southeast Asia — will generate 143 million more climate migrants by 2050.

The catch: Most people displaced because of the effect of weather and climate first migrate not internationally, but to towns and cities in their own countries.

• But as Abrahm Lustgarten reported in a sweeping story for the New York Times and ProPublica last year, as migrants crowd ill-equipped urban areas, they "stretch infrastructure, resources and services to their limits," which becomes both a source of misery and push for international migration.
• A model produced for the piece projected that migration from Central America will rise every year regardless of climate change, but that in the most extreme warming scenarios, more than 30 million migrants would head toward the U.S. border over the next 30 years.
• Climate change is especially challenging because the international refugee system — which was built in the aftermath of World War II — was set up to address conflict and political persecution, which means that no legal framework exists for climate refugees.
• Migration from the worst-hit regions is arguably one of the most necessary aspects of any adaptation to climate change, yet it will run headlong into the political obstacles around border control.

Flashback: Biden issued an executive order in February that calls for an examination of the international security implications of climate-related migration.

Be smart: Climate change's precise role in migration is tangled up with more immediate factors, like security and economic well-being. But we know millions of people will want to migrate to the U.S. in the future — and that many of them will try to come regardless of border policies.

• A survey released this week by Gallup found more than a quarter of the population of Latin America and the Caribbean — 120 million people — say they would like to permanently move to another country.
• 42 million of them say they want to move to the U.S.

Yes, but: Immigration is among the most politically divisive issues in the U.S., one that has repeatedly foiled efforts of presidents from both parties to find a solution.

The bottom line: "Here are questions every leader should be able to answer regardless of their politics," Gallup chairman and CEO Jim Clifton wrote this week. "How many more people are coming to the southern border? And what is the plan?"

• The answers: Almost certainly many more. And we don't know.