Effects of overpopulation
Changes in atmospheric composition and consequent global warming 
Irreversible loss of arable land and increases in desertification Deforestation and desertification can be reversed by adopting property rights, and this policy is successful even while the human population continues to grow.
Mass species extinctions. from reduced habitat in tropical forests due to slash-and-burn techniques that sometimes are practiced by shifting cultivators, especially in countries with rapidly expanding rural populations; present extinction rates may be as high as 140,000 species lost per year. As of 2007, the IUCN Red List lists a total of 698 animal species having gone extinct during recorded human history.
High infant and child mortality. High rates of infant mortality are caused by poverty. Rich countries with high population densities have low rates of infant mortality. 
Increased chance of the emergence of new epidemics and pandemics For many environmental and social reasons, including overcrowded living conditions, malnutrition and inadequate, inaccessible, or non-existent health care, the poor are more likely to be exposed to infectious diseases.
Starvation, malnutrition or poor diet with ill health and diet-deficiency diseases (e.g. rickets). However, rich countries with high population densities do not have famine.
Poverty coupled with inflation in some regions and a resulting low level of capital formation. Poverty and inflation are aggravated by bad government and bad economic policies. Many countries with high population densities have eliminated absolute poverty and keep their inflation rates very low.
Low life expectancy in countries with fastest growing populations
Unhygienic living conditions for many based upon water resource depletion, discharge of raw sewage and solid waste disposal. However, this problem can be reduced with the adoption of sewers. For example, after Karachi, Pakistan installed sewers, its infant mortality rate fell substantially. 
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
 Mitigation measures
While the current world trends are not indicative of any realistic solution to human overpopulation during the 21st century, there are several mitigation measures that have or can be applied to reduce the adverse impacts of overpopulation.