The number of people affected by the combined impact of the El Nino and La Nina weather patterns could exceed 100 million by the end of the year, the United Nations said on Wednesday.

The U.N. Food and Agriculture Organization (FAO) estimates that more than 60 million people, two thirds of them in East and Southern Africa, are facing food shortages because of droughts linked to El Nino, a climate phenomenon that occurs when water in the Pacific Ocean becomes abnormally warm.

The impact of La Nina, when waters in the Eastern Pacific cool after a phase of El Nino, is not as severe — but the weather pattern has also been linked to floods and droughts.