01:20 What Happens When U.S. Exits Climate Deal? What happens if the U.S. drops out of the Paris Climate Agreement? Scientists take a look.

At a Glance Because the U.S. is a huge contributor of greenhouse gas emissions, scientists say pulling out of the pact would worsen an already bad problem.

It would make it far more difficult to prevent crossing a dangerous global temperature threshold.

With reports coming out of Washington Wednesday that President Donald Trump intends to make good on a campaign promise to pull out of the landmark Paris climate change accord , scientists and environmentalists are chiming in, calling the move a "historical mistake."

Trump, who famously called global warming a Chinese hoax, said in a tweet Thursday that he would announce his decision at 3 p.m. EDT.

Last week, the leaders of the seven wealthiest democracies urged Trump to commit to the agreement.

"I hope they decide in the right way," said Italy's prime minister, Paolo Gentiloni . German Chancellor Angela Merkel said the leaders' talks were "very difficult, if not to say, very unsatisfactory."

The agreement was reached in December 2015 and officially took effect in November 2016. The accord was the first of its kind addressing global warming to be signed, ratified and put into force. Its intent was to keep the planet from rising above pre-industrial averages to two degrees Celsius or 3.6 degrees Fahrenheit by 2100.

The agreement committed rich and poor countries to take action to curb man-made greenhouse gases, which 97 percent of scientists who actively study climate change affirm “is a significant contributing factor in changing mean global temperatures.” Numerous studies published in peer-reviewed journals have also consistently shown rising global temperatures to be tied to human activity.

If reports that Trump is pulling out of the agreement proves to be correct in the coming days, what are the ramifications?

Last year, 375 top scientists signed an open letter advising Trump against pulling out of the accord.

In the letter, the scientists had said withdrawing would threaten “crossing climatic points of no return, possibly setting in motion large-scale ocean circulation changes, the loss of major ice sheets, and species extinctions.”

According to a report by the Associated Press, the Earth is likely to reach more dangerous levels of warming even sooner than predicted should the U.S. pull back from its pledge to cut carbon dioxide pollution.

In a statement , the Sierra Club said Wednesday the move would be a "historical mistake."

"Donald Trump has made a historic mistake which our grandchildren will look back on with stunned dismay at how a world leader could be so divorced from reality and morality," Sierra Club Executive Director Michael Brune said. "Trump has abandoned the standard of American leadership, turned his back on the what the public and the market demand, and shamelessly disregarded the safety of our families just to let the fossil fuel industry eke out a few more dollars in profits. This is a decision that will cede America’s role internationally to nations like China and India, which will benefit handsomely from embracing the booming clean energy economy while Trump seeks to drive our country back into the 19th century."

According to AP, scientists are saying the pullout will worsen an already bad problem and make it far more difficult to prevent crossing a dangerous global temperature threshold.

In fact, the decision could result in emissions of up to 3 billion tons of additional carbon dioxide in the air each year, the AP reports. When it adds up year after year, scientists said that is "enough to melt ice sheets faster, raise seas higher and trigger more extreme weather."

"If we lag, the noose tightens," Princeton University climate scientist Michael Oppenheimer, co-editor of the peer-reviewed journal Climatic Change, told AP.

Those who believe the Trump administration should pull out of the agreement, including several Republican senators, argue that the agreement opens up the government to diplomatic headaches and even possible legal action, Newsweek reports .

“Under the Paris Agreement, a nation honors its ‘non-binding’ emission reduction and climate finance commitments by turning those promises into enforceable obligations — domestic laws and regulations,” Christopher Horner, a senior fellow at the Competitive Enterprise Institute, wrote in a report published earlier this month. “Thus, the only way to unplug from the pressure machine is to withdraw from the agreement.”

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Still, scientists overwhelmingly agree the planet would warm up even faster than it is already should the U.S. pull out of the agreement signed by nearly 200 countries to reduce emissions. The goal of the agreement is to keep the planet from warming more than 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.

"Developed nations - particularly the U.S. and Europe - are responsible for the lion's share of past emissions, with China now playing a major role," Rutgers University climate scientist Jennifer Francis, told the AP. "This means Americans have caused a large fraction of the warming."

Some scientists argue that even if the U.S. abides by its commitment under the Paris agreement, the world will still pass that 2-degree mark. However, they also agree that it would occur at a much faster rate, which would result in "ecosystems being out of whack with the climate, trouble farming current crops and increasing shortages of food and water," the National Center for Atmospheric Research's Kevin Trenberth said.

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John Schellnhuber, director of the Potsdam Institute for Climate Impact Research, who is credited with coming up with the 2-degree goal, says the pull-out of the U.S. might not have a great impact.

"Ten years ago (a U.S. exit) would have shocked the planet," Schellnhuber said. "Today if the U.S. really chooses to leave the Paris agreement, the world will move on with building a clean and secure future."

Katharine Hayhoe, a Texas Tech climate scientist, disagrees.

"There will be ripple effects from the United States' choices across the world," she told the AP.

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