The scientists, including 30 Nobel Prize winners and numerous members of the US National Academy of Sciences, take issue with Trump's denial of global warming.

Thus it is of great concern that the Republican nominee for President has advocated U.S. withdrawal from the Paris Accord," their letter said, posted on responsiblescientists.org

"The consequences of opting out of the global community would be severe and long-lasting -- for our planet's climate and for the international credibility of the United States."

"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 said.

Most of the signatories are from the United States, with many from leading universities such as Harvard, Cambridge and Columbia.

More countries ratify deal

Meanwhile the Paris accord is rapidly moving towards ratification. Dozens of countries submitted their ratification to the UN on Wednesday, bringing the total to 60, UN Secretary General Ban Ki-moon said. Those countries represent 47.5 percent of the populations of UN members.

Final ratification requires 55 countries representing 55 percent of global carbon dioxide emissions.

"What once seemed impossible is now inevitable. I'm confident that by the time I leave office the Paris agreement will have entered into force," Ban, whose second five-year term ends on Dec. 31, told an event on the sidelines of the annual United Nations General Assembly.

Last December, nearly 200 nations agreed to slash greenhouse gases in order to keep global temperature rises to "well below" 2 degrees Celsius. The United States and China, the two largest producers of carbon emissions, ratified the accord this month.

Should either country rescind its Paris commitment, the chances of minimizing greenhouse gas emissions to a tolerable level will be greatly reduced.

bik/kl (Reuters, AFP, USNAS)