He first built his reputation at NASA as a scientist, studying the relationship between the living world and the atmosphere for the better part of two decades starting in 1982 -- first through computer modeling and later through leading the Terra satellite team. He only flew to space for three missions in 2002, 2006 and 2010, but he spent a sizeable 35 days in space that both had a meaningful impact (he helped build the International Space Station, for instance) and influenced his work back on Earth. From 2011 until his death, Sellers oversaw NASA climate and weather researchers with a strong sense of advocacy. After seeing how "fragile and infinitely precious" Earth was, he pressed for efforts to minimize global warming and preserve the planet.
Sellers sadly didn't fulfill his goal of seeing fundamental answers to climate change within his lifetime. However, he's leaving a large legacy behind him, including more than 70 papers and other work that helped scientists understand the Earth. Also, he's a point of pride for the British. He was appointed an Officer of the Order of the British Empire, and was one of the UK's most prominent spacefarers ahead of Tim Peake's 2015 launch. It's clear that the world will be poorer without him.