As the hallmarks of the climate crisis—heat, thirst, hunger, sea level rise, and conflict—send millions of frightened and desperate people into flight, migration experts are warning of an increasing reality for individuals and for nations: “Mobility is resilience.”

From Central America to the Sahel to Southeast Asia, people are fleeing, in search of cooler temperatures, water, food, dry land, and peace, writes ProPublica, in the first of a series of reports to be produced with the New York Times Magazine and the Pulitzer Center. If there is to be any hope of a humane and stable world, these migrants must not be corralled, caged, or blocked at borders, the publications state.

“The planet could see a greater temperature increase in the next 50 years than it did in the last 6,000 years combined,” ProPublica writes, citing a study just published in the Proceedings of the National Academy of Sciences. “By 2070, the kind of extremely hot zones, like in the Sahara, that now cover less than 1% of the Earth’s land surface could cover nearly a fifth of the land, potentially placing one of every three people alive outside the climate niche where humans have thrived for thousands of years.”

In the Sahel, UN researchers estimate that 65% of agricultural land is degraded. “In the nine countries stretching across the continent from Mauritania to Sudan, extraordinary population growth and steep environmental decline are on a collision course,” ProPublica reports. More than 100,000 have already died from drought, and worse is to come for a region that is “home to 150 million people and growing.”

In South Asia, which supports nearly 25% of the world’s population, the World Bank predicts unprecedented food insecurity. “While some 8.5 million people have fled already—resettling mostly in the Persian Gulf—17 to 36 million more people may soon be uprooted.”

And where they are headed is no safer: “If past patterns are a measure, many will settle in India’s Ganges Valley; by the end of the century, heat waves and humidity will become so extreme there that people without air conditioning will simply die.”

Rising seas, meanwhile, are further contributing to the movement of populations. By 2050, high tides could cover much of Vietnam, “as well as parts of China and Thailand, most of southern Iraq, and nearly all of the Nile Delta, Egypt’s breadbasket,” reports ProPublica. Coastal regions in the U.S. are also at risk.

What has to be recognized is that climate migrants don’t want to leave home, and will typically move first to larger urban areas in their own country. “It’s only when those places fail them that they tend to cross borders, taking on ever riskier journeys, in what researchers call ‘stepwise migration’,” notes ProPublica.

But the modelling shows that the ability of these cities to withstand climate change is not in their own hands. “Should the United States and other wealthy countries change the trajectory of global policy…by, say, investing in climate mitigation efforts at home but also hardening their borders—they would trigger a complex cascade of repercussions farther south”, where cities collapse economically while the countryside becomes less and less survivable.

Turning its migration model to Latin and South America, ProPublica writes that “since 2000, San Salvador’s population has ballooned by more than a third as it has absorbed migrants from the rural areas, even as tens of thousands of people continue to leave the country and migrate north.” By 2050, densely-populated El Salvador “will be 86% urban,” with much of the growth occurring in “slum-like suburbs,” where water, power, and jobs are scarce and gang activity is rampant.

Solutions are not easy to find. In Mexico, President Andrés Manuel López Obrador tried to make his country “a model for increasingly open borders,” in part to defy the jingoistic belligerence of the Trump regime. More than 400,000 were let into the country without documentation in December 2018, but the experiment backfired horribly, with the U.S. retaliating with economic hammers and Mexican citizens themselves reacting with distrust and unrest. Now, the COVID-19 pandemic has brought the situation to the boiling point as “migrants, with nowhere to go and no shelters able to take them in, roam the streets, unable to socially distance and lacking even basic sanitation.”

Still, “there is no more natural and fundamental adaptation to a changing climate than to migrate,” writes ProPublica, echoing Lorenzo Guadagno of the UN’s International Organization for Migration, who first said that “mobility is resilience.” The more power can be put into the hands of displaced people to decide for themselves where to live, the safer they will be.

If the U.S. and other wealthy countries both block migrants and “fail to help them at home,” up to 1.5 million will die annually from heat alone, according to ProPublica’s model. More still will die from hunger, or “in the conflicts that arise over tensions that food and water insecurity will bring.”

If, on the other hand, societies respond with compassionate foresight to both the climate crisis and migration, and help increase the world’s resilience to both phenomena, “food production will be shored up, poverty reduced, and international migration slowed—factors that could help the world remain more stable and more peaceful.”

But, ProPublica warns, “the window for action is closing.” With each degree of temperature increase, a billion more people are put at risk.

“For a long time, the climate alarm has been sounded in terms of its economic toll,” the publication states. “Now it can increasingly be counted in people harmed.”