A long-term geological record of global mean surface temperature (GMST) is important for understanding the history of our planet and putting present-day climate change into context. Such a record is necessary for constraining the relationship between climate and other aspects of the Earth system, including the evolution and extinction of life, and the chemistry of the atmosphere and oceans. Further, quantifying the relationship between GMST and atmospheric carbon dioxide (CO2) concentrations can refine our understanding of Earth’s climate sensitivity and improve future predictions under anthropogenic warming.


RATIONALE

Although several Phanerozoic (the last 539 million years) temperature reconstructions exist, during the intensively studied Cenozoic Era (the last 66 million years), they are colder and less variable than individual estimates from key time periods, particularly during ice-free (greenhouse) intervals. This discrepancy suggests that existing Phanerozoic temperature records may underestimate past temperature change, and merits further investigation using a new approach.


RESULTS

Here, we present PhanDA, a reconstruction of GMST spanning most of the Phanerozoic Eon. PhanDA was created using data assimilation, a method that statistically integrates geological data with climate model simulations. PhanDA indicates that Earth’s temperature has varied between 11° and 36°C over the past 485 million years. This range is larger than previous reconstructions; however, PhanDA agrees well with independent GMST estimates from the Cenozoic, providing confidence in its larger dynamical range.

PhanDA reveals key features in the relationship between GMST and the pole-to-equator temperature gradient, including polar amplification (i.e., larger temperature changes at high latitudes) and a shallowing of the gradient with increasing GMST. Tropical temperatures range between 22° and 42°C, refuting the idea of a fixed upper limit on tropical warmth and suggesting that ancient life must have evolved to endure extreme heat. We parse PhanDA into five climate states and find that overall, Earth has spent more time in warmer climate states than cold ones during the Phanerozoic.
There is a strong relationship between PhanDA GMST and CO2, indicating that CO2 is the dominant control on Phanerozoic climate. The consistency of this relationship is surprising because on this timescale, we expect solar luminosity to influence climate. We hypothesize that changes in planetary albedo and other greenhouse gases (e.g., methane) helped compensate for the increasing solar luminosity through time. The GMST-CO2 relationship indicates a notably constant “apparent” Earth system sensitivity (i.e., the temperature response to a doubling of CO2, including fast and slow feedbacks) of ∼8°C, with no detectable dependence on whether the climate is warm or cold.


CONCLUSION

PhanDA provides a statistically robust estimate of GMST through the Phanerozoic. We find that Earth’s temperature has varied more dynamically than previously thought and that greenhouse climates were very warm. CO2 is the dominant driver of Phanerozoic climate, emphasizing the importance of this greenhouse gas in shaping Earth history. The consistency of apparent Earth system sensitivity (∼8°C) is surprising and deserves further investigation. More broadly, PhanDA provides critical context for the evolution of life on Earth, as well as present and future climate changes.