July 6, 2022
A team of researchers has discovered human activity is the cause for drying out the climate in southwestern Europe.
William Harmon Norton Professor of Geology Rhawn Denniston
Using funding from a 2018 National Science Foundation grant, a multidisciplinary team of researchers, including Cornell College William Harmon Norton Professor of Geology Rhawn Denniston, identified how and why droughts have occurred in southern Portugal over the last millennium. The latest findings were just published in the Journal Nature Geoscience.
“Using stalagmites and cutting-edge climate model simulations, this study found that a big area of atmospheric high pressure in the eastern Atlantic Ocean–the Azores High Pressure System–has been responding to increased levels of carbon dioxide (C02) in the 20th century from human activities such as the burning of coal, oil, and natural gas, and as a result, is increasing the frequency of dry winters,” Denniston said.
The Azores High Pressure System is located in the Atlantic Ocean, west of Spain and Portugal. In the region’s Mediterranean climate, most precipitation falls during the winter rainy season and the Azores High strongly impacts how much rain falls.
“The stronger and bigger this Azores High Pressure System is, the less rain is going to fall in southern Spain or Portugal and the drier the area is,” Denniston said. “When the Azores High moves south, gets smaller, or becomes less intense, storms have an easier time reaching land and bringing more rain to Portugal. We’ve discovered that these large Azores High events are occurring more frequently in the 20th century than any other century in the last 1,200 years.”
The geologists on the team, led by Dr. Alan Wanamaker (Iowa State University) and his doctoral student, Diana Thatcher, previously used stalagmites as the proxy to understand the history of the Azores High and the resulting rainfall. The chemistry of the stalagmites preserves past climate variability as the stalagmites grow over time.
Denniston has been using stalagmites to study climate for decades. He analyzes the chemistry of tiny samples taken from within the layers of the stalagmite, which preserve the precipitation history. The stalagmite used in this analysis came from a cave on the western edge of Portugal.
“The ratio of isotopes in the stalagmites changes depending on the amount of rain,” Denniston said. “As the water drips into caves, stalagmites forming from the water preserve those isotopic ratios. By sampling the stalagmites, we are able to look into the past to see how precipitation varied. We date the stalagmites very precisely by measuring the ratio of uranium to thorium, which tracks the age.”
Next, climate modelers in the group, led by Dr. Caroline Ummenhofer of the Woods Hole Oceanographic Institution, and her student Nath …
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