In a Landmark Study, Scientists Discover Just How Much Earth’s Temperature Has Changed Over Nearly 500 Million Years

We live in a rapidly warming world. Immense volumes of human-generated greenhouse gases are nudging Earth’s climate to a warmer and warmer state, further changing our planet as sea levels rise, living things shift how they live, and phenomena like wildfires become more common. Geologists and paleontologists have been searching the fossil record for analogous times to get a sense of what Earth’s future might hold as human-driven climate change unfolds, but they lacked a comprehensive view of how our planet’s temperatures have risen and subsided through time. Now, by combining data about prehistoric temperatures with climate models, researchers have produced a 485-million-year-long history of Earth’s continuously shifting climate.

The new research, published Thursday in Science, got its start in 2018 as part of planning the “Deep Time” exhibition at the Smithsonian’s National Museum of Natural History. Smithsonian paleontologists Scott Wing and Brian Huber wanted to include a temperature curve in the show’s displays that would help visitors understand how Earth’s climate has changed over the past 539 million years, a span of time known as the Phanerozoic to experts. Despite the fact that researchers have been studying prehistoric climates for decades, however, no one had produced a reliable, comprehensive view of Earth’s surface temperatures during the entire 500-million-year-plus span. The knowledge gap was an opportunity to produce just such a picture of our planet’s ever-changing temperature.

In 2018, Wing, Huber and colleagues convened a workshop of climate scientists to share what they knew of Earth’s climate history and how such a big-picture view might be accomplished. “The idea of creating a robust and reproducible curve gained momentum,” says University of Arizona paleoclimatologist Emily Judd, who joined the project in 2020 to generate Earth’s deep climate record using both geological data and climate models.

Researchers estimate prehistoric temperatures in a variety of different ways. Some scientists look at ice cores with air bubbles trapped inside—the chemical makeup of those air bubbles holds clues about what the ancient climate was like at the time. Oxygen isotopes in prehistoric rock layers and fossils can also be compared to each other to estimate whether Earth was warmer or cooler at a given time. Even the anatomy of fossilized leaves can act as temperature proxies. A leaf with smooth edges and a long “drip tip” to allow water to run off its surface, for example, is indicative of a warm, humid habitat, while plants from cooler climates often have more jagged edges and no drip tips.

Different time periods and places offer different kinds of evidence. An initial step was gathering all the existing evidence together. “Together with several dozen members of the paleoclimate community, we built a database of more than 150,000 estimates of ancient temperatures,” Judd says; that project was published in 2022.

What the team had assembled was essentially as close as experts can get to visiting ancient landscapes and taking out a thermometer. “But it can be hard to make sense of these data in context,” Judd says, “like trying to figure out the picture of a thousand-piece jigsaw puzzle when you only have a handful of pieces to start with.” Researchers needed to assemble that metaphorical puzzle to get a more accurate record of Earth’s surface temperatures through the Phanerozoic.

To reveal the big picture, project collaborators of the University of Bristol in England made more than 850 climate model simulations of conditions during the Phanerozoic. By bringing the temperature estimates and climate simulations together, the researchers created a broader view of when the ancient Earth was relatively warmer or cooler.

The results covered Earth’s temperature over the past 485 million years, the majority of the Phanerozoic, covering a span of time that saw the early profusion of animal life in the seas, the emergence of plants onto land and several mass extinctions. (Earlier parts of the Phanerozoic require more data before they can be included in the analysis.) Earth’s average surface temperature spanned 51.8 degrees Fahrenheit to 96.8 degrees Fahrenheit through the Phanerozoic, with the planet being in the warmer temperature range more often, overall, than in cool temperatures.

“The main lesson we derive from this curve is that the Earth’s surface temperature has been quite variable,” study co-author Wing notes. Our planet’s average surface temperature did not stick close to a central point but instead has swung into very hot and very cold periods through the past 485 million years.

“This will be a very useful record,” says paleoclimatologist Benjamin Mills of England’s University of Leeds, who was not involved in the new study. The research is not only an improvement on previous methods, he notes, but also underscores the need to understand how incredibly sensitive Earth’s climate is to carbon dioxide, and to dig into the history of that sensitivity.

Warm temperatures throughout the past 485 million years are closely tied to carbon dioxide. When carbon dioxide in the atmosphere increased, temperatures climbed. The connection is especially stark around some mass extinctions, when Earth’s climate rapidly shifted in response to events like massive volcanic outpourings that released incredible volumes of carbon dioxide into the atmosphere. “The stable global temperature and the CO2 level before the extinction is very important, as this is the environment the organisms of the time were best adapted for,” Mills notes, with rapid changes to carbon dioxide and temperature creating strenuous conditions that Earth’s biodiversity struggled to keep up with.

The same pattern holds true today. Even though we are in a relatively rare temperature state—a mean surface temperature of 59 degrees Fahrenheit—with polar ice and relatively cool surface temperatures compared to ancient extremes, Judd says, human-produced greenhouse gases are causing rapid warming against the cooler background. We are warming the planet even against the cooler starting conditions humans evolved under.

The rate at which Earth’s climate is warming is especially alarming. “The rate of climate change plays a pivotal role in ecological outcomes,” Judd says. Gradual warming that takes place over millions of years puts pressure on organisms to move, evolve or go extinct, but adaptation to warmer temperatures is possible. What humans are doing now is more akin to moments like the end-Permian mass extinction, when massive volcanic outpourings dumped incredible amounts of carbon dioxide into the atmosphere and triggered the worst mass extinction of all time.

Judd notes that being mindful of Earth’s temperature is essential for our own survival. “Earth’s ability to endure dramatic temperature shifts does not guarantee the same for human societies,” she says, as we evolved and thrived under colder conditions, often in places near sea level that are becoming increasingly inundated as a warmer climate causes sea levels to rise. Even if other forms of life can cope with the changes we’re making, Judd adds, “Earth’s resilience does not directly translate to our own ability to adapt and thrive in the face of human-caused climate change.”

The record Judd and colleagues have created takes us to the present moment. The next decade will be crucial in shaping Earth’s climate, the United Nations reminded us all last year, advising that nations the world over cut their reliance on fossil fuels and invest in new science that may draw carbon dioxide from the atmosphere. No single, best solution exists. Earth is at a point where any and every possible action to curtail human-caused warming is essential. What happens next is up to us.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.