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AGU journal highlights: Nov. 11, 2008

1. Ups and downs of Greenland's ice sheet

Studies have indicated that the Greenland ice sheet is thinning, likely because of increasing average summer air temperatures in southern coastal regions. To investigate the total loss of the ice sheet's mass from 1958 through 2008, Rignot et al. study two parameters: ice discharge, which is obtained from remote sensing techniques; and surface mass balance, which is derived from a model of melting ice and snowpack accumulation. The authors find that surface mass balance and temporal variability in ice discharge are correlated for the time period studied. Using this correlation, they determine that the ice sheet lost mass during the warm period before 1970, although in the 1970s and 1980s the ice sheet accumulated almost as much mass as it had lost. However, ice sheet loss increased through the 1990s. In the past 11 years, the total mass deficit has tripled; as of 2007, about 267 billion tons of ice is lost each year. The authors also find that rather than melting, variations in ice discharge were the dominant factor in determining total ice loss.

Title:Mass balance of the Greenland ice sheet from 1958 to 2007
Authors: E. Rignot: Department of Earth System Science, University of California, Irvine, California, U.S.A.; also at Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.; J. E. Box: Byrd Polar Research Center, Ohio State University, Columbus, Ohio, U.S.A.; E. Burgess: Department of Geography, University of Utah, Salt Lake City, Utah, U.S.A.; E. Hanna: Department of Geography, University of Sheffield, Sheffield, U.K.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035417, 2008;

2. Shrinking Arctic ice boosts phytoplankton growth

Loss of Arctic sea ice has accelerated recently, culminating in a 2007 summer minimum ice extent that was 23 percent below the previous low. To quantify the impact of this unprecedented loss on marine primary production, Arrigo et al. couple satellite-derived sea ice extents, sea surface temperatures, and marine chlorophyll concentrations to a primary production model parameterized for Arctic waters. They find that annual primary production in the Arctic has increased yearly, with annual production in 2007 exceeding the 1998 to 2002 average by 23 percent. In particular, high increases in primary productivity were seen in the waters surrounding Siberia. Roughly 30 percent of the total increase in production is attributable to decreased minimum summer ice extent; the rest is likely due to a longer phytoplankton growing season. Should these trends continue, additional loss of ice during Arctic spring could boost productivity more than threefold above the 1998 to 2002 average, potentially altering marine ecosystem structure and carbon budgets.

Title:Impact of shrinking Arctic ice cover on marine primary production
Authors: Kevin R. Arrigo, Gert van Dijken, and Sudeshna Pabi: Department of Environmental Earth System Science, Stanford University, Stanford, California, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035028, 2008;

3. Comet-like tail of ion flux streams from Mars

A comet tail has two entities: a neutral tail with debris along the orbit, and a plasma tail with an induced magnetosphere driven by solar wind, solar X rays, and extreme ultraviolet radiation. Noting that solar wind and solar radiation are key components in upper atmospheric dynamics of terrestrial planets, Lundin et al. seek to determine if any similarities exist between the anatomy of a comet and a weakly magnetized planet such as Mars. Using data from an ion mass analyzer on board the European Space Agency's Mars Express spacecraft, the authors study cold ionospheric plasma and find that solar forcing sweeps ions from the dayside of Mars over the dawn/dusk terminator. These escaped particles expand into a comet-like tail, similar to that seen for Venus. Though initially of low energy, these ions accelerate to form energetic and structured ion fluxes within the Martian tail. Although other studies suggest that the formation of this tail is an asymmetric process, these new results show that low-energy ion escape is more symmetric.

Title:A comet-like escape of ionospheric plasma from Mars
Authors: R. Lundin, S. Barabash, M. Holmstrm, H. Nilsson, and M. Yamauchi: Swedish Institute of Space Physics, Kiruna, Sweden; M. Fraenz and E. M. Dubinin: Max Plank Institute fr Sonnensystemforschung, Lindau, Germany.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034811, 2008;

4. Surface waves effects in deep ocean

Long ocean waves (with wavelength greater than 1 meter (3.3 feet) and period longer than about 1 second) have long been known to excite acoustic waves at twice their frequency that can travel with little attenuation to the bottom of the ocean. This happens through a nonlinear interaction between wave components traveling in opposite directions. Farrell and Munk suggest that the same phenomenon extends to much higher frequencies (up to 13 Hz; wavelengths of the order of 1 centimeter (0.4 inches)), where surface tension becomes an important force. They claim to have seen the acoustic waves excited by such tiny ripples in deep-sea pressure recordings obtained 5.5 kilometers (3.4 miles) beneath the sea surface. If this interpretation is correct, it provides additional evidence that there is a significant change in the shape of the ocean wave spectrum at about 3 Hz. This research opens up a new avenue for the study of short-wavelength ocean waves in a frequency range where the force of the winds is particularly effective and alternative measurement methods prove arduous.

