AGU Journal highlights - 25 March 2004
The following highlights summarize research papers in Global Biogeochemical Cycles (GB), Geophysical Research Letters (GL), Journal of Geophysical Research--Atmospheres (JD), Journal of Geophysical Research--Planets (JB), and Paleoceanography (PA). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
1. First observation of a thin layer in Earth's magnetic field
Stereo observations taken by the four Cluster satellites of the Earth's magnetic field reveal details about thin layers that influence the physics of particles passing through the magnetopause [the outer limit of Earth's magnetic field]. Andre et al. examined the properties of a thin layer discovered on the edge of the magnetic field boundary. The authors report that the layer, on the Earth side of the field, is approximately 20 kilometers [10 miles] wide and includes currents and strong electrical fields that influence passing solar wind particles and energy. Previous spacecraft and existing simulations had noted the layer's existence but had not been able to determine any of its properties with the paucity of available information. The magnetopause passed within the Cluster satellite formation in 2002, providing data used to infer the electrical field current and changes and assume some of the microphysical processes that govern the space region.
Title: Thin electron-scale layers at the magnetopause
Mats Andre, A. Vaivads, S. C. Buchert, Swedish Institute of Space Physics, Uppsala, Sweden;
A. N. Fazakerley, A. Lahiff, University College London, London, United Kingdom.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018137, 2004
2. Opportunity landing site on Mars may be iron coated
Orbiter observations of the landing site of the second Martian explorer, "Opportunity," indicated that the surface is made up of an iron oxide material known as coarse hematite, which is associated with water deposits. A new report proposes, however, that the surface deposits may be made up of a thinner, denser form of hematite that could hinder the identification of the role of water on Mars. Kirkland et al. report that the thermal emissions from Opportunity's Sinus Meridiani landing area reveal contrasting spectral bands indicative of a smooth-surfaced material and similar to the emissions expected from fine-grained hematite. A coating of such closely spaced hematite particles, while of possible astrobiological interest, could present a concrete-like varnish over the surface that may explain why the lander has not detected wind streaks in the crust or minerals related to ancient water deposition.
Title: A different perspective for the Mars rover "Opportunity" site: Fine-grained consolidated hematite and hematite coatings
Laurel E. Kirkland, Lunar and Planetary Institute, Houston, Texas, and The Aerospace Corporation, El Segundo, California;
K. C. Kerr, P. M. Adams, The Aerospace Corporation, El Segundo, California.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL019284, 2004
3. GPS method to track ground movement after earthquakes
A rapid processing technique using Global Positioning System data to measure the land movement following an 8.0-magnitude earthquake in Japan may provide a simpler method to improve researchers' knowledge of the seismic cycle after such massive tremblors. Miura et al. used GPS coordinates before and after the 2003 Tokachi-Oki earthquake to estimate the slip distribution on the plate boundary. The subducting tectonic plates near that portion of northern Japan move approximately 80 millimeters [three inches] per year, making them two of the most seismically active areas in the world. The GPS data were attributed to a maximum co-seismic slip of approximately five meters [20 feet] near the quake's offshore epicenter, which agree with other displacement estimates. The authors note that the existing GPS network in Japan can continue to measure the co- and post-seismic slip distribution around the plate boundaries.
Title: The 2003 M8.0 Tokachi-Oki earthquake: How much has the great event paid back slip debts?
Satoshi Miura, Yoko Suwa, Akira Hasegawa, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Japan;
Takuya Nishimura, Geography and Crustal Dynamics Research Center, Geographical Survey Institute, Tsukuba, Ibaraki, Japan.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL019021, 2004
4. Burning cools huge swath of Amazonia
Smoke from biomass burning in the Amazonian rain forests obscures sunlight and lowers ground-level radiative heating over nearly three million square kilometers [one million square miles]. Procopio et al. analyzed seven years of surface heating changes associated with aerosol emissions from burning in the South American rain forests from the early 1990s and found significant cooling along the ground during the region's dry season. They note that the majority of the cooling over a massive swath of land on clear days is from sunlight absorbed by black carbon particles contained in aerosols associated with smoke. At the same time the ground is cooling, the atmosphere above the smoky layer is warmed by reflected sunlight, which can further modify climate effects, including atmospheric circulation, evaporation rate, cloud formation, and precipitation. The report is the first long-term study examining the surface effects from a thick blanket of aerosol haze in the region.
