Download New model better estimates mantle melt percentage

Eos, Vol. 95, No. 39, 30 September 2014
PAGE 360
Between 40 and 50 million years ago, the
Indian and Eurasian continental plates collided, eventually forming the Himalayas and
the Tibetan Plateau. If an average crustal thickness is assumed before the collision occurred,
recent research reveals that what is left of
the Eurasian and Indian plates after the collision is 30% less than what it should be—a
finding that has puzzled scientists because
the modern Eurasian plate is twice the normal crustal thickness in Tibet.
Some researchers have proposed that
portions of Eurasia were even thicker than
average prior to collision, but if that were the
case, an even larger amount of crust from
the Asian plate would have to have been
recycled into the mantle to explain the missing mass. However, Yakovlev and Clark have
a different hypothesis.
Using data from recently published paleomagnetic and geophysical studies, the authors found that the mass deficit could be
explained by a thin crust prior to collision.
The Asian plate would have grown to today’s
Petr Yakovlev
Thin precollision crust can explain
aspects of Indo-Asian convergence
Summer in the Kunlun Mountains, which mark the northern edge of the high (>4500 meters
tall) Tibetan Plateau.
thickness via shortening and underplating
from the Indian plate. These findings will
help further the understanding of the
Observing the Birkeland currents
AP
When the supersonic solar wind
(AMPERE), based on the 66 Iridium sathits the Earth’s magnetic field, a powellites orbiting the Earth, Anderson et al.
erful electrical connection occurs
have discovered that Earth’s response
with Earth’s field, generating millions
to onsets in forcing from the solar wind
of amperes of current that
occurs in two distinct stages.
JH
drive the dazzling auroras.
Currents first appear
U
These so-called Birkenear noon in the polar
land currents connect
regions and remain
the ionosphere to
steady for about half
the magnetosphere
an hour. Then the
and channel solar
second stage begins,
wind energy to
when strong curEarth’s uppermost
rents appear near
atmosphere. Solar
midnight and evenstorms release tortually join the initial
rential blasts of solar
currents near noon.
wind that cause much
Most of the solar wind
stronger currents and can
energy is deposited in the
overload power
polar atmosphere
grids and disrupt
by processes initiResults from the Active Magnetosphere
communications
ated in the second
and Planetary Electrodynamics Response
and navigation.
stage. The authors
Experiment (AMPERE) showing magnetic
For the first
note that scienfield signals. Different colored arrows inditime, scientists are
tists are working
cate data from different satellites.
making continto understand
uous, global measurements of the Birkehow the delay between the first and
land currents, opening a new window
second stages could give near-term
on our understanding of our home planwarning of impending space weather
et’s response to solar storms. Using the
disruptions. (Geophysical Research
Active Magnetosphere and Planetary
Letters, doi:10.1002/2014GL059941, 2014)
Electrodynamics Response Experiment
—JW
L
© 2014. American Geophysical Union. All Rights Reserved.
Indo-Eurasian collision and how it evolved.
(Tectonics, doi:10.1002/2013TC003469,
2014) —JW
Are flood damage models useful
outside their original context?
For researchers and decision makers trying
to understand the risks of extreme weather,
complex damage prediction models—which
in some cases consider physical, sociological,
and economic variables—are proving to be a
valuable tool. Yet at the same time, the effects
of extreme weather are highly context specific: A snow storm easily shrugged off by
Canadians can cripple the southwest United
States, for instance.
In their research, Schröter et al. question
whether existing flood damage models are
too context- specific to be widely useful, and if
not, which model constructions lend themselves to broad applicability. The authors used
eight different flood damage prediction models to calculate the damage done by three historic floods that took place in two different
regions of Germany.
The flood prediction models were of various degrees of complexity: One had only
1 input variable, another had 10. In all, the
eight models relied on 28 different input variables, ranging from flood water height to
household income and parameterizations
meant to represent human disaster response.
These model calculations were compared
against observations of flood damage.
Classically, complex flood prediction models struggle to predict damages for events and
locations other than those for which they
were tuned. The authors found that though
Eos, Vol. 95, No. 39, 30 September 2014
models that considered more input variables
performed better in new contexts, the increased complexity also brought increased
uncertainty. They found that probabilistic
models tended to do best in reliably predicting damage when applied to different contexts. (Water Resources Research, doi:10.1002/
2013WR014396, 2014) —CS
Lightning channel current
persists between strokes
The usual cloud-to-ground lightning occurs
when a large negative charge contained in
a “stepped leader” travels down toward the
Earth’s surface. It then meets a positive charge
that comes up tens of meters from the ground,
resulting in a powerful neutralizing explosion
that begins the first return stroke of the lightning flash. The entire flash lasts only a few
hundred milliseconds, but during that time,
multiple subsequent stroke–return stroke
sequences usually occur.
Some researchers have proposed that a residual current must flow through the ionized
channel of air left by the first stroke to allow the
propagation of the next stroke, called a dart
leader, but the prevailing—although unproven
—view today is that the current decays to zero
amperes before subsequent dart leaders and
their following return strokes occur.
To test the zero-current hypothesis, Ngin
et al. triggered lightning strikes using rockets
and wires. The authors found that rather than
decaying to zero, residual currents between
2 and 8 milliamperes remained prior to dart
leaders. However, aspects of the residual current remain unknown, including its source.