Title: What do deep sea pressure fluctuations tell about short surface waves?
Authors: W. E. Farrell: Science Applications International Corporation, San Diego, California, U.S.A.;
Walter Munk: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035008, 2008;

5. Potential predictability for some earthquakes?

Entropy is a measure of a system's available energy to do work. The idea that a complex system is driven to a state of maximum entropy production (MEP), following the second law of thermodynamics, has been suggested as a self-organizing mechanism for many phenomena, including atmospheric and mantle convection and the self-organization of sand grains by loading in sandpiles. Noting that natural seismicity in the Earth's crust is produced by stationary loading from plate tectonics, Main and Naylor seek to determine to what extent MEP governs energy budgets of natural and modeled seismic events. Assuming commonly observed feedbacks between remote boundary stress and strain rates at steady states, they find that several natural and model earthquake dynamics can be explained by the system organizing to a state of MEP. This suggests a theoretical degree of predictability for certain types of earthquakes, strongly limited by the proximity to criticality and by the practical difficulty of directly observing lithospheric stress fields at sufficient resolution.

Title: Maximum entropy production and earthquake dynamics
Authors: Ian G. Main and Mark Naylor: School of GeoSciences, University of Edinburgh, Edinburgh, U.K.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035590, 2008;

6. Magnetic fields reconnecting in near-Earth space

Interplanetary magnetic field lines can connect with a planet's magnetic field through a process called magnetic reconnection. Magnetic reconnection is a phenomenon of great importance to our solar system, and presumably to other star systems, because it converts energy stored in magnetic fields into particle kinetic energy. Further, reconnection changes the magnetic field topology, allowing effective exchanges of mass, momentum, and energy between differently magnetized plasma regions. In a solicited review, Paschmann discusses research on reconnection based on recent measurements not only within the Earth's magnetopause (the boundary between the magnetosphere and surrounding plasma) and magnetotail (the region on the nightside of the magnetosphere that trails Earth in a comet-like tail), but also within the solar wind (plasma ejected from the upper atmosphere of the Sun) and the Earth's magnetosheath (the region of space where the speed of the solar wind abruptly drops due to encountering the magnetopause). Within the solar wind and magnetosheath, boundary conditions are very different from those at the magnetopause and magnetotail. Among the measurements discussed, the author emphasizes that those taken close to the reconnection sites may offer the most exciting advances to understanding the reconnection phenomenon.

Title: Recent in-situ observations of magnetic reconnection in near-Earth space
Authors: Gtz Paschmann: Max-Planck-Institut fr extraterrestrische Physik, Garching, Germany
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035297, 2008;

7. Studying Saturn's oxygen ion supply

NASA's Cassini spacecraft is investigating Saturn, its satellites, and its magnetosphere. This long-term, direct observation of Saturn's plasma environment has allowed a rigorous investigation into magnetospheric composition, including ion populations that make up less than 5 percent of the total ion population. Because analysis of these minor species can provide clues to the origin and evolution of Saturn's rings and satellites, Martens et al. study molecular oxygen ions (O2+), a minor species they estimate to compose, on average, 0.3 to 0.4 percent of the total ion population in the inner magnetosphere. O2+ forms through sunlight-induced ionization of neutral oxygen, which is a product of sunlight-induced decomposition of water. Using data summed over 23 Cassini orbits, the authors discover a relative enhancement in the O2+ population beyond Saturn's main rings, which are the only known source of O2+, suggesting the potential for additional sources. They find that at distances close to Saturn's moon Rhea, O2+ signatures increase, possibly indicating that Rhea is a source of neutral oxygen to the inner magnetosphere.

Title: Observations of molecular oxygen ions in Saturn's inner magnetosphere
Authors: Hilary R. Martens, Daniel B. Reisenfeld, and John D. Williams: Department of Physics and Astronomy, University of Montana, Missoula, Montana, U.S.A.; Robert E. Johnson: Department of Engineering Physics, University of Virginia, Charlottesville, Virginia, U.S.A.; H. Todd Smith: Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035433, 2008;

8. Global climatology of drizzle

Understanding the extent and radiative characteristics of low clouds over the world's oceans, which can exert a significant cooling effect on global climate, is critical for understanding and predicting global climate. The role of drizzle, which links cloud-microphysical processes to boundary-layer dynamics and aerosol spectra, is particularly complex and can cause dramatic changes in the organization and extent of cloud coverage. Prior to the launch of NASA's CloudSat and the NASA/French Cloud-Aerosol- Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellites, assessing the frequency or extent of drizzle on a global scale was not possible. Now that these satellites have been launched, Leon et al. use CloudSat and CALIPSO data collected from July 2006 through June 2007 to analyze low clouds over the oceans. They find that drizzle is a globally pervasive feature of low clouds, consistent with previous observations collected during individual field campaigns. While drizzle is frequently detected for all low clouds regardless of location, it occurs more frequently and with higher intensity in midlatitudes than in the subtropics, suggesting that drizzle may play a particularly important role in determining the cloud characteristics in midlatitudes.

