AGU journal highlights - 4 January 2006I. Highlights, including authors and their institutions
The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.
1. Aurorae on Mars
Most visible on planets with a global dipole, such as Earth and Jupiter, aurorae occur throughout the solar system when charged particles intersect with magnetic field lines. Lacking a global dipole, Mars' strong crustal magnetic fields help generate localized ultraviolet auroral emissions, recently seen by the Mars Express (MEX) spacecraft. However, the exact physical mechanisms that govern Martian aurorae remain unknown. To study this phenomenon, Brain et al. analyzed data from the Mars Global Surveyor mission and found thousands of peaked electron energy spectra on the night side of Mars, similar to terrestrial auroral electrons. With energy fluxes 10 to 1,000 times higher than background signatures, these spectra occurred on and adjacent to magnetic field lines that connected shocked solar wind to crustal magnetic fields. The authors show that the emission intensities expected from the most energetic spectra approached those found in the aurorae observed by MEX. Because detection of these emissions varied with solar wind fluctuation, the authors predict that disturbed plasma environments are favorable for electron acceleration on magnetic field lines and that magnetic reconnection may play a role in the observation of auroral electrons.
Title: On the origin of aurorae on Mars
Authors: D. A. Brain, J. S. Halekas, L. M. Peticolas, R. P. Lin, J. G. Luhmann, D. L. Mitchell, and G. T. Delory: Space Science Laboratory, University of California, Berkeley, California, USA; S. W. Bougher: Department of Space Sciences, University of Michigan, Ann Arbor, Michigan, USA; M. H. Acuna: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; H. Reme: Centre d'Etude Spatiale des Rayonnements (CESR), Toulouse, France.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024782, 2005
2. Reducing sulfate aerosol air pollution will contribute to global warming
Sulfate aerosols, one of the major pollutants from coal burning, scatter incoming solar radiation back into space, producing a cooling effect on the Earth's surface by modifying the radiative properties and lifetimes of clouds and by potentially modifying precipitation patterns. To explore the role that manmade aerosols will play in the climate systems of the future, Brasseur and Roeckner used a coupled atmosphere-ocean model to hypothesize the climate feedbacks that will occur in two scenarios: 1) if current sulfate aerosol concentrations are maintained, and 2) if this pollution is immediately eliminated. They found that climate change resulting from greenhouse gas emissions would become much more pronounced if air quality were drastically improved in the future. Specifically, globally averaged surface air temperatures would increase in less than a decade by about 1 Kelvin [1 degree Celsius, 2 degrees Fahrenheit]. Precipitation would increase by three percent. Because an objective of the U.N. Framework Convention on Climate change is to stabilize greenhouse gas emissions to prevent human interference with the climate system, the authors suggest that strategies to limit climate warming below a specified threshold should be reconciled with strategies to reduce air pollution.
Title: Impact of improved air quality on the future evolution of climate
Authors: G. Brasseur and E. Roeckner: Max Planck Institute for Meteorology, Hamburg, Germany.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023902, 2005
3. Oilfield brines might be leaking into Nueces Bay, Texas
The discharge of fresh, brackish, or saline groundwater into bays and estuaries can threaten fragile coastal ecosystems by increasing eutrophication [enrichment by influx of nutrients] and turbidity [opaquneness due to suspended particles]. To explore new methods of detecting this water seepage, Breier et al. studied Nueces Bay, near Corpus Christi, Texas, an area believed to experience such groundwater discharge. Their 17-kilometer [11-mile] survey measured the salinity, temperature, and dissolved oxygen content in surface waters. At points along the survey track and slightly below the water's surface, they also measured dissolved radium isotope concentrations. Because aquifer and sediment pore waters become enriched in radium isotopes, these elements are natural tracers of groundwater discharge. In addition, the bay's bottom sediments were surveyed using continuous resistivity profiling, a technique that indicates salinity gradations within water and sediments. The survey revealed that vertical fingers of high and low resistivity extended up through seven meters [20 feet] of the bay's bottom sediments into the water. Located close to areas where surface waters had low dissolved oxygen content and high salinity, temperature, and dissolved radium, these fingers indicated the presence of either brackish submarine groundwater discharge or the leakage of oilfield brines from submerged petroleum wells or pipelines.
