Utrecht University: New geological method reconstructs precise summer and winter temperatures during greenhouse climate

An international research team led by Niels de Winter of Utrecht University has used a new method to reconstruct the monthly temperature differences during the Cretaceous period (78 million years ago). This era, the time of the dinosaurs, was characterised by high CO2 concentrations. ‘By knowing how large the differences between summer and winter were during warmer periods in the past, we can improve the models that predict extreme seasons in our future climate,’ said paleoclimatologist Niels de Winter. The paper was published on 10 June in the journal Communications Earth & Environment.

For these climate reconstructions, the researchers used very well-preserved fossils of mollusks that lived in southern Sweden 78 million years ago. The animals grew in the warm, shallow seas that covered Europe at that time. As they grew, these mollusks stored monthly variations in their environment and climate in their shell, similar to the rings in a tree.

New method
The team of researchers used the clumped isotope method for the first time to make reconstructions of seasonal temperature differences. Dr. Martin Ziegler leads the clumped isotope lab at Utrecht University: “With this method, we look at how specific atoms are arranged within the shells. This provides information about the temperature during the creation of a shell. Moreover, the method is based on physical principles and is therefore much more reliable.” De Winter, Ziegler and colleagues applied the new technique for the first time to very small samples of less than 100 micrograms from the fossil shells, making it possible to trace temperature changes on monthly time scales. They showed that the water temperature in Sweden during the greenhouse period varied between 15°C in winter and 27°C in summer, more than 10°C warmer than today. Together with colleagues from Bristol, the team of researchers compared the new reconstructions with simulations from climate models. The team discovered that the new reconstructions matched these simulations much better than reconstructions using previous methods.

Plankton versus shells
De Winter: “Previous methods for climate reconstruction often used deep sea sediments, which consist largely of the skeletons of plankton. But with such samples you never know exactly whether you are measuring the average annual temperature. If the plankton only live for a few months, or mainly grow during the annual algae bloom in spring, climate reconstructions based on these sediments give a distorted picture of the annual average. With this new method, where we use fossil shells, we now have a technique to check those other reconstructions for such seasonal deviations.”

New insights
By reconstructing the climate of the past, we can better estimate what our future climate will look like. De Winter explains: “Previously, it was thought that as the climate warmed, the difference between the seasons would decrease, much like there is less temperature difference between summer and winter in the tropics. On the contrary, our reconstructions and our climate model show that the summer temperature is warming just as fast as the winter temperature. So the temperature difference between seasons continues to exist.”