The findings also suggest that exoplanets located in habitable areas may be susceptible to glaciation.
At least twice in Earth’s history, almost the entire planet was nestled in a leaf of snow and ice. These dramatic events in the “Land of Snowballs” occurred rapidly in succession, about 700 million years ago, and evidence suggests that consecutive global glaciers set the stage for the subsequent explosion of complex life. and multicellular on Earth.
Scientists have considered multiple scenarios for what the planet may have tilted in each ice age. While no single driving process has been identified, it is assumed that anything that triggered the temporary freeze must have done so in a way that pushed the planet beyond a critical threshold, such as now reduce sunlight or atmospheric carbon dioxide to levels low enough to set. out of a global expansion of the ice.
But WITH Scientists now say the snowball lands were probably the product of “rate-induced glaciations.” That is, they found that the Earth can tilt in a global ice age when the level of solar radiation it receives changes rapidly over a geologically short period of time. The amount of solar radiation should not go down to a certain point; whenever the decrease in incoming sunlight occurs more rapidly than a critical rate, a temporary glaciation or Snowball Land will occur.
These findings, published in the article Proceedings of the Royal Society A, suggest that anything that triggered the Earth’s glaciation most likely involved processes that rapidly 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. Researchers have been interested in finding exoplanets within the habitable zone, at a distance from their star that would be within a range of temperatures that could withstand life. The new study suggests that these planets, like Earth, could also freeze temporarily if their climate changes abruptly. Even if they are within a habitable zone, Earth-like planets may be more susceptible to the global ice age than previously thought.
“You could have a planet that is fine within the classical habitable zone, but if the incoming insolation changes too quickly, you could get a Snowball Earth,” says lead author Constantin Arnscheidt, a graduate student in the Department of Earth, Science Atmospheric and Planetary at MIT. (EAPS). “What stands out is the idea that there are many more nuances in the concept of habitability.”
Arnscheidt has co-authored the work with Daniel Rothman, EAPS professor of geophysics and co-founder and co-director of the Lorenz Center.
A rush of snowballs
Regardless of the particular processes that have triggered past glaciations, scientists generally agree that the Snowball Lands arose from a “rampant” effect that led to feedback from the glacial albedo: as the ice sheet is reduced. sunlight, the ice expands from the poles to the equator. As more ice covers the planet, the planet becomes more reflective or higher than the albedo, which further cools the surface so that more ice expands. Eventually, if the ice reaches a certain point, it becomes a rampant process, resulting in global glaciation.
Global ice ages 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, the weathering is greatly reduced, so that carbon dioxide accumulates in the atmosphere, creating a greenhouse effect that will eventually thaw the planet out of its glaciation.
Scientists generally agree that the formation of Snowball Lands has something to do with the balance between incoming sunlight, feedback from the ice albedo, and the global carbon cycle.
“There are a lot of ideas about what caused these global glaciations, but they really come down to some implicit modification of solar radiation,” Arnscheidt says. “But it has generally been studied in the context of exceeding 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. .
“Over the course of this exercise, we realized that there was an immediate way to approach a serious issue by applying these ideas of rate-induced propensity, to the snowball, and to habitability,” Rothman states.
“Watch out for 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 the intake and storage of atmospheric carbon dioxide. The researchers were able to adjust each of these parameters to observe what conditions a Snowball Earth generated.
Ultimately, they found that a planet was more likely to freeze if incoming solar radiation decreased rapidly, at a faster rate than a critical rate, than at a critical threshold or at a particular level of sunlight. There is some uncertainty as to exactly what this critical rate would be, as the model is a simplified representation of the Earth’s climate. However, Arnscheidt estimates that the Earth would have to experience a 2 percent drop in incoming sunlight over a period of about 10,000 years to enter a global ice age.
“It is reasonable to assume that past glaciations were induced by rapid geological changes in solar radiation,” says Arnscheidt.
Certain mechanisms that may have rapidly darkened the sky for tens of thousands of years are still pending debate. One possibility is that widespread volcanoes may have released aerosols into the atmosphere, blocking sunlight from entering the world. Another is that primitive algae may have evolved mechanisms that facilitated the formation of light-reflecting clouds. The results of this new study suggest that scientists may consider processes like these, which rapidly reduce incoming solar radiation, as the most likely triggers for Earth’s frosts.
“Although humanity will not trigger a snowball glaciation in our current climate trajectory, the existence of a rhythm-induced tipping point“ on a global scale may still be a cause for concern, ”Arnscheidt notes. example, it teaches us that we need to worry about how fast we are changing the Earth’s climate, not just the magnitude of change. There could be other speed-induced tilt points that can be triggered by anthropogenic warming. Identifying them and limiting their critical percentages is a research-worthy goal. ”
Reference: “Pathways to Global Glaciation” by Constantin W. Arnscheidt and Daniel H. Rothman, July 29, 2020, Proceedings of the Royal Society A.
DOI: 10.1098 / rspa.2020.0303
This research has been funded, in part, by the MIT Lorenz Center.