Planetary 'Runaway Greenhouse' More Easily Triggered, Research Shows

Planetary 'Runaway Greenhouse'
More Easily Triggered, Research
Shows
July 30, 2013 — It might be easier
than previously thought for a planet
to overheat into the scorchingly
uninhabitable "runaway greenhouse"
stage, according to new research by
astronomers at the University of
Washington and the University of
Victoria published July 28 in the
journal Nature Geoscience.
In the runaway greenhouse stage, a
planet absorbs more solar energy
than it can give off to retain
equilibrium. As a result, the world
overheats, boiling its oceans and
filling its atmosphere with steam,
which leaves the planet glowing-hot
and forever uninhabitable, as Venus
is now.
One estimate of the inner edge of a
star's "habitable zone" is where the
runaway greenhouse process begins.
The habitable zone is that ring of
space around a star that's just right
for water to remain in liquid form on
an orbiting rocky planet's surface,
thus giving life a chance.
Revisiting this classic planetary
science scenario with new computer
modeling, the astronomers found a
lower thermal radiation threshold for
the runaway greenhouse process,
meaning that stage may be easier to
initiate than had been previously
thought.
"The habitable zone becomes much
narrower, in the sense that you can
no longer get as close to the star as
we thought before going into a
runaway greenhouse," said Tyler
Robinson, a UW astronomy
postdoctoral researcher and second
author on the paper. The lead author
is Colin Goldblatt of the University of
Victoria.
Though further research is called for,
the findings could lead to a
recalibration of where the habitable
zone begins and ends, with some
planets having their candidacy as
possible habitable worlds revoked.
"These worlds on the very edge got
'pushed in,' from our perspective --
they are now beyond the runaway
greenhouse threshold," Robinson
said.
Subsequent research, the
astronomers say, is needed in part
because their computer modeling was
done in a "single-column, clear-sky
model," or a one-dimensional
measure averaged around a planetary
sphere that does not account for the
atmospheric effect of clouds.
The findings apply to planet Earth as
well. As the sun increases in
brightness over time, Earth, too, will
move into the runaway greenhouse
stage -- but not for a billion and a
half years or so. Still, it inspired the
astronomers to write, "As the solar
constant increases with time, Earth's
future is analogous to Venus's past."
Other co-authors are Kevin J. Zahnle
of the NASA Ames Research Center in
Moffett Field, Calif.; and David Crisp
of the Jet Propulsion Laboratory in
Pasadena, Calif.