New research by the University of Utah and the Colorado School of Mines reconstructs how rainfall responded to extreme warming during this period using “proxies,” or clues left in the geological record in the form of plant fossils, soil chemistry and river deposits. The results challenge the commonly held view that wet places get wetter when the climate warms and drier places become drier, according to co-author Thomas Reichler, professor of atmospheric sciences at the U.
“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. “It has to do with the variability and the distribution of precipitation over time. 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.”
Rainfall was far more variable
Instead of focusing on the amount of precipitation each year, Reichler’s team explored when rain fell and how often. They found rainfall appears to be much less regular under extreme warming, often occurring in intense downpours separated by prolonged dry spells.
The researchers concluded polar regions were wet, even monsoonal, during the Paleogene, while many mid-latitude and continental interiors became drier overall.
The findings, published last month in the journal Nature Geoscience, are based on a comprehensive analysis of existing research. To conduct the study, Reichler teamed up with Colorado School of Mines geologists, who analyzed proxy data from the fossil record, while Reichler conducted the climate modeling with graduate student Daniel Baldassare.
This study looks back to the warmest time in Earth’s history, the early Paleogene, 66 to 48 million years ago, to understand how rainfall behaves when the planet gets very hot. This period began with the sudden demise of the dinosaurs and saw the rise of mammals in terrestrial ecosystems. This was the time when some of Utah’s notable landscapes, such as the hoodoos of Bryce Canyon and the badlands of the Uinta Basin, were deposited.
It was also a period of intense warming culminating in the well-studied event called the Paleocene-Eocene Thermal Maximum, or PETM, when levels of heat were 18 degrees Celsius (32 degrees Fahrenheit) warmer than they were just before humans began releasing greenhouse gas emissions into the atmosphere. Some scientists consider the climate of this period a possible worst-case scenario for future climate change.
Proxies in the fossil record
Since it is not possible to measure precipitation that occurred millions of years ago, scientists examine evidence in the geologic record to draw conclusions about ancient climates. In this case, Reichler’s Colorado colleagues looked at plant fossils and ancient soils.
“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. 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,” Reichler said.
Read the full story by Brian Maffly in @ The U.