1. Problems associated human overpopulation
and over-consumption

2. Inadequate fresh water for drinking
Inadequate fresh water for drinking. Some countries, like Saudi Arabia and Mexico use energy-expensive desalination to solve the problem of water shortages.
Depletion of natural resources, especially fossil fuels.

3. Increased levels of air pollution, water pollution, soil contamination and noise pollution.
Mass species extinctions and contracting biodiversity.
High infant and child mortality. 

4. Increased chance of the emergence of new 
epidemics and pandemics.
Overcrowded living conditions.
Malnutrition and starvation.
Inadequate, inaccessible, or non-existent health care.

5. Low life expectancy.
Unhygienic living conditions for many based upon water resource depletion, discharge of raw sewage and solid waste disposal.
Intensive factory farming to support large populations. It results in human threats including the evolution and spread of antibiotic resistant bacteria diseases, excessive air and water pollution, and new viruses that infect humans.

6. Elevated crime rate due to drug cartels and increased theft by people stealing resources to survive.
Conflict over scarce resources and crowding, leading to increased levels of warfare. From that develops a need for more laws and/or more restrictive laws to regulate these interactions and relations.
Democracy is threatened due to overpopulation, and could give rise to totalitarian style governments.