Over nearly half a billion years, Earth's global temperature has changed drastically, driven by carbon dioxide

The start of the Phanerozoic Eon 540 million years ago is marked by the Cambrian Explosion, a point in time when complex, hard-shelled organisms first appeared in the fossil record. Although researchers can create simulations all the way back to 540 million years ago, the temperature curve in the study focuses on the last 485 million years since there is limited geological data of temperature before then.

"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. We were limited with how far back we could go," said study co-author Jessica Tierney, a paleoclimatologist at the University of Arizona.

The researchers created the temperature curve using an approach called data assimilation. This allowed them to combine data from the geologic record and climate models to create a more cohesive understanding of ancient climates.

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

Refining scientists' understanding of how Earth's temperature has fluctuated over time provides crucial context for understanding modern climate change.

"If you're studying the last couple of million years, you won't find anything that looks like what we expect in 2100 or 2500," said Scott Wing, a co-author on the paper and a curator of paleobotany at the Smithsonian National Museum of Natural History. "You need to go back even further to periods when the Earth was really warm, because that's the only way we're going to get a better understanding of how the climate might change in the future."

The new curve reveals that temperature varied more greatly during the past 485 million years than previously thought. Over the eon, the global temperature spanned 52 to 97 degrees Fahrenheit. Periods of extreme heat were most often linked to elevated levels of the greenhouse gas carbon dioxide in the atmosphere.

"This research illustrates clearly that carbon dioxide is the dominant control on global temperatures across geological time," said Tierney. "When CO2 is low, the temperature is cold; when CO2 is high, the temperature is warm."

The findings also reveal that the Earth's current global temperature of 59 degrees Fahrenheit is cooler than Earth has been over much of the Phanerozoic. But greenhouse gas emissions from human-caused climate change are currently warming the planet at a much faster rate than even the fastest warming events of the Phanerozoic, the reseaerchers say. The speed of warming puts species and ecosystems around the world at risk and is causing a rapid rise in sea level. Some other episodes of rapid climate change during the Phanerozoic have sparked mass extinctions.

Rapidly moving toward a warmer climate could spell danger for humans who have mostly lived in a 10 degree Fahrenheit range for the global temperatures, compared to the 45 degree span of temperatures over the last 485 million years, the researchers say.

"Our entire species evolved to 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 researchers at the Smithsonian began in 2018. The team wanted to provide museum visitors with a curve that charted Earth's global temperature across the Phanerozoic, which began around 540 million years ago and continues into the present day.

The team collected more than 150,000 estimates of ancient temperature calculated from five different chemical indicators for temperature that are preserved in fossilized shells and other types of ancient organic matter. Their colleagues at the University of Bristol created more than 850 model simulations of what Earth's climate could have looked like at different periods of the distant past based on continental position and atmospheric composition. The researchers then combined these two lines of evidence to create the most accurate curve of how Earth's temperature has varied over the past 485 million years.

Another finding from the study pertains to climate sensitivity, a metric of how much the climate warms for the doubling of carbon dioxide.

"We found that carbon dioxide and temperature are not only really closely related, but related in the same way across 485 million years. We don't see that the climate is more sensitive when it's hot or cold," Tierney said.

In addition to Judd, Tierney, Huber and Wing, Daniel Lunt and Paul Valdes of the University of Bristol and Isabel Montañez of the University of California, Davis are coauthors on the study.

The research was supported by Roland and Debra Sauermann through the Smithsonian; the Heising-Simons Foundation and the University of Arizona's Thomas R. Brown Distinguished Chair in Integrative Science through Tierney; and the United Kingdom's Natural Environment Research Council.