To live on one of the Pacific atolls is to come face to face with climate change on a daily basis. Few people in the world had such personal experience as Tony de Brum of the realities of sea level rises and storm surges, of warmer seas, receding beaches and abandoned land. Fewer still have been able to turn that experience into international action to save the islands, and the rest of the planet with them.

De Brum, who has died aged 72, acted as ambassador on climate change for the Marshall Islands, a sparsely populated group of more than 1,000 tiny islands spread out over nearly 30 coral atolls. In 2015, he was instrumental in securing the Paris agreement on climate change, by which the world’s governments collectively agreed, for the first time, to hold global warming to no more than 2C above pre-industrial levels, which scientists say is the limit of safety, and with an aspiration to ensure warming does not exceed 1.5C.

The 1.5C provision was crucial for the low-lying Pacific island states: scientists estimate that, at 2C of warming, the atolls would be overswept by sea level rises and frequent storm surges. Only a lower limit would give them a lifeline, but the challenge of cutting greenhouse gas emissions in line with such a goal is immense, and while small developing countries were keen, many big economies balked at it. In the tense final days of the negotiations, De Brum helped to lead a coalition of developing and developed countries to forge a compromise and carry the day, resulting in the historic accord. He was hailed as “a real hero, a giant of history, a custodian of our shared future” by the Marshall Islands’ president, Hilda Heine, and by the former UN climate chief, Christiana Figueres, as “an inspiring leader, a pillar of determination”.

De Brum’s work on climate change, while it will form perhaps his most lasting global legacy, was only the third of the great causes he espoused in a five-decade career of public service, during which he served as foreign minister of the Marshall Islands for three separate terms, interspersed with periods as minister of finance, of health and the environment, and as assistant to the president.

Heads of delegations, including De Brum, arriving at the 2015 climate change talks in Paris. Photograph: Jacky Naegelen/Reuters

De Brum was born on Tuvalu, a former British colony 700 miles north of Fiji, as the war in the Pacific was nearing its final deadly stages. The family later moved to Likiep, in the Marshall Islands, at that time a trust territory of the US, which had wrested control from Japan. His father, Joe, became manager of Majuro airport; his mother, Cecilia, was a teacher.

When hostilities ended, the US began 12 years of nuclear testing in the area, with close to 70 blasts by 1958. As a boy, De Brum witnessed one of the consequences from a fishing boat. He later recalled: “I was fishing with my grandfather. He was throwing the net and suddenly the silent bright flash, and then a force, the shockwave. Everything turned red: the ocean, the fish, the sky, and my grandfather’s net. And we were 200 miles away from ground zero.” This was the Bravo Shot, the largest of the US nuclear tests carried out on Bikini Atoll, more than a thousand times as powerful as the bomb that destroyed Hiroshima.

The Marshall Islanders’ campaign for freedom from the US lasted through the 1970s, with independence coming in 1979. After studying at Chaminade University, then the University of Hawaii – he was one of the first Marshall Islanders to attend university, according to Heine – De Brum took part in the negotiations, and in the independent governments that followed.

His experience of nuclear testing led to a lifelong crusade for nuclear disarmament, culminating in filing lawsuits against all nine nuclear-armed countries in the international court of justice in 2014. “Nuclear weapons are a senseless threat to essential survival. There are basic human and ethical norms, not to mention longstanding treaties, which compel those who possess them to pursue and achieve their elimination,” he said. The legal bid failed.

Unassuming in manner, grey-haired and grey-suited, De Brum could have been indistinguishable from the hundreds of other functionaries milling through the halls of UN meetings. There was nothing grey about his personality, however: affable and friendly, he was always ready with a quip or a wink for journalists, and shared stories about his beloved islands, his childhood and his family. He was as happy to greet groups of schoolchildren, or students dressed as polar bears, as to mingle with world leaders.

De Brum’s work at the two-week-long Paris talks was a masterclass in diplomacy. The debate over whether the 1.5C target should be included had the potential to wreck the talks – it had wrecked the previous attempt, eight years earlier in Copenhagen. By agreeing a compromise, he showed a pragmatism often absent from the negotiations, and helped lead his fellow small island states and least developed countries in joining with the EU and various others in the “coalition of high ambition”.

Some dismissed the 1.5C aspirational target as unachievable, but De Brum’s faith was borne out by a new study, published soon after his death in Nature Geoscience, a prestigious peer-review journal. It concluded that keeping within 1.5C was possible, though it required effort.

De Brum is survived by his wife, Rosalie (nee Maddison), whom he married in 1969, three children, 10 grandchildren and five great-grandchildren, and by his father.

• Anton de Brum, politician, born 26 February 1945; died 22 August 2017