WASHINGTON (AP) — The climate phenomenon known as El Nino — and not climate change — was a key driver in low rainfall that disrupted shipping at the Panama Canal last year, scientists said Wednesday.

A team of international scientists found that El Nino — a natural warming of the central Pacific that changes weather worldwide — doubled the likelihood of the low precipitation Panama received during last year’s rainy season. That dryness reduced water levels at the reservoir that feeds freshwater to the Panama Canal and provides drinking water for more than half of the Central American country.

Human-caused climate change was not a primary driver of the Central American country’s unusually dry monsoon season, the World Weather Attribution group concluded, after comparing the rainfall levels to climate models for a simulated world without current warming.