AGU journal highlights - 28 January 2004
The following highlights summarize research papers in Geophysical Research Letters (GL), Global Biogeochemical Cycles (GB), and Journal of Geophysical Research--Planets (JE). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
1. Possible explanation for phenomenon observed prior to solar storms
A theoretical study that simulates the effects of solar materials as they reach Earth's magnetosphere describes a possible cause for large magnetic fluctuations observed during the onset of a space weather substorm. Ji and Wolf suggest that solar plasma ejected towards Earth can cause a temporary change in the normal incoming magnetic field, resulting in wild fluctuations that have previously been observed in near-Earth space early in the violent space weather phenomenon. The change slows the motion of magnetic waves coming towards Earth and causes a wrinkle of the magnetic field lines, termed a firehose instability, that affects the particle distribution in the region associated with the onset of a solar substorm. The authors conclude that the instabilities, which develop in only a few seconds, can explain the important and puzzling phenomenon.
Title: Firehose instability near substorm expansion-phase onset?
Shuo Ji and Richard A. Wolf, Rice University, Houston, Texas.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL017981, 2003
2. High frequency sea ice motion may affect Arctic dynamics
Satellite observations have confirmed that Arctic sea ice undergoes slight daily motion in winter. Kwok et al. report ice deformation estimates from RADARSAT imagery that observed the high Arctic several times per day. The new data provide evidence of oscillating ice movement that can add up to 10 centimeters [four inches] of ice thickness over six months during winter. Earlier studies had speculated that the ice motion would be inhibited by the strength of the ice pack during the Arctic winter. The authors note that the semi-diurnal ice motion observed during winter was not seen prior to the 1970s, which they suggest may be from a previous lack of detailed data or could be caused by the recent thinning of the Arctic ice cover. They conclude that current sea ice dynamics models should be modified to account for the small time-scale motions, which can have a significant contribution to the sea ice mass budget and affect the interactions among the Arctic Ocean and atmosphere.
Title: Sub-daily sea ice motion and deformation from RADARSAT observations
Ron Kwok, Glenn F. Cunningham, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California;
William D. Hibler III, University of Alaska, Fairbanks, Alaska.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL018723, 2003
3. The connection between drought and global warming
Researchers have identified a mechanism that may play a major role in determining whether rainfall increases or decreases in a region because of global warming. Neelin et al. analyzed tropical rainfall changes associated with El Nino variability and suggest that tropospheric warming linked with El Nino or global warming increases the amount of surface moisture contributing to cloud formation. Precipitation then rises as the moisture increases in the center of convective regions where small-scale atmospheric motion lead to cloud formation. The sum of the atmospheric processes leads to reductions in rainfall at the borders of convection zones that are near dryer regions. The authors used a climate model to simulate global warming and note that the mechanism is the leading cause of tropical drought and closely parallels a similar effect that causes El Nino drought areas.
Title: Tropical drought regions in global warming and El Nino teleconnections
J. D. (David) Neelin, H. Su, Institute of Geophysics and Planetary Science, University of California at Los Angeles, California;
C. Chou, Institute of Earth Sciences, Academia Sinica, Teipei, Taiwan, Republic of China.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL018625, 2003
4. Special section on Eastern Turkey Seismic Experiment
The 15 December 2003 issue of Geophysical Research Letters features a collection of papers about the Eastern Turkey Seismic Experiment, which was designed to analyze the early stages and consequences from the ongoing continental collision occurring along a major fault line in Turkey. Sandvol et al. summarize the research efforts to study the movements from shifts among three converging tectonic plates in the region over the past 10-20 million years. The seismic models, mapping, and geochemistry studies can be used to estimate the lithospheric [solid Earth] structure of the Anatolian plateau and to provide better earthquake hazard assessments in the highly active area. Some of the new findings in the papers include a report suggesting that widespread volcanism has changed the interpretation of the Anatolian plateau's depth.
Title: The Eastern Turkey Seismic Experiment: The study of a young continent-continent collision
Eric Sandvol, University of Missouri, Columbia, Missouri;
Niyazi Turkelli, Bogazici University, Istanbul, Turkey;
Muawia Barazangi, Cornell University, Ithaca, New York.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL018912, 2003
5. New method could allow space-based methane analysis
A newly reanalyzed 20-year record of atmospheric methane levels may allow researchers to better understand the cause of global methane trends and indicates a potential method for observing tropospheric [lower atmospheric] methane from space. Washenfelder et al. investigated high-resolution near-infrared spectra from the Kitt Peak National Solar Observatory in Arizona collected between 1977 and 1995. The authors used the known relationship between stratospheric methane and another stratospheric tracer gas to infer tropospheric methane levels from solar absorption data. The tropospheric methane estimates determined by their method agreed well with existing hydrocarbon measurements. Previous methane measurements are accurate, but are typically limited to smaller coverage areas. The researchers conclude that their technique may provide a new way to estimate tropospheric methane abundances from space, which would allow scientists an improved understanding of current atmospheric behavior and a technique to infer future methane trends.
Title: Tropospheric methane retrieved from ground-based near-IR solar absorption spectra
Rebecca A. Washenfelder, P. O. Wennberg, California Institute of Technology, Pasadena, California;
G. C. Toon, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL017969, 2003
6. Gravity waves can affect large-scale circulation patterns
Small-scale waves in the atmosphere and ocean likely have little impact on their large-scale flow, but may be able to significantly affect circulation patterns during unstable transition periods. Williams et al. analyzed the effects of spontaneously generated gravity waves on global flow patterns. The authors note that the ubiquitous waves' interactions with larger atmospheric and oceanic patterns are usually inconsequential, because of their small amplitude and fast speed. Their study suggests, however, that the waves, sustained by gravitational forces and Earth's rotations, can play a significant role in determining the large-scale atmospheric and oceanic behavior during transitions between large wave activity. The researchers used both model and laboratory experiments to study the affect on gravity waves' impact on ocean and tropospheric flow and suggest that such data can help refine weather prediction and climate models.
