30 November 2019 | New Scientist | 17Geology Climate change
Leah Crane Michael Marshall
THERE could be oceans’ worth
of water hiding deep within giant
planets. Minerals with water bound
up in their molecular structure can
remain stable at extremely high
pressures, a new study suggests.
This means they could act as
reservoirs for water even inside
planets much larger than our own.
Earth has a reservoir of water
like this. It is bound up in a mineral
called ringwoodite, deep
underground in our planet’s mantle.
But at higher pressures than those
in the mantle, we are unsure how
these sorts of water-bearing,
or hydrous, minerals behave.
Masayuki Nishi at Ehime
University in Japan and his
colleagues investigated using
a mineral made of aluminium,
oxygen and hydrogen. “We
succeeded in observing the
hydrous mineral under pressures
far higher than those in previous
studies,” says Nishi.
To mimic the heat and pressure
at the centre of large planets, the
researchers squeezed samples
of the mineral between two small
diamonds and heated them with
laser beams. They then used X-rays
to examine the crystal structure.
At high pressures, they found,
the aluminium hydroxide shifted
to a new phase with a sturdier
structure. The water bound up in
the mineral remained there even
under incredibly high pressures and
at temperatures well over 2000°C
(Icarus, doi.org/dfmh).
Nishi suspects that many hydrous
minerals can be stabilised under
much higher pressures than we
had thought possible, although
it is difficult to be sure because
testing how each mineral behaves
at high pressure is challenging.
Hydrous minerals might act as an
underground reservoir for surface
water on large terrestrial exoplanets
called super-Earths and help them
to maintain liquid oceans. ❚
Huge exoplanets
could host water
deep underground
EARTH’S climate may change far
more abruptly and dramatically
than we predicted. Regions of
the planet that are thousands of
kilometres apart may influence
each other, causing the global
climate to lurch into a new state.
Climatologists have long
suspected that parts of the
planet will change dramatically
and irreversibly if they are
warmed past a certain
tipping point.
One such place is the
Greenland ice sheet. Warmer
temperatures are melting the
ice, so its upper surface is now
at a lower altitude – where the
air is warmer and more melting
will occur.
It isn’t clear how much the
climate needs to warm relative
to pre-industrial levels to trigger
irreversible melting of this ice
sheet, but one study suggested
1.6°C would be enough.
That is alarming, but in recentyears scientists have realised
that the various tipping
elements can interact: one
tipping point could trigger
another, like dominoes.
For example, if the Greenland
ice sheet passes its tipping point
and starts melting irretrievably,
it will dump cold water into the
north Atlantic Ocean. This could
collapse a vast ocean current
called the Atlantic Meridional
Overturning Circulation(AMOC), causing rapid sea
level rise along the US eastern
seaboard and playing havoc
with the West African monsoon.
Now a mathematical analysis
of tipping points suggests that
in some cases it could be even
worse than that.The new study looked at what
can happen if two elements
influence each other.
It turns out there is a nasty
surprise: the two elements can
start changing irreversibly at a
lower temperature, so tipping
points may arrive sooner
(arxiv.org/abs/1910.12042).
“There might be a possibility
that certain feedbacks between
tipping elements lead to
earlier than expected tipping
of the connected system,”
says study co-author Jonathan
Donges of the Potsdam Institute
for Climate Impact Research
in Germany.
The study is an abstract
simulation rather than an
attempt to model real-world
tipping points like those that
could impact the Greenland
ice sheet or the AMOC. Even so,
the researchers think it could
be applicable to the real world.
In theory, that could mean the
Greenland ice sheet will pass its
tipping point and start melting
unstoppably before the global
climate has warmed by 1.6°C.
However, Donges cautions
that the model the team used
is “very stylised”.
Nevertheless, the analysis
is “very convincing”, says
Anna von der Heydt of Utrecht
University in the Netherlands.
She says such premature tipping
could well turn out to be real,
and it is important to find out.
A link between Greenland
and the AMOC is plausible, says
Juan Rocha at the Stockholm
Resilience Centre in Sweden.
“They are large, [geographically]
close, and their consequences
are strong enough as to affect
each other.” ❚Earth’s tipping points may
be closer than we thought
JASONEDWARDS/GET
TYIM
AGES
/NATIONAL^ GEO
GRAPHICIM
AGE^ COLLECTIONMeltwater flows
from the Greenland
ice sheet“Regions of the planet
that are thousands of
kilometres apart may
influence each other”