Earth’s climate has undergone great changes, from global volcanism to planet-cooling ice ages and dramatic changes in solar radiation. And yet life, for 3.7 billion years, has not stopped beating.
Now, a study by MIT researchers in Scientists progress confirms that the planet is home to a “stabilizing feedback” mechanism that works over hundreds of thousands of years to bring the climate back to the brink, keeping global temperatures within a stable and habitable range.
How does he accomplish this? One likely mechanism is “silicate weathering” – a geological process by which the slow and steady weathering of silicate rocks involves chemical reactions that eventually extract carbon dioxide from the atmosphere and into ocean sediments, trapping the gas in rocks.
Scientists have long suspected that silicate weathering plays a major role in regulating the Earth’s carbon cycle. The silicate weathering mechanism could provide a geologically constant force to control carbon dioxide and global temperatures. But there has never been direct evidence of the continued operation of such feedback, until now.
The new findings are based on a study of paleoclimate data that records changes in average global temperatures over the past 66 million years. The MIT team applied mathematical analysis to see if the data revealed characteristic patterns of stabilization phenomena that reduced global temperatures on a geologic timescale.
They found that indeed there appears to be a consistent pattern in which variations in Earth’s temperature are damped over time scales of hundreds of thousands of years. The duration of this effect is similar to the time scales over which silicate weathering is expected to act.
The results are the first to use real data to confirm the existence of a stabilizing feedback, the mechanism of which is likely silicate weathering. This stabilizing feedback would explain how the Earth remained habitable through dramatic climatic events in the geological past.
“On the one hand, it’s good because we know that the current global warming will eventually be canceled thanks to this stabilizing feedback,” says Constantin Arnscheidt, graduate student in the Department of Earth, Atmospheric and Planetary Sciences. (EAPS) from MIT. “But on the other hand, it will take hundreds of thousands of years, so not fast enough to solve our current problems.”
The study is co-authored by Arnscheidt and Daniel Rothman, professor of geophysics at MIT.
Scientists have already seen evidence of a climate-stabilizing effect in Earth’s carbon cycle: chemical analyzes of ancient rocks have shown that the flow of carbon in and out of Earth’s surface environment has remained relatively balanced, even in the face of dramatic changes in global temperature. Additionally, silicate weathering models predict that the process should have a stabilizing effect on global climate. And finally, the fact of Earth’s enduring habitability indicates an inherent geological control over extreme temperature variations.
“You have a planet whose climate has been subjected to so many dramatic external changes. Why has life survived all this time? One argument is that we need some kind of stabilization mechanism to maintain temperatures adapted to life,” says Arnscheidt. “But such a mechanism has never been shown from data to consistently control Earth’s climate.”
Arnscheidt and Rothman sought to confirm whether a stabilizing feedback was indeed at work, by examining data on global temperature fluctuations through geologic history. They worked with a range 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 study is only possible because there have been great strides in improving the resolution of these deep sea temperature records,” Arnscheidt notes. “Now we have data going back 66 million years, with data points at most thousands of years apart.”
Accelerate to a stop
To the data, the team applied the mathematical theory of stochastic differential equations, which is 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 were feedbacks acting on certain time scales,” says Arnscheidt.
Using this approach, the team analyzed the history of average global temperatures over the past 66 million years, considering the entire period on different time scales, such as tens of thousands of years through compared to hundreds of thousands, to see if stabilizing feedback patterns emerged in each timescale.
“To some extent, it’s like your car is speeding down the street, and when you brake, you slide for a long time before you stop,” Rothman says. “There is a time scale on which frictional resistance, or a stabilizing feedback, comes into play, as the system returns to a steady state.”
Without stabilizing feedbacks, fluctuations in global temperature should increase with time scale. But the team’s analysis revealed a regime in which the fluctuations did not increase, implying that a stabilizing mechanism prevailed in the climate before the fluctuations became too extreme. The time scale of this stabilizing effect – hundreds of thousands of years – coincides with what scientists predict for silicate weathering.
Interestingly, Arnscheidt and Rothman found that over longer timescales the data showed no stabilizing feedback. In other words, there does not appear to be a recurring decline in global temperatures on time scales greater than a million years. On these longer time scales, what has controlled global temperatures?
“There is a notion that chance may have played a major role in determining why, after more than 3 billion years, life still exists,” Rothman offers.
In other words, as the Earth’s temperatures fluctuate over longer time periods, these fluctuations may be small enough in a geological sense, to fall within a range in which a stabilizing feedback, such as silicate weathering, could periodically check the climate. and more precisely, in a habitable zone.
“There are two camps: some say chance is sufficient explanation, and others say there must be stabilizing feedback,” Arnscheidt says. “We are able to show, directly from the data, that the answer is probably somewhere in between. In other words, there has been some stabilization, but sheer luck probably also played a part. in keeping the Earth permanently habitable.”
Constantin Arnscheidt, Presence or absence of Earth system stabilizing feedbacks at different time scales, Scientists progress (2022). DOI: 10.1126/sciadv.adc9241
Provided by Massachusetts Institute of Technology
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