The urgent need to address the climate crisis was thrown into sharp relief yet again this week by a mountain of research that established an “unequivocal” link between human activity and warming global temperatures, according to a major new report from the Intergovernmental Panel on Climate Change.
Now, a pair of scientists at the Massachusetts Institute of Technology (MIT) reveal that modest warming events in Earth’s deep past often spiral into volatile climate extremes, a finding that has been written into the fossils of marine organisms over the Cenozoic period, which dates back 66 million years to the present.
This long view of our planet’s climate swings suggests that “as anthropogenic warming continues, Earth’s climate may become more susceptible to extreme warming events on time scales of tens of thousands of years,” according to a study published on Wednesday in Science Advances.
“Abrupt global warming events of the geologic past are of interest because they reveal fundamental aspects of how the Earth system works, and because they provide an observational window into the long-term consequences of anthropogenic climate change,” said Constantin Arnscheidt, a graduate student at MIT’s Department of Earth, Atmospheric and Planetary Sciences who led the study, in an email.
“In past studies the focus has been on specific large warming events,” he added. “Here, we wanted to understand the more general behavior throughout the Cenozoic (the past 66 million years), and so for the first time we considered all of the fluctuations involved rather than picking out the big ones.”
To accomplish this aim, Arnscheidt and co-author Daniel Rothman, a professor of geophysics at MIT and co-director of its Lorenz Center, relied on single-celled organisms called benthic foraminifera, which have been living and dying in Earth’s oceans for hundreds of millions of years.
These simple lifeforms grow hard shells that preserve information about temperatures and conditions at the time their bodies were deposited in sediments. As a result, their fossils offer an extremely useful window into the deep past, and are often used as a climate proxy in reconstructions of Earth’s paleoclimate.
Arnscheidt and Rothman assessed foraminifera records from around the world that date back to the cataclysmic asteroid impact that killed off the dinosaurs and many other species some 66 million years ago. In contrast to other studies, the team applied a statistical analysis of this entire Cenozoic period as opposed to focusing on major climate disruptions, such as the dramatic Eocene warming event that occurred about 55 million years ago.
This approach exposed “an intrinsic asymmetry that favors ‘hyperthermal-like’ extreme events of abrupt global warming,” according to the study. In other words, Earth’s Cenozoic climate has shown a clear bias toward warming events (hyperthermals) relative to cooling events, meaning that warming events were both more frequent and more extreme than cooling events.
“The fact that specific intervals in Earth history, such as the Eocene epoch, exhibit this bias is not surprising: this has been implicitly understood for a long time,” Arnscheidt said. “However, when we quantified the evolution of this asymmetry throughout the past 66 million years, we found that it displayed remarkable consistency over most of this period.”
“One simple way to explain the observed warming bias is to hypothesize that temperature fluctuations (on timescales of thousands to tens of thousands of years) themselves increase with temperature: this is called ‘multiplicative noise,’” he added. “The statistics of the observed fluctuations turn out to be mathematically consistent with the multiplicative noise hypothesis.”
Even modest global temperature increases during the Cenozoic seemed to lead to more stochastic and self-amplifying climate fluctuations than cooling events of similar magnitude. No doubt there are many complex mechanisms underlying these processes, but Arnscheidt and Rothman identified a few likely culprits, including biological and chemical processes that speed up at higher temperatures, as well as Earth’s orbit around the Sun.
“Many past global warming/hyperthermal events appear to coincide with changes in the eccentricity of Earth's orbit, but the mechanisms remain debated,” explained Arnscheidt. “The multiplicative theory provides a natural reason this could occur. If temperature fluctuations (on timescales of thousands to tens of thousands of years) indeed increase as it gets warmer, small changes in Earth's surface temperature due to orbital changes can, on average, generate abrupt warming events consistent with geologic observations.”
Interestingly, the team found that the Cenozoic’s long-term bias toward warming events disappeared about five million years ago, around the same time that ice cover crept down from the Arctic across much of the Northern Hemisphere. It’s possible that this recent period of glaciation helped to stabilize the global climate and reduce the incidence of extreme temperature fluctuations.
But Arnscheidt and Rothman warn that human-driven climate change may now be injecting all of that multiplicative noise and volatility back into the climate system. In light of the new IPCC report, which concludes that we are essentially locked into a global temperature spike of at least 1.5°C above pre-industrial averages, it’s more important than ever to heed the lessons etched into the paleoclimatological record of our planet.
“I think these results emphasize that Earth's long-term evolution is governed by complex, potentially amplifying mechanisms that we do not yet fully understand,” Arnscheidt said. “As humans continue to increase Earth's surface temperature, we will likely interact with these mechanisms, potentially at the peril of current and future generations.”