A groundbreaking study of Earth’s climate over the past 485 million years has revealed an extraordinary history of drastic temperature fluctuations, with hotter temperatures than scientists had previously recognized. This timeline offers an important perspective on the vast climate changes Earth has already endured and provides a stark warning about the unprecedented rate of warming driven by human activity today.
Published in the journal *Science* on Thursday, this comprehensive study is the most thorough reconstruction of Earth’s past temperatures ever produced. According to the researchers, the findings were made possible by combining over 150,000 fossil records with sophisticated climate models. This data highlights the close link between carbon dioxide levels and global temperatures, showing that for much of Earth’s history, the planet has been in a significantly warmer state.
At its most extreme, the Earth’s average temperature is thought to have reached 96.8 degrees Fahrenheit (36 degrees Celsius), a figure much higher than the 58.96 F (14.98 C) recorded in 2023. This new understanding of Earth’s climate history raises fresh concerns about modern climate change, said Emily Judd, a University of Arizona researcher and lead author of the study.
Judd explained that the timeline illustrates how rapid temperature changes were linked to many of Earth’s worst extinction events, including a mass extinction that wiped out nearly 90% of species, as well as the asteroid strike that wiped out the dinosaurs. “We know that these catastrophic events shift the landscape of what life looks like,” Judd said, adding that such drastic temperature changes have devastating consequences for life on Earth.
One of the most significant extinction events occurred 250 million years ago, when volcanic eruptions spewed carbon dioxide and other gases into the atmosphere, causing the Earth’s temperature to rise by more than 18 degrees Fahrenheit (10 degrees Celsius) over approximately 50,000 years. This event wiped out 90% of species. “When the environment warms that fast, animals and plants can’t keep pace with it,” Judd explained.
Yet, no point in the last 485 million years has seen Earth’s climate shift as rapidly as it is now. “In the same way as a massive asteroid hitting the Earth, what we’re doing now is unprecedented,” she emphasized.
485 Million Years of Climate Volatility
The study spans nearly the entire Phanerozoic eon, a period beginning with the appearance of multicellular organisms and continuing to the present day. This era includes significant climate swings, from hot to cold and back again, with some periods witnessing rapid spikes in temperature. Jess Tierney, a co-author of the study and a climate scientist at the University of Arizona, said that past models have generally depicted more gradual shifts in climate, but the new research reveals more abrupt and extreme changes.
Like previous research, the new timeline highlights the key role of carbon dioxide in driving temperature changes. “Carbon dioxide is really that master dial,” Tierney said, adding that the findings underscore the importance of addressing modern emissions from fossil fuels.
At the beginning of the timeline, 485 million years ago, Earth was in a “hothouse” state with no polar ice caps and average temperatures exceeding 86 F (30 C). Over time, atmospheric carbon dioxide levels dropped, and temperatures gradually declined. Around 444 million years ago, the planet experienced a “coldhouse” state, marked by ice sheets at the poles and global temperatures plummeting by more than 18 F (10 C). This abrupt cooling triggered Earth’s first major extinction, with roughly 85% of marine species disappearing as sea levels fell and ocean chemistry changed.
A more drastic shift took place 251 million years ago, near the end of the Permian period. Volcanic eruptions unleashed huge quantities of carbon dioxide into the atmosphere, leading to a sudden rise in temperatures by about 18 F (10 C). The oceans became dangerously hot and lost oxygen, causing marine ecosystems to collapse. “We know it to be the worst extinction in the Phanerozoic,” Tierney noted. “By analogy, we should be worried about human warming because it’s so fast.”
A Sobering Reminder for Today’s Climate Crisis
The study’s findings also make clear that the relatively mild climate humans have experienced throughout history is an exception rather than the norm. For most of the Phanerozoic, Earth’s average temperatures exceeded 71.6 F (22 C), with little or no ice present at the poles. Coldhouse conditions like the present day have occurred only about 13% of the time during this period.
This is a sobering revelation, Judd noted. Although life has survived in much hotter climates than what humans are currently creating, humans evolved during one of the coldest periods in Earth’s history, with global temperatures averaging around 51.8 F (11 C). As a result, many aspects of modern life are tied to the relatively cool climate we’ve known. Without drastic action to reduce greenhouse gas emissions, scientists predict that global temperatures could rise to 62.6 F (17 C) by the end of this century — temperatures not seen since the Miocene epoch, over 5 million years ago.
“We built our civilization around those geologic landscapes of an icehouse,” Judd explained. “So even though climate has been warmer, humans haven’t lived in a warmer climate, and there are a lot of consequences that humans face during this time.”
Reconstructing Earth’s Climate: A Decade of Research
The researchers’ work began almost ten years ago when curators at the Smithsonian were designing a new fossil exhibit for the National Museum of Natural History. The exhibit aimed to connect ancient climate shifts with those happening today, but the scientists quickly realized that a comprehensive timeline of Earth’s temperatures during the Phanerozoic didn’t exist.
Scott Wing, a co-author of the study and a Smithsonian curator, explained that the team wanted to create a more scientifically rigorous temperature record for this period. The researchers began by compiling a database of climate proxies — fossil evidence that indicates past temperatures. For instance, the chemical makeup of extinct creatures’ teeth and ancient algae fats can provide clues about ocean temperatures at different times.
Despite collecting over 150,000 data points, the team still faced challenges. Most of the evidence came from marine environments, which cover only 70% of Earth’s surface, and the data offered only snapshots of temperatures at specific points in time. Even with this vast dataset, Judd said the process was akin to “trying to assemble a jigsaw puzzle with only 1 percent of the pieces.”
To fill in the gaps, the team used climate models and a technique called data assimilation, which merges real-world evidence with simulations to generate more accurate predictions. “It’s a way of mathematically integrating those handful of puzzle pieces with those possible pictures and finding out, what’s the picture those pieces belong to?” Judd said.
A Warning for the Future
While the new temperature timeline offers unprecedented insights into Earth’s past, it also raises questions. If average global temperatures exceeded 35 degrees Celsius during some periods, certain regions could have been much hotter. Wing pointed out that even the most heat-tolerant modern species might struggle to survive in such conditions. “It’s an indication of all the things we don’t know about how greenhouse climates work,” he said.
Michael Mann, a prominent climate scientist at the University of Pennsylvania, praised the study for its ambition but expressed caution about some of its findings. “I am skeptical about the specific, quantitative conclusions,” Mann said, suggesting that the study might overestimate future warming.
Wing acknowledged that more research is needed. The team hopes to refine their timeline by adding more land-based data and to use their findings to improve models of future climate change. For now, Judd hopes the research will serve as a wake-up call, reminding humanity of the urgent need to act before climate change wreaks havoc on ecosystems and societies.
“My concern is what human life looks like. What it means to survive,” Judd concluded.