What the last 485 million years tell us about global climate change

For much of the last 485 million years, the global climate fluctuated wildly between extremely hot and extremely cold weather, findings that researchers said have major implications for today’s pattern of human-caused climate change.

Just as today’s rising temperatures are linked by most climate scientists to carbon dioxide emissions along with other greenhouse gases entering the atmosphere, the huge temperature variations occurring over the past 485 million years are directly correlated with changing carbon dioxide levels, the study concluded. Its lead author, Emily Judd, is a former University of Arizona postdoctoral researcher. Its other six co-authors include a current UA research professor, Jessica Tierney.

But the spikes in carbon dioxide that occurred during distant past periods were due mainly to natural forces in the form of volcanic eruptions, from actual volcanoes and from plate tectonics behavior, the researchers said. The behavior of plate tectonics is a geologic phenomenon in which large, moving plates underlying the oceans collide with, pull apart or slide past each other at their boundaries. The resulting movement causes magma, lighter than the surrounding rock, to rise to the surface, triggering volcanic eruptions.

Overall, the study’s findings indicate carbon dioxide has played a dominant role in controlling Earth’s temperature for nearly the last half-billion years, said lead author Judd.

“This finding affirms the finding of countless other studies that have come before ours demonstrating the important link between greenhouse gas concentrations and global temperatures,” said Judd, now a climate research analyst at the National Institute for Water and Atmospheric Research in Auckland, New Zealand. She is also a former postdoctoral researcher for the Smithsonian Institution.

The temperatures over the 485-million-year period have varied far more than researchers previously thought, with global mean surface temperatures ranging from 52 to 97 degrees Fahrenheit. The peak temperatures were far higher than those of today, in which annual global mean surface temperatures hit a modern-day record last year of nearly 59 degrees Fahrenheit.

The study analyzed temperatures over a period covering most of what’s known as the Phanerozoic Eon, which dates back about 539 million years. It covers a period of geologic time when life diversified, populated land and endured multiple mass extinctions.

“Evolution simply can’t keep up”

The hottest end of the temperature range found over the study period was not only far hotter than today’s now-elevated temperatures, it was also a lot hotter than those we’re expected to see under forecasts for global temperatures at the end of this century. In fact, the study’s findings show the current mean global temperature is cooler than the Earth has been over much of the 485-million-year period studied.

But greenhouse gas emissions of today, caused by the burning of fossil fuels, are currently warming the planet at a much faster rate than during any of the fastest-warming periods analyzed by the new study, the researchers said.

“In the same way as a massive asteroid hitting the Earth, what we’re doing now is unprecedented,” Judd told the Washington Post.

The current speed of warming puts species and ecosystems around the world at risk and is causing a rapid rise in sea level, many scientists have said. Some other episodes of rapid climate change earlier in the Phanerozoic sparked mass extinctions, the researchers on this study concluded.

“Humans, and the species we share the planet with, are adapted to a cold climate,” said UA Professor Jessica Tierney, a co-author of the new study. She specializes in researching past climate change. “Rapidly putting us all into a warmer climate is a dangerous thing to do.”

Overall, one of the biggest lessons that the geologic record has taught about climate change’s consequences is that the absolute global temperature is far less important than how quickly CO2 concentrations and temperatures change, Judd said.

“When CO2 and temperatures change slowly, organisms can keep pace with the environmental change, evolving, developing adaptations and/or migrating. However, when CO2 and temperatures change rapidly, as is happening today due to anthropogenic emissions, evolution can’t keep pace with the environmental change.” Anthropogenic emissions is a scientific term commonly used to refer to human-caused warming.

“We’ve seen this before at the end of the Permian (about 252 million years ago), due to widespread volcanism, and at the end of the Cretaceous (about 66 million years ago), due to an asteroid impact — when the climate and the environment change rapidly, evolution simply can’t keep up,” Judd said.

What is happening today is particularly catastrophic because organisms that exist today evolved to tolerate pre-industrial conditions, she said.

“Just like humans, they’re cold adapted. They (and we) are not equipped to handle the warmer conditions and the rate of change is too fast for evolution to keep pace,” she said.

It is also crucial to remember that Earth’s and “life’s” ability to endure dramatic temperature shifts does not guarantee the same for human societies, she said.

“Humans evolved to tolerate colder conditions and we have established our populations close to water sources and often near sea level. As we observe the Earth warming at a rapid pace within human time scales, we are faced with challenges such as more frequent and intense storms, more frequent and intense droughts (in some regions) and floods (in some regions), rising sea levels, and, ultimately, a reduction in habitable and arable land,” Judd said.

“Not the end of the world but …”

Generally, “if you’re studying the past 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 of the new study and curator of paleobotany for 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.”

And you have to go back 14 million years to reach a time when there was a comparable amount of CO2 in the earth’s atmosphere, said Brian Huber, a Smithsonian researcher who worked on the study.

The fastest example from the geological records of warming temperatures due to CO2 emissions was probably at least 10 times slower than today’s human-caused rates of warming increases, Wing said. That period was triggered by an event 56 million years ago known as the Paleocene-Eocene Thermal Maximum, he said.

