Public Release:  AGU journal highlights -- July 2, 2009

American Geophysical Union

1. Ancient supervolcano's eruption caused decade of severe winters

Previous studies have suggested that Indonesia's Toba supervolcano, when it erupted about 74,000 years ago, triggered a 1,000-year episode of ice sheet advance, and also may have produced a short-lived "volcanic winter," which drastically reduced the human population at the time. Previous climate model simulations of the eruption have been unable to produce the glaciation, and there are no climate observations to support the volcanic winter. To investigate additional mechanisms that may have enhanced and extended the effects of the Toba eruption, as well as the volcanic winter, Robock et al. conduct six climate model simulations using state-of-the-art models that include vegetation death effects on radiation budgets, and stratospheric chemistry feedbacks that might affect the lifetime of the volcanic cloud. The authors use a wide variety of aerosol injection volumes, ranging from 33 to 900 times that of the 1991 Mount Pinatubo injection. They find that none of the models initiate glaciation. Nonetheless, they produce a decade of severe volcanic winter, which would likely have had devastating consequences for humanity and global ecosystems, supporting the idea that the Toba eruption produced a genetic bottleneck in human evolution.

Title: Did the Toba volcanic eruption of ~74 ka B.P. produce widespread glaciation?

Authors: Alan Robock and Georgiy Stenchikov: Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey, USA;

Caspar M. Ammann and Samuel Levis: National Center for Atmospheric Research, Boulder, Colorado, USA;

Luke Oman: Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland, USA;

Drew Shindell: NASA Goddard Institute for Space Studies, New York, New York, USA.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011652, 2009; http://dx.doi.org/10.1029/2008JD011652


2. Understanding fault movement during Wenchuan earthquake

The magnitude 7.9 earthquake on 12 May 2008 in Wenchuan, China, killed more than 87,000 people, injured 370,000, and caused extensive damage. Because some areas in the fault zone were inaccessible, scientists did not immediately have a detailed understanding of the fault movement during this event. To learn more, Hao et al. combine field investigations of the Longmen Shan fault zone with satellite observations to obtain details of the crustal deformation over the entire fault zone. They also create a model to simulate the motion of the fault zone. The model shows that thrust fault slips were dominant along the segment of the Yingxiu-Beichuan fault near the epicenter, while dextral fault slips dominated along the northeast segment of that fault. On the basis of observations and simulations, the authors conclude that the simultaneous ruptures of the Yingxiu-Beichuan and Guanxian-Anxian faults caused the Wenchuan earthquake. The results should improve scientists' understanding of the motion of the Longmen Shan fault zone during this disastrous earthquake.

Title: Coseismic surface-ruptures and crustal deformations of the 2008 Wenchuan earthquake Mw7.9, China

Authors: Ken Xiansheng Hao, Hiroyuki Fujiwara, and Taku Ozawa: National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan;

Hongjun Si: Kozo Keikaku Engineering Inc., Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037971, 2009; http://dx.doi.org/10.1029/2009GL037971


3. First direct measurement of Lunar backscatter from solar wind

When the solar wind, made up mostly of ionized hydrogen, hits the Moon, most of it is absorbed, but some is reemitted as energetic neutral atoms. This lunar backscatter and neutralization had been predicted but not directly measured until now. Using NASA's Interstellar Boundary Explorer satellite, launched on 19 October 2008, McComas et al. have for the first time observed energetic neutral atoms scattered off the Moon from the incoming solar wind ion beam. They find that about 10 percent of solar wind ions hitting the Moon result in the emission of neutral atoms, corresponding to about 150 metric tons of hydrogen emitted from the Moon per year. They also show that the energy spectrum and numbers of neutral atoms emitted from the Moon trace the variations in the incident solar wind. The authors suggest that the findings could shed additional light on the solar wind's interactions with other objects in the solar system, such as dust grains, asteroids, and moons of other planets, and could provide clues to the evolution of dust and rocky moons in other planetary systems.

