Two dozen people have already died from hunger and drinking contaminated water in drought-stricken Papua New Guinea, but the looming El Niño crisis could leave more than four million people across the Pacific without enough food or clean water.

The El Niño weather pattern – when waters in the eastern tropical Pacific ocean become warmer, driving extreme weather conditions – may be as severe as in 1997-98, when an estimated 23,000 people died, forecasters believe.

In Papua New Guinea’s Chimbu province in the highlands region, a prolonged drought has been exacerbated by sudden and severe frosts which have killed off almost all crops. The provincial disaster centre has confirmed 24 people have died from starvation and drinking contaminated water.

Provincial disaster co-ordinator Michael Ire Appa told RadioNZ he feared the death toll could even be higher.

“The drought has been here for almost three months now and in areas that were affected by the drought there’s a serious food shortage, including water, and some of the districts have not reported, so there may be more [deaths] than that,” he said.

Two highlands provinces have already declared a state of emergency.

Oxfam Australia’s climate change policy advisor Dr Simon Bradshaw said many parts of PNG would run out of food in two or three months, but in some areas there was as little as a month’s food left, and few ways to get more in.

“In the highland areas people are almost exclusively reliant on subsistence farming, farming of sweet potatoes. We do know that water is becoming very scarce, that’s of course impacting food production, and PNG is almost entirely dependent on its own food – I think 83% of its food is produced in-country – so any hit on food production poses immediate challenges in terms of food security.”

Over the coming months, the El Niño pattern will bring more rain, flooding and higher sea levels to countries near the equator, raising the risk of inundation for low-lying atolls already feeling the impacts of climate change.

At the same time, the countries of the Pacific south-west – which have larger populations – will be significantly drier and hotter.

El Niño years typically have a longer, more destructive cyclone season.

“El Niño has the potential to trigger a regional humanitarian emergency and we estimate as many as 4.1 million people are at risk from water shortages, food insecurity and disease across the Pacific,” Sune Gudnitz, head of the Pacific region office of the United Nations Office for the Coordination of Humanitarian Affairs said.

“Countries including Papua New Guinea, Fiji, Tonga and the Solomon Islands are already feeling El Niño’s impact with reduced rainfall affecting crops and drinking-water supplies. Drought conditions would further complicate the humanitarian situation in countries that are just emerging from the devastation caused by tropical cyclones Pam, Maysak and Raquel.”

Many countries across the region are entering the El Niño period in a vulnerable state. Drought has been officially declared in 34 provinces in Indonesia, while in Vanuatu – still recovering from the devastation of cyclone Pam, which struck in March – authorities are warning reduced rainfall will damage food security, health and livelihoods.

In some parts of Fiji, water is already being trucked into villages that have run out. And Tonga, which has suffered a drought for nearly a year, has been forced to ship water supplies to the country’s outer islands.

Countries where food insecurity affects large proportions of the population were of special concern, Bradshaw said.

“With an El Niño event, you usually get about one-fifth less rainfall across the country as well as significant changes to the timing of the rainy season, a lot more rain concentrated in January, and that, combined with deforestation, increases the risk of landslides, flash floods, damage to infrastructure and destruction of crops. Timor Leste is somewhere we’re watching particularly closely because of the existing challenges, and the effect the El Niño will have on top of that.”

Bradshaw said the impact of the El Niño would compound the difficulties faced by Pacific countries struggling to cope with the effects of climate change.

He said recent research suggested El Niño patterns – usually seen every three to seven years – could now occur twice as frequently, and that “normal” conditions might become more similar to those of El Niño.

“We’ve had two unusually hot years, and now we’ve got a very strong El Niño event, so I think it would be fair to say, unfortunately, that we’re in uncharted waters. What we’ve seen is somewhat unprecedented and climate change is increasingly going to put us in that position.”

The countries most affected by the combined effects of climate change and El Nino are – for reasons of geography, economy, governance and remoteness – often the least equipped to deal with their impacts.

“We’ve seen an unprecedented run of extreme and erratic weather, which has had very real impacts,” Bradshaw said. “Of course, those impacts are felt first and hardest by the world’s poorest communities, but these countries are also the least responsible for climate change. They’ve contributed negligibly to global greenhouse emissions.

“I think it drives home the fact that climate change affects us all; it affects poorer countries first and hardest, but we have a responsibility as a wealthy, developed nation to be both doing far more to reduce our own emissions, but also to be providing greater support with adaptation and resilience-building to poorer countries.”

Bradshaw said the effects of the El Niño, combined with climate change, should drive all countries towards a strong agreement at climate change talks in Paris in December.