According to the New York Times, the encyclical, which will be sent to the world's 5,000 Catholic bishops and published online in five languages on Thursday, is expected to call for an end to the 'tyrannical' exploitation of nature by mankind.
Climate change activists are treating the as a milestone that could place the Roman Catholic Church at the forefront of a new coalition of religion and science.
Its release comes during a year with three major international policy meetings, most notably a United Nations climate change conference in Paris in December.
This month, the Vatican sent notifications to bishops around the world with instructions for spreading the pope's environmental message to the more than one billion Catholics worldwide.
"We clearly need a fundamental change of course, to protect the earth and its people - which in turn will allow us to dignify humanity, Cardinal Peter Turkson of Ghana, who oversaw the drafting of the encyclical, said at a conference on climate change this spring at the Vatican.
Vatican officials say that the encyclical is a theological document, not a political one, and have refused to divulge the contents. But there is already much speculation about how Francis will comment on humans' role in causing climate change, a link he has spoken about in the past. The Vatican's scientific academy recently attributed climate change to "unsustainable consumption" and called it "a dominant moral and ethical issue for society."
This stance has rankled some conservative Catholics, as well as climate change sceptics, who have suggested that Francis is being misled by scientists and that he could veer into contentious subjects like population control.
