Almost 80 years ago, Mahatma Gandhi said that "the world has enough for everyone's needs, but not everyone's greed." He was concerned about our human greed that was consuming the planet's resources as world population was rapidly growing.

Over 50 years ago, I joined the Zero Population Growth organization, begun by Paul Ehrlich, a Stanford professor, who argued that it was time for a population-control movement. He believed that the world would face shortages of water, food and more if population growth was not reduced.

Because population growth of the world has increased steadily since the Medieval times, people remained interested in its progress. It was on July 11, 1987 that the world's population reached 5 billion people and it was called "Five Billion Day." This prompted the United Nations Population Fund to create World Population Day, now celebrated annually on July 11. Since then, according to the UN Population Fund, there were an estimated 7 billion people on Oct. 31, 2011. As for current numbers, the World Population Clock shows there are over 325 million people in the United States, and 7.7 billion people world-wide.

Shortly after joining the Zero Population Growth movement in 1968, I learned that the topic was one of immediate concern for at least 25 centuries. For example, Confucius, the philosopher (551 - 479 BC) said "excessive (population) growth may reduce output per worker, repress levels of living for the masses and engender strife." Aristotle (384 - 322 BC) said "one would have thought that it was even more necessary to limit population than property. The neglect of this subject, which in existing states is so common, is a never-failing cause of poverty among the citizens; and poverty is the parent of both revolution and crime." Tertullian, writer and theologian (160 - 220 AD) said that "the strongest witness is the vast population of the Earth to which we are a burden and she scarcely can provide for our needs."

One can see that the concern about too much population growth on our planet has been with us from far in our past and continues until this day. In short, the question remains: how many people will our planet hold? The shortest and best answer is "we don't know." Each year, according to current estimates, roughly 83 million people are being added to the world's population. Taking into account that fertility levels are expected to continue to decline, our planet's population is expected nonetheless to reach 8.6 billion in 2030, 9.8 billion in 2050 and 11.2 billion in 2100.

Because of the millions of people now added to our planet's population annually, and the many related social issues, the United Nations seeks to promote awareness of those issues, especially the importance of family planning, gender equality, poverty, maternal health and human rights. This year's July 11 Population Day is focusing on people under 25, reproductive issues, and health. All of these issues may be best addressed remembering Gandhi's encouragement to prioritize need rather than greed in each of our lives.