The answer is simple, just let Mother Nature take its course. Mankind's populations are aging rapidly and the population pyramid is overturning. An aging population has the consequence of a progressively narrowing reproduction age-bracket. Underlying this process in a falling birth rate and mortality rate. The birth rate has been in decline since the early 1960's as a result of urbanization, and the mortality rate is falling as a result of technology and our victory in the war with microscopic organisms. The UN estimates that Zero Population Growth (ZPG) will occur in two or three decades. The end result will be the extinction of our species within 100 years if an economic collapse doesn't get us first. Bottom line: overpopulation is a non-issue.