* Pope writes keenly awaited encyclical on the environment
* U.N. leader and pope discuss effects of climate change
* Sceptics say view one-side, deny climate change man-made
By Philip Pullella
VATICAN CITY, April 28 (Reuters) - The Vatican and the United Nations teamed up to warn the world of the effects of climate change on Tuesday, coming down firmly against sceptics who deny human activities help change global weather patterns.
U.N. Secretary General Ban Ki-Moon discussed climate change with the pope before opening a one-day conference of scientists and religious leaders called "The Moral Dimensions of Climate Change and Sustainable Development".
The pope, who is due to make a major address on sustainable development at the United Nations in September, has said he believes man is primarily responsible for climate change and is writing an encyclical on the environment.
Ban, opening the conference of some 60 scientists, religious leaders and diplomats hosted by the Pontifical Academy of Sciences, urged industrialised countries to invest in clean energy and reduce their carbon footprints.
"Mitigating climate change and adapting to its effects are necessary to eradicate extreme poverty, reduce inequality and secure equitable, sustainable economic development," he said.
The gathering's final joint declaration said "Human-induced climate change is a scientific reality, and its decisive mitigation is a moral and religious imperative for humanity."
The Paris summit on climate change in December "may be the last effective opportunity to negotiate arrangements that keep human-induced warming below 2 degrees C," adding that the "current trajectory may well reach a devastating 4degrees C or higher," it said.
Ban said he and the pope discussed Francis' keenly awaited encyclical, which will be addressed to all of the world's 1.2 billion Roman Catholics and which the pope has said he hopes will influence the Paris conference.
"It (the encyclical) will convey to the world that protecting our environment is an urgent moral imperative and a sacred duty for all people of faith and people of conscience," Ban said.
Jeffrey Sachs, Colombia University professor and director of the U.N. Sustainable Solutions Network, told reporters companies that invest in fossil fuels stand to lose money.
"Everybody needs to understand that policies are going to change to make it unprofitable if you wreck the planet," he said. "Those companies that continue exploring and developing fossil fuel resources for which there is no safe use are going to pay a very heavy cost for that"
The Heartland Institute, a Chicago think tank that says climate change is not human-induced, sent a delegation to Rome to contest the premise of the conference.
Heartland member Christopher Monckton of Britain, told reporters that the pope "should listen to both sides of the scientific argument ... not only people of one, narrow, poisonous political and scientific viewpoint".
(Reporting by Philip Pullella; Editing by Tom Heneghan/Hugh Lawson)