Title: Multiyear analysis of Amazonian biomass burning smoke radiative forcing of climate
A. S. Procopio, Institute of Physics and Institute of Astronomy, Geophysics and Atmospheres, University of Sao Paulo, Sao Paulo, Brazil;
P. Artaxo, Institute of Physics, University of Sao Paulo, Sao Paulo, Brazil;
Y. J. Kaufman, L. A. Remer, J. S. Schafer, B. N. Holben, NASA/Goddard Space Flight Center, Greenbelt, Maryland.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018646, 2004
5. Warming trend seen in western North American temperature extremes
Newly developed digital air temperature archives indicate an intense warming in the lowest wintertime minimum temperatures recorded over western and central North America. Scott M. Robeson estimated the monthly temperature extremes from highly detailed new records to evaluate trends in climate change across the continent. He found that minimum air-temperature distribution over western North America during wintertime, including western Canada and Alaska, has warmed at rates exceeding three degrees Celsius [five degrees Fahrenheit] over the past 50 years. Traditional temperature analyses use average air temperatures over a larger region and are often unable to capture the full range of climate changes. Robeson notes that while the highest and lowest maximum air temperatures over western North America are warming equally, much of eastern North America has shown little change in its minimum and maximum temperatures over the past half-century.
Title: Trends in time-varying percentiles of daily minimum and maximum temperature of North America
Scott M. Robeson, Indiana University, Bloomington, Indiana.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL019019, 2004
6. Unusual lightning patterns in Tibetan Plateau
Satellite observations show unusual patterns of lightning flashes over the Tibetan Plateau during springtime that may not be predicted by current climate models. Toumi and Qie report that the seasonal frequency of lightning on the plateau, a region that plays a critical role in determining the Asian monsoon climate circulation, is better determined by ground-level heat flux than by other means of lightning prediction. They suggest that climate models for the region must account for the seasonal heat flux variations in order to accurately predict the observed lightning phenomena; otherwise they will likely underestimate the frequency of thunderstorms. The authors tracked daily lightning frequency data for nearly seven years in the region and compared their results with precipitation, cloud and temperature information taken simultaneously. The researchers conclude that total heat flux is the best predictor for seasonal lightning flash density and provides a necessary component to approximate the climate and water cycle at the plateau.
Title: Seasonal variation of lightning on the Tibetan Plateau: A Spring anomaly?
Ralf Toumi, Imperial College, London, United Kingdom;
Xiushu Qie, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, China.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018930, 2004
7. Revising manmade carbon levels in oceans
A new technique to estimate the manmade contribution to global carbon levels indicates that current studies may overestimate the amount of carbon stored in the global ocean. Hall et al. use a model that accommodates a range of mixing scenarios in the transport of manmade carbon from the ocean surface to the depths, which removes an assumption of weak mixing that affects many current studies. By analyzing measurements of independent trace chemicals, the researchers show that the strong mixing scenario is the most realistic and suggest that such a scenario results in lower manmade carbon estimates. The authors then applied their technique to the Indian Ocean--although they believe that their estimates for reduced carbon should apply globally--obtaining a range of present-day manmade carbon mass in the ocean. Their carbon levels projections of around 20 billion tons agrees with the upper range of currentestimates, but their lower threshold is significantly lower.
Title: Estimates of anthropogenic carbon in the Indian Ocean with allowance for mixing and time-varying air-sea CO2 disequilibrium
Timothy M. Hall, NASA Goddard Institute for Space Studies at Columbia University, New York, New York;
Darryn W. Waugh, Thomas W. N. Haine, Johns Hopkins University, Baltimore, Maryland;
Paul E. Robbins, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California;
Samar Khatiwala, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.
