Using data assimilation to combine model outcomes and reconstructions of seasonality during past warm periods

Anthropogenic global change necessitates the improvement of our understanding of the dynamics of warmer climates in the past. Combining information from Earth’s climate history with numerical simulations of past climate helps us to identify gaps in our knowledge of climate mechanisms and improves projections for future climate^1,2. Data assimilation is a valuable tool to reconcile information from climate reconstructions and models in a consistent statistical framework³. These data assimilation efforts have focused mostly on climate variability on geological timescales (thousands to millions of years). However, seasonal changes in climate parameters such as temperature and precipitation are a defining characteristic of climate zones and have a dominant impact on the impact of climate on nature and human society⁴.

This work lays the foundations for applying data assimilation techniques to compare and combine reconstructions and model information on a seasonal scale. We use seasonal-scale temperature reconstructions from incrementally grown fossil mollusc shells which record seasonality in their living environment during modern⁵, Pliocene⁴ and Cretaceous periods⁶. We combine these data with model outcomes from the same periods to arrive at a combined estimate of seasonal temperature variability and discuss the methodological choices that lead to this result. Using this data analysis product allows us to more easily interrogate the outcomes from climate models with various boundary conditions using proxy-based information on select climate variables. The aim is to lay the foundation for data assimilation for estimating short-term climate variability in the geological past from skeletal carbonate archives and comparing model and reconstruction outcomes.

 

References

• Hakim, G. J. et al. The last millennium climate reanalysis project: Framework and first results. Journal of Geophysical Research: Atmospheres 121, 6745–6764 (2016).
• Tierney, J. E. et al. Past climates inform our future. Science 370, (2020).
• Dirren, S. & Hakim, G. J. Toward the assimilation of time-averaged observations. Geophysical Research Letters 32, (2005).
• de Winter, N. J. et al. Amplified seasonality in western Europe in a warmer world. Science Advances 10, eadl6717 (2024).
• Caldarescu, D. E. et al. Clumped isotope thermometry in bivalve shells: A tool for reconstructing seasonal upwelling. Geochimica et Cosmochimica Acta 294, 174–191 (2021).
• de Winter, N. J. et al. Absolute seasonal temperature estimates from clumped isotopes in bivalve shells suggest warm and variable greenhouse climate. Commun Earth Environ 2, 1–8 (2021).