Estimating past global temperature is important for understanding the history of life on Earth and for predicting future climate. Now, a new reconstruction of Earth's temperature history over the past 485 million years – based on a method that combines diverse physical proxy data with climate model predictions – reveals a much wider range of climate variability across the Phanerozoic eon than previously understood. The findings highlight atmospheric carbon dioxide (CO2) as the dominant factor controlling climate variability throughout this period, offering new insights into the Earth's climate sensitivity across long timescales. A record of global mean surface temperature (GMST) throughout the Phanerozoic – the last 540 million years during which animals and plants evolved – is crucial. Over the last 500 million years, climate shifts across the Phanerozoic have been closely tied to biological evolution, tectonic activity, and atmospheric and ocean chemistry. Studying this temperature evolution helps clarify the mechanisms driving long-term climate changes. Additionally, GMST data from the Phanerozoic provides important context for current human-induced warming. However, temperature reconstructions from this period have traditionally relied on disparate, incomplete proxy data or predictions from Earth systems models, which are difficult to confirm. According to Emily Judd and colleagues, both methods have suggested that global Phanerozoic temperatures have been generally colder and less variable than other studies of recent geological periods have shown. Such discrepancies raise doubts about the current view of Earth's long-term climatic history.
Judd and colleagues present PhanDA – a state-of-the-art reconstruction of GMST spanning the last 485 million years. PhanDA is based on paleoclimate data assimilation (DA), a technique that statistically blends geological proxy data on past conditions with Earth system model simulations to create a more complete record of surface air temperatures across the Phanerozoic. According to Judd et al., the PhanDA reconstruction reveals that Earth's temperature has varied more dynamically than previously thought, exhibiting a large range of GMST, spanning 11 degrees Celsius (°C) to 36° C. The authors also identified five distinct climate states and show that more of Earth's history was spent in warmer rather than colder climates. Moreover, the findings demonstrate a strong link between global temperatures and atmospheric carbon dioxide levels, suggesting that CO2 has been the primary driver of climate changes over this period. However, Judd et al. note that the nature of this link is likely complex and will require further research to constrain. In a Perspective, Benjamin Mills discusses the study and its limitations in greater detail.
