At least twice in Earth’s history, almost the entire planet was locked in a layer of snow and ice. These dramatic “Snowball Earth” events occurred in rapid succession, somewhere around 700 million years ago, and evidence suggests that consecutive global glaciations set the stage for the subsequent explosion of complex, multicellular life on Earth.
Scientists have considered multiple scenarios for what may have tipped the planet in each ice age. While not a single conduction process has been identified, it is assumed that what caused the temporary freezes must have been done in a way that pushed the planet beyond a critical threshold, such as reducing incoming sunlight or atmospheric carbon dioxide. at levels low enough to set off a global ice expansion.
But MIT scientists now say Snowball Earths was likely the product of “velocity-induced glaciations.” That is, they discovered that Earth can fall into a global ice age when the level of solar radiation it receives changes rapidly in a geologically short period of time. The amount of solar radiation does not have to fall to a particular threshold point; As the decrease in incoming sunlight occurs faster than a critical rate, a temporary glaciation or Snowball Earth will occur.
These findings, published today in the Proceedings of the Royal Society A, They suggest that what triggered the Earth’s ice ages likely involved processes that quickly reduced the amount of solar radiation reaching the surface, such as widespread volcanic eruptions or biologically induced cloud formation that could have significantly blocked the sun’s rays.
The findings can also be applied to the search for life on other planets. The researchers have been interested in finding exoplanets within the habitable zone, a distance from their star that would be within a temperature range that could support life. The new study suggests that these planets, like Earth, could also freeze temporarily if their climate abruptly changes. Even if they are within a habitable zone, Earth-like planets may be more susceptible to global glaciations than previously thought.
“It could have a planet that stays well within the classical habitable zone, but if the incoming sunlight changes too fast, it could get a Snowball Earth,” says lead author Constantin Arnscheidt, a graduate student in the Department of Science. Earth, Atmospheric and Planetary MIT. (EAPS). “What this highlights is the notion that there are much more nuances to the concept of habitability.”
Arnscheidt is co-author of the article with Daniel Rothman, EAPS professor of geophysics and co-founder and co-director of the Lorenz Center.
A runaway snowball
Regardless of the particular processes that triggered past glaciations, scientists generally agree that the Snowball Lands arose from a “runaway” effect involving an ice albedo feedback: as incoming sunlight is reduced , the ice expands from the poles to the equator. As more ice covers the globe, the planet becomes more reflective, or higher in albedo, which further cools the surface so that more ice expands. Eventually, if the ice reaches a certain degree, this becomes an uncontrolled process, resulting in global ice age.
Global glaciations on Earth are temporary in nature, due to the planet’s carbon cycle. When the planet is not covered in ice, the levels of carbon dioxide in the atmosphere are somewhat controlled by the weathering of rocks and minerals. When the planet is covered in ice, weathering is greatly reduced, whereby carbon dioxide accumulates in the atmosphere, creating a greenhouse effect that eventually unfreezes the planet of its ice age.
Scientists generally agree that the formation of Snowball Earths has something to do with the balance between incoming sunlight, ice albedo feedback, and the global carbon cycle.
“There are many ideas about the cause of these global glaciations, but they all come down to an implicit modification of the incoming solar radiation,” says Arnscheidt. “But it’s generally been studied in the context of crossing a threshold.”
He and Rothman had previously studied other periods in Earth’s history where speed, or the speed at which certain changes in climate occurred, played a role in triggering events, such as past mass extinctions.
“In the course of this exercise, we realized that there was an immediate way to make a serious point by applying such speed-induced tipping ideas to Snowball Earth and livability,” says Rothman.
“Be careful with the speed”
The researchers developed a simple mathematical model of the Earth’s climate system that includes equations to represent the relationships between incoming and outgoing solar radiation, the Earth’s surface temperature, the concentration of carbon dioxide in the atmosphere, and the effects of weathering in absorption and storing atmospheric carbon dioxide. The researchers were able to adjust each of these parameters to observe what conditions a Snowball Earth generated.
Ultimately, they discovered that a planet was more likely to freeze if the incoming solar radiation decreased rapidly, at a rate that was faster than a critical index, rather than reaching a critical threshold or a particular level of sunlight. There is some uncertainty as to what exactly that critical rate would be, since the model is a simplified representation of Earth’s climate. However, Arnscheidt estimates that Earth would have to experience a drop of about 2 percent in incoming sunlight over a period of approximately 10,000 years to fall into a global ice age.
“It is reasonable to assume that past glaciations were induced by rapid geological changes to solar radiation,” says Arnscheidt.
The particular mechanisms that may have rapidly darkened the skies for tens of thousands of years are still up for debate. One possibility is that widespread volcanoes have thrown aerosols into the atmosphere, blocking incoming sunlight worldwide. Another is that primitive algae may have developed mechanisms that facilitated the formation of clouds that reflect light. The results of this new study suggest that scientists may consider processes like these, which rapidly reduce incoming solar radiation, as more likely triggers for Earth’s ice ages.
“Although humanity will not trigger a snowball glaciation in our current climate trajectory, the existence of such a ‘velocity-induced tipping point’ on a global scale may remain a cause for concern,” says Arnscheidt. “For example, it teaches us that we must be careful with the speed at which we are modifying Earth’s climate, not just with the magnitude of the change. There could be other velocity-induced tipping points that could be caused by anthropogenic warming. Identifying them and limiting their critical rates is a valuable goal for future research. ”
This research was funded, in part, by the MIT Lorenz Center.
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