Title: Detecting submarine groundwater discharge with synoptic surveys of sediment resistivity, radium, and salinity
Authors: J. A. Breier, C. F. Breier, and H. N. Edmonds: Marine Science Institute, The University of Texas at Austin, Port Aransas, Texas, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024639, 2005
4. Measuring Canopy Cover with Satellites
One purpose of the Ice, Cloud and land Elevation Satellite (ICESat) is to measure the vertical structure of continental surfaces based on topographic relief and vegetation using received waveforms recorded by the Geoscience Laser Altimeter System (GLAS). In low-relief areas with tree cover, the waveforms and the derived elevation products provide useful biophysical parameters, including maximum canopy height, the outer-canopy ruggedness, and a measure of canopy cover. However, for areas where within-footprint topographic relief is large compared to vegetation height, interpretation of the waveforms is complex. Harding and Carabajal sought to determine the contribution of canopy and ground to received waveforms by comparing ICESat measurements with high-resolution digital elevation maps from the Kitsap Peninsula in Washington state, an area where canopy heights have been surveyed in great detail using airborne laser altimetry. Using the airborne data and a model of the GLAS instrument response, they computed synthetic waveforms to clarify how the ground contributes to ICESat data signals. They suggest that this approach can provide a means to validate GLAS waveforms, elevation products, and footprint geolocation.
Title: ICESat waveform measurements of within-footprint topographic relief and vegetation vertical structure
Authors: David J. Harding: Planetary Geodynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; Claudia C. Carabajal: NVI, Inc.; Space Geodesy Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023471, 2005
5. Clouds can form within polluted air
The interactions between aerosols and clouds are believed to significantly impact precipitation and influence Earth's energy budget by changing the albedo [reflectivity] and absorption of the Earth-atmosphere system. Though clouds are thought to be a cooling influence, because they enhance the reflectivity of the planet, soot and other aerosols are thought to absorb incoming solar radiation, decrease cloud cover, and raise temperatures. To further understand potential impacts of cloud and aerosols on albedo, Hart et al. used the Geoscience Laser Altimeter System (GLAS) on board the Ice, Cloud, and land Elevation Satellite (ICESat) to measure cloud and aerosol particle concentrations in the highly polluted atmosphere over the northern Indian Ocean. Using an algorithm that can distinguish between cloud and aerosol layers in the atmosphere up to an altitude of 40 kilometers [25 miles], the authors found that clouds can form within, and well below, thick layers of aerosol loading, suggesting that a complex system governs albedo changes in the atmosphere over such regions. The authors note that for the first time, global monitoring of cloud and aerosol interaction will be accomplished with spaceborne lidar [laser-based radar].
Title: Height distribution between cloud and aerosol layers from the GLAS spaceborne lidar in the Indian Ocean region
Authors: William D. Hart, Steven P. Palm, Dennis L. Hlavka: Science Systems and Applications, Inc., NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; James. D. Spinhirne: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023671, 2005
6. Electron diffusion regions observed at magnetic field reconnection sites
Magnetic reconnection occurs when the interplanetary magnetic field (IMF) reconnects with Earth's magnetosphere, merging field lines and converting magnetic energy into particle energy on timescales shorter than magnetic field diffusion times. This reconnection occurs in an area called the electron diffusion region, a site previously unobserved by scientists. However, by analyzing one hour of Cluster satellite data, Mozer et al. were able to observe 19 electron diffusion regions where Earth's magnetosphere meets the IMF, by searching for areas with non-zero parallel electric field lines, high electromagnetic energy, and electron beam acceleration, among other factors. Their analysis shows for the first time that these regions directly convert magnetic energy to electron energy, allowing for the measurement of electron accelerations, field aligned currents, and post acceleration fates. The authors were also able to show that the spatial dimensions of large electric fields in the electron diffusion regions have average thicknesses consistent with theory. Further analysis revealed that these regions were boundaries between open and closed magnetic field geometries that contained different plasma and magnetic field line flows.
Title: New features of electron diffusion regions observed at sub-solar magnetic field reconnection sites
Authors: F. S. Mozer, S. D. Bale, and J. P. McFadden: Space Sciences Laboratory, University of California, Berkeley, California, USA; R. B. Torbert: Physics Department and Apace Science Center, University of New Hampshire, Durham, New Hampshire, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024092, 2005
7. Polar stratospheric clouds over Antarctica
Polar stratospheric clouds (PSCs) appear regularly in the winter polar stratosphere. Past studies have documented two types of PSCs--type I contains particles of nitric and sulfuric acids, while type II contains water ice. Palm et al. documented the first satellite observations of Antarctic PSCs, taken from the last two days of September 2003, using the Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud, and land Elevation Satellite (ICESat). The authors showed that of 30 ICESat tracks studied, 16 detected PSC signatures, leading them to infer that the satellite was observing different portions of a single large PSC that reached from the troposphere-stratosphere boundary up to 21 kilometers [ 13 miles] in altitude and had a horizontal extent of more than 3500 kilometers [2,200 miles]. In the deepest regions of the PSC, temperatures were cold enough to support water ice (type II) PSCs. Outside this cold region, type I PSCs were observed. GLAS data also showed a substantial amount of tropospheric cloudiness below the PSC, implying that the layers of the atmosphere interact during polar stratospheric cloud formation. The authors suggest that further research using satellite lidar will help understand this interaction.
