Pope Francis called on Thursday on the world’s 1.2 billion Catholics to join the fight against climate change. In a papal encyclical letter, Pope Francis declared that the science of climate change is clear and that the Catholic Church views climate change as a moral issue that must be addressed in order to protect the Earth and everyone on it.

The encyclical is called "Laudato Si", which translates into "Praised Be" - a reference to a prayer from the pope's namesake St. Francis, the patron saint of ecology.

The letter comes ahead of the next UN climate change conference in Paris, in December, where governments will reach a universal climate change agreement that must keep the average global temperature from rising beyond 2C degrees and secure the ability of all countries to adapt to the climate change that is already in the global system.

Countries gather before that in New York, in September, at the UN General Assembly to agree a new set of sustainable development goals. These inter-linked efforts must put all nations on track towards a sustainable future that decouples human growth from pollution and environmental degradation.

A spokesman for UN Secretary-General Ban Ki-moon issued the following statement:

The Secretary-General welcomes the papal encyclical released today by His Holiness Pope Francis which highlights that climate change is one of the principal challenges facing humanity, and that it is a moral issue requiring respectful dialogue with all parts of society. The Secretary-General notes the encyclical’s findings that there is "a very solid scientific consensus" showing significant warming of the climate system and that most global warming in recent decades is "mainly a result of human activity".

The Secretary-General reaffirms that humanity has a significant obligation to care for and protect our common home, the planet Earth, and to show solidarity with the poorest and most vulnerable members of society who are suffering most from climate impacts. The Secretary-General therefore urges governments to place the global common good above national interests and to adopt an ambitious, universal climate agreement in Paris this year.

The Secretary-General welcomes the contributions of all religious leaders and people of influence in responding to the climate challenge and in strengthening sustainable development. He looks forward to welcoming Pope Francis at the United Nations in September to address the UN General Assembly.

The UN’s top climate change official UNFCCC Executive Secretary Christiana Figueres said:

"Pope Francis’ encyclical underscores the moral imperative for urgent action on climate change to lift the planet’s most vulnerable populations, protect development, and spur responsible growth. This clarion call should guide the world towards a strong and durable universal climate agreement in Paris at the end of this year. Coupled with the economic imperative, the moral imperative leaves no doubt that we must act on climate change now."

Executive Director of the UN Environment Programme Achim Steiner issued the following statement following the release of the encyclical:

"The UN Environment Programme welcomes Pope Francis' unambiguous call to action in the face of global environmental degradation and climate change. This encyclical is a clarion call that resonates not only with Catholics, but with all of the Earth’s peoples. Science and religion are aligned on this matter: The time to act is now.

"We share Pope Francis' view that our response to environmental degradation and climate change cannot only be defined by science, technology or economics, but is also a moral imperative. We must not overlook that the world’s poorest and most vulnerable suffer most from the changes we are seeing. 

"Humanity’s environmental stewardship of the planet must recognize the interests of both current and future generations. With the adoption of the Sustainable Development Goals in September and a climate agreement in December, we have the opportunity to positively alter the course of history, creating a better and more equitable world for all. Given what we know about the state of our planet, and the choices we can make today, there can be no equivocating in the face of grave consequences. We all must recognise the need to reduce our environmental impact, and consume and produce in a sustainable way. As Secretary General Ban Ki moon has repeatedly stated, ‘We are the first generation that can end poverty, and the last generation that can act to avoid the worst impacts of climate change.’"

UNESCO Director General Irina Bokova said:

"Pope Francis calls for a new vision of humanity in relation to the planet that is our home. This is a call for courage and unity, where every woman and man has opportunities and skills to contribute, especially the most marginalized, where sustainability means much more than green laws and policies – it means new ways of thinking and behaving as global citizens, it means a new focus on the ocean and biodiversity. We need this vision and courage more than ever to reach a new climate change agreement this year in Paris."

UN Development Programme Administrator Helen Clark said:

"I welcome Pope Francis' very important contribution to the climate change debate through his encyclical on the environment and the poor. The poor and the marginalized in our societies are the ones who are the most vulnerable to climate change, and are also the ones hardest hit by its impacts.

UNDP works with developing countries to avoid what Pope Francis describes as an "economy of exclusion," and strives to enable progress and growth which benefits everyone. As we look forward later this year to the creation of sustainable development goals and the expected climate change agreement, we must seize this once in a generation opportunity to chart a new course for sustainable development which benefits everyone and protects our planet."