Ithaca, N.Y. -- As the earth's population surpasses the 6 billion mark, some researchers are predicting how the planet's burgeoning population will affect the environment and quality of life for humans in another 100 years.

David Pimentel, a Cornell University professor of ecology and agricultural sciences, sees several possible scenarios for the 22nd century: A planet with 2 billion people thriving in harmony with the environment; or, at the other extreme, 12 billion miserable humans suffering a difficult life with limited resources and widespread famine. Pimentel says the next century is crucial because the human population could pass sustainable limits.

'We must avoid letting human numbers continue to increase and surpass the limit of the earth's natural resources and forcing natural forces to control our number by disease, malnutrition and violent conflicts over resources,' Pimentel writes in his report, 'Will Limits of the Earth's Resources Control Human Numbers?' which appeared in the first issue of the journal Environment, Development and Sustainability.

Pimental says the only way to manage the earth's population is to reduce the number of children per couple. He estimates that if people average 1.5 children per couple, the optimal earth population of 2 billion could be achieved in 100 years. Even slightly more children per couple will make the earth's number's swell in short order: 'If we adopted a policy of 2.1 children starting tomorrow, the world population will continue to increase and 60 years from now we will have close to 12 billion people,' he said.

Pimentel says that in order for every person on earth to have adequate resources of food, shelter and clothing, the ideal population on the earth should be about 2 billion -- approximately the number of people living on the planet in the 1950s. These fortunate 2 billion will be free from poverty and starvation, living in an environment capable of sustaining human life with dignity, the report suggests.