Title: Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon

Authors: D. J. McComas and F. Allegrini: Southwest Research Institute, San Antonio, Texas, USA;

P. Bochsler, H. Kucharek, and E. Möbius: Space Science Center, University of New Hampshire, Durham, New Hampshire, USA;

P. Frisch: Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois, USA;

H. O. Funsten: Los Alamos National Laboratory, Los Alamos, New Mexico, USA;

M. Gruntman: Astronautics and Space Technology Division, University of Southern California, Los Angeles, California, USA;

P. H. Janzen and D. B. Reisenfeld: Department of Physics and Astronomy, University of Montana, Missoula, Montana, USA;

N. A. Schwadron: Department of Astronomy, Boston University, Boston, Massachusetts, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038794, 2009; http://dx.doi.org/10.1029/2009GL038794


4. Reducing uncertainty in estimates of global sea level rise

Accurate information about long-term sea level changes is important for the study of global climate change. Most estimates of global sea level rise over the past century are based on long tide gauge records. However, these records produce a wide range of estimates of global sea level because they do not generally take into account all of the vertical motions of the land on which the tide gauges are located. Wöppelmann et al. show that they can reduce the uncertainty in global sea level estimates by using GPS observations to correct for vertical land motions. They analyze GPS observations from 227 stations around the world taken from January 1997 to November 2006 to compute vertical velocities and uncertainties in vertical land motion, and use these data to correct long tide gauge records. Their calculations lead to an estimate of global sea level rise of 1.61 ± 0.19 millimeters (0.0634 ± 0.0075 inches) per year over the past century, in good agreement with other recent estimates. The authors believe the method could help scientists improve understanding of sea level rise and variability.

Title: Rates of sea-level change over the past century in a geocentric reference frame

Authors: G. Wöppelmann and C. Letetrel: LIENSS, UMR6250, Université de La Rochelle, La Rochelle, France;

A. Santamaria: Instituto Geográfico Nacional, Madrid, Spain and LAREG/IGN et GRGS, Marne-la-Vallée, France;

M.-N. Bouin: LAREG/IGN et GRGS, Marne-la-Vallée, France and Météo-France, CMM, CNRM, Brest, France;

X. Collilieux and Z. Altamimi: LAREG/IGN et GRGS, Marne-la-Vallée, France;

S. D. P. Williams: Proudman Oceanographic Laboratory, Liverpool, UK;

B. Martin Miguez: Intergovernmental Oceanographic Commission of UNESCO, Paris, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038720, 2009; http://dx.doi.org/10.1029/2009GL038720


5. Boost in freshwater content of Arctic Ocean

Fresh water flowing into or out of the Arctic Ocean plays an important role in ocean circulation and may be a factor in the response of the world ocean to climate change. To study recent change in freshwater content of the Arctic, McPhee et al. analyze data from an extensive aerial hydrographic survey carried out in March and April 2008. Their study shows a dramatic increase in the amount of fresh water in the Arctic Ocean as well as a significant change in the distribution of fresh water, as compared with average winter values. In particular, the authors find that freshwater volume in the Canada and Makarov basins on the Pacific side of the Lomonosov Ridge increased by about 8,500 cubic kilometers (about 2,000 cubic miles), while the freshwater volume on the Eurasian area decreased by about 1,100 cubic kilometers (about 260 cubic miles). The freshening of the Arctic occurred in conjunction with the recent dramatic loss of Arctic sea ice, the authors note. They find that these changes have altered Arctic Ocean circulation, with a large increase in northward transport of fresh water in the Canada Basin.

Title: Rapid change in freshwater content of the Arctic Ocean

Authors: M. G. McPhee: McPhee Research Co., Naches, Washington, USA;

A. Proshutinsky: Woods Hole Oceanographic Institute, Woods Hole, Massachusetts, USA;

J. H. Morison and M. Steele: Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, USA;

M. B. Alkire: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037525, 2009; http://dx.doi.org/10.1029/2009GL037525


6. Data gaps in records hinder detection of climate trends

The Earth's climate is driven by exchange of energy among the Sun, Earth, and space. This is affected by greenhouse gases, clouds, surface properties, and aerosols. Several instruments monitor climate, and to help agencies evaluate potential start times of new missions, Loeb et al. investigate how gaps in the record affect scientists' ability to fully understand climate. Focusing on cloud radiative effects, the authors simulate a 30-year climate data record based on the first five years of data from the Clouds and the Earth's Radiant Energy System (CERES) instrument on board NASA's Terra satellites. On the basis of analyses of accuracy and precision errors, the authors find that a gap of any length anywhere in the record will significantly increase the time required to detect a trend above natural variability because data collected prior to and after the gap cannot be combined accurately enough to ensure trend detection at the required level. To avoid gaps, the authors recommend that at least six months of global or one year of tropical overlapping measurements between successive instruments are needed.

