Answer: 
The main physical characteristic affecting North Africa and Southwest Asia's settlement patterns is water. 
Step-by-step explanation: 
- The northern part of the continent of Africa and the southwestern part of Asia are predominantly deserts characterized by scarcity of water. 
 
- These two regions of the world are counted among the driest on the planet. 
 
- Desertification due to rapid deforestation in the past caused these regions to turn into huge sandy deserts. 
 
- The countries in the southwest Asia are comparatively wealthy and have been implementing various techniques of desalination of seawater to tackle the scarcity of water in the region.