The UK-based Optimum Population Trust says fast-rising population levels lead to growing emissions.
The website is urging wealthy people to offset their own CO2 emissions by funding contraception programmes.
It says taking such action is better value than spending money on wind turbines, solar power or hybrid cars.
Critics would argue the analysis is too simplistic, a BBC correspondent says.
The BBC's environment analyst Roger Harrabin says they could contend that reducing the number of people born in the US would make a big difference in achieving reductions in CO2 levels.
Our correspondent added that carbon emissions from people in much of sub-Saharan Africa are so low that they can barely be counted.
According to the OPT, every £4 spent on family planning saves one tonne of CO2.
It estimates that a similar reduction would require an £8 investment in tree planting, £15 in wind power, £31 in solar energy and £56 in hybrid vehicle technology.
It is promoting a scheme in which wealthy people can offset their own carbon emissions by funding contraceptive programmes in the developing world.
