Summers warm more than winters, fossil shells reveal

In a warmer climate, summers warm much faster than winters in northwestern Europe. That is the conclusion of research into fossil shells by an international team of earth scientists. With this knowledge we can better map the consequences of current global warming in the North Sea area.

The researchers measured the chemical composition of fossil shells. These shells were built by mollusks such as oysters and clams that lived in the North Sea in the Pliocene, about three million years ago. The shells grew layer by layer, much like tree rings, and stored very detailed information during their lifetime.

Snapshot of the seasons

During the Pliocene, the Earth was on average 2.5 to 3 degrees Celsius warmer than it is now. In their study, published in Science Advances, the researchers took a ’snapshot’ of the climate at that time to gain insight into the difference between the seasons in a warmer climate.

Our research increases confidence in the quality of future climate predictions based on models.

Dr. Martin Ziegler

Summers heat up more than winter

The key insight is that summers warm much more than winters in a warmer climate such as the Pliocene. While winters became about 2.5 degrees warmer, temperatures during summer were about 4.3 degrees higher. The researchers see a similar result in models projecting future climate, which predict roughly the same amount of warming for the year 2100.

Rare heavy isotopes

In this study, they looked at the order of isotopes in the fossil shells - the so-called ’clumped isotope analysis’ method. The degree of order in the carbon and oxygen isotopes in fossil calcite shells depends on the temperature. Therefore, the measurements can be used to reconstruct the temperature at which the shells were formed. "Our research shows a better agreement between reconstructions and climate models, which also increases confidence in the quality of future climate predictions based on models," says coauthor Martin Ziegler.

The study gives us a glimpse of what the climate in Europe will be like if we continue our current trend towards a warmer world. Niels de Winter (VU Amsterdam): "We will likely experience stronger temperature differences between summer and winter, and the chance of heatwaves during the summer will increase."


De Winter, N., (2024), Science Advances, DOI: 10.1126/sciadv.adl6717