CREDIT: AP Photo/Scott Heppell

Presidential contender Donald Trump gestures to the media on the 17th fairway on the first day of the Women’s British Open golf championship on the Turnberry golf course in Turnberry, Scotland, Thursday, July 30, 2015.

If Donald Trump is elected president, America’s approach to energy and the environment will be drastically different than it is today.

Trump made that clear last week, when he laid out his full energy policy proposal for the first time in Bismarck, North Dakota. In that speech, Trump said he would roll back president Obama’s climate change regulations, build the Keystone XL pipeline, and “cancel” the landmark Paris climate agreement. In a nutshell, Trump promised to undo almost every major policy developed in the last decade intended to slow human-caused global warming.

What Trump did not do in his speech, however, was mention the words “climate change.” He did not say whether he believed the phenomenon was occurring, and he didn’t speculate on how his policies would solve or worsen the problem.

So, ThinkProgress asked five climate scientists to weigh in on the potential climate impacts of Trump’s energy plan. Their reactions ranged from concerned to flat-out horrified.

Higgins: A “greater risk to society.”

Paul Higgins, the director of the American Meteorological Society’s policy program, registered on the concerned end.

If Trump “cancelled” the Paris climate agreement and the Clean Power Plan (Obama’s regulatory effort to limit carbon emissions from power plants), Higgins said it is almost certain that more greenhouse gases would be emitted, resulting in higher concentrations of carbon dioxide in the atmosphere and, therefore, a warmer planet.

“Undoing these efforts would mean that future emissions of carbon dioxide would be larger and future atmospheric concentrations would be higher,” Higgins said. “Higher CO2 concentrations would mean larger changes in climate and faster rates of change. Larger and faster changes in climate, in turn, pose greater risk to society.”

These risks are potentially serious, Higgins said. He noted that scientists do not know definitively how much carbon dioxide humans can emit safety. Currently, the going theory is that humans could emit anywhere from 100 billion tons to 500 billion tons of carbon before warming reached catastrophic levels.

This uncertainty, Higgins said, underscores the need to keep global carbon emissions as low as possible. Trump’s policies would likely ensure higher emissions.

“We do not know how much carbon we can emit safely and we cannot know in advance at what point human-caused climate changes will lead to catastrophic societal consequences,” he said. “The more carbon we emit, the higher the carbon dioxide concentrations in the atmosphere will be and the larger the changes in climate we’ll face.”

Trenberth: “Incredible ignorance.”

Kevin Trenberth, a distinguished senior scientist at the National Center for Atmospheric Research, had stronger words for the presumptive Republican nominee’s energy plan.

“[My] quick reaction is that his comments show incredible ignorance with regard to the science and global affairs,” said Trenberth, a leading expert on how climate change impacts the hydrological cycle.

Trenberth praised the “remarkable” achievements of the Clean Power Plan and the Paris climate agreement, both of which Trump promised to eliminate. The Clean Power Plan is the Obama administration’s regulatory effort to limit carbon emissions from power plants, which are America’s largest source of carbon emissions.

But, Trenberth noted, neither the Clean Power Plan nor the Paris climate agreement are enough on their own to slow the pace of warming. That, he said, is exactly why they need to remain in place.

“So much more is actually needed, and for Trump to make the pandering statements to the coal people and threaten to undermine all of these major achievements is a statement to me that he absolutely must not be elected and it would be a disaster if he were,” he said.

Francis: “Greed and selfishness.”

Jennifer Francis, a research professor at Rutgers University’s Institute of Marine and Coastal Science, also acknowledged that neither the Clean Power Plan nor the Paris climate agreement would slow climate change to a manageable pace on their own. But they are “absolutely critical steps in the right direction,” she said.

Indeed, no one regulation in any one country can be significant enough to stop global human-caused climate change. The idea behind them, however, is that all countries need to act together to fix the problem, and that the United States must take the lead considering it’s currently the world’s second-largest emitter and historically by far its largest.

Francis said that reneging on the Paris climate agreement and rolling back the Clean Power Plan would be detrimental for future generations — and to her personal patriotism.

“Turning [these policies] back would not only diminish the quality of life for our children and their children, but it would also be a sorry message to the rest of the world that U.S. leadership does not base its decisions on facts or science but rather on greed and selfishness,” she said. “I would be much less proud to be an American if Trump gets his way.”

Mann: “An existential threat.”

Michael Mann, director of the Earth System Science Center at Pennsylvania State University, kept his comments brief.

“In my assessment,” he said, “it is not an overstatement to say that Donald Trump’s climate change views and policy proposals constitute an existential threat to this planet.”

Hayhoe: “Doing something … is far cheaper than not.”

Katharine Hayhoe, director of the Climate Science Center at at Texas Tech University, took a technical approach to Trump’s climate plan — specifically, his proposal to “cancel” the Paris climate agreement. The Paris agreement is a pact between nearly 200 countries to limit global warming to 2 degrees Celsius.

If the U.S. dropped out of the agreement and continued with no climate policy, Hayhoe calculated that the U.S. would contribute .66 degrees Celsius of warming by 2100. That would result in warming of 2.66 degrees Celsius, which some scientists consider a dangerous level of climate change.

This scenario, however, assumes that all other countries would continue to participate in the Paris agreement if the U.S. drops out. This is unlikely, Hayhoe noted, because U.S. policy influences the policies of other countries.

“If the U.S. does nothing on climate, the chances of every other nation succeeding in its targets are small, due to the impact of the U.S. economy on everything from trade to technology transfer,” she said. “On the other hand, even if everyone else works hard to achieve the Paris target and the U.S. doesn’t, the U.S. will also be influenced by other nations.”

Hayhoe, however, said she was hopeful that a Donald Trump presidency wouldn’t impact the Paris climate agreement, due to increasing public support for action that limits the detrimental impacts of climate change. In January, a New York Times/CBS poll found that approximately two-thirds of Americans “support the United States joining a binding international agreement to curb growth of greenhouse gas emissions.”

“As the impacts grow ever more evident, severe, and costly, what was obvious to the 195 nations who met in Paris will become obvious to every human on this planet: doing something about climate change is far cheaper than not,” Hayhoe said.