Title: Spontaneous generation and impact of inertia-gravity waves in a stratified, two-layer shear flow
Paul D. Williams, P. L. Read, Clarendon Laboratory, University of Oxford, United Kingdom;
T. W. N. Haine, Johns Hopkins University, Baltimore, Maryland.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL018498, 2003
7. Surface photochemistry may recycle nitrogen compounds back into atmosphere
Sunlight causes nitric acid naturally deposited on ground surfaces and vegetation to quickly break down and can lead to enhanced nitrogen oxide concentrations in the overlying atmosphere. Zhou et al. suggest that photolysis [light-caused chemical decomposition] on the ground and plant surfaces degrades nitric acid into its component parts much faster than similar chemical reactions in the atmosphere. They also suggest that the process could explain the high nitrogen compounds often observed in remote regions. The authors note that the compounds emitted during the chemical breakdown may recycle nitric acid back into the troposphere and represent a new source of nitrogen oxides, which are a precursor for ozone. They conclude that their laboratory and field results can help researchers better understand the atmospheric chemistry in rural environments.
Title: Nitric acid photolysis on surfaces in low-NOx environments: Significant atmospheric implications
Xianliang Zhou, Honglian Gao, Yi He, Gu Huang, Wadsworth Center, New York State Department of Health, Albany, New York, and State University of New York at Albany, New York;
Steven B. Bertman, Western Michigan University, Kalamazoo, Michigan;
Kevin Civerolo, New York State Department of Environmental Conservation, Albany, New York;
James Schwab, Atmospheric Sciences Research Center, State University of New York at Albany, New York.
Source: Geophysical Research Letters (GL) paper: 10.1029/2003GL018620, 2003
8. First analysis of carbon sequestration in saltwater ecosystems
Salt marshes and mangrove swamps can store a higher amount of carbon and release fewer greenhouse gases than equally sized peatlands and other freshwater wetlands. Chmura et al. compiled data from more than 150 sites along saltwater coasts and found that sediment within sheltered mangrove swamps and salt marshes can maintain a very high carbon content and may thus serve as a viable potential sink for carbon sequestration. They also found that although the sequestration rates drop with increasing temperatures, an abundant quantity of sulfate in saltwater wetlands hinders the production of methane and other greenhouse gases in the ecosystems. Although there are still no accurate predictions for the size of saline wetlands in the Atlantic, Pacific, and Indian oceans, there were no previous empirical estimates for the carbon storage potential of such wetlands worldwide.
Title: Global carbon sequestration in tidal, saline wetland soils
Gail L. Chmura, Centre for Climate and Global Change Research, McGill University, Montreal, Quebec, Canada;
Shimon C. Anisfeld, Yale University, New Haven, Connecticut;
Donald R. Cahoon, James C. Lynch, National Wetlands Research Center, U.S. Geological Survey, Lafayette, Louisiana.
Source: Global Biogeochemical Cycles (GB) paper: 10.1029/2002GB001917, 2003
9. Global warming can cause long-term ocean circulation changes
Increases in carbon dioxide of human origin may have significant long-term adverse effects on ocean chemistry, potentially causing substantial reductions in the density of important waters. Matear and Hirst report that a global doubling or tripling of atmospheric carbon dioxide levels from pre-industrial levels, followed by an extended leveling-off period, could remove dissolved oxygen from the deep ocean and affect ocean circulation patterns. The authors note that global warming resulting from the elevated carbon dioxide levels would affect the upper ocean layer over time, changing the depth of organic particulate-filled subsurface waters and reducing the overturning circulation that recycles dense deepwater throughout the ocean. The researchers simulated atmospheric carbon dioxide increases, followed by several centuries of constant carbon dioxide levels using an integrated ocean-climate model, and found that the ocean's response led to progressive changes that would alter the its ventilation and change its overall circulation.
Title: Long-term changes in dissolved oxygen concentrations in the ocean caused by protracted global warming
Richard J. Matear, CSIRO Marine Research and the Antarctic Cooperative Research Center, Hobart, Tasmania, Australia;
A. C. Hirst, CSIRO Atmospheric Research, Aspendale, Victoria, Australia.
Source: Global Biogeochemical Cycles (GB) paper: 10.1029/2002GB001997, 2003
10. Dark Martian dust likely glass particles from impacts
Many of the dark materials seen on Mars and several of the features found at the Pathfinder and Viking Martian landing sites may be glass or larger once-molten materials caused by impacts. Schultz and Mustard counter the widely held view that the mobile dark materials are volcanically derived windblown particles. The authors note that many of the materials observed on Mars do not conform to the absorption properties expected of a granular composition and suggest that part of the Martian sediment is made up of glass particles caused by meteor impacts. Field studies on Earth have shown that such impacts over millions of years have also caused similar melt-glass materials on the Moon and around terrestrial craters. The researchers conclude that the Martian atmosphere has retained the glass particles from historical impacts for billions of years, although many of the particles are buried or continually blown across the landscape by winds.
Title: Impact glasses on Mars
Peter H. Schultz, John F. Mustard, Brown University, Providence, Rhode Island.
Source: Journal of Geophysical Research-Planets (JE) paper: 10.1029/2002JE002025, 2003
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