Title: Impact of data gaps on satellite broadband radiation records

Authors: Norman G. Loeb, Bruce A. Wielicki, Takmeng Wong, and Peter A. Parker: NASA Langley Research Center, Hampton, Virginia, USA.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011183, 2009; http://dx.doi.org/10.1029/2008JD011183


7. Glaciers cause seismic activity in Iceland

In volcanic regions, repeating long-period (lp) earthquakes occur often and are sometimes thought to signal an imminent eruption. Recently, however, some of these earthquake events have been found to be associated with ice movement rather than with volcanic activity. To accurately assess volcanic hazards, scientists need to correctly identify the source of earthquake activity. Jónsdóttir et al. analyze climatic and seismic data from Katla volcano, Iceland. Their study, covering more than 13,000 lp events since 2000, indicates that earthquake activity was seasonal and clearly correlated with climatic changes associated with increased ice movement. They also note that the seismic activity has been continuous for years, with no sign of volcanic eruption. They conclude that the lp events recorded in the region were caused by glacial movements, not volcanic activity, as previously thought. Although the results are specific to the Katla volcano region, the authors suggest that global warming could lead to increasing glacier-induced earthquake activity at other glacier-covered volcanoes.

Title: Glacial long period seismic events at Katla volcano, Iceland

Authors: Kristín Jónsdóttir, Roland Roberts, Veijo Pohjola, Björn Lund, Zaher Hossein Shomali, Ari Tryggvason, and Reynir Böðvarsson: Department of Earth Science, Uppsala University, Uppsala, Sweden.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038234, 2009; http://dx.doi.org/10.1029/2009GL038234


8. Features of Medieval Warm Period in China's Tianshan Mountains

During the Medieval Warm Period (MWP), which lasted from the ninth to the fourteenth century, the climate in Europe was unusually warm. Although the MWP climate is best documented in Europe, various studies have suggested that a climate anomaly also occurred in most of China during the same time period. Zhang et al. investigate the climate during the MWP on the northern slope of the Tianshan Mountains in Xinjiang in northwestern China. To reconstruct climate in the region, they analyze three sediment profiles sampled from sites at different elevations and vegetation zones in the region, looking at pollen counts and types, grain size, and other data. They also consider other records, including plant seeds, tree rings, and historical documents. On the basis of all of these lines of evidence, the authors conclude that the climate was humid on the northern slope of the Tianshan Mountains during the MWP. They note that other studies have found that the MWP was warm and dry in the southern part of Xinxiang, so the MWP was manifested differently in different regions of China.

Title: "Medieval Warm Period" on the northern slope of central Tianshan Mountains, Xinjiang, NW China

Authors: Yun Zhang and Zhao Chen Kong: State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China;

Shun Yan: Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, China;

Zhen Jing Yang: Institute of Hydrologic and Environmental Geology, Chinese Academy of Geological Sciences, Zhengding, China;

Jian Ni: State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China; and Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037375, 2009; http://dx.doi.org/10.1029/2009GL037375


9. Radioactive isotope in ice core record reflects solar activity over 600 years

Climate scientists and solar physicists have increasingly been using beryllium-10 levels to gain information about past solar activity. Beryllium-10, a radioactive isotope, is produced in cosmic ray showers. Because stronger solar activity deflects cosmic rays away from Earth, low levels of beryllium-10 correlate with higher solar activity. However, few extensive beryllium-10 records exist. Berggren et al. describe a new 600-year beryllium-10 record from the North Greenland Ice Core Project (NGRIP), one of only two beryllium-10 records with annual resolution over several centuries. Comparing the NGRIP record with the Dye-3 record from another site in Greenland, they find that both show similar long-term trends with some short-term differences between the two sites. The authors also compare the NGRIP and Dye-3 data with sunspot records and with neutron data, which tracks cosmic ray intensity. Their results confirm that ice core beryllium-10 reflects solar cycle variations as well as longer-term trends in solar activity. They note that while recent beryllium-10 values are low, they do not indicate unusually high solar activity compared with the last 600 years.

