The strongest El Nino on record is likely to increase the threat of hunger and disease for tens of millions of people in 2016 aid agencies say.
The weather phenomenon is set to exacerbate droughts in some areas while increasing flooding in others.
Some of the worst impacts are likely in Africa with food shortages expected to peak in February.
Regions including the Caribbean, Central and South America will also be hit in the next six months.
This periodic weather event, which tends to drive up global temperatures and disturb weather patterns, has helped push 2015 into the record books as the world's warmest year.
Image copyright WFP Image caption Lula Derar lives Gurgur village in Ethiopia and says this is a very challenging time
"By some measures this has already been the strongest El Nino on record, it depends on exactly how you measure it," said Dr Nick Klingaman from the University of Reading.
"In a lot of tropical countries we are seeing big reductions in rainfall of the order of 20-30%. Indonesia has experienced a bad drought, the Indian monsoon was about 15% below normal and the forecasts for Brazil and Australia are for reduced monsoons."
As both droughts and floods continue, the scale of the potential impacts is worrying aid agencies. Around 31m people are said be facing food insecurity across Africa, a significant increase over the last year.
Around a third of these people live in Ethiopia where 10.2m are projected to require humanitarian assistance in 2016.
The UK's Department for International Development says it is providing emergency support for 2.6m people and 120,000 malnourished children. They say they will provide 8m people with food or cash support from January 2016.
