Climate simulations created by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) indicate that the planet could warm roughly 3°C by the end of the century under intermediate carbon emissions scenarios. But how such a rise might manifest in seasonal and regional temperature changes can be difficult to test.
Focusing on western Europe, a group of researchers looked back 3.3 million years ago, when Earth was about as warm as it is predicted to be in the year 2100, for clues. Fossilized shells from the time give a season-by-season account of regional temperatures and show that summer and winter may not warm at the same rate. The new study was published in Science Advances.
A Blurry Picture
Temperature projections can be tested on multiyear scales relatively easily. Paleoclimatologists often turn to sediment deposits to search for buried fossils or certain isotopes that can indicate temperature conditions. These deposits can show changes over a long period of time because sediment buildup can take “literally ages,” said Niels de Winter, a paleoclimatologist at Vrije Universiteit Amsterdam and first author on the study.
“But what’s really impactful for us as humans is to see the impacts of two or three degrees warming on extreme weather events in our seasons,” said de Winter. Smaller-scale fluctuations in temperature such as these can be harder to predict.
“We’re able to do a snapshot of the climate in this warm period [3.3 million years ago].”
Shorter records, such as from the rings of a tree, may be used to tease out seasonal conditions, but trees don’t preserve well over millions of years. Fossilized shells, however, do.
While living, some marine critters record the seasonal temperature changes around them in each new ringed growth of their calcium carbonate shell. “They record environmental change on a very short timescale,” de Winter said. “And so we’re able to do a snapshot of the climate in this warm period [3.3 million years ago].”
Shells are also abundant. De Winter and his colleagues analyzed fossil mollusk shells from the collection of the Royal Belgian Institute of Natural Sciences in Brussels. They chose shells gathered from harbor excavations in Antwerp, Belgium, near the North Sea.
The researchers measured oxygen and carbon isotopes along cross sections of the shells. To what degree the heavier isotopes of oxygen and carbon are bonded together rather than to other lighter isotopes is dependent on the temperature of the water in which each part of the shell grew. From this, a technique known as clumped isotope thermometry, the researchers could pull out seasonal temperature differences.
“This is an elegant and unique method of data-model comparison,” wrote Harry Dowsett, a research geologist with the U.S. Geological Survey who was not involved with the study, in an email.
“This is a pretty worrisome prediction.”
The fossilized shells recorded winters that were about 2.5°C warmer than current sea surface temperatures and summers that were about 4.3°C warmer. The findings corroborate the general IPCC climate simulations and suggest that in the future, as average temperatures rise by roughly 3°C, the summers could warm much more than winters will.
“This is a pretty worrisome prediction,” de Winter said. These numbers suggest that Europe may face prolonged heat in the summer in the coming decades—conditions the area is already struggling to deal with. A record heat wave affected the region in July 2023, topping the heat waves in 2022 that caused 60,000 reported deaths.
“This work is an excellent example of how deep-time paleoclimate research has a direct bearing on our understanding and preparing for future climate change,” Dowsett said.
Clouds and Insulation
Scientists are still trying to understand why summers may warm faster than winters. The reason could, in part, be clouds—simulations show a reduction in cloud cover in northwestern Europe during the summer as westerly winds decrease, which may increase warming. Conversely, winter cloud cover did not significantly change.
Sea ice may also play a factor, de Winter said. “It insulates the winter.” Arctic ice cools the ocean, making heat transfer from the ocean into the atmosphere less efficient and keeping air temperatures relatively low in the winters.
In the future, de Winter plans on expanding the project to analyze shells all over northwestern Europe to further understand how the region will experience a warmer climate.
—Sierra Bouchér, Science Writer
Citation: Bouchér, S. (2024), Fossilized shells reveal the seasonality of a warmer climate, Eos, 105, https://doi.org/10.1029/2024EO240315. Published on 26 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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