It was that role that led him to a face-to-face encounter with the pope in May 2014.
Ramanathan was at the Vatican to lead a discussion about the ways that society affects nature. Without much advance notice, he learned that he would get an opportunity to say two sentences to the pope as he passed through a parking lot between meetings.
Ramanathan quickly distilled his thoughts on climate change into two sentences. Then he realized he had a problem.
"They said the pope responds better when people speak in Spanish -- and I don't speak Spanish," Ramanathan recalled Thursday after the encyclical was released.
Although he did find someone to translate his two sentences, he had trouble remembering them.
"When the pope appeared and smiled, I forgot them," he said. "I could barely do it in English. When the pope smiles, you forget yourself."
After Ramanathan recovered, this is what he told the pontiff:
"We have a collection of experts from around the world who are concerned about climate change. The changes are already happening and getting worse, and the worst consequences will be felt by the world's 3 billion poor people."
His parking-lot comments came as the Pontifical Academy of Sciences finalized a statement on climate change.
"The massive fossil fuel use at the heart of the global energy system deeply disrupts the Earth's climate and acidifies the world's oceans," the statement said. "The warming and associated extreme weather will reach unprecedented levels in our children's lifetimes and 40% of the world's poor, who have a minimal role in generating global pollution, are likely to suffer the most."
Francis incorporated some of the academy's conclusions into his encyclical, which addressed topics ranging from global warming to pollution to consumerism.
"Doomsday predictions can no longer be met with irony or disdain," Francis wrote. "We may well be leaving to coming generations debris, desolation and filth. The pace of consumption, waste and environmental change has so stretched the planet's capacity that our contemporary lifestyle, unsustainable as it is, can only precipitate catastrophes."
Those words deeply touched the 70-year-old scientist, who has been conducting research on the climate since the 1970s.
"It is the most thought-provoking and heart-tugging statement on climate change that I have ever seen," Ramanathan said. "I thought, my goodness, this is like Rachel Carson's 'Silent Spring,' the book that changed America."