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Early Earth may have been prone to deep freezes (Forwarded)
Office of News Services
University of Colorado-Boulder
Eric Wolf, 303-735-3545
Brian Toon, 303-492-1534
Jim Scott, 303-492-3114
December 5, 2011
Early Earth may have been prone to deep freezes, says CU-Boulder study
Two University of Colorado Boulder researchers who have adapted a
three-dimensional, general circulation model of Earth's climate to a time
some 2.8 billion years ago when the sun was significantly fainter than
present think the planet may have been more prone to catastrophic glaciation
than previously believed.
The new 3-D model of the Archean Eon on Earth that lasted from about 3.8
billion years to 2.5 billion years ago, incorporates interactions between
the atmosphere, ocean, land, ice and hydrological cycles, said CU-Boulder
doctoral student Eric Wolf of the atmospheric and oceanic sciences
department. Wolf has been using the new climate model -- which is based on
the Community Earth System Model maintained by the National Center for
Atmospheric Research in Boulder -- in part to solve the "faint young sun
paradox" that occurred several billion years ago when the sun's output was
only 70 to 80 percent of that today but when geologic evidence shows the
climate was as warm or warmer than now.
In the past, scientists have used several types of one-dimensional climate
models -- none of which included clouds or dynamic sea ice -- in an attempt
to understand the conditions on early Earth that kept it warm and hospitable
for primitive life forms. But the 1-D model most commonly used by scientists
fixes Earth's sea ice extent at one specific level through time despite
periodic temperature fluctuations on the planet, said Wolf.
"The inclusion of dynamic sea ice makes it harder to keep the early Earth
warm in our 3-D model," Wolf said. "Stable, global mean temperatures below
55 degrees Fahrenheit are not possible, as the system will slowly succumb to
expanding sea ice and cooling temperatures. As sea ice expands, the planet
surface becomes highly reflective and less solar energy is absorbed,
temperatures cool, and sea ice continues to expand."
Wolf and CU-Boulder Professor Brian Toon are continuing to search for the
heating mechanism that apparently kept Earth warm and habitable back then,
as evidenced by liquid oceans and primordial life forms. While their
calculations show an atmosphere containing 6 percent carbon dioxide could
have done the trick by keeping the mean temperatures at 57 degrees F,
geological evidence from ancient soils on early Earth indicate such high
concentrations of CO2 were not present at the time.
The CU-Boulder researchers are now looking at cloud composition and
formation, the hydrological cycle, movements of continental masses over time
and heat transport through Earth's system as other possible modes of keeping
early Earth warm enough for liquid water to exist. Wolf gave a presentation
on the subject at the annual American Geophysical Union meeting held Dec.
5-9 in San Francisco.
Toon said 1-D models essentially balance the amount of sunshine reaching the
atmosphere, clouds, and Earth's terrestrial and aquatic surfaces with the
amount of "earthshine" being emitted back into the atmosphere, clouds, and
space, primarily in the infrared portion of the electromagnetic spectrum.
"The advantage of a 3-D model is that the transport of energy across the
planet and changes in all the components of the climate system can be
considered in addition to the basic planetary energy balance."
In the new 3-D model, preventing a planet-wide glaciation requires about
three times more CO2 than predicted by the 1-D models, said Wolf. For all
warm climate scenarios generated by the 3-D model, Earth's mean temperature
about 2.8 billion years ago was 5 to 10 degrees F warmer than the 1-D model,
given the same abundance of greenhouse gases. "Nonetheless, the 3-D model
indicates a roughly 55 degrees F mean temperature was still low enough to
trigger a slide by early Earth into a runaway glacial event, causing what
some scientists call a Snowball Earth,' " said Wolf.
"The ultimate point of this study is to determine what Earth was like around
the time that life arose and during the first half of the planet's history,"
said Toon. "It would have been shrouded by a reddish haze that would have
been difficult to see through, and the ocean probably was a greenish color
caused by dissolved iron in the oceans. It wasn't a blue planet by any
means." By the end of the Archean Eon some 2.5 billion year ago, oxygen
levels rose quickly, creating an explosion of new life on the planet, he
Testing the new 3-D model has required huge amounts of supercomputer
computation time, said Toon, who also is affiliated with CU-Boulder's
Laboratory for Atmospheric and Space Physics. A single calculation for the
study run on CU-Boulder's powerful new Janus supercomputer can take up to
The CU-Boulder study was funded by a NASA Earth and Space Science Fellowship
to Wolf as well as a grant from the NASA Exobiology and Evolutionary Biology
Toon will be presented with AGU's Roger Revelle Medal for innovative work on
the effects of aerosols on clouds and climate at the 2011 San Francisco
meeting. The Revelle Medal is presented annually to a scientist who has
shown outstanding accomplishments or contributions toward the understanding
Earth's climate systems.
An illustration of what early Earth may have looked like several billion
years ago. (Copyright 2011 Don Dixon/cosmographica.com)
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