(Adds quotes from Vatican, China-Germany details, quotes from economist)
WASHINGTON, June 1 (Reuters) - President Donald Trump on Thursday will announce his decision on whether to keep the United States in a landmark global pact to fight climate change, after a source close to the matter said he was preparing to pull out of the Paris accord.
Trump said in a Twitter post on Wednesday night that he would make the announcement at 3 p.m. (1900 GMT) on Thursday in the White House Rose Garden, ending his tweet with "MAKE AMERICA GREAT AGAIN!"
An American withdrawal, promised by Trump during the 2016 U.S. presidential campaign, would deepen a rift with U.S. allies and put the United States in the same league as Syria and Nicaragua as the world's only non-participants in the landmark 195-nation accord agreed upon in Paris in 2015.
The source, speaking on condition of anonymity, said on Wednesday Trump was favoring an exit, however, and was working out terms of the planned withdrawal with U.S. Environmental Protection Agency Administrator Scott Pruitt, an oil industry ally and climate change doubter. The source said any withdrawal announced by Trump could have conditions or caveats that were under discussion.
The Vatican, which under Pope Francis' insistence has strongly backed the Paris accord, would see a U.S. exit as a "a huge slap in the face," a senior Vatican official said on Thursday.
"It will be a disaster for everyone," Bishop Marcelo Sanchez Sorondo told the Rome newspaper La Repubblica.
At their meeting last month, the pope gave Trump a signed copy of his 2015 encyclical letter that called for protecting the environment from the effects of climate change and backed scientific evidence that it is caused by human activity.
U.S. supporters of the pact said any pullout by Trump would represent an abdication of American leadership on a leading issue of our time and would show that the United States cannot be trusted to follow through on international commitments.
The pact was the first legally binding global deal to fight climate change. Virtually every nation voluntarily committed to steps aimed at curbing global emissions of "greenhouse" gases. These include carbon dioxide generated from burning of fossil fuels that scientists blame for a warming planet, sea level rise, droughts and more frequent violent storms.
Scientists have said a U.S. withdrawal from the deal could speed up the effects of global climate change, leading to heat waves, floods, droughts, and more frequent violent storms.
During the campaign, Trump said the accord would cost the U.S. economy trillions of dollars with no tangible benefit. Trump also has expressed doubts about climate change, at times calling it a hoax to weaken U.S. industry.
The Republican vowed at the time to "cancel" the Paris deal within 100 days of becoming president on Jan. 20, part of an effort to bolster U.S. oil and coal industries.
Since taking office, he has come under pressure from some advisers, close U.S. allies, corporate CEOs, Democrats and some fellow Republicans to keep the United States in the accord.
Some executives from coal companies have weighed in, arguing the U.S. should stay in with reduced emissions cuts targets, with an eye toward ensuring that Washington keeps some influence over the future of the global energy mix.
The United States, under former President Barack Obama, had committed to reduce its emissions by 26 percent to 28 percent from 2005 levels by 2025. The U.S. accounts for more than 15 percent of global emissions and is the world's second largest emitter of greenhouse gases, exceeded only by China.
Last year was the warmest since records began in the 19th century. A panel of leading climate scientists found in 2014 at least a 95-percent probability that man-made greenhouse gas emissions have caused most of the warming since around 1950.
China, which overtook the United States as the world's biggest emitter of greenhouse gases in 2007, and the European Union will seek on Friday to save the Paris agreement. Chinese Premier Li Keqiang will meet top EU officials at a summit in Brussels.
In a statement backed by all 28 EU states, the European Union and China were poised to commit to full implementation of the agreement, EU and Chinese officials said.
Trump has already moved to dismantle Obama-era climate change regulations, including the U.S. Clean Power Plan aimed at reducing emissions from main coal-fired power plants.
"Putting national resources further into coal while China takes the lead in solar is like investing in building a better horse-drawn carriage back when Henry Ford was investing in mass producing cars, " said University of California at Berkeley economist Solomon Hsiang. "It's simply bad business."
Some U.S. states, including California, Washington and New York, have vowed to continue to reduce greenhouse gas emissions and continuing engaging in the international climate agreement process.
(Reporting by Valerie Volcovici, Timothy Gardner, Jeff Mason, and Roberta Rampton; Additional reporting by Robin Emmott and Robert-Jan Bartunek in Brussels; Writing by Will Dunham; Editing by Nick Zieminski)