1. ice / North and South Poles / melt
-> If the ice in the North and South Poles melt, the sea level will rise.
2. sea level / rise
-> If the sea level rises, there will be floods in many parts of the world.
3. there / floods / many parts / world
-> If there are floods in many parts of the world, many people will lose homes and lands.
4. many people / lose homes and lands
-> If many people lose homes and lands, homelessness will be a big problem.