Looking into Earth’s deep past helps scientists understand what may happen as the planet warms today. One time period stands out in this search: the Paleogene Period, which began about 66 million years ago.
During this era, Earth held much more carbon dioxide in the air than now, and global temperatures rose far higher.
By studying this ancient world, scientists can learn how heat changes rainfall, landscapes, and living systems.
A new study by researchers from the University of Utah and the Colorado School of Mines takes a closer look at how rain behaved during extreme warming.
Instead of focusing only on how much rain fell each year, the study asks a different question: when did rain fall, and how often did dry spells appear?
Earth during ancient heat
The Paleogene Period began after the extinction of the dinosaurs and lasted for millions of years. Mammals began spreading across land, and many familiar rock formations started to form.
In places like Utah, landscapes such as Bryce Canyon hoodoos and the Uinta Basin badlands developed during this time.
Carbon dioxide levels during the Paleogene reached two to four times modern levels. Global heat climbed even higher during a major event called the Paleocene Eocene Thermal Maximum, or PETM.
Studying heat without instruments
During this time, temperatures rose about 32 degrees Fahrenheit (18 degrees Celsius) higher than just before modern industrial warming.
Some scientists view this period as a possible preview of extreme future climate conditions.
To understand rainfall during this hot era, scientists cannot rely on direct measurements.
Rain gauges did not exist millions of years ago. Instead, researchers turn to indirect clues left behind in rocks, soils, and fossils.
Using clues from ancient landscapes
These natural clues are called proxies. Proxies help scientists estimate past climate conditions based on physical evidence.
“From the shape and size of fossilized leaves, you can infer aspects of climate of that time because you look at where similar plants exist today with those leaves,” said Thomas Reichler, professor of atmospheric sciences at the University of Utah.
“So this would be a climate proxy. It’s not direct measurement of temperature or humidity; it’s indirect evidence for climate of that time.”
River channels also tell an important story. Strong floods carve riverbeds differently than steady light rain.
“When there is intermittent, strong precipitation then followed by long periods of drought, that precipitation is forming the riverbed in different ways because there’s very high amounts of water flowing down and carving it out or transporting the rocks much more vigorously than were it a little drizzle every day,” said Reichler.
Rain became less predictable
The research reveals a surprising pattern. Many scientists once believed that warming makes wet places wetter and dry places drier. This study challenges that idea.
“There are good reasons, physical reasons for that assumption. But now our study was a little bit surprising in the sense that even mid-latitudes regions tended to become drier,” Reichler said.
The key finding involves timing. Rainfall during extreme warming became irregular. Long dry periods appeared, followed by short bursts of heavy rain.
This pattern resembles strong monsoon systems rather than steady seasonal rainfall.
Polar regions during the Paleogene stayed wet, sometimes even monsoonal. Many mid-latitude regions and inland areas experienced longer dry spells, even when total rainfall stayed similar.
“If there are relatively long dry spells and then in between very wet periods as in a strongly monsoonal climate conditions are unfavorable for many types of vegetation,” Reichler said.
Timing changed rain impact
The study shows that dry conditions often came from shorter wet seasons rather than less rain overall.
Rain arrived in intense bursts, leaving long gaps without water. These patterns started well before the PETM and continued long after.
Such findings matter for modern climate science. Climate models often focus on yearly averages. This research suggests that averages can hide dangerous extremes.
Comparisons with paleoclimate simulations show that modern models may underestimate how uneven rainfall becomes under extreme heat.
Longer droughts combined with stronger storms affect ecosystems, rivers, floods, and water planning.
Learning from ancient climate
Reichler cautions that reconstructions using proxies carry uncertainty. Even so, fossil records provide the strongest window into climates far hotter than today.
Looking at Earth’s ancient climate teaches an important lesson. In a warming world, reliable rainfall may matter more than total rainfall.
Understanding that difference could shape how societies prepare for floods, droughts, and future water challenges.
The study is published in the journal Nature Geoscience.
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