How Earth’s temperature has changed over the past 500 million years

A recent study presents a new curve of global mean surface temperature, revealing that Earth’s temperature has varied more than previously thought over much of the Phanerozoic Eon — a period marked by the diversification of life, colonization of land, and multiple mass extinctions. 

The curve also confirms that Earth’s temperature is strongly correlated with the amount of carbon dioxide in the atmosphere.

Studying the temperature of Earth’s past

The Phanerozoic Eon began approximately 540 million years ago with the Cambrian Explosion, a time when complex, hard-shelled organisms first appeared in the fossil record. 

Although researchers can create simulations dating back to this era, the temperature curve in the study focuses on the last 485 million years due to limited geological temperature data from earlier periods.

“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.

Developing the temperature curve

The researchers developed the temperature curve using an approach called data assimilation. This method 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 University of Arizona.

“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.

Looking way back to predict the future

Study co-author Scott Wing is a co-author on the paper and a curator of paleobotany at the Smithsonian National Museum of Natural History. 

“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 Wing.

“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.”

CO2 is the biggest driver of temperature 

The new curve reveals that temperatures varied more significantly during the past 485 million years than previously thought. 

Over this period, global temperatures ranged from 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,” Tierney said. “When CO₂ is low, the temperature is cold; when CO₂ is high, the temperature is warm.”

Rapidly warming temperatures on Earth

The findings also show that Earth’s current global temperature of 59 degrees Fahrenheit is cooler than it has been over much of the Phanerozoic. 

However, 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 that eon, the researchers say. 

This rapid warming poses risks to species and ecosystems worldwide and is causing a swift rise in sea levels. Some other episodes of rapid climate change during the Phanerozoic have sparked mass extinctions.

Humans evolved in a colder climate

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

“Our entire species evolved to an ‘ice house’ climate, which doesn’t reflect most of geological history,” noted Tierney.

“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.”

Earth’s global temperature across the Phanerozoic 

The collaboration between Tierney and researchers at the Smithsonian began in 2018.

The team aimed 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 researchers collected more than 150,000 estimates of ancient temperatures calculated from five different chemical indicators preserved in fossilized shells and other types of ancient organic matter. 

Their colleagues at the University of Bristol created over 850 model simulations of what Earth’s climate could have looked like at different periods in the distant past, based on continental positions and atmospheric composition. 

Accurate curve of Earth’s temperature variation

By combining these two lines of evidence, the team created the most accurate curve of how Earth’s temperature has varied over the past 485 million years.

Another significant finding from the study pertains to climate sensitivity, a metric of how much the climate warms with 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 concluded.

The study is published in the journal Science.

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