Public Release: 

AGU Journal highlights -- March 21, 2007

American Geophysical Union

1. Solar blast from the past dwarfed modern ozone destruction

A burst of protons from the Sun in 1859 destroyed several times more ozone in Earth's atmosphere than did a 1989 solar flare that was the strongest ever monitored by satellite, a new analysis finds. When energetic protons from the Sun penetrate Earth's stratosphere, they ionize and dissociate nitrogen and oxygen molecules, which then form ozone-depleting nitrogen oxides. Thomas et al. developed a scale factor between known nitrate enhancements from recent solar proton events. By using data on nitrate enhancements in Greenland ice cores following the September 1859 burst, they used the scale factor to determine that the total energy released by that solar proton event was 6.5 times larger than the amount released in the 1989 event. Models using this energy total showed that 3.5 times more ozone was destroyed in the 1859 episode than in that of 1989. Because ozone regulates the amount of harmful ultraviolet radiation reaching Earth, the authors emphasized that understanding intense solar proton events will be important to predicting potential damage to the biosphere.

Title: Modeling atmospheric effects of the September 1859 solar flare

Authors: B. C. Thomas: Department of Physics and Astronomy, Washburn University, Topeka, Kansas, U.S.A.;

C. H. Jackman: Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;

A. L. Melott: Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029174, 2007

2. Deep-diving West Coast plate may have triggered massive central U.S. quake

Scientists studying extraordinary earthquakes in the central U.S. in 1811-12 along the New Madrid seismic zone have revealed a possible new driving mechanism for intraplate seismicity. The earthquakes were unprecedented in the historical record within stable continental plate interiors. Forte et al. analyze viscous flow models of the mantle based on high-resolution seismic tomography. They find that remnants of the ancient Farallon plate, a slab of crust swallowed beneath the western North American continental margin nearly 70 million years ago, continue to descend into the deep mantle under central North America. The descent induces mantle flow towards the Earth's deep interior directly below the New Madrid seismic zone. That flow, in turn, may strain the overlying crust, causing seismic ruptures, the authors hypothesize. Their results may shed light on present-day seismic hazards in the central Mississippi River Valley.

Title: Descent of the ancient Farallon slab drives localized mantle flow below the New Madrid seismic zone

Authors: A. M. Forte and R. Moucha: Départment des Sciences de la Terre et de l'Atmosphère, Centre de Recherche en Géochimie et en Géodynamique, Université du Québec à Montréal, Montréal, Quebec, Canada;

J. X. Mitrovica: Department of Physics, University of Toronto, Toronto, Ontario, Canada;

N. A. Simmons and S. P. Grand: Jackson School of Geological Sciences, University of Texas at Austin, Austin, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027895, 2007

3. Dry winters in north Mediterranean stoke hot European summers

Shortages of winter rainfall over southern Europe precede hot summers further north on the continent, a new study shows. From an analysis of meteorological records spanning the past 58 years, Vautard et al. determine that parched conditions around the northern Mediterranean create a mass of anomalously warm, dry air that spreads northward in early summer. The researchers detected the pattern while searching for previously undiscovered weather scenarios that could help explain heat waves such as the one experienced by Europe in the summer of 2003. That hot spell severely impacted human health, vegetation productivity, and air quality. Many models predict a boost in frequency of extreme weather events, such as heat waves, as human influence on climate grows. Because dry soil inhibits convective cloud formation, the authors suggest that soil water contents in Europe's Mediterranean regions play a critical role in the maintenance of climate across Europe. Numerical-simulation experiments of regional atmospheric circulation support that conclusion. The investigators also find that previous winter and early spring rainfall frequency in the Mediterranean regions correlates with summer temperature in continental Europe.

Title: Summertime European heat and drought waves induced by wintertime Mediterranean rainfall deficit

Authors: R. Vautard: Laboratoire des Sciences du Climat et de l'Environnment, L'Institut Pierre-Simon Laplace, Gif sur Yvette, France; and Laboratoire de Météorologie Dynamique, École Polytechnique, Palaiseau, France;

P. Yiou, N. de Noblet, N. Viovy, P. Ciais, and M. Kageyama: Laboratoire des Sciences du Climat et de l' Environnment, L'Institut Pierre-Simon Laplace, Gif sur Yvette, France;

F. D'Andrea and J. Polcher: Laboratoire de Météorologie Dynamique, École Polytechnique, Palaiseau, France;

C. Cassou: European Center for Research and Advanced Training in Scientific Computation/Centre National de la Recherche Scientifique, Toulouse, France;

Y. Fan: National Centers for Environmental Prediction, Camp Springs, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028001, 2007

