Microsoft’s Chief Environmental Officer Lucas Joppa announced he will depart the company after more than 12 years to join a private equity firm, according to a LinkedIn post.

Joppa called the move the “hardest professional decision” of his life, but one he makes with “full confidence in Microsoft’s ability to achieve its sustainability ambitions” without him.

He has led the company’s sustainability efforts since 2018, having previously held roles of chief environmental scientist and computational ecologist.

“What I have learned over the past decade is that building an environmentally sustainable future requires maturing sustainability markets at an almost incomprehensible speed, scope, and scale,” Joppa wrote. “And that is exactly what I’ll be focusing on in my new role.”

He said moving forward “sustainability and software will remain at the center of my world.” In September, he will begin his new role as senior managing director and chief sustainability officer at an unnamed investment firm with “a mission to put sustainability at the center of everything it does.”

“For those of you who know me well, you know I have had a longstanding desire to help build a company from scratch, and that is what I am setting off to do,” he wrote.