By Environment Correspondent Alister Doyle
OSLO (Reuters) - A 2015 global pact for fighting climate change will benefit in some ways at least if U.S. President Donald Trump carries out a threat to pull out, backers say, in a shift from gloom about the fate of a deal that took two decades to negotiate.
The Paris Agreement requires consensus for all decisions, meaning the withdrawal of a recalcitrant United States would make it easier for emitters such as China and the European Union to design details of a trillion-dollar shift from fossil fuels.
Trump has called man-made climate change a hoax and made a so-far unfulfilled campaign pledge to "cancel" the agreement, saying he wants to promote the domestic fossil fuel industry.
In a step to undo environmental regulations introduced under former President Barack Obama, Trump will sign an order on Tuesday aimed at making it easier for companies to produce energy in the United States.
"There will be some advantages for other countries and there will also be extraordinary disadvantages," if the U.S. ends up quitting Paris, said Christiana Figueres, an architect of the agreement who was the U.N.'s climate chief in Paris.
"It's not a black and white scenario," she said.
She said the ideal outcome, both for the United States and other nations, was for Washington to stay and make deep cuts in greenhouse gas emissions. There were better investment prospects for renewable energies such as solar power than coal, she added.
The fear has long been that a pullout of the world's top economy would drain other nations' willingness to cut greenhouse gas emissions under an agreement ratified by nations as diverse as China, Saudi Arabia and African countries.
But there is an emerging rival view that Paris might be better off.
"The Achilles heel of the Paris Agreement is that it's built on consensus," said Johan Rockstrom, director of the Stockholm Resilience Center at Stockholm University.
"It's very difficult to have a negative giant in the room" able to obstruct all decisions, he said, adding that he had swung in recent weeks to reckon that a U.S. pullout would be better overall from an earlier view that it would be a "big failure".
The Paris Agreement has few binding obligations. It lets all nations set their own goals for fighting climate change and has no penalties for non-compliance.
Governments have set a 2018 deadline to work out a rule book for the Paris Agreement, filling in details, for instance, of how nations will report and monitor their curbs on emissions.
Maldives Environment Minister Thoriq Ibrahim - chair of the Alliance of Small Island States whose members fear they are at risk from rising sea levels - urged continued U.S. participation in Paris.
"The closer you look at it, the clearer it becomes that it (the Paris Agreement) also promotes important strategic, economic, and security benefits as well," he said.
Norwegian Prime Minister Erna Solberg also said it would be better for the United States to stay. "But I don't think that it's the end of the Paris Agreement if the United States decides to leave," she said.
Oliver Geden, of the German Institute for International and Security Affairs, disagreed. A U.S. withdrawal would badly undermine the Paris Agreement and any decisions would be largely irrelevant without the world's biggest economy, he said. "The momentum could fade away pretty soon," he predicted.
Still, almost 200 nations agreed after Trump's election in November that the world had an "urgent duty" to combat an "alarming and unprecedented" rate of global warming.
"The pace and scale of change already underway in the global economy is remarkable and irreversible," said Stephanie Pfeifer, head of the Institutional Investors Group on Climate Change, a forum with 18 trillion euros ($20 trillion) in assets under management.
"Renewables have already overtaken coal as a global power source, electric vehicles are the growth segment of the auto industry and jobs are being created very rapidly in clean energy," she said.
The U.S. target under Paris, set by Obama, is to cut emissions by between 26 and 28 percent below 2005 levels by 2025 as part of global efforts to avert more downpours, droughts, heatwaves and floods.
Trump's pro-coal policies are likely to make U.S. emissions cuts less ambitious.
By staying in the Paris Agreement, Trump would also undermine a demand in its Article 4 that successive national climate plans have to be ever deeper cuts. A U.S. pullout would make it easier for backers to argue that the principle has not been violated.
The Paris Agreement sets an over-riding goal of limiting global warming to "well below" two degrees Celsius (3.6 Fahrenheit) above pre-industrial times. The United Nations says that current pledges are insufficient to meet that goal.
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(editing by David Stamp)