Peter Thomson is picking up debris along New York’s East River. When he comes across a particularly stubborn bag, lodged tight in the ground, he wrestles with it for a while. Eventually, he asks for a knife to free it: he’s not prepared to leave it behind.

The self-described fifth-generation Fijian is observing Nelson Mandela Day’s tradition of 67 minutes of community service, acknowledging Mandela’s 67 years of activism. He has ventured onto the rocks near a dangerous stretch of water known as Hell Gate, which reminds him of the white water at the entrance to every Pacific island. His bodyguard hovers above, near a group of younger New York City volunteers who stay safely on the grass.

A couple of weeks later we meet in Thomson’s office, perched just above the East River, a few kilometres downstream. Our interview is sandwiched between an unexpected meeting with representatives of Kazakhstan and Thomson’s weekly Friday afternoon meeting with the president of the United Nations Security Council.

He is the outgoing president of the UN General Assembly. Unlike the Security Council, which is beholden to the latest missile test from Pyongyang or siege in Mosul, the General Assembly is focused on more slow-burning issues: refugees, antimicrobial resistance, nuclear weapons, climate change and oceans. As president, Thomson is responsible for rallying the UN’s 193 member states. Not an easy task in a year when Donald Trump, Vladimir Putin and Kim Jong-un have other ideas.

Thomson had been in office just eight weeks the morning his wife woke him to tell him that Trump had been elected. Looking back now, he describes the increasingly bilateral behaviour of the United States and Russia as “not something I would bet my grandchildren’s future on”. In his South Pacific accent, Thomson often speaks in terms of generations.

“It’s dark days, quite frankly,” he says of the Trump administration. Trump is expected to make his first appearance in the UN General Assembly hall in September, when Fiji will pass the reins to Slovakia. While Fiji is the first Pacific Island country to hold the presidency of the General Assembly, Thomson, who is also an Australian citizen, is the second Australian to hold the role after former leader of the Labor Party Herbert Vere Evatt in 1948.

On climate change, Fiji and the US are poles apart. Under Prime Minister Frank Bainimarama, Fiji has taken a leading role on climate change at the UN. It was the first country to sign the Paris agreement in February 2016 and in November  will host the yearly UN Climate Change Conference, remotely, from Bonn, Germany.

By contrast, the US is pushing ahead with its intention to withdraw from the Paris agreement unless “the United States can identify terms that are more favourable to it, its businesses, its workers, its people and its taxpayers”.

The planned withdrawal of the US, the biggest carbon emitter in history, is just the latest in a long series of delays in international climate negotiations. Many already believe the Paris deal is not ambitious enough and is simply a starting point for long overdue international co-operation. Thomson remains optimistic, pointing to recent innovations and renewed commitments from American cities. “People realise that this is an existential issue, not just for Pacific Island countries or river delta situations but for everybody.”

Yet it is oceans where Thomson seems to have found his purpose for the latter part of his presidency, and the coming years, after a High-Level Summit on Refugees and Migrants at the beginning of his presidency failed to gain traction. In July, Fiji co-hosted a UN conference on oceans with Sweden, although damage from cyclone Winston meant the conference was relocated from Fiji to New York, a reminder of growing climate-related threats. The meeting aimed to address problems such as plastics gradually outweighing fish in the oceans.

The distant descendent of a master mariner, Thomson says the oceans have been in his blood since his mother sailed from Fiji’s second-largest island, Vanua Levu, to its largest island, Viti Levu, to give birth to him. “As soon as she was ready we got back into the tiny little boat and spent three days sailing back up to Labasa.”

His island upbringing also means that he “feels for Australians about what’s happening in the Great Barrier Reef”. If you’re brought up in the islands “you know what a pristine coral reef used to look like and … you know what a dead reef looks like and how devastating that is … It’s like seeing the beauty of a tropical rainforest turn to a desert.”

Thomson also fears other consequences of warming oceans, including the mass migration of life to more temperate zones. Not just fish, he adds, but also micro fauna, “the fundamentals of life”. People will follow, he says, migrating to cities such as Sydney and Melbourne.

By the time he was selected to be Fiji’s UN representative in 2010, Thomson was an author living in exile on Sydney’s northern beaches. It was a long way from the beginning of his career, working for the Fijian government, digging pit latrines and building seawalls, after undertaking development studies at Cambridge.

His first book, Kava in the Blood, won New Zealand’s E. H. McCormick Award for Best First Book of Nonfiction. It details how Thomson – the son of the former British colonial administrator of Fiji – found himself suddenly out of favour back home. Stripped of his Fijian citizenship, he became both an Australian and New Zealand national.

Yet after the 2006 coup, Thomson’s luck changed. A change in Fijian dual citizenship laws allowed Thomson to regain his Fijian passport, and so reinstated he was plucked by the Bainimarama government to represent the country at the United Nations.

From there, Thomson followed a somewhat textbook path to the presidency, with roles including chairman of the Group of 77, which represents 134 developing countries at the UN. He also negotiated a change of name of the Asian Group at the UN to the Asia-Pacific Group. Both were somewhat unusual roles for the white son of a former colonial administrator.

In many ways, Thomson has continued the mantle of his predecessor, Danish politician Mogens Lykketoft. Lykketoft’s presidency also included a rush to restore the name of the office after a corruption scandal engulfed 68th president, John Ashe of Antigua and Barbuda. Ashe died of traumatic asphyxiation in June 2016 in an apparent weight-lifting accident days before he was due to appear in court on charges of bribery and corruption, related to his presidency.

Scandals aside, Thomson spends much of his time making dry speeches about the UN’s Sustainable Development Goals. He concedes that two years in, the goals are still not widely recognised.

At dinner parties with friends from Australia and New Zealand he says he is often met with confusion when he mentions the goals, known by their initialism. “What are the SDGs – sexually transmitted diseases?”

The goals cover everything from gender inequality to sustainable consumption, a favourite topic of Thomson’s, who says he and his wife try to do their bit.

“Everybody is getting so selfish. I don’t know if it’s mounting fear or what they see on the internet and the behaviour of the American president. It’s just like, jettison your principles, look after yourself and get ready, I suppose, for whatever is coming.”

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