8 June 2013 | NewScientist | 43rainstorm could make you lose half your
atmosphere in one area,” says Pierrehumbert,
and that might suck in more air from all
around, changing the dynamics of the entire
atmosphere.
Such oddities, paired with water vapour’s
tendency to amplify other sources of warming,
mean that some of the most resilient Goldilocks
worlds may be those with only trace amounts
of water. Astrophysicist Sara Seager at the
Massachusetts Institute of Technology and
her colleagues have been modelling “desert”
planets with insufficient water to support
global oceans or a steamy atmosphere. They
conclude that a desert planet with just 1 per
cent atmospheric water vapour could
maintain habitable surface temperatures as
close as 0.5 AU to a sunlike star, well inside the
orbit of Venus (arxiv.org/abs/1304.3714).
Kasting, Pierrehumbert and their colleagues
have recently secured NASA funding to jointly
develop a fully three-dimensional climate
model, one that will build in more robust and
realistic cloud feedbacks and hydrological
cycles for a wide variety of potentially
habitable rocky planets. Other groups, notably
those of François Forget at Pierre and Marie
Curie University in Paris, France, and of Jochem
Marotzke at the Max Planck Institute for
Meteorology in Hamburg, Germany, are also
on the case. “Understanding full atmospheric
circulation and its effects on clouds and water
vapour is the real frontier we’re all moving
toward,” Pierrehumbert says. “This is what will
ultimately have the largest effect on drawing
the habitable zone’s boundaries.”
Yet at least in Earth’s case, the greatest
uncertainty may now lie with us humans.
Thanks to our burning of fossil fuels, the
atmosphere’s CO 2 content is currently crossing
the 400 parts per million threshold, up from a
pre-industrial average of 280 ppm. In its most
recent consensus report, published in 2007,
the United Nations Intergovernmental Panel
on Climate Change pegged the most probable
eventual warming from a doubling of CO 2 –
a level we are likely to reach in the second half
of this century – at 3 °C.
Compared with the fate of Venus, that
sounds innocuous. But this temperature rise
is roughly equivalent to shifting Earth’s orbit
inwards by 1 per cent, to 0.99 AU – precisely
where the habitable zone’s inner boundary lies
in the latest model. Might further greenhouse
emissions push us over the edge, eventually to
follow Venus’s destiny?
Various studies in recent decades haveconcluded that this outcome is highly
unlikely. Even if we managed to burn most of
the planet’s economically recoverable fossil
fuel reserves, not merely doubling
atmospheric CO 2 but increasing it by a factor
of 8 or 16, the worst outcome would be only
a moderately moist greenhouse.Apocalypse no
In work yet to be published, geochemist
Colin Goldblatt at the University of Victoria in
British Columbia, Canada, has revisited the
question using the same updated information
on greenhouse-gas absorption that was used
to revise the habitable zone boundaries.
His conclusion is that if Earth’s present-day
atmosphere were to enter a very hot and moist
greenhouse state, the extra water vapour
would absorb more infrared sunlight than
previously appreciated. That could reduce
cloud and ice cover, further increasing heat
absorption. In turn, that could trigger a
runaway greenhouse – but only if we also
chose to use the energy from all the Earth’s
coal, oil, and gas for no other purpose than to
cook up even more CO 2 from limestone.
That reassurance does nothing to lessen
the shorter-term impacts of climate change
on human society in the coming decades and
centuries. But palaeoclimate records suggest
that, over scales of hundreds of millions of
years, our planet is remarkably resilient to
changes to its thermostat. “Despite highlevels of atmospheric CO 2 in the remote past,
Earth hasn’t left the habitable zone,” says
Goldblatt.
That’s probably down to Earth’s carbonate-
silicate cycle. This feature appears to be
unique, in the solar system at least, thanks to
our planet’s singular combination of oceans
and tectonic activity. As rising temperatures
start to steam water from the oceans,
increased rainfall washes more CO 2 out of the
atmosphere, ultimately sequestering it in
rocks for many millions of years before it is
eventually burped back out by volcanoes.
It is Earth’s natural temperature regulator –
and perhaps its greatest claim to be the best of
all possible worlds. It could also, ironically, be
life’s ultimate undoing, perhaps even before
the sun would otherwise become too bright
for comfort. According to calculations
performed by Kasting and his student Ken
Caldeira in 1992, perhaps a billion years from
now Earth’s interior will have cooled enough
to substantially reduce volcanism, slowing the
carbonate-silicate cycle and leaving so much
carbon locked in rock that Earth’s atmosphere
will cease to support most forms of
photosynthesis (Nature, vol 360, p 721).
Till then, though, is life on Earth safe? Given
our vast uncertainties about how climates
work – and now even about Earth’s position in
the Goldilocks zone – Goldblatt reckons it is
probably unwise to take too much for granted.
“It’s like playing tag on top of a cliff on a foggy
day. No one’s fallen off yet, but you don’t know
how close the edge is.” nLee Billings is a freelance writer based in
New York City” In Earth’s case, the biggest uncertainty in
habitability may now lie with us humans”The shifting zone
A star’s habitable zone is the region around it in which an Earth-like planet can have liquid water. New
calculations have shifted that zone outwards, altering our view of the habitability of many exoplanets, and
putting Earth at risk from an ageing sun sooner than we thought200% 100% 25%3000K7000K5800KOur sunSurface temperature of starStarlight intensity at planet compared with Earth today
(Starlight intensity depends on both the star's surface temperature and the planet's distance from the star)EarthHabitable zones also shift
outwards over time because
stars heat up as they ageNew inner boundaryOld inner boundary
New outer boundaryOld outer boundaryKepler 62eKepler 22bGliese 667Cc Gliese 581g Gliese 581dHD 40307g Kepler 62fTau Ceti fMarsVenus130608_F_Goldilocks.indd 43 30/5/13 14:22:19