In January 2016, water in the central Pacific Ocean warmed to 2.6 degrees above normal. For a resident of Dhaka, that distant event might seem irrelevant, but it was the first link in a chain that led to a prediction: come autumn, the city would likely face a major cholera outbreak.

The researchers took a page from weather forecasters, who use pressure over the ocean to predict storms onshore. They looked at cholera incidence in Dhaka from 1995 to 2010 and built two models. The first describes how the disease spreads from person to person and is fueled by contaminated water. The second divides the city into neighborhoods and tracks how outbreaks move from one area to another. Both models incorporate the same sensor: the January ocean temperature anomaly.

When the models were run for the August–December 2016 season, both raised the alarm. The first gave an 87% probability of a major outbreak, the second 92%. For comparison, in calm years that figure never exceeded 29%. The gap is huge, and it comes down to that warm water in the Pacific.

Ocean temperature acted like a barometer falling well before a storm. In January 2016, the anomaly was 2.60 degrees; in January 1998, when Dhaka experienced its worst cholera outbreak in two decades, it was 2.56 degrees. Almost an exact match. The ocean seemed to repeat its earlier signal.

Such a forecast gives health systems months of lead time, not days. Hospitals can be prepared in advance, rehydration solutions stockpiled, and the public warned. The method is not limited to cholera or Dhaka: any climate-sensitive disease could gain its own early indicator. A thermometer in the Pacific turns out to be a useful tool for doctors on the other shore.