The Paraná River in South America acts like an instrument that records solar activity. People have measured the river's daily flow since 1904. Astronomers, meanwhile, counted sunspots. It turns out that when there are many sunspots, the river floods more.

The researchers smoothed both datasets over 11 years to remove short-term fluctuations. The resulting correlation was 0.78, which is roughly the strength of the link between a person's height and weight: strong but not perfect. On shorter timescales, from year to year, the river follows the rhythm of El Niño, the warm water off the coast of Peru. In 1997, a strong El Niño caused the river to flood an area larger than Greece.

So the Sun does not simply heat Earth uniformly. It influences where rain falls. When solar activity is high, southeastern South America gets more rain, while East Africa gets less. The Paraná River, like a large gauge, reflects this difference. If the connection is confirmed with other data, it could be used to warn people about floods in advance.