The World Bank says people need assistance to adapt to climate change so a surge in climate migration is avoided.
The Bank has warned in a new study, that without action, tens of millions of people will be forced to move because of water shortages, crop failures, sea-level rise, storm surges and other climate threats.
But it said if global warming is kept to the limits set in the Paris Agreement on climate change, and people are helped to adapt, then the numbers migrating will be much lower.
The head of the World Bank Kristalina Georgieva said there is a small window now, before the effects of climate change deepen, to prepare for this new reality.
The study said economies need to be less vulnerable to climate change, farmers helped to adapt their growing techniques, and city infrastructure made more resilient.
Cities would also need to create enough jobs, and boost health and education services, to meet the needs of the growing number of people migrating to urban areas.
One of the creators of the report, environmental specialist Kanta Kumari Rigaud, says without that preparation, cities could face serious repercussions, including increased tensions and conflict.