July 29 (UPI) -- At least twice during Earth's history, the entirety of the planet has become encased in ice, though scientists say they have struggled to determine what triggered the Snowball Earths.
Previously, researchers have estimated that when decreases in sunlight crossed a certain threshold, runaway ice expansion blanketed the earth.
However, new findings -- published Wednesday in the journal Proceedings of the Royal Society A -- suggest a specific threshold isn't necessary to account for Snowball Earths.
"Our study shows that a sufficiently fast drop in effective incoming solar radiation can trigger snowball glaciation," lead study author Constantin Arnscheidt told UPI in an email.
Runaway ice expansion happens as a result of a destabilizing feedback mechanism called the ice-albedo feedback. The albedo is the reflectivity of the Earth -- the more ice there is, the more reflective Earth is.
As ice expands southward, more and more sunlight gets reflected. This pattern has a cooling effect, which in turn, encourages ice expansion and a stronger albedo -- completing the feedback loop.
Researchers used a relatively simple mathematical model of Earth's climate to show that this destabilizing feedback loop can be triggered when incoming solar radiation decreases quickly enough -- "faster than a critical rate," according to the study's authors.
"The model captures essential aspects of the relationships between incoming and outgoing solar radiation, the surface temperature of the Earth, the concentration of carbon dioxide in the atmosphere, and the processes that control atmospheric carbon dioxide levels on long timescales," said Arnscheidt, a graduate student in MIT's department of earth, atmospheric and planetary sciences.
The model showed that a threshold of diminished radiation isn't needed for the ice-albedo feedback loop. A precipitous drop in sunlight, whether caused by widespread volcanic eruptions or biologically induced cloud formation, will suffice.
Scientists still can't be certain what caused Earth to twice experience snowball glaciation, they said.
Because weathering rates and a variety of other geochemical variables can influence the carbon cycle and glaciation patterns, there are a number of possible triggers that could explain Snowball Earths.
"It's somewhat easier to trigger snowball glaciations via the rate-induced glaciation route than via other ways," said Arnscheidt. "This means that we should consider changes in solar radiation as more likely triggers for past Snowball Earth events than changes in the carbon cycle."
The findings may be useful in the search for habitable exoplanets.
"Rate-induced glaciation implies that planets well within the habitable zone can experience transient snowball glaciations if conditions change too quickly," Arnscheidt said.
"Because most life likely would not survive a snowball glaciation ... this highlights that there's much more nuance to the concept of habitability than just the classical habitable zone," Arnscheidt said.