Two University of Colorado Boulder researchers who have
adapted a 3D, 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 3D 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, says
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% 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 1D 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, says Wolf.
“The inclusion of dynamic sea ice makes it harder to keep
the early Earth warm in our 3D model,” Wolf says. “Stable, global mean
temperatures below 55 F 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% carbon dioxide could have done
the trick by keeping the mean temperatures at 57 F, geological evidence from
ancient soils on early Earth indicate such high concentrations of carbon
dioxide 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.
Toon says 1D 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 3D 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 3D model, preventing a planet-wide glaciation requires
about three times more carbon dioxide than predicted by the 1D models, says
Wolf. For all warm climate scenarios generated by the 3D model, Earth’s mean
temperature about 2.8 billion years ago was 5 to 10 F warmer than the 1D model,
given the same abundance of greenhouse gases. “Nonetheless, the 3D model
indicates a roughly 55 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,'” says 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,” says 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 says.
Testing the new 3D model has required huge amounts of supercomputer
computation time, says 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 three
months.