A new study conducted jointly by the University of Arizona and the Smithsonian Institution provides the most detailed view yet of how Earth’s surface temperatures have changed over the past 485 million years.
Published in Science magazine study It shows a curve of the Earth’s average surface temperature, indicating that the Earth’s temperature fluctuated more than previously thought throughout much of the Phanerozoic Era, a geological period during which life diversified, the land became populated, and there were multiple mass extinctions. The curve also confirms that the Earth’s temperature is strongly correlated with the amount of carbon dioxide in the atmosphere.
The Phanerozoic Era, 540 million years ago, began with the Cambrian explosion, the time when complex, hard-shelled organisms first appeared in the fossil record. Although the researchers were able to create simulations that went back 540 million years, geological data on temperatures before that is limited, so the temperature curves in their study focus on the past 485 million years.
“It’s hard to find rocks that old that preserve temperature indicators — even 485 million years old, there aren’t that many of them — so there’s a limit to how far back we can go,” said Jessica Tierney, a paleoclimatologist and professor of geological sciences at the University of Arizona and co-author of the study.
The researchers created the temperature curves using a technique called data assimilation, which combines data from the geological record and climate models to gain a more consistent understanding of ancient climates.
Jessica Tierney, a professor in the Department of Geosciences at the University of Arizona, studies past climate changes to understand how the Earth system works and what the future holds.
“This method was originally developed for weather forecasting,” says lead author Emily Judd, a former postdoctoral researcher at the Smithsonian’s National Museum of Natural History and the University of Alberta. “Instead of using it to predict future weather, we’re using it here to infer ancient climates.”
Improving scientists’ understanding of how Earth’s temperatures have fluctuated over time provides important context for understanding modern climate change.
“If we look back millions of years, we’re not going to find anything that we would expect in 2100 or 2500,” said study co-author Scott Wing, curator of paleobotany at the Smithsonian’s National Museum of Natural History. “We need to go back further, to times when the Earth was really warm, because that’s the only way we’re going to get a better understanding of how the climate will change in the future.”
The new curves show that temperatures have fluctuated more over the past 485 million years than previously thought. During that time, global temperatures have ranged from 52 to 97 degrees Fahrenheit. Periods of extreme heat were most often associated with rising concentrations of carbon dioxide, a greenhouse gas, in the atmosphere.
“This study clearly shows that carbon dioxide controls global temperatures throughout geological time,” Tierney said. “Less carbon dioxide means cooler temperatures, and more carbon dioxide means warmer temperatures.”
The study also found that Earth’s current global temperature is 59 degrees Fahrenheit, lower than it was during most of the Phanerozoic Era. However, due to greenhouse gas emissions caused by human-induced climate change, the Earth is currently warming at a much faster rate than the most rapid warming events that occurred during the Phanerozoic Era, the researchers said. This rate of warming is endangering species and ecosystems around the world and causing rapid rises in sea levels. Rapid climate changes during the Phanerozoic Era have also led to mass extinctions.
Researchers say a rapidly warming climate could mean dangers for humans, who currently live within a 10 degree Fahrenheit range, compared with a 45 degree Fahrenheit temperature fluctuation over the past 485 million years.
“Humanity as a whole has evolved for ‘glacial’ climates, but this does not reflect most of geological history,” Tierney said. “We are changing the climate to a place that was never anticipated for humans. The Earth has warmed and can continue to warm, but humans and animals cannot adapt that quickly.”
Tierney and Smithsonian researchers began collaborating in 2018. The team wanted to provide museum visitors with a curve showing global temperatures during the Phanerozoic Era, which began about 540 million years ago and continues to the present.
The team collected more than 150,000 estimates of ancient temperatures calculated from five different chemical indicators of temperature preserved in fossilized shells and other ancient organic matter. Their colleagues at the University of Bristol created more than 850 model simulations that show what Earth’s climate was like at different times in the distant past, based on the location of the continents and the composition of the atmosphere. The researchers then combined these two pieces of evidence to create the most accurate curve yet showing how Earth’s temperature has changed over the past 485 million years.
Another finding of the study concerns climate sensitivity, a measure of how much a doubling of carbon dioxide would warm the climate.
“We found that not only are carbon dioxide and temperature really closely related, but that they’ve been related in the same way for 485 million years – there’s no sensitivity to when the climate is hotter or colder,” Tierney said.
In addition to Judd, Tierney and Wing, co-authors on the study include Bryan Huber of the Smithsonian Institution, Daniel Lunt and Paul Valdes of the University of Bristol, and Isabel Montañez of the University of California, Davis.
The research was supported by Roland and Debra Sauermann of the Smithsonian Institution, Tierney’s Heising Simons Foundation and the Thomas R. Brown Distinguished Professor of Integrated Science at the University of Arizona, and the Natural Environment Research Council in the UK.