Source: Global Biogeochemical Cycles (GB) paper 10.1029/2003GB002120, 2004
8. Earliest evidence of methyl bromide
An ice core taken from the Antarctic reveals the earliest evidence of airborne methyl bromide, extending the historical record of the compound to over 300 years ago. Saltzman et al. found trace levels of methyl bromide in air bubbles from a shallow West Antarctic ice core dating to the year 1671. The compound, which contributes significantly to stratospheric ozone destruction, is included in the Montreal Protocol to phase out industrial production of stratospherically damaging chemicals. Methyl bromide is, however, the only chemical included in the Protocol that also has major natural sources. The ice core results demonstrate that human activities have significantly increased atmospheric levels of the gas and highlight the uncertainty remaining in understanding the modern atmospheric methyl bromide budget.
Title: Methyl bromide in preindustrial air: Measurements from an Antarctic ice core
Eric S. Saltzman, Murat Aydin, University of California, Irvine, California;
Warren J. De Bruyn, Chapman University, Orange, California;
Daniel B. King, Drexel University, Philadelphia, Pennsylvania;
Shari A. Yvon-Lewis, National Oceanographic and Atmospheric Administration, Miami, Florida.
Source: Journal of Geophysical Research-Atmospheres (JD) paper: 10.1029/2003JD004157, 2004
9. Unique radar properties of lava domes
Researchers have identified unique properties of silicic lava domes that can allow the features to be easily identified by radar observations on Earth and across the solar system. Plaut et al. report that the circular lava domes, formed by volcanic eruptions and magma flow, exhibit distinct surface roughness characteristics that reflect differently than other geologic surfaces when analyzed by specific radar observation techniques. The authors note that the topographic radar signature of the domes shows them to be among the roughest natural surfaces ever measured. They suggest that radar backscatter measurements can be used to accurately identify such domes in remote areas on Earth, in order to monitor changes or growth of the dome's surface area, and on other planets where the dome surfaces may provide clues to the composition of the erupted lavas.
Title: Unique radar properties of silicic lava domes
Jeffrey J. Plaut, Jakob J. van Zyl, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California;
Steven W. Anderson, Black Hills State University, Spearfish, South Dakota;
David A. Crown, Planetary Science Institute, Tucson, Arizona;
Ellen R. Stofan, Proxemy Research, Rectortown, Virginia.
Source: Journal of Geophysical Research-Planets (JE) paper: 10.1029/2002JE002017, 2004
10. Ice shelf disintegration in North Atlantic?
A new report suggests that the Heinrich icebergs in the North Atlantic could record an ice shelf collapse similar to the recent breakup of the Larsen ice shelf in Antarctica. Hulbe et al. propose that ice shelves formed along the eastern Canadian coast during cold extremes of long-term climate oscillations would have been vulnerable to rapid disintegration as warming began. Such vulnerabilities were observed in the collapse of the Larsen A and B ice shelves when surface meltwater filled fractures at the ice surface. The new hypothesis stands in contrast to prior ones suggesting that climate cooling might have preserved icebergs for long ocean journeys or that extreme ice sheet surges could have increased the number and debris content of icebergs at sea. The authors speculation that the Heinrich events were a consequence of global climate forcing also accounts for the nearly instantaneous change in ice-carried debris deposition indicated in the sedimentary record from ice cores in Portugal and Iceland.
Title: Catastrophic ice shelf breakup as the source of Heinrich event icebergs
Christina L. Hulbe, Portland State University, Portland, Oregon;
Douglas R. MacAyeal, University of Chicago, Chicago, Illinois;
George H. Denton, University of Maine, Orono, Maine;
Johan Kleman, Stockholm University, Stockholm, Sweden;
Thomas V. Lowell, University of Cincinnati, Cincinnati, Ohio.
Source: Paleoceanography (PA) paper: 10.1029/2003PA000890, 2004
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