Study: A plunge in incoming
sunlight may have triggered 'snowball Earths'
Source: Massachusetts Institute of
Technology
Summary: Global ice ages may have been
triggered by sharp declines in incoming sunlight, research finds.
At
least twice in Earth's history, nearly the entire planet was encased in a sheet
of snow and ice. These dramatic "Snowball Earth" events occurred in
quick succession, somewhere around 700 million years ago, and evidence suggests
that the consecutive global ice ages 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 into
each ice age. While no single driving process has been identified, it's assumed
that whatever triggered the temporary freeze-overs must have done so in a way
that pushed the planet past a critical threshold, such as reducing incoming
sunlight or atmospheric carbon dioxide to levels low enough to set off a global
expansion of ice.
But
MIT scientists now say that Snowball Earths were likely the product of
"rate-induced glaciations." That is, they found the Earth can be
tipped into a global ice age when the level of solar radiation it receives changes
quickly over a geologically short period of time. The amount of solar radiation
doesn't have to drop to a particular threshold point; as long as the decrease
in incoming sunlight occurs faster than a critical rate, a temporary
glaciation, or Snowball Earth, will follow.
These
findings, published in the Proceedings of the Royal Society A,
suggest that whatever triggered the Earth's ice ages most likely involved
processes that quickly reduced the amount of solar radiation coming to the
surface, such as widespread volcanic eruptions or biologically induced cloud
formation that could have significantly blocked out the sun's rays.
The
findings may also apply to the search for life on other planets. Researchers
have been keen on 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 ice
over temporarily if their climate changes abruptly. Even if they lie within a
habitable zone, Earth-like planets may be more susceptible to global ice ages
than previously thought.
"You
could have a planet that stays well within the classical habitable zone, but if
incoming sunlight changes too fast, you could get a Snowball Earth," says
lead author Constantin Arnscheidt, a graduate student in MIT's Department of
Earth, Atmospheric and Planetary Sciences (EAPS). "What this highlights is
the notion that there's so much more nuance in the concept of
habitability."
Arnscheidt
has co-authored the paper 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 Snowball Earths arose from a "runaway" effect
involving an ice-albedo feedback: As incoming sunlight is reduced, 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 for more
ice to expand. Eventually, if the ice reaches a certain extent, this becomes a
runaway process, resulting in a 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, 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 vastly reduced, so that carbon
dioxide builds up in the atmosphere, creating a greenhouse effect that
eventually thaws the planet out of its ice age.
Scientists
generally agree that the formation of Snowball Earths has something to do with
the balance between incoming sunlight, the ice-albedo feedback, and the global
carbon cycle.
"There
are lots of ideas for what caused these global glaciations, but they all really
boil down to some implicit modification of solar radiation coming in,"
Arnscheidt says. "But generally it's been studied in the context of
crossing a threshold."
He
and Rothman had previously studied other periods in Earth's history where the
speed, or rate at which certain changes in climate occurred had a role in triggering
events, such as past mass extinctions.
"In
the course of this exercise, we realized there was an immediate way to make a
serious point by applying such ideas of rate-induced tipping, to Snowball Earth
and habitability," Rothman says.
"Be
wary of speed"
The
researchers developed a simple mathematical model of the Earth's climate system
that includes equations to represent relations between incoming and outgoing
solar radiation, the surface temperature of the Earth, the concentration of
carbon dioxide in the atmosphere, and the effects of weathering in taking up
and storing atmospheric carbon dioxide. The researchers were able to tune each
of these parameters to observe which conditions generated a Snowball Earth.
Ultimately,
they found that a planet was more likely to freeze over if incoming solar
radiation decreased quickly, at a rate that was faster than a critical rate,
rather than to a critical threshold, or particular level of sunlight. There is
some uncertainty in exactly what that critical rate would be, as the model is a
simplified representation of the Earth's climate. Nevertheless, Arnscheidt
estimates that the Earth would have to experience about a 2 percent drop in
incoming sunlight over a period of about 10,000 years to tip into a global ice
age.
"It's
reasonable to assume past glaciations were induced by geologically quick
changes to solar radiation," Arnscheidt says.
The
particular mechanisms that may have quickly darkened the skies over tens of
thousands of years is still up for debate. One possibility is that widespread
volcanoes may have spewed aerosols into the atmosphere, blocking incoming
sunlight around the world. Another is that primitive algae may have evolved
mechanisms that facilitated the formation of light-reflecting clouds. The
results from this new study suggest scientists may consider processes such as
these, that quickly reduce incoming solar radiation, as more likely triggers
for Earth's ice ages.
"Even
though humanity will not trigger a snowball glaciation on our current climate
trajectory, the existence of such a 'rate-induced tipping point' at the global
scale may still remain a cause for concern," Arnscheidt points out.
"For example, it teaches us that we should be wary of the speed at which
we are modifying Earth's climate, not just the magnitude of the change. There
could be other such rate-induced tipping points that might be triggered by
anthropogenic warming. Identifying these and constraining their critical rates
is a worthwhile goal for further research."
This
research was funded, in part, by the MIT Lorenz Center.
Story
Source:
Materials provided by Massachusetts Institute of Technology. Original written by
Jennifer Chu.
.png)
.jpg)
.gif)
