A French doubter who authored a book arguing that solar activity — not greenhouse gases — was driving global warming, de Larminat sought a spot at a climate summit in April sponsored by the Vatican’s Pontifical Academy of Sciences. Nobel laureates would be there. So would U.N. Secretary General Ban Ki-moon, U.S. economist Jeffrey Sachs and others calling for dramatic steps to curb carbon emissions.

After securing a high-level meeting at the Vatican, he was told that, space permitting, he could join. He bought a plane ticket from Paris to Rome. But five days before the April 28 summit, de Larminat said, he received an e-mail saying there was no space left. It came after other scientists — as well as the powerful Vatican bureaucrat in charge of the academy — insisted he had no business being there.

“They did not want to hear an off note,” de Larminat said.