The Encyclopedia Britannica termed this period “a short interval of maximum temperature lasting approximately 100,000 years during the late Paleocene and early Eocene epochs.” That interval was characterized by the highest global temperatures of what’s known as the Cenozoic Era, spanning the period 65 million years ago to the present.

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The study also found that “coldhouse” climates, periods when Earth had large, permanent icesheets and comparatively low CO2 concentrations, as it does today, have been relatively rare over the 485-million-year period, said Judd. They’ve accounted for just 13% of that time, Judd said.

But this finding comes with a few caveats, she said.

“First, our record shows the long-term evolution of climate across nearly a half-billion years. We reconstructed temperatures 85 times over this interval, which means that each temperature estimate averages about 5 million years. This means that shorter-term climate variability, like glacial-interglacial cycles, gets averaged out in our record,” Judd said.

Second, the finding that for most of the past 485 million years’ the climate was warmer than today’s “does not imply that we do not need to worry about ongoing anthropogenic warming,” Judd said, using a scientific term commonly employed to refer to human-caused warming.

“Unfortunately, it seems this finding has been twisted by climate deniers, skeptics, and delayers to dismiss concerns over the ongoing human-caused climate crisis. I want to stress that this viewpoint is reductive, harmful, and incorrect,” she told the Arizona Daily Star.

Still, this study, besides making clear that today’s human-caused changes to global temperatures are unprecedented, also shows that what’s going on now with climate “is not the end of the world,” said Wing.

“The planet and the life of this planet have survived major changes in temperature,” said Wing. “It’s more that we need to be concerned about ourselves and our culture and our societies and how they withstand the incredibly rapid change. We’re trying to help the scientific community to understand how the Earth changes and trying to provide context for the general public.”

While what’s going on now isn’t the end of the world, “it is a disaster of our own making that truly is unprecedented,” Wing said.

Fossils give clues to ancient temperatures

The study is not the first to look at how temperatures and climate have changed over millions of years. But it covers the longest period to be studied that relies on consistent research methods and data sets, Wing said.

The start of the Phanerozoic Eon 540 million years ago is marked by the Cambrian Explosion, a point in time when complex, hard-shelled organisms first appeared in the fossil record. The study presents a curve of global mean surface temperatures covering the 485 million years. Although researchers can create simulations that look back 540 million years, the temperature curve in this study focuses on the last 485 million years, since there is limited geological data of temperature before then.

“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 Tierney, a paleoclimatologist and professor of geosciences at UA.

The researchers created the temperature curve using an approach called data assimilation, a UA news release said. This 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,” Judd said. “Instead of using it to forecast future weather, here we’re using it to hindcast ancient climates.”

Some climate models for each geologic period studied vary in the amount of CO2 emissions they assume were occurring and in the temperature distributions, said Brian Huber, another Smithsonian researcher who worked on the study.

“What data assimilation does, it provides the best fit between data temperature estimates, and climate model output,” said Huber, the natural history museum’s curator of foraminifera, which are single-cell organisms that make up a shell that becomes part of a fossil record.

Collaboration between Tierney and researchers at the Smithsonian leading to this study began in 2018. The team wanted 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. But Wing and Huber were surprised to find a reliable temperature curve for this period did not yet exist. This is largely due to the fragmentary nature of the fossil record, the Smithsonian said.

Fossil specimens offer some clues about ancient temperatures — for example, the chemistry of fossilized shells offers insights into oceanic temperatures in the distant past — but these are only isolated snapshots of one region at a single time. This makes it difficult to decipher what ancient temperatures looked like on a global scale.

“It is like trying to visualize the picture of a 1,000-piece jigsaw puzzle, when you only have a handful of pieces,” Judd said.

The team collected more than 150,000 estimates of ancient temperatures, calculated from five different chemical indicators for temperature that are preserved in fossilized shells and other types of ancient organic matter. Their colleagues at the University of Bristol in Great Britain created more than 850 model simulations of what Earth’s climate could have looked like at different periods of the distant past.

The researchers then combined the two lines of evidence to create the most accurate curve of how Earth’s temperature has varied over the past 485 million years, the UA news release said.

Lessons in real time

The challenges of more frequent and intense storms, among the other results of rapid, current climate change, were brought home very recently for researcher Huber of the Smithsonian through the experience of his sister, he said.

She had to come up to the Washington, D.C. area where he lives from her home in the Black Mountain area of western North Carolina near Asheville, after Hurricane Helene dropped 21 inches of rain where she lived.

Many scientists have fingered climate change as one of the culprits for both Helene and for Hurricane Milton, which hit Florida Wednesday and Thursday.

They say warmer temperatures have warmed the Gulf of Mexico, ultimately fueling the rapid amplification of both events from smaller tropical storms to massive hurricanes. The World Weather Attribution, a network of scientists that calculates the role of climate change in extreme weather events using real world data and climate models, has already published analyses that found warming oceans had made both storms’ rainfall and winds more intense.

“No trees hit her house. But she will be without sewer and water for weeks to come,” Huber said of his sister. “The entire sewer and water treatment plant there washed away. She has no electricity. She needs to be online to do her work and there’s no way for her to get on the Internet.”