Pope Francis has called for an ethical and economic revolution to prevent catastrophic climate change and growing inequality.

Outlining the Vatican's first detailed teaching on the environment, the Pontiff urges humanity, but especially wealthier countries, to address in a meaningful way the growing threat of global warming.

Pope Francis, himself a trained chemist, calls for urgent action to develop policies to reduce greenhouse gases, including substituting fossil fuels and developing renewable energy sources.

This morning's 106-page letter from Pope Francis to his 5,000 bishops around the world - officially labelled an 'encyclical' - is only the second document of its kind that he has written since his election over two years ago.

It says that "the polluter should pay principle" should be recognised by the world's governments.

Justice dictates that the main emitters of greenhouse gases should recompense the victims of global warming in countries suffering from floods, droughts and other severe weather events to which global warming contributes, the letter states.

Saying he was "drawing on the results of the best scientific research available," he calls climate change "one of the principal challenges facing humanity in our day".

In several passages in the six-chapter encyclical, Pope Francis confronts head on both climate change deniers and those who say it is not man-made. 

"A very solid scientific consensus indicates that we are presently witnessing a disturbing warming of the climatic system," he says.

"Humanity is called to recognise the need for changes of lifestyle, production and consumption, in order to combat this warming or at least the human causes which produce or aggravate it."
              
"It is true that there are other factors - such as volcanic activity, variations in the earth's orbit and axis, the solar cycle - yet a number of scientific studies indicate that most global warming in recent decades is due to the great concentration of greenhouse gases - carbon dioxide, methane, nitrogen oxides and others - released mainly as a result of human activity," he says.
              
Pope Francis calls for policies to "drastically" reduce polluting gases.

Technology based on fossil fuels "needs to be progressively replaced without delay" and sources of renewable energy developed.

In a passage certain to upset conservatives, he says "a legal framework which can set clear boundaries and ensure the protection of ecosystems has become indispensable". 

One of the major themes of the encyclical is the disparity of wealth.  

"We fail to see that some are mired in desperate and degrading poverty, with no way out, while others have not the faintest idea of what to do with their possessions, vainly showing off their supposed superiority and leaving behind them so much waste which, if it were the case everywhere, would destroy the planet," he says.

He criticises those who "maintain that current economics and technology will solve all environmental problems, and argue, in popular and non-technical terms, that the problems of global hunger and poverty will be resolved simply by market growth."

Former president Mary Robinson has said the Papal Encyclical establishes climate change and safeguarding the earth for humanity as the moral issue of our time. 

In a statement she said it is significant that Pope Francis uses one of the highest forms of doctrinal teaching (an encyclical), to communicate that preserving a safe climate system "represents one of the principal challenges facing humanity in our day." 

The Mayo-born barrister, who now leads the Mary Robinson Foundation - Climate Justice, also says the document is a call for climate justice from one of the most influential moral voices on our planet today.

She commends the pope for showing "his profound understanding of the connection between nature, justice for people living in poverty, human dignity and the need to act in solidarity in the face of climate change."

She added that transformative leadership is needed to overcome what she called "the climate challenge" and said the encyclical answers that call in both its sentiment and in the scale of its reach.  

She underlined its call on us all to act in solidarity, to protect the most vulnerable people and to safeguard the earth and its resources for current and future generations. 

"I hope many of all faiths and none will hear it and be compelled to act in this significant year of 2015,"  the statement concluded, referring to the four-yearly summit of heads of state planned for Paris at the end of this year which will try to reach a global deal on climate change.

The last effort in Copenhagen in 2009 failed to do so.