What stalagmites reveal about the climate
Dominik Fleitmann liest in Stalagmiten, wie sich Klima und Umwelt über Tausende bis Millionen von Jahren verändert haben. Die Archive aus der Höhle zeigen auch, wie Klimaveränderungen menschliche Lebensräume geprägt haben. Und welche Folgen der heutige Klimawandel haben könnte.
24 September 2026 | Catherine Weyer
Dominik Fleitmann, how did you come to study the climate using stalagmites?
It was through my doctoral research. Using stalagmites as climate archives is a relatively young field. Previously, we mainly knew this kind of approach from ice cores, tree rings, or lake sediments. But stalagmites also give us a very precise view into the past.
How far back have you been able to look?
We studied a stalagmite from Saudi Arabia that was 8 million years old. When we discovered it, our instruments were not yet capable of dating it that far back. That only became possible later.
What can you see in a stalagmite?
An enormous amount. For one thing, we can determine its age with a precision of up to about ten years. To do this, we use radioactive elements: uranium is incorporated into the stalagmite as it grows and gradually decays into thorium. We can measure this process and use it to determine the age with high precision.
We can also measure oxygen and carbon isotopes, which provide information about climate, temperature, rainfall, and vegetation. We can also extract even tiny amounts of water trapped inside the stalagmite as it grew.
How do you obtain these stalagmites?
We look for suitable caves and draw on other researchers' knowledge. We then travel to the site with a team to take samples. This can mean drilling out a sample or using a hammer to remove an entire stalagmite. Of course, we obtain all the necessary permits beforehand. But that does not necessarily mean everything goes smoothly.Thanks to these climate archives, you also know that today’s deserts were repeatedly green in the past. When was the most recent period?
The last green phase in North Africa and Arabia lasted from around 10,000 to 6,000 years ago. We can still find traces of it today. For example, we find hippopotamus and crocodile bones among the present-day dunes. The period lasted long enough for entire ecosystems to become established.
Why does it matter when the desert was green?
The green desert allowed Homo sapiens to spread northward. At the time, the Indian monsoon brought more rainfall, which also reached farther north. Today’s deserts became savannas, providing an ideal habitat for our ancestors and allowing them to spread from Africa into Asia and Europe. When this green belt disappeared, it created a natural barrier. The people who had already migrated through the savanna continued to spread across Europe and Asia, while others remained in Africa.
Do stalagmites ever surprise you?
A stalagmite almost always has surprises. In one from Turkey, for example, we found tiny pieces of wood. They came from people who had entered the cave and used torches. We also found human bone fragments from people buried there. The stalagmite was 6,000 years old.
Can you also identify natural events in the stalagmites?
Yes. We can reconstruct flooding events, for example. When floodwater carries large amounts of sediment into a cave, some of it is deposited on the stalagmite. Volcanic eruptions and earthquakes can also leave traces behind.
You study the climate and how it changes every day. Are you worried about the future?
I am very concerned. And not only because of the direct effects of rising temperatures, for example, in Switzerland. I am worried about the cascading effects that could follow. We have seen in the past that changes in climate can trigger migration on a scale that becomes very difficult to control. Food shortages and water scarcity have also driven revolutions. The prospect of growing conflicts over scarce resources because of climate change worries me greatly.
Has there ever been a period in Earth’s history with a climate similar to the one we are heading toward?
There have been periods in Earth’s history that were similarly warm or even hotter. But there were no humans then. And that is one of the major challenges of climate change: it is unclear whether our society, as we know it, can continue to exist if the natural ecosystems it depends on are irreversibly destroyed. At several points in Earth’s history, up to 90 percent of all species became extinct. The Earth continued to exist afterward. But it was a very different planet – and in an extreme scenario, it could one day be one without humans.
This interview has been edited and condensed for publication. Listen to the full episode of the University of Basel podcast “Unisonar” (in German) and subscribe to the podcast on your preferred podcast platform.
