Pope Francis on Tuesday urged the United Nations to take a "very strong stand" on climate change at a landmark summit this year in Paris on global warming.
The pope spoke at a Vatican-hosted conference of mayors and governors from major world cities who signed a declaration urging global leaders to take bold action at the U.N. summit, saying it may be the last chance to tackle human-induced global warming.
"I have great hopes in the Paris summit," he said. "I have great hopes that a fundamental agreement is reached. The United Nations needs to take a very strong stand on this."
Last month, the pope issued an encyclical on climate change, the first ever dedicated to the environment. The call to his church's 1.2 billion members could spur the world's Catholics to lobby policymakers on ecology issues and climate change.
The Vatican conference linked climate change and modern slavery because, according to an introductory paper, "global warming is one of the causes of poverty and forced migration".
The conference is the Vatican's latest attempt to influence the Paris summit in December, the purpose of which is to reach a global agreement to combat climate change after past failures.
Mayors from South America, Africa, the United States, Europe and Asia signed a declaration stating that the Paris summit "may be the last effective opportunity to negotiate arrangements that keep human-induced warming below 2 degrees centigrade."
Leaders should come to a "bold agreement that confines global warming to a limit safe for humanity while protecting the poor and the vulnerable...," the declaration, which the pope also signed, reads.
High-income countries should help finance the cost of climate-change mitigation in low-income countries, it says.
In a rejection of so-called climate-change deniers, the declaration says: "Human-induced climate change is a scientific reality, and its effective control is a moral imperative for humanity."
On Tuesday morning, California Governor Edmund "Jerry" Brown, whose state is suffering a severe drought, urged mayors to "fight the propaganda" of big business interests that deny that climate change is human induced.
New York Mayor Bill de Blasio called Pope Francis "the most powerful voice on this earth for those whose voice is not being heard," and added: "He did not convene us here to accept the status quo but to indict it".
De Blasio announced that New York City would commit to reduce carbon emissions by 40 percent by 2030 on top of a previous commitment to reduce them by 80 percent by 2050.
Tony Chammany, the mayor of Kochi, India, said coastal areas were already feeling the effects of rising sea levels. "It is now or never, there may never be a replay," he said.