Global mean surface temperature across the last 485 million years. From Judd et al., 2024

In the last 540 million years, five mass extinction events shaped the history of the Earth. Those events were related to extreme climate change. On a global scale the main forces behind this phenomenon are: solar forcing, atmospheric CO₂ concentration, and plate tectonics. 

Earth’s climate has oscillated between two basic states: the “Icehouse”, and the “Greenhouse”, and superimposed on this icehouse–greenhouse climate cycling, there are a number of geologically abrupt events known as hyperthermals, when atmospheric CO₂ concentrations may rise above 16 times (4,800 ppmv). The “Icehouse” is characterized by polar ice, with alternating glacial–interglacial episodes in response to orbital forcing. The ‘Cool Greenhouse” displays some polar ice and alpine glaciers, with global average temperatures between 21° and 24°C. Finally, the ‘Warm Greenhouse’ lacks of any polar ice, and global average temperatures might have ranged from 24° to 30°C.

Heterohelix globulosa foraminifera isolated from the K-Pg boundary clay at Geulhemmerberg in the Netherlands. Image credit: Michael J. Henehan/PNAS

Rocks, sediments, ice core records and fossils offers a glimpse of past enviroments. Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments. Pollen and other palynomorphs proved to be an extraordinary tool to paleoenvironmental reconstruction too. They provide information on regional changes in vegetation through time. Stomatal frequency of land plants, which has been shown in some species to vary inversely with atmospheric pCO₂, has been used to estimate paleo-pCO₂ for multiple geological time periods. The leaf margins are another source of paleotemperature information. In warmer climates, a greater proportion of leaves tend to have smooth edges. On the other hand, in cooler climates, most leaves have serrated edges. Therefore usign the relative proportions of fossil leaves edges climate scientists can estimate ancient temperatures. 

Leaves from the Eocene Florissant flora of Colorado dominated by species (or morphotypes) with toothed leaves. Photo; National Park Service/NPS

Reconstructions of Earth’s history have considerably improved our knowledge of episodes of rapid emissions of greenhouse gases and abrupt warming. Several episodes of global climate change were similar in magnitude to the anthropogenically forced climate change that has occurred during the past century. Consequently, the development of different proxy measures of paleoenvironmental parameters has received growing attention in recent years. Quantifying the relationship between of global mean surface temperature (GMST) and atmospheric carbon dioxide (CO₂) concentrations can refine our understanding of Earth’s climate sensitivity and improve future predictions under anthropogenic warming.

A new study by the Smithsonian and the University of Arizona offers the most detailed overview of the planet climate history. The team applies a database called PhanTASTIC (Phanerozoic Technique Averaged Surface Temperature Integrated Curve Project) that combined temperature records from various sources. This database is integrated with climate models, and by using an approach called data assimilation, the researchers produce a more accurate curve of Earth’s temperature over the last 485 million years.

PhanDA reconstructed GMST for the past 485 million years. Orange dashed lines show the timing of the five major mass extinctions of the Phanerozoic. From Judd et al., 2024.

The team compiled more than 150,000 published data points from five proxies for ancient ocean temperature and other types of ancient organic matter, and generated more than 850 model simulations of Earth’s climate at different periods of deep time based on continental position and atmospheric composition.

The integrated data indicates that Earth’s temperature has varied between a minimum of 11°C [Late Pleistocene; 129 to 11.7 thousand years ago (ka)] and a maximum of 36°C (Turonian; 93.9 to 89.39 Ma) over the past 485 million years. The curve obtained reveals a strong correlation between atmospheric CO₂ concentrations and GMST, identifying CO₂ as the dominant control on variations in Phanerozoic global climate and suggesting an apparent Earth system sensitivity of ~8°C.

Today the most politically unstable countries are also places where environmental degradation affected food production and water supply. Other human societies have succumbed to climate change – like the Akkadians – while others have survived by changing their behavior in response to environmental change. We have the opportunity to protect the future of our own society by learning from the mistakes of our ancestors. The clock is ticking.

References:

Judd, E. J., Tierney, J. E., Lunt, D. J., Montañez, I. P., Huber, B. T., Wing, S. L., & Valdes, P. J. (2024). A 485-million-year history of Earth’s surface temperature. Science, 385(6715), eadk3705. DOI: 10.1126/science.adk3705

David Evans, Navjit Sagoo, Willem Renema, Laura J. Cotton, Wolfgang Müller, Jonathan A. Todd, Pratul Kumar Saraswati, Peter Stassen, Martin Ziegler, Paul N. Pearson, Paul J. Valdes, Hagit P. Affek. Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry. Proceedings of the National Academy of Sciences, 2018; 201714744 DOI: 10.1073/pnas.1714744115