A new study, co-led by the University of Arizona and the Smithsonian, provides the most detailed glimpse yet into how Earth’s surface temperature has changed over the past 485 million years. Published in Science, the study presents a global mean surface temperature (GMST) curve, revealing that Earth’s temperature has fluctuated more than previously believed throughout much of the Phanerozoic Eon. This period, marked by life diversifying and enduring multiple mass extinctions, also shows a strong correlation between atmospheric carbon dioxide (CO2) levels and temperature changes.
Understanding how GMST has varied over this half-billion-year period is crucial, as evolutionary patterns of flora and fauna significantly influenced climate evolution. Judd et al. constructed a record of GMST by combining proxy data with climate models, presenting a range of temperatures from 11° to 36°C. This wide span includes an “apparent” climate sensitivity of around 8°C—roughly two to three times today’s level. These insights provide essential context for understanding the processes that have shaped Earth’s climate over this interval.
The Phanerozoic Eon began about 540 million years ago, marked by the Cambrian Explosion, when complex, hard-shelled organisms first appeared. While climate simulations extend back to 540 million years, the study focuses on the past 485 million years due to limited geological temperature data before that period. "It’s hard to find rocks that are that old and have temperature indicators preserved in them—even at 485 million years ago, we don’t have that many," explained Jessica Tierney, a paleoclimatologist and professor at the University of Arizona.
The temperature curve was constructed using a method known as data assimilation, which combines geological data with climate model simulations. “This method was originally developed for weather forecasting,” said Emily Judd, lead author of the paper. “Instead of using it to forecast future weather, here we’re using it to hindcast ancient climates.”
The GMST curve reveals that Earth’s temperature ranged from 52° to 97°F (11° to 36°C) over the last 485 million years, with periods of extreme heat linked to elevated CO2 levels in the atmosphere. "This research illustrates clearly that carbon dioxide is the dominant control on global temperatures across geological time," Tierney noted. "When CO2 is low, the temperature is cold; when CO2 is high, the temperature is warm."
Presently, Earth’s average temperature of 59°F (15°C) is cooler than much of the Phanerozoic, yet human-caused climate change is warming the planet at an unprecedented rate. The researchers highlight that this rapid warming is causing sea levels to rise and putting ecosystems at risk. “Our entire species evolved in an 'ice house' climate, which doesn’t reflect most of geological history,” Tierney said. “We are changing the climate into a place that is really out of context for humans. The planet has been and can be warmer—but humans and animals can't adapt that fast.”
The collaboration between Tierney and Smithsonian researchers began in 2018 with the goal of providing museum visitors with a temperature curve charting Earth’s global temperature over the Phanerozoic. The team collected over 150,000 temperature estimates from fossilized shells and organic matter, combined with 850 climate model simulations from the University of Bristol. This resulted in the most accurate reconstruction of Earth’s temperature over the past 485 million years.
PhanDA, the name given to this temperature reconstruction, indicates that Earth’s climate has varied more dynamically than previously thought. The relationship between GMST and CO2 was found to be strikingly consistent, suggesting that CO2 is the primary driver of climate during the Phanerozoic. This consistency also led researchers to discover that Earth’s climate sensitivity—how much the climate warms with a doubling of CO2—has remained relatively stable at around 8°C.
Co-authors of the study include Brian Huber of the Smithsonian, Daniel Lunt and Paul Valdes of the University of Bristol, and Isabel Montañez from the University of California, Davis. The research was supported by the Smithsonian, the Heising-Simons Foundation, and the University of Arizona’s Thomas R. Brown Distinguished Chair in Integrative Science.
