As the sea creeps steadily inland in countries such as Bangladesh, and as dwindling rains put already marginal farmland out of play in Ethiopia and other places, a wave of migration triggered by a changing climate is taking shape on the horizon.

But most “climate migrants” will not be heading abroad to start new lives; instead they will settle elsewhere in their home countries. A new World Bank report released this week declares that if nothing is done to curb global warming and factor migration into development planning, by mid-century this internal population shift could involve more than 140 million people in three regions examined: sub-Saharan Africa, south Asia and Latin America. “Climate change is already a driver of internal migration, and it will become more so in the future,” says John Roome, senior director for climate change at The World Bank Group.

The potential for such a surge in areas comprising 55 percent of the developing world’s population raises questions of environmental justice because those who have contributed least to global warming are forced to shoulder most of the burden. It is incumbent on developed countries like the U.S. to step up, says María Cristina García, a professor of American Studies at Cornell University who was not involved in the report. Developed countries can help by both working to limit greenhouse gas emissions and funding efforts to help developing nations plan for climate migration challenges, García says.

Some people will need to migrate despite any measures that might be taken—but “this doesn’t have to be a crisis,” Roome says. Properly managed migration could even bring more economic opportunities to some poor communities, the World Bank report’s authors contend. But planning needs to start now.

“Those People Do Not Get Counted”

The study of climate migration is still relatively new, and projections of just how many people might be driven from their homes as the world warms are hard to pin down. Predictions of climate change impacts carry an inherent uncertainty, and the reasons people decide to migrate—or are forced to—are often complex.

To get a clearer picture of how this story might unfold, the authors of the report modeled how slow-burn climate effects (such as coastal land lost to sea level rise, along with water scarcity and crop failure caused by changing rainfall and higher temperatures) affected population patterns in the three regions covered by the report. They focused on internal migration because most people pushed from home—whether for economic, climate-related or other reasons—are displaced within their own countries.

The models looked at how populations might shift in the future if greenhouse gas emissions abate, and compared that scenario with what could happen if emissions continue on their current trajectory. The models also incorporated instances in which development planning alleviated economic inequality—and when that equality gap widened. When emissions were left to soar and development was left unequal, internal migration registered highest: an estimated 86 million people in sub-Saharan Africa, 40 million in south Asia and 17 million in Latin America by 2050. But tackling either issue substantially reduced migration numbers to as few as 31 million across all three regions.

Migration “hot spots”—places people are likely to leave, as well as their probable destinations—emerged in each region. “The impact of climate on migration is not uniform across countries or even within countries,” Roome says. For example, people may increasingly leave Ethiopia’s northern highlands, where agriculture depends on seasonal rains that are now unreliable. Others are likely to flee coastal areas in Bangladesh, where saltwater infiltrating the drinking supplies of 20 million people may already be causing an increase in diarrheal diseases. The report also found climate change could cause people to leave urban centers that have long attracted migrants drawn to the promise of better-paying jobs. Bangladesh’s capital, Dhaka, a sprawling city of more than 17 million people, is threatened by sea level rise and ever-higher storm surges; Addis Ababa, Ethiopia’s capital of three million, could see increasingly unreliable rains and thus an unstable water supply.

The report’s overall findings are no surprise for researchers who have studied climate migration over the last decade or so, says García, who is writing a book on climate migration. Overall migration is usually associated with refugees fleeing to other countries in times of war or other crises, with the U.N. and other agencies keeping track of those pushed outside their own borders. But internal migration is less thoroughly tracked. “We just don’t have a lot of hard data” on it, says Alice Thomas, climate displacement program manager at the nonprofit Refugees International. “Often it just happens kind of slowly over time, and those people do not get counted.

Who Bears the Brunt?

The report’s authors caution that it is not meant to be a precise forecast, but rather a guide to what might happen and an aid to planning for a potential upheaval. This kind of modeling is useful “not because any one scenario is going to give us the answer” but because it illuminates the various forces influencing migration, says report co-author Alex De Sherbinin, deputy manager of the NASA Socioeconomic Data and Applications Center at Columbia University.

“It’s really about taking a longer-term view of the issue,” says report co-author Kanta Kumari Rigaud, a lead environmental specialist with the World Bank. For example, Kumari says countries can foster industries that are less subject to climate fluctuations in order to help communities adapt and prevent the need for people to leave. Ethiopia has done this, the report notes, by pushing to diversify its economy—three quarters of its population currently depends on agriculture but the government, with help from the World Bank, has implemented more sustainable land management.

Various efforts to bolster the economy and reduce poverty have led to a $50-billion gain in gross domestic product over the last decade as well as higher school enrollment and improved sanitation. Governments can also step in earlier to provide support when population movements will eventually become unavoidable, the report suggests, instead of waiting until families have exhausted all their resources battling drought or rising seas. Social services could help line people up with jobs in more climatically stable areas, for example. This could raise the economic prospects for families and countries as a whole. “When it’s planned, it should be a win–win situation for everybody,” Thomas says.

Of course, this kind of planning requires a dedicated effort—one that even developed nations like the U.S. have struggled to implement. Thomas notes thousands of people left Puerto Rico for the mainland after Hurricane Maria last fall, largely because they had so little support on the ground. “Even in wealthy countries we don’t have the right laws and policies in place,” Thomas says. “Those measures will take a long time to put in place.”

But it is poorer countries “that are paying the price and have the populations that are being forced to suffer the most,” Thomas says. The U.S. and other wealthier nations have nominally committed to efforts, including the Green Climate Fund, to help developing countries study and plan for climate impacts. But many of the wealthier players have yet to fulfill their promises; the U.S., under the administration of Pres. Donald Trump, has balked at providing more money for such programs.

Even if seriously concerted efforts are made to lower carbon dioxide emissions and promote more equitable development, millions will still be displaced because of the inevitable warming that is already baked in. The authors and other experts hope the new report will help spur action and research into the problem. “Hopefully,” Roome says, “this report can raise awareness of the issue and create a little bit of this political will.”