1.2 billion Catholics now firmly in the camp of those who say climate change is man-made
Pope Francis blesses the crowd after a mass at the San Giuseppe all'Aurelio parish on Dec. 14, 2014, in Rome. (Andreas Solaro/AFP)

Pope Francis will this week warn that global warming threatens the future of the planet and say there is both an urgent need and moral imperative to reduce fossil fuel consumption, Vatican sources said Tuesday.

A landmark Church statement on the environment, due to be officially released on Thursday, places the leader of the world's 1.2 billion Catholics firmly in the camp of those who say climate change is mainly man-made at a critical time in the global debate over what, if anything, can be done to slow or reverse it.

The papal intervention, seen as a potential game-changer by Green groups, comes six months before international leaders gather in Paris to try and seal a global deal on steps to reduce carbon emissions.

Francis will outline his views in the form of an encyclical, a statement of fundamental principles designed to guide Catholic teaching on a subject that is issued as a letter from the pope to bishops around the world.

But he will also make it clear that his appeal is addressed to everyone on the planet and that he sees a clear link between defending the environment and delivering social justice.

The encyclical, entitled "Laudato Si" (Be Praised), will reference the views of climate change sceptics who are particularly influential in the United States by acknowledging that natural cases such as volcanic activity have contributed to the phenomenon of rising average temperatures.

But it will leave no doubt that the Pope believes that the causes are mainly human, and that creates a compelling practical case and a moral imperative for action to phase out fossil fuels and develop alternative energy sources.

- A major impact -

"Great natural forces are not under our control; human causes are," said a source familiar with the final text to be released on Thursday, a draft of which was leaked to Italian weekly L'Espresso.

"There is strong scientific evidence that the human factors are already having much impact and causing great damage not only to nature itself but also to the lives of people across the globe, especially the poor."

Francis's thinking on the issue is in line with the view outlined in a 2013 UN report which concluded that there was a 95 percent chance that global warming was the result of human activity.

But by expressing it he is seen as potentially influencing millions of people who have yet to think seriously about the subject, generating fresh pressure on the nearly 200 governments who will be represented in Paris in December.

"The encyclical is going to have a major impact," predicted Christiana Figueres, head of the UN Framework Convention on Climate Change (UNFCCC).

"Pope Francis is personally committed to this issue like no other pope before him and he is deeply eloquent and deeply passionate about this and the encyclical will communicate that."

The document will likely burnish Francis's reputation as a socially radical pope but its contents will come as no surprise.

The Argentinian gave a taste of his views on the issue last month when he warned "the powerful of the earth" they would answer to God if environmental damage undermined the drive to end global hunger.

"The planet has enough food for all, but it seems that there is a lack of willingness to share it with everyone," Francis said at a mass to mark the opening of the general assembly of the Catholic charitable organisation Caritas.

"We must do what we can so that everyone has something to eat, but we must also remind the powerful of the earth that God will call them to judgement one day and there it will be revealed if they really tried to provide food... and preserve the environment so that it could produce this food."