The US alongside China, the world's two largest carbon-emitting nations, had played a major role in building momentum for the Paris deal.
Climate scientists who inform political decisions also expressed alarm, with one noting that Trump's campaign rhetoric on global warming had "strolled into a fact-free zone".
So the realization that Trump now holds the cards on U.S. energy and climate policy has sent a collective shudder through climate scientists and policymakers across the globe.
The WMO says the warming trend is the result of man-made greenhouse gases, and has made extreme weather events more likely. Others held out hope that Trump would change his stance and honor USA commitments under the Paris Agreement. They were also asked to increase financial and technical support to countries and communities most vulnerable to the effects of climate change.
Small island nations which fear they will be swallowed by rising seas are among the biggest supporters of the Paris deal and other global efforts to curb emissions, mainly from fossil fuels.
More than 100 countries, including the US, have formally joined the agreement, which seeks to reduce emissions and help vulnerable countries adapt to rising seas, intensifying heat waves, the spreading of deserts and other changes from man-made warming.
In addition to the Clean Power Plan, which Trump has vowed to kill, there are several environmental and energy rules under litigation including the Interior Department's fracking rule, public lands leasing for drilling, the Dakota Access Pipeline, Arctic drilling, Atlantic drilling and others.
The U.S. under the Bush administration declined to join the previous climate deal, the 1997 Kyoto Protocol, which greatly reduced its impact on global emissions.
And the global average amount of carbon dioxide in the atmosphere reached 400 parts per million in 2015.
And for global action, Trump's surprise win - which stunned diplomats here - is already shaking up discussions at this year's climate conference here, where the focus is on putting the Paris deal into action.
And beyond that Clean Power Plan, other new climate policies would be needed to reach the USA target for emissions, experts say.
He has called for stripping regulations to allow unfettered production of fossil fuels - a key source of emissions - and rescinding the Clean Power Plan, an Obama administration strategy to fight climate change. What really killed coal in the United States is much cheaper natural gas from hydraulic fracturing or fracking, said former astronaut Jay Apt, now co-director of the Carnegie Mellon Electricity Industry Center. "Even that record is likely to be beaten in 2016", said Taalas.
UN chief Ban Ki-moon congratulated Trump on his victory and said people everywhere looked to the United States to work for the common good.
While shell-shocked American climate activists in Marrakech cried and embraced, USA negotiators declined to speak to reporters about the election outcome.
That's a danger that goes beyond the US; pulling out would politically signal to other countries that they too can back out of the deal, which would pose a huge setback as the world attempts to slow a rapidly warming planet.
Withdrawing from the Paris Agreement, which 100 countries, including the U.S., have ratified, would take four years - an entire presidential term. However, Trump could also decide to simply ignore the Obama administration's Paris pledge to reduce USA emissions by 26-28 percent from 2005 levels by 2025. There is no punishment for countries that miss their targets.
If we add up all the nationally determined contributions (NDCs) to reducing emissions of greenhouse gases that countries have put forward so far, we are still headed for more than 2 degrees.
"Expect some long-awaited, rigorous examination of the theory/models", John Christy, of the University of Alabama, Huntsville, wrote in an email.