When Dr. Dongyu Zheng examines ancient rocks, he is reading a story written over hundreds of millions of years. Now, that story has revealed something surprising: Earth's past warm periods were not as scorching as scientists once believed.
Researchers at the University of Leeds have found that ancient Earth was probably much cooler than previous studies suggested. Their study, published in the journal Nature Communications, shows that during the Phanerozoic period—the last 540 million years of Earth's history—the planet's warmest eras reached only about 10 degrees Celsius above preindustrial levels. Earlier estimates had suggested temperatures could have climbed 20 or even 30 degrees higher than today.
The team, led by Dr. Zheng of Chengdu University of Technology, used a different method than previous researchers. Most past studies relied on oxygen isotopes found in ocean sediments. Instead, Dr. Zheng's team used the Chemical Index of Alteration (CIA)—a measurement that looks at how rock minerals change when exposed to different temperatures over time. This gave them a clearer picture of ancient heat.
"The evolution and flourishing of life were not sporadic accidents, but were closely linked to Earth's ability to regulate its climate over geological time," Dr. Zheng said.
Natural processes like rock weathering have helped keep Earth's temperature stable for millions of years, the researchers found. These feedback systems act like a thermostat, slowly pulling heat back into balance. This regulation allowed life to survive and evolve through periods that were warm but never extreme.
But there is a warning hidden in these ancient stones. Professor Benjamin Mills, a co-author of the study and professor of Earth System Evolution at Leeds, said the findings suggest that burning all fossil fuel reserves could warm the planet by 10 degrees Celsius. That would push Earth beyond anything it has experienced in the past 540 million years.
"We shouldn't be complacent when viewing ancient hot climates that supported diverse ecosystems," Professor Mills said. "Earth's natural regulation systems are slow, and humans must perform our own climate regulation to keep the planet in a habitable range."
The research also found that Earth's long-term sensitivity to carbon dioxide may be lower than some recent models predicted. While this might sound like good news, the team emphasizes that human-caused warming is happening far faster than any natural climate shift in Earth's history. Ancient hot periods developed gradually over millions of years. Humans have already warmed the planet by about 1.2 degrees Celsius in just over a century.
The team combined their rock data with climate simulations to reconstruct past global temperatures, building a database of tens of thousands of measurements. This new approach offers scientists a better tool for understanding both Earth's past and its possible future.