The Pope will do battle with climate change sceptics by insisting that global warming is caused by human activity and that changes in lifestyle and consumption are urgently required, a leaked copy of his eagerly awaited encyclical showed yesterday.

The Pope, who trained as a scientist before the priesthood, also warns that a careless attitude towards the environment leads to deprivation and exclusion for the poorest people in society.

In throwing his moral weight behind those who argue that climate change is man-made, he is making a direct attempt to influence the outcome of a UN conference in Paris in December that will aim to convince nations to cut their greenhouse gas emissions.

He will also open a new front in his battle with conservative forces by throwing down the gauntlet to a powerful lobby that rejects the need to curtail the use of fossil fuels to save the planet.

“Humanity is called to recognise the need for changes in lifestyle, production and consumption to combat this warming or, at least, the human causes that produce and accentuate it,” the Pope writes in the 192-page teaching document Laudato Si (Be Praised), which takes its title from a hymn composed by his namesake, St Francis of Assisi.

An encyclical is the highest level of teaching document that a pope can issue, short of an apostolic constitution. The latest will be published on Thursday, but an advance copy was published yesterday on the website of the Italian weekly L’Espresso.

Father Federico Lombardi, the Vatican spokesman, said that the leaked document was an “intermediate version” of the encyclical and that some provisions may be different or missing from the final version.

In the draft, the Pope writes that numerous scientific studies demonstrate that global warming is the result of greenhouse gases emitted “above all as a result of human activity”.

He adds: “If the current trend continues, this century could witness extraordinary climate changes and unprecedented destruction of ecosystems, with grave consequences for all of us.”

The Pope has a master’s degree in chemistry. He writes that environmental degradation causes poverty and migration, but people fleeing poverty are unprotected under international law. Those holding economic and political power often seem dedicated to masking the problem and concealing its symptoms, he suggests.

The encyclical will enrage conservative critics such as Stephen Moore, a Roman Catholic who is the chief economist at The Heritage Foundation, the conservative Washington think tank, who has said: “This is a pope who clearly has some Marxist leanings.” The pontiff’s ideology, he believes, “would make people poorer and less free”.

The Pope plans to take his case to the UN general assembly in September.

There has not been an encyclical dedicated to climate change before, although Pope Francis’s predecessor, Benedict XVI, called for cuts in greenhouse gases and installed solar panels in the Vatican.