One hundred years from now, democratically determined population-control practices and sound resource-management policies could have the planet's 2 billion people thriving in harmony with the environment. Lacking these approaches, a new Cornell study suggests 12 billion miserable humans will suffer a difficult life on Earth by the year 2100.

"Of course, reducing population and using resources wisely will be a challenging task in the coming decades," said David Pimentel, professor of ecology and evolutionary biology and lead author of the report titled "Will Limits of the Earth's Resources Control Human Numbers?" in the first issue of the journal Environment, Development and Sustainability.

"It will be much more difficult," Pimentel said, "to survive in a world without voluntary controls on population growth and ever diminishing supplies of the Earth's resources."

Even at a reduced world population of 2 billion in A.D. 2100, life for the average Earth dweller will not be as luxurious as it is for many Americans today. But the lifestyle won't be as wasteful of resources, either, the Cornell ecologist predicts. Some observers are seeing early signs that nature is taking a hand at reducing human populations through malnutrition and disease. According to the report, global climate change is beginning to contribute to the food and disease problems.

"With a democratically determined population policy that respects basic individual rights, with sound resource-use policies, plus the support of science and technology to enhance energy supplies and protect the integrity of the environment," the report concludes, "an optimum population of 2 billion for the Earth can be achieved."

Then the fortunate 2 billion will be free from poverty and starvation, living in an environment capable of sustaining human life with dignity, the report suggests, adding a cautionary note: "We must avoid letting human numbers continue to increase and surpass the limit of Earth's natural resources and forcing natural forces to control our number by disease, malnutrition and violent conflicts over resources."

Among the key points in the report:

• The world population is projected to double in about 50 years.
  
• Even if a worldwide limit of 2.1 children per couple were adopted tomorrow, Earth's human population would continue to increase before stabilizing at around 12 billion in more than 60 years. The major reason for continued growth is "population momentum," due to the predominantly young age structure of the world population.
  
• The U.S. population has doubled during the past 60 years to 270 million and, at the current growth rate, is projected to double again, to 540 million, in the next 75 years. Each year our nation adds 3 million people (including legal immigrants) to its population, plus an estimated 400,000 illegal immigrants.
  
• Increasing U.S. and global population will place restrictions on certain freedoms: freedom to travel and commute to work quickly and efficiently, freedom to visit and enjoy natural areas, freedom to select desired foods and freedom to be effectively represented by government.
  
• Today, more than 3 billion people suffer from malnutrition, the largest number and proportion of the world population in history, according to the World Health Organization. Malnutrition increases the susceptibility to diseases such as diarrhea and malaria.
  
• One reason for the increase in malnutrition is that production of grains per capita has been declining since 1983. Grains provide 80 percent to 90 percent of the world's food. Each additional human further reduces available food per capita.
  
• The reasons for this per capita decrease in food production are a 20 percent decline in cropland per capita, a 15 percent decrease in water for irrigation and a 23 percent drop in the use of fertilizers.
  
• Biotechnology and other technologies apparently have not been implemented fast enough to prevent declines in per capita food production during the past 17 years.
  
• Considering the resources likely to be available in A.D. 2100, the optimal world population would be about 2 billion, with a standard of living about half that of the United States in the 1990s, or at the standard experienced by the average European.