The agency said the battle against global warming could be helped if the world slowed population growth by making free condoms and family planning advice more widely available.
The agency did not recommend countries set limits on how many children people should have, but said: "Women with access to reproductive health services ... have lower fertility rates that contribute to slower growth in greenhouse gas emissions."
"As the growth of population, economies and consumption outpaces the Earth's capacity to adjust, climate change could become much more extreme and conceivably catastrophic," the report said.
The world's population will likely rise from the current 6.7 billion to 9.2 billion in 2050, with most of the growth in less developed regions, according to a 2006 report by the United Nations.
The U.N. Population Fund acknowledged it had no proof of the effect that population control would have on climate change. "The linkages between population and climate change are in most cases complex and indirect," the report said.
It also said that while there is no doubt that "people cause climate change," the developing world has been responsible for a much smaller share of world's greenhouse gas emissions than developed countries.
Still, Thoraya Ahmed Obaid, the U.N. Population Fund's executive director, told a news conference in London on Wednesday that global warming could be catastrophic for people in poor countries, particularly women.
"We have now reached a point where humanity is approaching the brink of disaster," she said.
In three weeks, a global conference will be held in Copenhagen aimed at reaching a deal to replace the 1997 Kyoto Protocol, which required 37 industrial countries to cut heat-trapping greenhouse gas emissions.
On Wednesday, one analyst criticized the U.N. Population Fund's pronouncements as alarmist and unhelpful.
"It requires a major leap of imagination to believe that free condoms will cool down the climate," said Caroline Boin, a policy analyst at International Policy Network, a London-based think tank.
She also questioned earlier efforts by the agency to control the world's population.
In its 1987 report, the U.N. Population Fund warned that once the global population hit 5 billion, the world "could degenerate into disaster." At the time, the agency said "more vigorous attempts to slow undue population growth" were needed in many countries.
According to Boin, "Numerous environmental indicators show that with development and economic growth we are able to preserve more natural habitats. There is no causal relationship between population density and poverty."
In this month's Bulletin, the World Health Organization's journal, two experts also warned about the dangers of linking fertility to climate change.
"Using the need to reduce climate change as a justification for curbing the fertility of individual women at best provokes controversy and at worst provides a mandate to suppress individual freedoms," wrote WHO's Diarmid Campbell-Lendrum and Manjula Lusti-Narasimhan.
