The global carbon cycle exerts substantial control over Earth’s climate. Still, life on Earth has kept on beating for the last 3.7 billion years. Understanding Earth’s history, the long-term effects of anthropogenic climate change, and planetary habitability depend on how the climate is maintained on geologic time scales.
MIT scientists have confirmed that the Earth has a “stabilizing feedback” mechanism that functions over millions of years to draw the climate back from the edge and maintain global temperatures within a steady, habitable range.
According to scientists, that possible mechanism is silicate weathering. It is a geological process by which the slow and steady weathering of silicate rocks involves chemical reactions that ultimately draw carbon dioxide out of the atmosphere and into ocean sediments, trapping the gas in stones.
For a long, scientists presumed that silicate weathering has a significant role in controlling the Earth’s carbon cycle. Its mechanism might provide a geologically constant force in keeping carbon dioxide — and global temperatures — in check. However, no direct evidence has been observed for the continual operation of such feedback until now.
For this study, scientists studied paleoclimate data that recorded changes in average global temperatures over the last 66 million years. Later by applying a mathematical analysis, the team determined whether the data revealed any patterns characteristic of stabilizing phenomena that reined in global temperatures on a geologic timescale.
They discovered that, over hundreds of thousands of years, there does indeed seem to be a constant pattern in which the Earth’s temperature oscillations are tamed. This effect’s duration is comparable to the timeframes predicted for silicate weathering.
Scientists looked at the data of global temperature fluctuations through geologic history. They wanted to confirm whether stabilizing feedback has indeed been at work. They used a variety of global temperature records obtained by previous researchers, including preserved Antarctic ice cores and data on the chemical makeup of prehistoric marine fossils and shells.
Constantin Arnscheidt, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS), said, “This whole study is only possible because there have been great advances in improving the resolution of these deep-sea temperature records. Now we have data going back 66 million years, with data points at most thousands of years apart.”
The mathematical analysis that scientists used in the study included stochastic differential equations. These equations are commonly used to reveal patterns in widely fluctuating datasets.
Arnscheidt explains, “We realized this theory makes predictions for what you would expect Earth’s temperature history to look like if there had been feedbacks acting on certain timescales.”
Thanks to this approach, scientists could analyze the history of average global temperatures over the last 66 million years. They also determined whether any patterns of stabilizing feedback emerged within each timescale.
Daniel Rothman, professor of geophysics at MIT, said, “To some extent, it’s like your car is speeding down the street, and when you put on the brakes, you slide for a long time before you stop. There’s a timescale over which frictional resistance, or stabilizing feedback, kicks in when the system returns to a steady state.”
Temperature fluctuations should increase with time without stabilizing feedback. However, the team’s investigation identified a regime in which changes did not improve, suggesting that a stabilizing mechanism was in place before fluctuations became excessive. The hundreds of thousands of years predicted by scientists for silicate weathering coincide with the timescale for this stabilizing effect.
Scientists found that the data did not reveal any stabilizing feedback on longer timescales. That is, there doesn’t appear to be any recurring pull-back of global temperatures on timescales longer than a million years. Over these longer timescales, then, what has kept global temperatures in check?
Rothman said, “There’s an idea that chance may have played a major role in determining why life still exists after more than 3 billion years.”
In other words, as the Earth’s temperatures fluctuate over longer stretches, these fluctuations may just happen to be small enough in the geologic sense to be within a range that a stabilizing feedback, such as silicate weathering, could periodically keep the climate in check, and more to the point, within a habitable zone.