What will happen if people in this world are increasing day by day? 

Firstly, We can think about the enviroment, the garbage will increase, too. And then we have a new problem about the enviroment, it will be polluted. We don't have enough energy or the food for everyone.  

Secondly, some humans don't have awareness about the enviroment, they litter everywhere, they don't know what should they do to protect the enviroment.

Thirdly, Maybe people only standard of living and don't protect the enviroment. And then the natural resoureces will be run out. The wild animals will be extinction

Finally, People are increasing and they want to have a place to stay. So they have to burn the forest,........ And then the global will be warm up, the ice will melt and so on.