Title: Observations of Antarctic Polar Stratospheric Clouds by the Geoscience Laser Altimeter System (GLAS)
Authors: Stephen P. Palm: Science Systems and Applications Inc., NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; Michael Fromm: Naval Research Laboratory, Washington D.C., USA; James Spinhirne: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023524, 2005
8. A new climate study compiles over 100,000 model scenarios
Recent climate modeling trends have suggested that scientists should use weighted model ensembles to better predict future climate. However, results from such studies are susceptible to biases from subjective decisions that give different models different weights. Instead, Piani et al. proposed that a "perturbed physics" ensemble might better simulate present and future climate dynamics, independent from sampling strategy. Using a computing project called climateprediction.net, the authors searched for constraints on the response to increasing greenhouse gas levels among present-day observable climate variables, by compiling a grand ensemble of experiments that distributed computational resources to more than 100,000 volunteer participants. These participants conducted one or more experiments, each comprised of three separate simulations using predetermined settings of parameter values and initial conditions. The first simulation calculated natural climate variability, the second calculated climate change due to current manmade greenhouse emissions, and the third calculated climate change if current emission levels were to double over the next 15 years. Using this massive data set, the authors used statistical analyses to determine that temperatures will likely rise by more than 3 Kelvin [3 degrees Celsius, 5 degrees Fahrenheit] before the climate reaches equilibrium with human emissions.
Title: Constraints on climate change from a multi-thousand member ensemble of simulations
Authors:C. Piani, D. J. Frame, D. A. Stainforth, and M. R. Allen: Department of Physics, Oxford University, United Kingdom.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024452, 2005
9. Estuarine bathymetry does not rely on sediments
Estuaries are places where tidal currents mix sea water and sediments with those from rivers. Over future decades, impacts of climate change--including sea level rise--will modify these processes. Estuaries are not only important for supporting a rich and healthy ecology, but are also critical for navigation, recreation, and industry. Accurate forecasts of estuarine bathymetric [sea bottom] evolution are essential for sustaining these activities. Past studies traditionally derived predictions of bathymetric evolution from existing patterns of net inflow or outflow of sediments. However, new theories indicate that estuarine bathymetries are controlled by the dynamics and mixing of tides and river flows, with the prevailing sediment content a consequence, rather than a determining factor. Prandle et al. considered this new paradigm reversal by referring to data on depth, length, sediment concentrations, and rates of vertical and axial mixing from estuaries in the United Kingdom, spanning ria [inlets], coastal plain, and bar-built landforms. Comparisons of these data with theoretical values, in terms of tidal amplitude and river flow, supported these new hypotheses. Their close correspondence allows the assessment of likely sensitivities of an estuary to particular climate change components.
Title: Estuaries are not so unique
Authors: David Prandle and Andrew Lane: Proudman Oceanographic Laboratory, Liverpool, United Kingdom; Andrew J. Manning: HR Wallingford Ltd, Estuaries and Dredging Group, Oxfordshire, United Kingdom; Centre for Coastal Dynamics and Engineering, School of Earth Ocean and Environmental Sciences, University of Plymouth, Plymouth, United Kingdom.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024797, 2005
10. Model comparisons reveal insights on thermohaline circulation slowdown
Model experiments have suggested that the addition of freshwater to the northern Atlantic will effect thermohaline [global heat- and salt-driven vertical and horizontal] circulation in a complex, nonlinear way, leading to widespread global climate change. Eleven such models, previously selected as part of the international network of Earth system models of intermediate complexity (EMICs), were rigorously compared by Rahmstorf et al. In their comparisons, the authors found that climate systems in all models responded to increased northern Atlantic freshwater in a delayed manner and that at some point in time, the increased freshwater input will reach a threshold where deepwater formation is no longer possible. The models differed in where the present-day climate exists with respect to this threshold: results vary from less than 0.1 Sverdrup [measure of ocean volume transport] to more than 0.5 Sverdrup of freshwater needed to stop deepwater formation. Because about 0.1 Sverdrup of freshwater would be added to the northern Atlantic if the entire Greenland icecap were to melt over the next thousand years, the authors suggest that further analysis is needed to determine how far the present climate may be from deepwater formation shutdown.
