Piers Sellers, a British-born climate scientist for Nasa who remained optimistic about the fate of the Earth despite the grim climate change models he oversaw and who gained American citizenship to fulfil a childhood dream of becoming an astronaut, has died in Houston. He was 61.

The death was confirmed in a statement by Nasa Administrator Charles Bolden. Dr Sellers, who had been diagnosed with Stage 4 pancreatic cancer in October 2015, went public with his diagnosis in a New York Times column in January 2016. He wrote that while he had hoped he would see solutions to the problem of climate change in his lifetime, he was devoted to continuing his climate research until he died.

“There is no convincing, demonstrated reason to believe that our evolving future will be worse than our present, assuming careful management of the challenges and risks,” he wrote, sounding a note of optimism in spite of increasingly drastic changes in the global temperature and precipitation patterns that he studied. “History is replete with examples of us humans getting out of tight spots.”

Dr. Sellers had worked on global climate problems from 1982 to 1996 at Nasa's Goddard Space Flight Center in Greenbelt, Maryland, and at the University of Maryland at College Park. He wrote more than 70 papers, using computer models to delve into the relationship between the biosphere — the region of the Earth inhabited by humans and other organisms — and the atmosphere. He was also a lead scientist for Terra, a Nasa satellite launched in 1998 that monitors the state of the Earth’s climate and environment.

All the while, he applied to become a Nasa astronaut.

The corps is open only to U.S. citizens, but Dr. Sellers said he wanted, at the very least, to keep his name before the astronaut selection board. He acquired dual citizenship in 1991, and five years later became part of Nasa's largest astronaut training group. The class of 44 so cramped Nasa's training facilities it was humorously nicknamed “the Sardines.”

In three space flights from 2002 to 2010, Dr. Sellers logged 35 days in space, including 41 hours in six space walks. Some lasted as long as seven hours, including a walk in 2006 during which Dr. Sellers tested new emergency repair techniques and materials using caulk guns, putty knives and his “favorite spatula” from home, according to the Times.

“The best practice for this is to have an old house in Houston,” he joked during the mission. Later in the walk, he lost his spatula, which mission control spotted drifting away from the shuttle and tracked, unconcerned by any possible kitchen-utensil collisions, by radar.

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Dr. Sellers returned to the Goddard center in 2011, overseeing scientists researching climate and weather — including the causes and effects of climate change — as deputy director of the center’s sciences and exploration directorate and as acting director of its Earth sciences division.

He made a point of using his experiences as an astronaut to further people’s awareness of climate issues.

Asked what it was like going into space, he told a United Nations interviewer in 2011 that even as a climate scientist, he was surprised to see how thin the atmosphere was. “That really brought home to me how easily mankind can affect its own environment. The stuff we breathe, there’s not much of it. It’s a very thin atmosphere. We better pay attention.”

Piers John Sellers was born in Crowborough, a town in the south of England, on April 11, 1955. The son of a British army officer, he was raised on military posts around the world.

England has no manned space travel program. (The first official British astronaut, Tim Peake, flew as part of the European Space Agency in December 2015.)

Dr. Sellers said he dreamed of going into space ever since he saw images of Russian cosmonaut Yuri Gagarin orbiting the Earth in 1961 and U.S. astronaut Neil Armstrong walking on the moon in 1969.

Through a British air force cadet program, Dr. Sellers learned to fly gliders before he could drive. He received a bachelor’s in ecological science from the University of Edinburgh in 1976 and a doctorate in biometeorology from the University of Leeds in 1981. He moved to the United States one year later, starting work at Goddard as a research meteorologist.

His marriage to the former Amanda Lomas ended in divorce. Survivors include two children from his marriage, Imogen Shelton of Austin and Thomas Sellers of Houston; his mother, Lindsay Sellers of Guildford, England; four brothers; and a grandson.

Dr. Sellers became just the third Briton to go into space when he flew on the shuttle Atlantis in 2002, carrying out three space walks as part of the assembly of the International Space Station. Three months later, the shuttle Columbia disintegrated on reentry, killing all seven astronauts on board, including three with whom Dr. Sellers had trained.

His 2006 mission on Discovery was partly designed to test safety measures that would prevent similar accidents. His final flight, on Atlantis in 2010, delivered a new module to the International Space Station.

Months later, Dr. Sellers was appointed an Officer of the Order of the British Empire for his services to science. He proclaimed himself “gobsmacked.”

“I’ve no regrets,” he wrote in the Times in January. “As an astronaut I spacewalked 220 miles above the Earth. Floating alongside the International Space Station, I watched hurricanes cartwheel across oceans, the Amazon snake its way to the sea through a brilliant green carpet of forest, and gigantic nighttime thunderstorms flash and flare for hundreds of miles along the Equator. From this God’s-eye-view, I saw how fragile and infinitely precious the Earth is. I’m hopeful for its future.”

Ever loyal to his profession as a scientist, he concluded: “And so, I’m going to work tomorrow.”