Earth scientists in the US and UK have compiled a comprehensive record of latitudinal sea surface temperature (SST) gradients over the past 95 million years of Earth’s history using δ18O data obtained from planktonic foraminifera shells.
Earth scientists have compiled a comprehensive record of latitudinal sea surface temperature (SST) gradients over the past 95 million years of Earth’s history. The results of this study help bridge the gap between empirical observations and existing climate models.
Delta-O-18 (δ18O) is the ratio of 18O to 16O in a foraminifera shell, which reveals the water temperature at the time of shell formation.
In climate modeling, temperature differences between high and low latitudes are a useful indication of the Earth’s ability to redistribute heat. They are also a useful reference for testing the validity of climate models. The results of this study help bridge the gap between empirical observations and existing climate models.
There are so many interrelated elements in climate science. What we’re doing here is trying to improve the foundation by testing some of the underlying dynamics of climate models that are used to predict future climate.
Daniel Gaskell, Department of Earth and Planetary Sciences, Yale University
A brief introduction to climate modeling
Climate modeling refers to the quantitative modeling of the Earth’s climate using numerical simulations of climatic factors. These simulations are used to model the current state of the climate as well as to predict future states.
Climate modeling is a complex process that involves defining initial variables, translating atmospheric, geological, human and other processes into mathematical equations, and iterating over time using computers.
Energy balance models represent the oldest and most basic climate models. They simply follow the balance of energy as it enters and leaves Earth’s atmosphere. These models only track surface temperature. They are considered “zero-dimensional” because they treat the Earth as a single entity.
Above, one-dimensional models take other variables such as latitude differences into account when simulating energy transfers. Radiative correction models, for example, simulate energy transfers at different heights in the Earth’s atmosphere.
Global Climate Model (GCM) simulations go even further. In addition to modeling heat transfer, they simulate the physical systems that govern the climate, such as air and water flows in the atmosphere or on the surface of the oceans.
GCM models are three-dimensional. In fact, they have become veritable “earth system models” that can simulate greenhouse gas emissions, atmospheric chemistry, ocean ecology, land use effects, carbon cycles and nitrogen and a whole host of other factors.
Climate modeling using plankton shells
The research team was led by Daniel Gaskell and Pincelli Hull of Yale University’s Department of Earth and Planetary Sciences. Contributions were made by Purdue University, University College London and the University of Southampton.
The team used fossil records of planktonic foraminifera to map sea surface temperatures over the past 95 million years of Earth’s history.
Foraminifera are single-celled, mostly marine organisms that form outer shells. These shells are mostly made of calcium carbonate (CaCO3). The Delta-O-18 (δ18O) of shells, the ratio of 18O to 16O, fluctuates with temperature and therefore provides a reliable record of climatic fluctuations.
Central to the study were the differences in sea surface temperature between Earth’s low and high latitudes – the latitudinal temperature gradient. This helps to understand how climate models work.
However, until now it has been difficult to produce reliable data on sea surface temperature. Foraminifera shells have presented researchers with the ideal solution.
Their shells bear an imprint of the seawater conditions at the time of their formation.
Daniel Gaskell, Department of Earth and Planetary Sciences, Yale University
The team found that latitudinal sea surface temperature gradients varied from 26.5°C to 15.3°C over a mean global sea surface temperature range of 15.3°C to 32.5°C.
The constant variations of Delta-O-18 in planktonic foraminifera from low and high latitudes – and from the sea floor – suggest fundamental constraints of the climate system related to sea temperatures, latitude, atmospheric CO2continental configurations and other factors.
The models are working better than we thought… This means that we understand this aspect of the climate system quite well and it implies that some of the more extreme scenarios are not as likely to occur.
Daniel Gaskell, Department of Earth and Planetary Sciences, Yale University
References and further reading
Gaskel, D. and. Al.(2022) The latitudinal temperature gradient and its climatic dependence as inferred from the δ18O of foraminifera over the last 95 million years. PNAS [online] Available at: https://www.pnas.org/doi/full/10.1073/pnas.2111332119
Shelton, J. (2022) The fundamentals of climate models are sound – the proof is in the plankton [online] Available at: https://news.yale.edu/2022/03/07/core-aspects-climate-models-are-sound-proofs-plankton
