Secretary-General Antonio Guterres released the Surging Seas In A Warming World report at the Pacific Islands Forum (PIF) in Tonga on Tuesday, using the summit as a platform for his advocacy.
"The ocean is overflowing," he told delegates.
"I am in Tonga to issue a global SOS - Save Our Seas - on rising sea levels. A worldwide catastrophe is putting this Pacific paradise in peril."
Humanity is treating the sea like a sewer.— António Guterres (@antonioguterres) Plastic pollution is choking sea life.Greenhouse gases are causing ocean heating, acidification & accelerating rise in sea levels.Pacific islands, dramatically impacted by the consequences of climate change, are showing the way to… pic.twitter.com/CEAjeNVpz5August 26, 2024
The findings paint a dire picture for key settlements around the Pacific: Rarotonga, Suva, Apia, Nuku'alofa and more.
Modelling suggests each of those capitals would see annual floods - which currently average five or fewer - soar by the 2050s.
Rarotonga would encounter 75 flooding days in an average year. Suva would see 25, Apia 35 and Nuku'alofa 35.
"Some locations in the Pacific small island developing states could experience floodings for almost half of the entire year," the report reads.
Tongan communities are no stranger to the impact of flooding. Many villages were inundated in a 2022 tsunami caused by the massive eruption of undersea volcano Hunga Tonga-Hunga Ha'apai.
On Monday, Mr Guterres visited a string of villages to the east of Nuku'alofa to hear directly about their experiences.
His audience in a church hall in Kolonga was just a few dozen people, but on Tuesday he wanted leaders in advanced countries to hear his clarion call.
Mr Guterres unapologetically called out developed economies, specifically the G20 which includes Australia, for failing to wean off fossil fuels.
"They must keep the promises made at COP28 to triple renewables capacity, double energy efficiency and end deforestation by 2030 ... and end new coal projects and new oil and gas expansion now," he said.
Nowhere in the world is more affected than the Pacific by sea level rise, and the more savage and unpredictable storms that climate change will bring.
The average elevation of the region is under two metres above sea level.
Major industries - fishing, tourism, agriculture and more - are tied to the ocean or rely on stable climate.
Almost 90 per cent of Pacific Islanders live within five kilometres of the coast, and most infrastructure is within 500m of shorelines.
Sea level rise projections have also been plotted out for developed cities.
Sydney and Melbourne felt eight and nine centimetres of sea level rise in the 30 years to 2020, but would encounter double that in the next 30 years, should warming continue at a path of three degrees.
Mr Guterres also warned even a 2C rise could lead to a tipping point of the loss of the Greenland ice sheet and much of the West Antarctic ice sheet.
"That would mean condemning future generations to unstoppable sea level rise of up to 20 metres over a period of millennia," he said.
"At three degrees of warming - our current trajectory - the rise in sea level would happen much more quickly: over centuries."
This science is why the United Nations has pegged its Paris Agreement goals to just 1.5C of warming.
The next major Paris Agreement milestone comes in 2025 when governments are obliged to present new emissions-cutting plans, known as NDCs, at the COP28 climate change conference.
Mt Guterres also called for developed economies to improve climate financing to developed nations, to help them adapt.
5°C