The United Nations Secretary-General António Guterres has said that big polluters have a clear responsibility to cut emissions – or risk a worldwide catastrophe.

“The Pacific is today the most vulnerable area of the world,” he told the BBC at the Pacific Island Forum Leaders Meeting in Tonga. “There is an enormous injustice in relation to the Pacific and it’s the reason I am here.”

“The small islands don’t contribute to climate change but everything that happens because of climate change is multiplied here.”

But eventually the "surging seas are coming for us all," he warned in a speech at the forum, as the UN releases two separate reports on rising sea levels and how they threaten Pacific island nations.

The World Meteorological Organization’s State of the Climate in the South West Pacific report says this region faces a triple whammy of an accelerating rise in the sea level, a warming of the ocean and acidification – a rise in the sea’s acidity because it’s absorbing more and more carbon dioxide.

“The reason is clear: greenhouse gases – overwhelmingly generated by burning fossil fuels – are cooking our planet,” Mr Guterres said in a speech at the forum.

“The sea is taking the heat – literally.”

This year’s theme - transformative resilience – was tested on the opening day when the new auditorium was deluged by heavy rains and buildings evacuated because of an earthquake.

"It's such a stark reminder of how volatile things are within our region, and how important it is that we need to prepare for everything," Joseph Sikulu, Pacific director at 350, a climate change advocacy group, told the BBC.

Not far from the venue was a street parade, with dancers representing the region, including Torres Strait islanders, Tongans and Samoans. At the start of the parade, a big banner reads: "We are not drowning, we are fighting". Another says: "Sea levels are rising – so are we".

It echoes a challenge that threatens to wipe out their world - the UN Climate Action Team released a report called "Surging Seas in a Warming World" showing that global average sea levels are rising at rates unprecedented in the past 3,000 years.

According to the report, the levels have risen an average of 9.4cm (3.7in) in the past 30 years but in the tropical Pacific, that figure was as high as 15cm.

"It's important for leaders, especially like Australia and Aotearoa, to come and witness these things for themselves, but also witness the resilience of our people," Mr Sikulu said.

"A core part of Tongan culture is our ability to be able to continue to be joyful throughout our adversity, and that's how we practise our resilience and to see and witness that, I think is going to be important."

This is the second time Secretary-General Guterres has participated in the Pacific Islands Forum Leaders Meeting. The annual meeting brings together leaders from 18 Pacific Islands, including Australia and New Zealand.

As leaders convened for the official opening ceremony, heavy rain caused extensive flooding. Shortly afterwards, a magnitude 6.9 earthquake hit the Tonga region, highlighting just how vulnerable it is.

In 2019, Mr Guterres travelled to Tuvalu where he sounded the alarm about rising sea levels. Five years on, he says he has seen real changes.

“We see everywhere an enormous commitment to resist, a commitment to reduce the negative impact of climate change,” he told the BBC. “The problem is, the Pacific Islands also suffer another big injustice - the international financial instruments that exist to support countries in distress were not designed for countries like this.”

Mr Guterres on Monday visited local communities whose livelihoods are threatened by rising sea levels. They’ve been waiting for seven years for a decision to be made on the funding of a sea wall.

“The bureaucracy, the complexity, the lack of sense of urgency because it’s a small island, far away,” he said, citing the failings of the international financial system, especially when it comes to small, developing island states.

“There are promises of increases of money available for adaptation in developing countries but the truth is we are far from what is needed, from the solidarity that is needed for these countries to be able to exist.”

Many Pacific islanders here at the conference single out the biggest regional donor and emitter - Australia.

Earlier this year, Prime Minister Anthony Albanese said Australia would be ramping up its extraction and use of gas until “2050 and beyond,” despite calls to phase out fossil fuels.

“There is an essential responsibility of the big polluters,” Mr Guterres said, when asked by the BBC what message he has for regional emitters like Australia.

Without that, the world will breach the threshold of 1.5C that was established in the Paris Agreement in 2015. That agreement aims to limit global warming to "well below" 2C by the end of the century, and "pursue efforts" to keep warming within the safer limit of 1.5C.

“Only by limiting warming to 1.5 degrees Celsius do we have a fighting chance of preventing the irreversible collapse of the Greenland and West Antarctic ice sheets – and the catastrophes that accompany them,” Mr Guterres said.

“That means cutting global emissions 43% compared to 2019 levels by 2030, and 60% by 2035.”

Last year though, global emissions rose 1%.

“There is an obligation of the G20 that represent 80% of emissions – there’s an obligation for them to come together, to guarantee a reduction of emissions now," Mr Guterres said.

Singling out the G20 as well as companies who contribute to much of the world's global emissions, he added: "They have a clear responsibility to reverse the current trend. It’s time to say 'enough'."