McElroy was a towering intellectual whose pioneering research shaped how we understand the atmospheric chemistry of more than half the planets in our solar system, including Earth. At Harvard, McElroy was a driving force behind the University’s commitment to environmental and climate science. McElroy joined Harvard in 1970 at the age of 31, making him among the youngest tenured faculty at the University. Chris Nielsen, the executive director of the Harvard-China Project, recalled McElroy, in his late-60s, pouring over textbooks to teach himself the fundamentals of energy sciences. That curiosity propelled the deeply collaborative and interdisciplinary ethos of the Harvard-China Project.