Earth’s climate has undergone some major changes, from global volcanism to planet-cooling ice ages and dramatic shifts in solar radiation. And yet life has continued to beat for the past 3.7 billion years.
Now, a study by MIT researchers in Scientific progress confirms that the planet harbors a “stabilizing feedback” mechanism that works over hundreds of thousands of years to pull the climate back from the brink, keeping global temperatures within a stable, habitable range.
How does it achieve this? One likely mechanism is “silicate weathering” — a geological process in which the slow and steady weathering of silicate rocks involves chemical reactions that eventually pull carbon dioxide out of the atmosphere and into ocean sediments, trapping the gas in rocks.
Scientists have long suspected that silicate weathering plays an important role in regulating Earth’s carbon cycle. The mechanism of silicate weathering could be a geologically constant force to control carbon dioxide — and Earth’s temperature. But until now there has never been direct evidence for the continuous operation of such feedback.
The new findings are based on a study of paleoclimate data that records changes in Earth’s average temperature over the past 66 million years. The MIT team applied mathematical analysis to see if the data revealed patterns typical of stabilizing phenomena that controlled global temperatures on a geological time scale.
They found that there does indeed appear to be a consistent pattern in which Earth’s temperature fluctuations are dampened over timescales of hundreds of thousands of years. The duration of this effect is similar to the timescales over which silicate weathering is predicted to occur.
The results are the first to use actual data to confirm the existence of a stabilizing feedback, the mechanism of which is likely silicate weathering. This stabilizing feedback would explain how Earth has remained habitable through dramatic climate events in the geologic past.
“On the one hand, it’s good because we know that the current global warming will eventually be outweighed by this stabilizing feedback,” said Constantin Arnscheidt, a graduate student in MIT’s Department of Earth, Atmosphere, and Planetary Sciences ( EAPS). “But on the other hand, it will take hundreds of thousands of years for it to happen, so not fast enough to solve our current problems.”
The study is co-authored by Arnscheidt and Daniel Rothman, professor of geophysics at MIT.
Stability in data
Scientists have previously seen hints of a climate-stabilizing effect in Earth’s carbon cycle: Chemical analyzes of ancient rocks have shown that the flux of carbon in and out of the Earth’s surface has remained relatively balanced even through dramatic fluctuations in global temperatures. In addition, models of silicate weathering predict that the process should have a stabilizing effect on the global climate. And finally, the fact of the Earth’s continued habitability indicates an inherent geological control over extreme temperature fluctuations.
“You have a planet whose climate is subject to so many dramatic external changes. Why has life survived all this time? One argument is that we need some sort of stabilizing mechanism to keep temperatures suitable for life,” says Arnscheidt. “But data has never shown that such a mechanism has consistently controlled Earth’s climate.”
Arnscheidt and Rothman sought to confirm whether a stabilizing feedback was indeed at work, by looking at records of global temperature fluctuations throughout geological history. They worked with a series of global temperature records compiled by other scientists, from the chemical composition of ancient marine fossils and shells, as well as preserved Antarctic ice cores.
“This whole research is only possible because great progress has been made in improving the resolution of these deep-sea temperature records,” Arnscheidt notes. “Now we have data going back 66 million years, with data points that are at most thousands of years apart.”
Come to a quick stop
To the data, the team applied the mathematical theory of stochastic differential equations, commonly used to reveal patterns in highly fluctuating data sets.
“We realized that this theory makes predictions about what you would expect Earth’s temperature history to look like if there had been feedback on certain time scales,” Arnscheidt explains.
Using this approach, the team analyzed the history of Earth’s average temperature over the past 66 million years, considering the whole period over different time scales, such as tens of thousands of years versus hundreds of thousands, to see if patterns of stabilizing feedback emerged within any time scale.
“To some extent, it’s like your car is hurtling down the street, and when you brake, you slide long before you stop,” says Rothman. “There’s a time scale over which friction drag, or a stabilizing feedback, kicks in when the system returns to a steady state.”
Without stabilizing feedbacks, global temperature fluctuations should increase over time. But the team’s analysis revealed a regime in which fluctuations did not increase, implying that a stabilizing mechanism existed in the climate before fluctuations became too extreme. The timescale for this stabilizing effect — hundreds of thousands of years — coincides with what scientists predict for silicate weathering.
Interestingly, Arnscheidt and Rothman found that on longer time scales, the data revealed no stabilizing feedback. That is, there does not seem to be a recurring drop in global temperatures on timescales of more than a million years. So what has kept global temperatures under control on these longer time scales?
“There’s an idea that chance played a big part in determining why life still exists after more than 3 billion years,” Rothman offers.
In other words, since Earth’s temperatures fluctuate over longer stretches, these fluctuations may just happen to be small enough in a geological sense, to fall within a range that a stabilizing feedback, such as silicate weathering, could control climate periodically, and more to the point, within a habitable zone.
“There are two camps: some say random chance is a good explanation, and others say there should be a stabilizing feedback,” says Arnscheidt. “We can show directly from data that the answer is probably somewhere in between. In other words, there was some stabilization, but sheer luck probably also played a role in keeping Earth continuously habitable.”
Constantin Arnscheidt, Presence or Absence of Earth System Stabilizing Feedbacks at Different Time Scales, Scientific progress (2022). DOI: 10.1126/sciadv.adc9241
Offered by Massachusetts Institute of Technology
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