Title: A 600-year annual beryllium-10 record from the NGRIP ice core, Greenland

Authors: A.-M. Berggren and A. Aldahan: Department of Earth Sciences, Uppsala University, Uppsala, Sweden;

J. Beer and J. Abreu: Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland;

G. Possnert: Tandem Laboratory, Uppsala University, Uppsala, Sweden;

P. Kubik and M. Christl: Laboratory for Ion Beam Physics, ETH, Zurich, Switzerland;

S. J. Johnsen and B. M. Vinther: Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038004, 2009; http://dx.doi.org/10.1029/2009GL038004


10. Isotope data suggest new history of the Arctic

Recent studies have suggested that beginning about 44 million years ago the Arctic underwent a transition from lake to ocean conditions. On the basis of new evidence from rhenium (Re) and osmium (Os) isotopes in the sedimentary record, Poirier and Hillaire-Marcel propose a new chronology of that change. To trace the Arctic's shift from a freshwater to a marine environment, the authors analyze Re and Os concentrations and Os isotope ratios in sedimentary material collected from the Lomonosov Ridge in the Arctic. Their analysis suggests that the Arctic was an enclosed freshwater lake until about 38 million years ago, when tectonic activity caused the Fram Strait to widen, connecting the Arctic to global ocean circulation and bringing about a fairly rapid transition from lake to ocean conditions. Previous studies had suggested that the transition from lake to ocean conditions was completed only about 17.5 million years ago, after a 26-million-year hiatus. The authors conclude that if the results are confirmed by further studies, they will give scientists a new perspective on the tectonic evolution of the Arctic.

Title: Os-isotope insights into major environmental changes of the Arctic Ocean during the Cenozoic

Authors: Andre Poirier and Claude Hillaire-Marcel: GEOTOP, Université du Québec à Montréal, Montreal, Quebec, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037422, 2009; http://dx.doi.org/10.1029/2009GL037422


11. Mixing in Earth's outer core causes geomagnetic dipole to collapse

For the past 160 years, the Earth's magnetic dipole has been weakening at a rate of nearly six percent per century. To gain an improved understanding of the mechanisms contributing to the dipole moment collapse, Liu and Olson perform numerical calculations that show how convective mixing flows in the Earth's liquid outer core can lead to a decreasing dipole moment. Their simulations show that as fluid mixes in the outer core, magnetic energy is transferred from the dipole to smaller scales, producing patches of reversed magnetic field at the core-mantle boundary and weakening the dipole. They demonstrate that the rate of dipole moment decay is weakly sensitive to the particular mixing flow pattern but varies with the magnetic Reynolds number, a measure of the velocity of the flow. In particular, the authors find that a mixing flow in the outer core with magnetic Reynolds number in the range of 200�, which they suggest is a physically reasonable range, could account for the historically measured rate of decrease of the geomagnetic dipole moment.

Title: Geomagnetic dipole moment collapse by convective mixing in the core

Authors: Lijun Liu: Seismological Laboratory, California Institute of Technology, Pasadena, California, USA;

Peter Olson: Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038130, 2009; http://dx.doi.org/10.1029/2009GL038130


12. Energetic particles slow solar wind at the edge of the solar system

In 2007, the Voyager 2 spacecraft crossed the solar wind termination shock, the boundary with interstellar space where the solar wind--the stream of charged particles emitted by the Sun--collides with the interstellar medium and slows from supersonic to subsonic speeds. In the 40 days prior to crossing the termination shock, Voyager 2 measured a gradual decrease in the solar wind speed from about 380 to 300 kilometers per second (236 to 186 miles per second), as well as an exponential increase in the intensity of energetic ions in the region. Using a simple two-fluid model and plasma and energetic particle conservation laws, Florinski et al. show that the ions, with energies of a few megaelectron volts, could produce enough pressure to create a precursor shock that caused the observed deceleration of the solar wind. The source of these ions has not been determined, but the study sheds light on this distant region at the edge of the solar system.

Title: An energetic-particle-mediated termination shock observed by Voyager 2

Authors: V. Florinski, J. A. le Roux, G. P. Zank: Department of Physics and Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, Alabama, USA;

R. B. Decker: Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038423, 2009; http://dx.doi.org/10.1029/2009GL038423

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