A new study led by the University of Arizona and the Smithsonian provides the most detailed look yet at how Earth's surface temperature has changed over the past 485 million years.

The study, published in the journal Science, presents a curve of global mean surface temperature that shows the Earth's temperature has varied more than previously thought over much of the Phanerozoic Era, a period of geological time during which life diversified, land was colonised and several mass extinctions occurred. The curve also confirms that the Earth's temperature is strongly correlated with the amount of carbon dioxide in the atmosphere.

The start of the Phanerozoic Eon 540 million years ago is marked by the Cambrian Explosion, a time when complex, hard-shelled organisms first appear in the fossil record. Although researchers can create simulations that look back 540 million years, the temperature curve in the study focuses on the last 485 million years because there is little geological data on temperatures 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 in how far back we could go," said study co-author Jessica Tierney, a paleoclimatologist and professor of geosciences 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 geological record and climate models to create a more coherent 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 predict future weather, here we're using it to hindcast ancient climates."

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

"If you look at the last few million years, you're not going to find anything that looks like what we expect in 2100 or 2500," said Scott Wing, co-author of the paper and 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 shows that temperature has varied more over the past 485 million years than previously thought. Over the eon, the global temperature ranged from 52 to 97 degrees Fahrenheit. Periods of extreme heat were most often associated with elevated levels of the greenhouse gas carbon dioxide in the atmosphere.

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

The findings also show that the Earth's current global temperature of 59 degrees Fahrenheit is cooler than it has been for much of the Phanerozoic. But greenhouse gas emissions from human-induced climate change are currently warming the planet at a much faster rate than even the fastest warming events of the Phanerozoic, the researchers say. This rate of warming is endangering species and ecosystems around the world and causing rapid sea-level rise. Some other episodes of rapid climate change in the Phanerozoic have led to mass extinctions.

The rapid transition to a warmer climate could spell danger for humans, who have mostly lived in a 10 degree Fahrenheit range for global temperatures, compared to a 45 degree range for temperatures over the past 485 million years, the researchers say.

"Our entire species has evolved to an 'icehouse' climate that doesn't reflect most of geological history," Tierney said. "We are changing the climate to a place that is really out of context for humans. The planet has been warmer and can get warmer - but humans and animals can't adapt that quickly.

The collaboration between Tierney and researchers at the Smithsonian began in 2018. The team wanted to provide museum visitors with a curve showing Earth's global temperature throughout the Phanerozoic Era, which began about 540 million years ago and continues to the present day.

The team collected more than 150,000 estimates of ancient temperatures, calculated from five different chemical indicators of temperature preserved in fossilised shells and other types of ancient organic matter. Their colleagues at the University of Bristol created more than 850 model simulations of what the Earth's climate might have been like at different times in 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 the Earth's temperature has changed over the past 485 million years.

Another finding from the study relates to climate sensitivity, a measure of how much the climate warms for every doubling of carbon dioxide.

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