The Phanerozoic Eon, which began 540 million years ago, is known for the Cambrian Explosion - the emergence of complex organisms with hard shells. However, due to limited geological data, the study focuses on the last 485 million years, as earlier records are scarce. "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.
To develop the temperature curve, the research team employed a method called data assimilation, which merges geological data with climate models. "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."
The findings provide essential context for understanding modern climate change, as historical temperature fluctuations offer insights into future warming trends. "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 Scott Wing, a co-author on the paper and a 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 temperature curve revealed that global temperatures over the Phanerozoic varied between 52 to 97 degrees Fahrenheit, with periods of extreme heat often linked to elevated atmospheric carbon dioxide levels. "This research illustrates clearly that carbon dioxide is the dominant control on global temperatures across geological time," said Tierney. "When CO2 is low, the temperature is cold; when CO2 is high, the temperature is warm."
Although Earth's current global temperature of 59 degrees Fahrenheit is cooler than most of the Phanerozoic, the researchers warn that human-induced climate change is causing the planet to warm at an unprecedented rate. This rapid warming poses significant risks to ecosystems and species, potentially leading to a rise in sea levels and more extreme climate events. Historically, similar rapid climate changes have led to mass extinctions.
The study also highlights that humans have primarily lived within a narrow range of global temperatures, compared to the wide fluctuations observed throughout Earth's history. "Our entire species evolved to an 'ice house' climate, which doesn't reflect most of geological history," Tierney said. "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."
The collaboration between researchers from the University of Arizona and the Smithsonian began in 2018, aiming to create a detailed temperature chart spanning the Phanerozoic Eon. More than 150,000 ancient temperature estimates were collected from fossilized shells and other organic materials, and researchers at the University of Bristol produced over 850 model simulations based on past continental positions and atmospheric compositions. These efforts culminated in the most accurate curve of Earth's temperature changes over the past 485 million years.
The study also addressed climate sensitivity - the measure of how much the climate warms when atmospheric carbon dioxide levels double. "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 said.
In addition to Judd, Tierney, Wing, and colleagues from the Smithsonian, co-authors include Daniel Lunt and Paul Valdes from the University of Bristol, and Isabel Montanez from the University of California, Davis.
The research was supported by Roland and Debra Sauermann through the Smithsonian; the Heising-Simons Foundation; and the University of Arizona's Thomas R. Brown Distinguished Chair in Integrative Science, with additional support from the UK's Natural Environment Research Council.
Image Caption: Fig. 3. Climate states across the Phanerozoic. (A) Time series of Earth's climate state, with each stage assigned a state in accordance with the quantiles defined in panel (B) and the height of each rectangle scaled by the median GMST of that stage. (B) Kernel density plot of the distribution of median GMST values, with each the five quantiles defined (the range of which is indicated in the parentheses below each state's label). (C) Pie chart showing the proportion of time spent in each climate state across the Phanerozoic. (D) The latitudinal surface air temperature gradient associated with each of the climate states (colored bands showing the 16th to 84th percentiles, colored lines showing the median value)
Research Report:A 485-million-year history of Earth's surface temperature
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