When international negotiators reached a first-of-its kind climate change agreement in Paris this month, the United Nations’ normally low-key leader, Ban Ki-moon, celebrated onstage, arms raised in victory and more exuberant than many had ever seen him before.

Nearly nine years had passed since, in his first days as secretary-general, Ban surprised world leaders by making global warming a top item on his agenda. Now, on the eve of his final year in office, the cheers in Paris marked the culmination of his nonstop campaign, pressed with world leaders at summit after summit and in locales including melting glaciers and islands at risk of disappearing.

It was an emotional moment, and looking back at the road to Paris in an interview with The Associated Press, Ban paid tribute to many people, including the leaders of the United States, China, India and France. He also spoke proudly of his own role.

No other leader in the world “has been raising, without fail, all the time, climate change,” Ban said. “I have spent real passion … and most of my time and energy on this issue.”

You could have blown me away. “He had a deep commitment then, and he has stayed with it, and stayed with it, and stayed with it.

It was quite a shift for the former South Korean foreign minister, whose main focus before becoming the eighth U.N. secretary-general in 2007 was his country’s standoff with North Korea.

Ban traced his interest in climate change to his yearlong campaign to lead the United Nations, which took him to many countries and broadened his vision of global issues.

Two weeks before he was sworn in as secretary-general, Ban told Tim Wirth, then president of the United Nations Foundation, that one of his two highest priorities would be climate change, along with empowering women.

“You could have blown me away,” Wirth said of Ban’s choice of tackling global warming. “He had a deep commitment then, and he has stayed with it, and stayed with it, and stayed with it.”

At the time, climate change was not a popular topic.