Population Matters said the pontiff had more power than anyone because a change in his teaching could prevent millions of births around the world.
In the 192 page letter, Laudato Si (Be Praised), Pope Francis said if there was no change the earth would look "like an immense pile of filth".
"Doomsday predictions can no longer be met with irony or disdain," he said in a direct message to climate change and global warming deniers.
"We are faced not with two separate crises, one environmental and the other social, but rather with one complex crisis which is both social and environmental."
But Simon Ross, Chief Executive of Population Matters, accused the Pope of "sidestepping" the issue of population growth in The Times.
He said: "The one thing that would make the biggest contribution to the Pope's twin goals of poverty alleviation and environment conservation would be abandoning the Catholic church's condemnation of modern family planning."
He added that the United Nations has projected that the global population could reach 11 billion by 2100.
Mr Ross said European Catholic's often ignored the Catholic Church's ban on contraception but this was less the case in Asia, Africa and Latin America.
Speaking to Premier, Cardinal Vincent Nichols, the Catholic leader of faithful in England and Wales urged people to "think again from a perspective of the wellbeing of the earth and our brothers and sisters in other countries who are desperately poor".
But Lord Lawson of Blaby, formed founder of the climate-sceptic Global Warning Policy Foundation said the encyclical was "a mixture of junk science, junk economics and junk ethics."
