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A little bit of global warming may go a long way. A recent mathematical analysis of the climate of the Cenozoic—our current geologic era, starting at the demise of the dinosaurs 66 million years ago—says that natural processes may amplify small amounts of warming, turning them into “hyperthermal” events that can last for thousands of years or longer. This finding suggests that human-induced climate change could make our planet susceptible to more extreme warming events in the future.
Scientists have studied several major Cenozoic warming events in detail, including the Paleocene-Eocene Thermal Maximum, in which global temperatures jumped by more than 5°C and remained elevated for tens of thousands of years. Such events can help scientists understand how the planet responds to climate changes and predict how it might react to current human-caused changes.
Constantin Arnscheidt and Daniel Rothman of the Lorenz Center at the Massachusetts Institute of Technology, however, decided to examine the climate–carbon cycle history of the entire period. Their study was published in Science Advances.
“We wanted to understand the more general behavior of sub-million-year climate–carbon cycle fluctuations throughout the Cenozoic,” said Arnscheidt, a graduate student and the study’s lead author. “And so, for the first time, we considered all of the fluctuations involved rather than picking out the big ones.”
The researchers used a database of benthic foraminifera found in deep-ocean sediments. The single-celled organisms are protected by shells of calcium carbonate. Changes in surface temperature, surface inorganic carbon, ocean chemistry, and other climate factors alter the carbon and oxygen isotope ratios in the shells, making it possible for scientists to use them as climate proxies.
Arnscheidt and Rothman used statistical methods to analyze the database. “Climate fluctuations on a wide range of timescales are the result of many complex processes that are impossible to model exactly,” said Arnscheidt. “Stochastic models, which have long been employed to understand shorter-term climate variability, capture essential aspects of this behavior by including random-noise terms.”
Their results showed an imbalance between global warming and global cooling, with a strong bias toward extreme warming events. There were more warming than cooling events, they produced a greater swing in temperatures, and they lasted longer. This trend continued until the start of the Pliocene, about 5.3 million years ago, when the global climate cooled considerably and ice sheets began covering North America.
The bias in the statistics was consistent with the principle of “multiplicative noise,” in which the extent of changes in a system depends on its state. In this case, if temperature variations over periods of thousands or tens of thousands of years increase as the climate gets warmer, “this would result in a warming bias precisely like the one observed,” Arnscheidt said.
A warming bias would suggest that a little bit of global warming may trigger natural biological or geochemical processes (which the researchers say still need to be identified) that operate more efficiently under warmer conditions. These processes pump additional carbon and other warming compounds into the atmosphere and increase the temperature even more, leading to extreme and long-lasting warming events.
The initial impulse for warming events could come from changes in the eccentricity of Earth’s orbit, which varies over a period of about 100,000 years. Scientists have observed that some warming events appear to align with this cycle but haven’t been able to explain how the changing eccentricity could cause large climate swings. The new model suggests that although the initial change in climate caused by the cycle might be small, the multiplier effects could turn it into a major event.
Exploring Climate’s Operational Boundaries
“The paper does push us to explore much more Earth’s response to orbital forcing in the different climate states,” said Thomas Westerhold, director of the Center for Marine Environmental Sciences at the University of Bremen, Germany, who led the development of the foraminifera database but was not involved in this project. “The climate system seems to have operational boundaries that once they are passed, the system moves into a different state….We need to know where those boundaries are that once crossed, we cannot simply make undone.”
The study doesn’t say that multiplicative effects will boost the effects of anthropogenic climate change anytime soon, Arnscheidt noted. It does, however, suggest that if current warming continues, the climate could become more susceptible to extreme warming events like those seen in the geologic record.
“Fundamentally, this study highlights that there is much yet to be learned about the mechanisms governing Earth’s long-term climate evolution and that human climate forcing today may have far-reaching effects on the long-term future,” Arnscheidt said.
—Damond Benningfield (firstname.lastname@example.org), Science Writer
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Benningfield, D. (2021), Small climate changes could be magnified by natural processes, Eos, 102, https://doi.org/10.1029/2021EO163169. Published on 16 September 2021.
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