The world population of 5.5 billion is more than twice what the Earth can support over the long term, a study says.
Paul Ehrlich of Stanford University and Gretchen Daily of UC Berkeley have concluded that the Earth's maximum "carrying capacity" is about 1.5 billion to 2 billion people.
"We're unsustainable now," Ehrlich said in an interview last week. "We want to reduce the impact so there's hope of getting over the top and bringing the population back down."
The study also said that the world's population would double again before efforts to control it take hold.
The best way to boost the Earth's carrying capacity and to reduce population is to promote political and social policies that remove the enormous inequities between rich and poor, men and women and industrial and developing countries, the researchers said.
Removing inequities among nations encourages the kind of international cooperation needed to control environmental problems, such as global warming, that can hinder agricultural production, Ehrlich said.
He also said that women with jobs are less likely to have children. "There's a lot of evidence that equity between the sexes can lower the fertility rate," Ehrlich said.
Other reforms, such as a fairer distribution of land among farmers in developing countries, can help increase the Earth's carrying capacity, Daily said.
"If people own their own land, there's more incentive to pass it on the way they got it," rather than to abuse it, Daily said. Land that is well cared for can support more people than land that is abused, she said.
Jane Lubchenco of Oregon State University said that Ehrlich and Daily are making an important contribution to population research. "What they're doing is way ahead of anybody else," she said.
Ehrlich and Daily said the recommendation for greater social and political equity is based on scientific findings, not their personal political philosophy.
The researchers presented their findings at the annual meeting of the Ecological Society of America and the American Institute of Biological Sciences.
Ehrlich, author of the groundbreaking 1968 book, "The Population Bomb," said that increasing Earth's carrying capacity would require that Americans consume less, but could lead to an improvement in their quality of life.
"What if over the next 50 years we redesigned America for people instead of cars," he said. "People would walk to work, everyone's health would improve." Fuel consumption and pollution would also drop, he said.