Powerful encyclical covering environment expected to add weight to December’s climate change summit.

Stand by for an unholy row as the Pope comes out as perhaps the world's most effective environmental campaigner.

On Thursday, Pope Francis will issue the first-ever encyclical on the environment. The most authoritative document a Pontiff can publish, this encyclical is also the first to try to influence a global political process. It is unapologetically aimed at helping to forge global agreement to tackle climate change at a special summit in Paris in December.

Circulated to the church's 5000 bishops and 400,000 priests, it is a call to action for the world's 1.2 billion Catholics. Prayer vigils and pilgrimages, protests and policy briefings have all been prepared, as environmentalists and the faithful alike seek to maximise its leverage.

The Pope himself will front the campaign, travelling to the US in September to speak at the United Nations and metaphorically entering the lions' den by addressing Congress - about a third of whose members are Catholics, but which remains the world's most resistant legislative body to taking action on global warming.

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It marks a big landmark for the church. Only once before, say experts, has an encyclical sought to influence a political process, rather than react to it - when Pius XI proclaimed his "burning anxiety" at events in Nazi Germany. But that was addressed to one country, not the whole world. And publishing it months before the Paris meeting shows - as Catherine Pepinster, editor of The Tablet, puts it - that Pope Francis "is willing to intervene in a quite deliberate way in the global political process".

Appropriately for the first scientist Pope (he trained as a chemist), the encyclical is also thought to be the first to be rooted in science, based on the reports of the Intergovernmental Panel on Climate Change. Paul VI issued a statement on a "hospitable Earth" to the groundbreaking 1972 Stockholm conference. John Paul II stressed a duty to safeguard the environment, and Benedict XVI became known as the "green pope" for his writings. But Pope Francis's encyclical goes far further, by enshrining environmental protection as central to the faith. Though not infallible, its teaching becomes part of Catholic doctrine: disregarding it requires powerful evidence and argument.

Old Argentinian colleagues say Francis did not seem particularly environmentally conscious, and were surprised when he chose the name of the greenest saint. But he has a longstanding concern for the poor, and has been much influenced by reports filtering up through the church from the grassroots of how climate change is already devastating their lives (the decision to publish was taken after visiting the Philippines after Typhoon Haiyan). His green Greek Orthodox counterpart, Patriarch Bartholomew, has also been very influential.

The encyclical - called Laudato sii (Be Praised) - is expected to address the whole environment, not just climate change, and concentrate on morality, much more than science, focusing on human development and concern for the poor.