4. Calculating a sharper view of Moon geochemistry

A method of processing lunar image data significantly improves how finely scientists can discern a key geochemical feature of the Moon's surface, a new study finds. In past studies, researchers have used data from a gamma-ray spectrometer aboard NASA's Lunar Prospector to investigate thorium's distribution on the lunar surface. Their scrutiny revealed that the Moon has distinct geochemical provinces, a factor that influences theories of the Moon's formation history and evolution. To improve spatial resolution, Lawrence et al. compared two different methods for enhancing the spatial contrast, resolution, and information density of the image data. One technique relies on iterative filtering to extract data from signal noise. The other seeks to smooth data and noise into a more cohesive image. The authors found that the data-smoothing method better represents the thorium abundances in actual features, improving image resolution by at least 50 percent.

Title: Global spatial deconvolution of Lunar Prospector Th abundances

Authors: D. J. Lawrence, R. C. Elphic, W. C. Feldman, J. J. Haggerty and T. H. Prettyman: Group ISR-1, Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A.;

R. C. Puetter: Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, California, U.S.A.; also at PixonImaging LLC, San Diego, California, U.S.A.;

P. D. Spudis: Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028530, 2007

5. Lake Superior summer temperatures rising faster than regional air temperatures

A new analysis of data from buoys, weather stations, and historical ice records indicates that summer surface temperatures of Lake Superior have increased approximately 2.5°C since 1979, roughly twice the rate of regional atmospheric warming. Austin and Colman hypothesize that declining winter ice cover is causing the lake to absorb more solar radiation than it did in past years. The increased absorption, in turn, causes earlier stratification of the lake at a rate of roughly half a day per year. Large mid-latitude lakes often freeze over in winter, mix thoroughly during spring and fall, and stratify in summer due to solar heating. The earlier start to Lake Superior's stratified season significantly increases the period over which the lake warms during the summer months, resulting in higher summer temperatures. Though little-studied, the response of large lakes to climate change will likely have an important regional effect.

Title: Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: a positive ice-albedo feedback

Authors: Jay A. Austin: Large Lakes Observatory and Department of Physics, University of Minnesota, Duluth;

Steven M. Colman: Large Lakes Observatory and Department of Geology, University of Minnesota, Duluth.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029021, 2007

6. Whatever the warming, ocean acidifies from carbon-dioxide buildup

A new study indicates that future changes in ocean acidification caused by atmospheric carbon dioxide emissions are largely independent of the amount of climate change caused by those emissions. The ocean plays a major role in the uptake of carbon dioxide emitted from fossil-fuel burning, helping to moderate future climate change. However, the addition of the gas to the ocean alters marine chemistry by increasing acidity (decreasing pH), posing a threat to shelled organisms and the predators that feed off them. Cao et al. seek to quantify the effect of climate change on ocean acidity and on the calcium-carbonate minerals that form shells and skeletons. Using an Earth system model, the researchers find that ocean pH declines by 0.31 units by the end of this century if atmospheric carbon dioxide concentrations continue on a trajectory that ultimately stabilizes at 1,000 parts per million. This increase in acidity occurs regardless of how much of a global-warming-related temperature rise takes place as carbon dioxide builds up to that concentration.

Title: Effects of carbon dioxide and climate change on ocean acidification and carbonate mineral saturation

Authors: Long Cao and Atul. K. Jain: Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois, U.S.A.;

Ken Caldeira: Department of Global Ecology, Carnegie Institution, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028605, 2007

7. Mineral physics illuminates lower mantle hypothesis

Scientists have shown increased interest in a mantle layer known as D", presumed to be just above the core-mantle boundary, since laboratory experiments in 2004 revealed a possible new high-temperature, high-pressure, crystal packing structure derived from the common mantle mineral perovskite. Noting that recent research has also hypothesized the existence above and below the D" layer of discontinuities in the velocities of seismic waves, Hernlund and Labrosse investigate whether such discontinuities are theoretically predicted by mineral physics. Using independent constraints for a lower bound on temperature in the Earth's deep mantle and for the temperature of the Earth's inner core boundary, the authors examine the nature of the transition between perovskite phases, as hypothesized by the existence of the D" layer and the observed seismic velocity anomalies. They find it consistent only with part of the range of uncertainties in current knowledge of the pressure-temperature behavior of minerals at such depths.

Title: Geophysically consistent values of the perovskite to post-perovskite transition Clapeyron slope

Authors: J. W. Hernlund: Institute de Physique du Globe de Paris, Paris, France;

S. Labrosse: Laboratoire des Sciences de la Terre, École Normale Supérieure de Lyon, Lyon, France

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028961, 2007


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