Climate change will transform more than 143 million people into “climate migrants” escaping crop failure, water scarcity, and sea-level rise, a new World Bank report concludes.

Most of this population shift will take place in Sub-Saharan Africa, South Asia, and Latin America—three “hot spots” that represent 55 percent of the developing world’s populations.

This worst-case scenario is part of a ground-breaking study focused on the impacts of slow-onset climate, as opposed to more visibly dramatic events such as extreme storms and flooding. The report, Groundswell—Preparing for Internal Climate Migration, also shifts the focus from cross-border migration, which has drawn global attention as refugees and migrants flee war, poverty and oppression, to in-country migration, which involves many more millions of people on the move in search of viable places to live. The 143 million represent 2.8 percent of the three regions’ population.

Sea-level rise is already prompting the migration of people from Pacific and Oceania island chains and low-lying coastal areas that flood regularly, and areas suffering extreme drought has sent others in search of sustainable farmland. Much of the coming migration will shift populations of people over the next three decades from rural areas to urban areas. Not surprisingly, the poorest people in the poorest countries will be hardest hit, the report finds.

The study’s authors say there is still reason for optimism: if the world acts in time to reduce greenhouse gas emissions and engages in “robust development planning,” the flood of “climate migrants” could be reduced by 80 percent to a mere 40 million people.

Inside the Floating Hospital Helping Flood Victims in Bangladesh

“We have a small window now, before the effects of climate change deepen, to prepare the ground for this new reality,” Kristalina Georgieva, the World Bank chief executive officer, said in a statement. “Steps cities take to cope with the upward trend of arrivals from rural areas and to improve opportunities for education, training and jobs will pay long-term dividends.” (See 100 practical ways to reverse climate change.)

3 Case Studies

The report profiled three countries as case studies—Ethiopia, Bangladesh and Mexico—and warns that fast-growing cities will have to diversify economically and create climate-resilient jobs to successfully absorb population growth.

View Images Climate change is expected to impact Mexico by encouraging more people to move to Mexico City from outlying areas that are more at risk for changes. Photograph by Guillermo Arias, AFP/Getty

There are exceptions. Declining rainfall in Ethiopia’s northern highlands, for example, may drive people out of the country in search of new areas where they can grow rainfed crops. And lack of rainfall in Addis Ababa, Ethiopia’s largest city, may slow its growth.

Alternatively, sea-level rise and storm surges will prompt growth in the major cities of Bangladesh, including the capital city of Dhaka. Bangladesh, the study predicts, will experience greater shifts and changes to population from climate change than any other event.

Mexico, the wealthiest of the trio profiled, is less vulnerable to climate change and better prepared than Ethiopia and Bangladesh. But “it needs to pay close attention to pockets of poverty,” the study’s authors found. The central plateau around Mexico City and Guatemala City, which may offer better climate conditions, may attract climate migrants.