It's a cycle. Overpopulation requires more arable land for agriculture. When there isn't any, it's created, such as the San Joaquin Valley. This valley was basically desert. This was done by damming a few rivers and irrigating the land. That being said you can only convert so many deserts for agriculture, you start to run out of water, even if you still receive your normal annual rain fall. That would be a drought caused by over population. Same thing is is happening in Phoenix. The water table continues to drop and the aquifers are becoming harder and harder to reach. Not due to climate change but by more people.
So dealing with overpopulation needs to go hand in hand with changing how we generate and use energy.