Utilizing a method known as data assimilation, the research team merged geological records and climate models to create a detailed curve of global mean surface temperature (GMST) across the Phanerozoic Eon, a period spanning the last 540 million years. According to the team, this curve helps illustrate the relationship between Earth's temperature and carbon dioxide levels.

"This method was originally developed for weather forecasting," said Emily Judd, lead author and a former postdoctoral researcher at the Smithsonian and the University of Arizona. "Instead of using it to forecast future weather, here we're using it to hindcast ancient climates."

The study provides essential context for modern climate change. "If you're studying the past couple of million years, you won't find anything that looks like what we expect in 2100 or 2500," explained Scott Wing, the Smithsonian's curator of paleobotany. "You need to go back even further to periods when the Earth was really warm to better understand how the climate might change in the future."

The study reveals temperature shifts from 52 to 97 degrees Fahrenheit (11-36 degrees Celsius) during the Phanerozoic. The research highlights that periods of extreme heat were closely tied to high levels of atmospheric carbon dioxide.

Jessica Tierney, a paleoclimatologist at the University of Arizona and co-author, emphasized the role of carbon dioxide in regulating global temperatures. "This research illustrates clearly that carbon dioxide is the dominant control on global temperatures across geological time," Tierney said. "When CO2 is low, the temperature is cold; when CO2 is high, the temperature is warm."

The research also showed that the Earth's current GMST of 59 degrees Fahrenheit (15 degrees Celsius) is lower than the average for much of the Phanerozoic. However, current greenhouse gas emissions are warming the planet at a rate much faster than any previous event in this eon, potentially leading to severe ecological impacts.

Tierney noted the risks, stating, "Humans, and the species we share the planet with, are adapted to a cold climate. Rapidly putting us all into a warmer climate is a dangerous thing to do."

The research stems from work that began in 2018 when Smithsonian scientists were developing the "David H. Koch Hall of Fossils - Deep Time," which highlights how Earth's climate has evolved over the last half-billion years. However, the researchers found that a comprehensive temperature curve for this period was lacking. This prompted the creation of the PhanTASTIC (Phanerozoic Technique Averaged Surface Temperature Integrated Curve) Project.

The team reconstructed past climates using data from more than 150,000 published sources, incorporating ocean temperature records preserved in fossilized shells. They also worked with researchers from the University of Bristol, who provided 850 climate simulations to refine the global temperature model.

"This isn't the final curve," said Brian Huber, the Smithsonian's curator of foraminifera. "Researchers will continue to uncover additional clues about the deep past, which will help revise this curve down the road."

Co-authors of the study include Daniel Lunt and Paul Valdes from the University of Bristol, as well as Isabel Montanez from the University of California, Davis. The research was supported by funding from several organizations, including the Smithsonian, the Heising-Simons Foundation, and the United Kingdom's Natural Environment Research Council.

Research Report:A 485-million-year history of Earth's surface temperature

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