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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