What ancient stalactites can tell us about how forest fires will burn on a hotter Earth

In case of a forest fire, escape into the cave. This is the official emergency directive from the Oregon Caves National Monument and Preserve, a forested area that protects a labyrinth of passageways dissolved in a rare marble formation high in the Siskiyou Mountains. There hasn’t been a fire on the reservation in a century. But the danger of a wildfire in the dry forest is noticeable. If a fast-spreading forest fire were to burn through, the cave would be the safest place for park rangers to hide.

However, the 1.7-million-year-old cave is not completely isolated from fires burning on the surface. When a fire burns above, the heat and smoke can change the chemistry of the water seeping through the rock. If it drips into the cave, it can leave burn marks in mere layers of mineral residue. Over millennia, strange cave structures called speleothems form, protruding from any surface on which water flows, including stalagmites on the cave floor and stalactites on the ceiling.

“It’s a snapshot in time,” Katie Wendt, a paleoclimatologist at Oregon State University, told me when I accompanied her on a recent expedition into the cave. She is part of a growing group of researchers using cave records of wildfires to expand our view of fire activity going back hundreds of thousands of years, to a time when temperatures on Earth were even hotter than today. That, in…