Inside a cave in southern Brazil, scientists found something remarkable: a stone that remembers floods stretching back 7,500 years. The discovery is helping researchers understand what causes devastating rains in the region — and what might happen next.
The cave is called Malfazido Cave, located in the municipality of Doutor Ulysses in Paraná state. Within it grow tall rock formations called stalagmites, which rise from the cave floor. These formations act like tree rings, building up in thin layers over thousands of years. When floods pour water into the cave, they leave behind tiny deposits of fine sediment that get trapped inside the growing stone. Scientists can read these layers like pages in a book, revealing when floods happened going back millennia.
A team of Brazilian researchers, led by geologist Julio Cauhy, spent years studying these layers. They identified 921 distinct flood events preserved in the stalagmites. What they found was striking: the 20th century experienced some of the highest frequency of extreme rainfall events in the entire 7,500-year record. "Until now, all of our knowledge was limited to instrumental records, which generally cover the past 100 years at most in Brazil," Cauhy told Agência FAPESP. "Speleothems can grow continuously and rapidly, as in the case of Malfazido Cave, producing a high-resolution record. In other words, it's possible to determine the interannual or even annual frequency of these events."
The scientists also uncovered what drives these floods. Two main factors keep appearing in their data. First, when West Antarctica experiences cooler-than-average summers, southern Brazil tends to see more extreme rainfall. This happens because temperature differences between the polar regions and warmer areas to the north change how winds move moisture across the continent, pulling water from the Amazon rainforest southward. Second, over the past 1,000 years, the researchers found a clear link between extreme rainfall and El Niño events — the periodic warming of Pacific Ocean waters that disrupts weather worldwide.
These findings have urgent relevance today. The World Meteorological Organization has predicted a moderate-to-strong El Niño in the coming months, and Brazil's disaster monitoring center has already warned of potential heavy rains and landslides in the country's south-central region. This comes just one year after floods ravaged more than 470 municipalities in Rio Grande do Sul state in May 2024, during the last El Niño year.
The research, published in April in the journal Communications Earth & Environment, represents the first detailed reconstruction of extreme weather events going this far back in Brazil's history. Cauhy conducted part of the work at the Max Planck Institute for Chemistry in Germany. With better understanding of what triggers these disasters, communities may be better prepared for what lies ahead.