It is increasingly clear that the prolonged drought conditions, record heat, sustained forest fires and frequent and more extreme storms experienced in recent years are a direct result of rising global temperatures brought on by the addition of man-made carbon dioxide in the atmosphere. And a new MIT study of extreme weather events in Earth’s ancient history suggests today’s planet may become more volatile as it continues to warm.
The study, published today in Scientists progress, examines the paleoclimatic record of the past 66 million years, during the Cenozoic era, which began shortly after the extinction of the dinosaurs. Scientists have found that during this period, fluctuations in Earth’s climate experienced a surprising “warming bias”. In other words, there have been many more warming events – periods of prolonged global warming, lasting thousands to tens of thousands of years – than cooling events. Additionally, warming events tended to be more extreme, with larger temperature changes, than cooling events.
The researchers say that a possible explanation for this warming bias may lie in a “multiplier effect,” whereby a modest degree of warming – for example from volcanoes releasing carbon dioxide into the atmosphere – naturally accelerates certain biological processes and chemicals that enhance these fluctuations, leading, on average, to even more warming.
Interestingly, the team observed that this warming bias disappeared around 5 million years ago, around the time when ice caps began to form in the northern hemisphere. It is not known what effect the ice has had on the Earth’s response to climate change. But as the current arctic ice recedes, the new study suggests that a multiplier effect may return, and the result may be further amplification of human-induced global warming.
“The northern hemisphere’s ice caps are shrinking and could potentially disappear as a long-term consequence of human actions,” says lead author of the study, Constantin Arnscheidt, a graduate student in the Department of Earth Sciences , the atmosphere and the planets of MIT. “Our research suggests that this may make Earth’s climate fundamentally more sensitive to extreme and long-term global warming events such as those seen in the geological past.”
The co-author of the Arnscheidt study is Daniel Rothman, professor of geophysics at MIT, and co-founder and co-director of the Lorenz Center at MIT.
A volatile surge
For their analysis, the team consulted large databases of sediments containing deep-water benthic foraminifers – single-celled organisms that have existed for hundreds of millions of years and whose hard shells are preserved in the sediments. The composition of these shells is affected by ocean temperatures as organisms develop; shells are therefore considered to be a reliable indicator of ancient Earth temperatures.
For decades, scientists have analyzed the composition of these seashells, collected around the world and dated at different times, to track the fluctuation of the Earth’s temperature over millions of years.
“When using this data to study extreme weather events, most studies have focused on large individual temperature peaks, typically a few degrees Celsius of warming,” says Arnscheidt. “Instead, we tried to look at the aggregate stats and consider all the fluctuations involved, rather than picking the biggest ones.”
The team first performed a statistical analysis of the data and observed that over the past 66 million years, the distribution of global temperature fluctuations did not resemble a standard bell curve, with symmetrical tails representing an equal probability of extreme heat and extreme cold. fluctuation. Instead, the curve was noticeably out of balance, skewed towards hotter than cold events. The curve also exhibited a significantly longer tail, representing hot events that were more extreme, or of higher temperature, than the more extreme cold events.
“This indicates that there is some kind of amplification over what you would otherwise expect,” says Arnscheidt. “Everything points to something fundamental that is behind this surge, or bias towards the warming events.”
“It’s fair to say that the Earth system is becoming more volatile, in a warming sense,” Rothman adds.
A warming multiplier
The team wondered if this warming bias could be the result of “multiplicative noise” in the climate-carbon cycle. Scientists have long understood that higher temperatures, up to a point, tend to accelerate biological and chemical processes. Since the carbon cycle, which is a key driver of long-term climate fluctuations, is itself made up of such processes, increases in temperature can cause larger fluctuations, tilting the system towards extreme warming events.
In mathematics, there is a set of equations that describe such general amplifying or multiplicative effects. The researchers applied this multiplicative theory to their analysis to see if the equations could predict the skewed distribution, including the degree of its skewness and the length of its tails.
Ultimately, they found that the data and the observed bias towards warming could be explained by the multiplicative theory. In other words, it is very likely that over the past 66 million years, periods of modest warming have on average been further enhanced by multiplier effects, such as the response of biological and chemical processes that still have warmed the planet.
As part of the study, the researchers also looked at the correlation between past warming events and changes in Earth’s orbit. For hundreds of thousands of years, the Earth’s orbit around the sun regularly becomes more or less elliptical. But scientists have wondered why many past warming events seem to coincide with these changes, and why these events exhibit warming in excess of what the change in Earth’s orbit could have caused on its own.
Thus, Arnscheidt and Rothman incorporated Earth’s orbital changes into the multiplicative model and their analysis of Earth’s temperature changes, and found that the multiplier effects could predictably amplify, on average, modest increases in temperature due to changes in Earth’s orbit.
“The climate is warming and cooling in synchronization with orbital changes, but the orbital cycles themselves predict only modest changes in climate,” says Rothman. “But if we consider a multiplicative model, then a modest warming, associated with this multiplier effect, can lead to extreme events which tend to occur concurrently with these orbital changes.”
“Humans are forcing the system in a new way,” adds Arnscheidt. “And this study shows that when we increase the temperature, we are likely to interact with these natural enhancer effects.”
This research was funded, in part, by the School of Science at MIT.