Title: Thermohaline circulation hysteresis: A model intercomparison
Authors:Stephan Rahmstorf and Andrey Ganopolski: Potsdam Institute for Climate Impact Research, Potsdam, Germany; Michel Crucifix: Hadley Centre for Climate Prediction and Research, Met Office, Exeter, United Kingdom; Hugues Goosse: Indtitut d'Astronomie et de Geophysique Georges Lemaitre, Universite Catholique de Louvain-la-Neuve, Belgium; Igor Kamenkovich: Joint Institute for the Study of the Atmosphere and the Oceans, University of Washington, Seattle, Washington, U.S.A; Reto Knutti: National Center for Atmospheric Research, Boulder, Colorado, USA; Gerrit Lohmann: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; Robert Marsh: Southampton Oceanography Centre, Southampton, United Kingdom; Lawrence A. Mysak and Zhaomin Wang: McGill University, Montreal, Quebec, Canada; Andrew J. Weaver: University of Victoria, Victoria, British Columbia, Canada.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023655, 2005
11. Compiling climate models yields predictions with lower errors
Though most climate models predict that the northern Atlantic thermohaline circulation will weaken during the 21st century, due to increasing levels of greenhouse gas concentrations, the models vary significantly in the degree to which this weakening will occur, indicating a large uncertainty in response to increased freshwater input. To reduce these uncertainties, Schmittner et al. analyzed nine different climate models selected for study by the upcoming Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Noting that past studies have highlighted the superiority of multi-model ensembles over any single model, each model was weighted according to its ability to accurately simulate important climate dynamics (salinity, temperature, mixed layer depth, and mass transports) that are well known from observations. This procedure yielded an optimal estimate for the evolution of the northern Atlantic thermohaline circulation, which predicts that the circulation will weaken by as much as 25 percent over the next century. Though errors still remained large, the authors note that when compared to the average behavior of unweighted models, their model ensemble reduces any uncertainties present. They predict that future strides in oceanographic data collection will further reduce these errors.
Title: Model Projections of the North Atlantic thermohaline circulation for the 21st century assessed by observations
Authors: A. Schmittner: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA; M. Latif and B. Schneider: Ocean Circulation and Climate Dynamics, Leibniz-Institut fuer Meereswissenschaften, Kiel, Germany.
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024368, 2005
12. Calcite analyses will aid in the search for life on Mars
During the first hundred million years of its history, Mars likely had a denser atmosphere, milder temperatures, and liquid water on its surface, leading astrobiologists to hypothesize that life once existed on that planet. However, the Viking landers missions found that Martian soil was depleted in organic molecules. Stalport et al. suggest that while these organic compounds might have short residence times in Martian soil, inorganic compounds could yield clues about the possibility that life had existed on Mars. Noting that calcite can be formed by biomineralization, the authors sought to identify differences between 12 terrestrial calcite samples formed through biotic, abiotic, and diagenetic [biological, nonbiological, and compaction, etc.] mechanisms, the latter occurring as biomineralized [minerals absorbed into biological organisms] calcite metamorphoses into calcite that has lost fossil traces. Along with chemical compositions, biotic and abiotic calcites were analyzed for their ability to withstand high temperatures. The authors found that biotic and diagenetic calcites contained quartz, feldspar, and magnesium impurities, while abiotic calcite contained no impurities. Moreover, abiotic calcite was the most resistant to thermal degradation. The authors expect that their results will help guide on-site or sample-return studies of Martian minerals, to determine whether Martian carbonates contain biological signatures.
Title: Search for past life on Mars: Physical and chemical characterization of minerals of biotic and abiotic origin: 1. Calcite
Authors: Fabien Stalport, Patrice Coll, Rafael Navarro Gonzalez, Francois Raulin, and Patrick Ausset: Laboratoire Interuniversitaire des Systemes Atmospheriques, Universite Paris XII, Creteil, France; Michel Cabane and Cyril Szopa: Service d'Aeronomie du CNRS, SA/IPSL, Universite Paris VI, Paris, France; Alain Person: Laboratorie de Geologie Sedimentaire, Universite Paris VI, Paris, France; Rafael Navarro Gonzalez: Formerly at Laboratorio de Quimica de Plasmas y Estudios Planetarios, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico; Marie Jo Vaulay: Interfaces, Traitements, and organization et Dynamique des Systemes (ITODYS), Universite Paris VII, Paris, France; Chris P. McKay: Space Science Division, NASA Ames Research Center, Moffett Field, California, USA; John Zarnecki: Planetary and Space Sciences Research Institute, Open University, Milton Keynes, UK.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023403, 2005.
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