Title: Climatology of drizzle in marine boundary layer clouds based on one year of data from CloudSat and CALIPSO
Authors: David C. Leon, Zhien Wang, and Dong Liu: Atmospheric Science Department, University of Wyoming, Laramie, Wyoming, U.S.A.
Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD009835, 2008, in press;

9. Observations imply water vapor-climate feedback

Warming temperatures evaporate water, increasing humidity. This increase in humidity has the potential to further warm the atmosphere because water vapor is a potent greenhouse gas. This water vapor feedback has the capacity to about double the direct warming from greenhouse gas increases. Using satellite data, Dessler et al. observe and quantify the behavior of atmospheric water vapor and the water vapor feedback during variations of the Earth's climate between 2003 and 2008. They find that global averaged surface air temperatures on Earth varied by 0.6 degrees Celsius (1.1 degrees Fahrenheit) during the years analyzed, with specific humidity over most of the troposphere increasing with rising global surface temperature averages. Relative humidity increased in some regions and decreased in others, with the global average remaining nearly constant at most altitudes. The water vapor feedback implied by these observations is strongly positive, similar to that seen by climate models. The magnitude of the feedback is similar to that obtained if the atmosphere maintained constant relative humidity everywhere.

Title: Water-vapor climate feedback inferred from climate fluctuations, 2003
Authors: A.E. Dessler, Z. Zhang, and P. Yang: Department of Atmospheric Sciences, Texas A&M University, College Station, Texas, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035333, 2008;

10. A seasonal look at soil moisture

Modifications of temperatures and precipitation patterns due to global climate change will cause significant transformations of ecosystems and plant physiological functions, from evapotranspiration and groundwater recharge to carbon storage and biochemical cycling. Quantifying variability in near-surface soil moisture is critical to understanding how these changes will influence ecosystems. However, the dynamic interactions between vegetation and soil moisture remain largely unresolved because it is difficult to monitor and quantify subsurface hydrologic fluxes at relevant scales. To bridge the gap between remotely sensed data and in situ point observations, Jayawickreme et al. use electrical resistivity techniques to monitor the influence of climate and vegetation on root-zone moisture for a forest-grassland ecotone in Michigan. By analyzing data collected from October 2006 through September 2007, the authors find large seasonal differences in root-zone moisture dynamics as well as large differences in effective rooting depth and moisture distributions for the two vegetation types. Such results highlight the likely influences of land transformations on groundwater recharge, streamflow, and land-atmosphere exchanges.

Title: Subsurface imaging of vegetation, climate, and root-zone moisture interactions
Authors: Dushmantha H. Jayawickreme: Department of Geological Sciences, Michigan State University, East Lansing, Michigan, U.S.A.; now at Department of Biology, Duke University, Durham, North Carolina, U.S.A.; Remke L. Van Dam and David W. Hyndman: Department of Geological Sciences, Michigan State University, East Lansing, Michigan, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034690, 2008;

11. Wildfires boost ozone beyond health standards

In 2007, about 5.3 million hectares (13 million acres) burned across the United States, with California accounting for approximately 10 percent of the burned area. Many of California's fires were ignited by broken power lines in fall 2007. Severe drought, hot weather, and unusually strong Santa Ana winds helped to rapidly spread these fires. To quantify the impact of these fires on regional air quality and, in particular, the creation of surface ozone by pollutants released by these fires, Pfister et al. analyze observations of ozone concentrations during the fires with global chemistry transport model simulations. These simulations include synthetic tracers that provide information about the amount of ozone produced from the fires. The authors find that the global model fits well with local observations and that a clear increase in observed ozone is found when the model predicts a strong impact of pollution from the fires. This increase in ozone can elevate the frequency of ozone concentrations that exceed current U.S. health standards, potentially causing violations to air pollution codes also during photochemically less active seasons.

See press release at

Title: Impacts of the fall 2007 California wildfires on surface ozone: Integrating local observations with global model simulations.
Authors: G. G. Pfister, C. Wiedinmyer, and L. K. Emmons: Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034747, 2008;


Contact: Peter Weiss
American Geophysical Union

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