The authors note that the sensitivity of their
measurements is unprecedented and may
help improve models describing the evolution
of the ionized lightning channels and the process known as current cutoff. (Geophysical
Research Letters, doi:10.1002/2014GL059601,
2014) —JW
Models of ozone changes
do not agree with real measurements
Ozone (O3), an important component of the
Earth’s atmosphere, is created by natural and
anthropogenic processes. It is also a greenhouse gas, contributing to climate warming.
Modeling the changes in concentrations of
greenhouse gases, including O3, helps scientists project the effects of these gases in the
future. However, some scientists have raised
questions about the accuracy of long-term
trends of ground-level O3 calculated by models, prompting Parrish et al. to take a closer
look.
The authors looked at three model calculations of O3 concentrations, focusing on their
changes over the past several decades. They
then compared these model calculations with
actual measurements taken by ground- and
air-based instruments over the same time
period.
The authors found that the models significantly underestimated the change in lower
troposphere O3 at northern midlatitudes over
the past 5–6 decades, implying that its contribution to radiative forcing may currently be
greater than previously thought. These disagreements, the authors note, indicate that
“limited confidence” can be placed on current
estimates of radiative forcing by O3 and that
resolving the discrepancies will improve scientists’ ability to predict O3 concentrations in
the future. (Journal of Geophysical Research:
Atmospheres, doi:10.1002/2013JD021435, 2014)
—JW
New model better estimates
mantle melt percentage
Studying structures on Earth that arise
because of mantle melt—hot spots, mid-ocean
ridges, and continental rifts—helps scientists
understand the evolution and dynamics of
the planet. To quantify the fraction of mantle
melting, scientists analyze seismic waves that
travel through the mantle and crust, or they
conduct a magnetotelluric (MT) survey, which
measures the natural variations of the mantle’s electrical and magnetic fields.
The interpretation of seismic and electrical
measurements in terms of melt fraction does
not always agree; in several cases, MT surveys propose a higher melt percentage than
seismic work. Pommier and Garnero note
that this may be because MT surveys rely on
laboratory-acquired conductivity data and
do not account for the fact that in the Earth,
melting processes yield changes in melt
chemistry during partial melting, leading to
changes in melt electrical conductivity.
The authors explored this discrepancy with
a petrology-based electrical model that accounts for the way the chemical composition
—and thus electrical conductivity—of mantle
rock changes as it melts. The authors gathered data from previous studies to build their
model and compared their modeled measurements with MT and seismic estimates from
melt-related structures around the world (e.g.,
Hawaii, the Mid-Atlantic Ridge, and the Afar
Ridge).
Their model shows that less melt is needed
to explain the electrical conductivity values
than what MT studies suggest, reconciling the
discrepancy with lower melt fraction estimates
from seismic studies. The results emphasize
a more constrained, joint electrical- seismic
approach to determine melt in the mantle and
thus further our understanding of the Earth’s
inner dynamics. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2013JB010449,
2014) —JW
Observing the onset
of a magnetic substorm
Magnetic substorms, the disruptions in geomagnetic activity that cause brightening of
auroras, may sometimes be driven by a different process than generally thought, a new
study shows.
Hwang et al. report observations using the
Cluster spacecraft and ground-based mag-
© 2014. American Geophysical Union. All Rights Reserved.
netometers associated with the onset of a
substorm. They saw two consecutive sudden
jumps in the current sheet normal component of the magnetic field in the plasma sheet
(the surface of dense plasma that lies approximately in Earth’s equatorial plane), separated
by about 5 minutes. The first magnetic field
enhancement, along with a series of other
magnetic structures and a region of rarefied
plasma, propagated outward away from Earth;
the second magnetic field enhancement (dipolarization front) rapidly propagated toward
Earth.
The researchers argue that the observed
sequence of events suggests that a disruption
in the current sheet originated near Earth and
moved toward the magnetotail, where it facilitated magnetic reconnection (the breaking
and reconnecting of magnetic field lines,
which releases energy), creating conditions
for substorm enhancement. This differs from
the more commonly accepted scenario in
which a substorm begins with magnetic reconnection in the magnetotail. (Journal of Geophysical Research: Space Physics, doi:10.1002/
2013JA019657, 2014) —EB
Ocean warming could drive
heavy rain bands toward the poles
In a world warmed by rising atmospheric
greenhouse gas concentrations, precipitation
patterns will change because of two factors:
First, warmer air can hold more water, and,
second, changing atmospheric circulation
patterns will shift where rain falls. According
to previous model research, middle- to highlatitude precipitation is expected to increase
by as much as 50%. However, the reasons
models predict this are hard to tease out.
Using a series of highly idealized model
runs, Lu et al. found that ocean warming
should cause atmospheric precipitation bands
to shift toward the poles. The changes in atmospheric circulation brought on by a warming
ocean should cause an increase in the intensity and frequency of extreme precipitation
events at middle and high latitudes as well
as a reduction in the same near the equator.
The changes would mean that for high-latitude
regions, now rare storms would become much
more common.
The authors tested the effect of ocean
warming on atmospheric circulation and precipitation using a highly idealized “aquaplanet” model, a representation of the Earth
that was just sea and sky but no land. They
ran the model at a range of spatial resolutions
and found that the changes in precipitation
that stem from changing circulation patterns
may outweigh changes that derive from
other factors. (Geophysical Research Letters,
doi:10.1002/2014GL059532, 2014) —CS
—ERNIE BALCERAK, Staff Writer; COLIN SCHULTZ,
Writer; and JOANNA WENDEL, Staff Writer