Few doubt the world's booming population contributes to rising carbon emissions.
But as a U.N. climate conference in Poznan, Poland considers how to reduce heat-trapping, greenhouse gases, the talk is all about setting emissions targets and funding renewable energy projects. Stabilizing population is not even on the table.
U.N. officials contend that pushing policies on population growth could undermine already difficult negotiations that are fraught with finger pointing between rich and poor nations over who is to blame for global warming.
The developing world would oppose introducing population into the mix on the grounds that it would hold them accountable for a problem they blame on the West. The Vatican along with Catholic and Muslim countries, meanwhile, are opposed over fears population policies would increase support for abortion and birth control.
"A lot of people say population pressure is a major driving force behind the increase in emissions, and that's absolutely true," the U.N.'s top climate official Yvo de Boer said. "But to then say 'OK, that means that we need to have a population policy that reduces emissions,' takes you onto shaky ground morally."
With the world's population expected to reach more than 9 billion by 2050 - three times what it was in 1960 - researchers argue that slowing population would make it easier to solve the climate crisis.
Understanding the role population plays, they contend, would also allow countries to develop policies to reduce their carbon footprint as well as protect citizens from the rising seas and worsening storms associated with global warming.
"If we don't address the population issue and population continues to grow the way it is, ... we will fail to solve the climate crisis," Ryerson said.
Brian O'Neill, a population expert with the National Center for Atmospheric Research, said there is substantial evidence showing a strong correlation between a country's economic growth and its emissions.
"Slowing population growth as best we know now will substantially reduce emissions and it will make society better able to cope with climate change and it will improve the well being of people more broadly," O'Neill said.
O'Neill and others also said it's not only the sheer numbers of people that mattered. He and his colleagues released findings this year that found urbanization in China and India could result in increased emissions of as much as 70 percent by 2100 while aging in the United States could contribute to as much as a 40 percent decline. Smaller household sizes also have led to increased emissions.
However, other researchers argue using population policies to address climate change is shortsighted and would lead to coercive family planning policies like the one-child policy in China.
"The countries that have relatively high population growth like Africa tend to have the smallest carbon footprint," said Betsy Hartmann, director of the population and development program at Hampshire College in Massachusetts. "It really doesn't make sense to think that reducing family size is going to reduce global warming. Family sizes are already coming down."
Rajendra Pachauri, chairman of the U.N.'s Intergovernmental Panel on Climate Change, said addressing population growth must also consider a country's consumption rates - a reference to the fact that India's per capita emissions are a 15th that of the United States and China's one-sixth.
"It is for country to decide how to limit growth whether you reduce consumption levels, use more renewable energy," Pachauri said. "It's really a national decision. It is very difficult to provide for something like this in a multilateral agreement."
"Population doesn't need to be part international negotiations on mitigation. You don't have say country X will cap its emissions and population," he said.
"But countries will begin to see that a more rapidly rising population will make it hard for them to curb emissions," said Engelman, the author of "More: Population, Nature and What Women Want."