The Marshall Islands has warned of a Pacific "climate catastrophe" that will wipe it off the map without decisive action on global warming, saying the next 12 months are critical.

Tony de Brum, minister in assistance to the Marshall Islands' president, is in Australia making the case for a major climate declaration at the 16-member Pacific Islands Forum it will host in September which he has insisted US Secretary of State John Kerry must attend.

De Brum hopes the so-called Majuro Declaration can be presented to the United Nations General Assembly to help renew global efforts on emissions reduction as the Pacific region confronts rising seas and growing numbers of so-called climate refugees.

"Our hosting of the forum comes at the cusp of the most important geopolitical period for the region since World War II," de Brum told reporters at a briefing in Sydney Thursday, adding the next 12 months "are critical to addressing climate change".

"Business as usual will lead to a climate catastrophe and time is running out.

"We feel very strongly that if (Kerry) does not attend it would be a slap in the face and like the United States would be reversing its so-called pivot to the Pacific," he added.

De Brum said the tiny Pacific atoll of 55,000 people, which stands at an average of just two metres above sea level, was already feeling the impacts of global warming with an unprecedented seven-month drought in the north and a devastating king tide earlier this year triggering disaster declarations.

"During my lifetime I have seen an island in the lagoon of Majuro atoll, the capital centre of the Marshall Islands, disappear from the surface of the Earth," he said.

"We do not have scientists measuring this that and the other, we have experienced first-hand the effects of climate change... It is not something that is down the road or at the turn of the century."

De Brum said the Marshalls government was already ferrying food and drinking water to 13 outer island communities due to drought-linked shortages that were threatening the export of copra, the dried-out flesh of coconuts from which oil is extracted, which underpinned its economy.

There had also been a "marked increase" in what he described as climate refugees from neighbouring Kiribati and Tuvalu and he said the government expected similar movements out of the Marshalls itself in coming years, with a two-metre sea level rise predicted by the World Bank before the end of the century.

"This would fundamentally alter the world as we see it and be the end of my country, the end of Kiribati, the end of Tuvalu and many other countries like it."