On Tuesday, Stephen Hawking and 374 other prominent scientists issued a personal plea to Republican presidential candidate Donald Trump: Don’t back out of the Paris climate agreement.

Doing so could have “severe and long-lasting” consequences for global climate and US credibility, warns an open letter penned by members of the National Academy of Sciences, including 30 Nobel laureates. The letter doesn’t exactly mention Mr. Trump by name, but refers to him as "the Republican nominee for President," who has said he would either alter or abandon the deal if elected – although his hypothetical ability to do so grew dimmer on Wednesday, as 20 more world leaders signed on to the agreement in New York.

“A ‘Parexit’ would send a clear signal to the rest of the world: ‘The United States does not care about the global problem of human-caused climate change. You are on your own,’ ” the letter reads. “Such a decision would make it far more difficult to develop effective global strategies for mitigating and adapting to climate change.”

The Paris Agreement, which seeks to cut global emissions and limit global average temperature rise to below two degrees Celsius, has been fast-tracked through the United Nations since its inception in late 2015. That’s largely due to the upcoming US presidential election, the outcome of which could make or break the deal.

“I will be looking at that very, very seriously, and at a minimum I will be renegotiating those agreements, at a minimum. And at a maximum I may do something else,” Trump told Reuters in May. “But those agreements are one-sided agreements and they are bad for the United States.”

Trump’s approach reflects a broader GOP strategy when it comes to energy talks: ambiguity. Many Republican politicians do not often specifically mention climate change by name, instead focusing on US energy independence and the economic impact of such deals. That strategy has been employed by other GOP heavy-hitters such as Ted Cruz and Marco Rubio, who are reluctant to make economic changes to combat climate change, still a sharply partisan issue.

Yet more and more Americans recognize the reality of climate change, making outright denial more politically risky: in a recent poll by the Energy Policy Institute at the University of Chicago and the Associated Press-NORC Center for Public Affairs Research, for example, 77 percent of respondents said climate change is happening.

And for most Americans – not just Congress – the cost of climate action is a tough pill to swallow. In the University of Chicago and AP-NORC poll, 42 percent of all respondents said they would not pay even $1 per month for preventing climate change, although 65 percent believe it is a problem the government should address.

Other studies suggest that Americans' tepid commitment to actually combatting climate change may be a matter of poor communication: UC San Diego researchers found that people were more likely to donate when environmental appeals focused on collective action, rather than personal guilt.

The US has already ratified the Paris accord, but it won’t be locked in until 55 countries representing 55 percent of greenhouse gas emissions join the agreement. After Wednesday's signings at the United Nations, 60 countries have officially signed on, but only 48 percent of global emissions have been accounted for.

If Trump wins the election and the accord is not ratified by inauguration day, he could try to revoke US participation. Technically, he could stall climate progress even if the deal is set before then – but he would also be subject to public “name and shame” reviews by the UN.

And the agreement's drafters may have predicted such a scenario: written into one article is a requirement that any nation wishing to withdraw first wait for four years, Reuters reports.

Ultimately, it may not matter all that much even if Trump does bail on the Paris accord, some experts say. The US is already on track to meet its promises under the deal, since an increase in natural gas use has displaced industrial coal and reduced emissions.

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“He might go back on the agreement in Paris but the net effect for the US is not likely to make much of a difference, if at all, in terms of our trajectory,” an anonymous White House official told the Guardian in May.

This report includes material from Reuters.