Public Release: 

AGU journal highlights -- Aug. 27, 2009

American Geophysical Union

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL) or the Journal of Geophysical Research - Atmospheres (JGR-D).

In this release:

  1. Global warming to resume in decade ahead
  2. Terrain suggests recent ice age in Mars
  3. Understanding the human role in global dimming
  4. Satellite promises to improve air quality forecasts
  5. Method may glimpse tropical storm traits 20 days ahead
  6. Surface heat patterns influence outermost atmosphere
  7. Oscillation modes identified in Earth's core

Anyone may read the scientific abstract for any of these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2009GL038932. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below.

Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at .

1. Global warming to resume in decade ahead

Numerical models have difficulty producing reliable predictions of Earth's surface temperature for the near future. To improve temperature forecasts, Lean and Rind decompose the observed temperature change in recent years into components due to anthropogenic causes and natural causes, including El Niño events, volcanic activity, and solar activity. The study explains why 1998 was especially warm and why surface temperatures have not increased since then: 1998 was influenced by a super El Niño, and since 2000 declining solar irradiance and La Niña cooling have cancelled the warming due to increased greenhouse gas emission; no major El Niño has occurred to produce enhanced warming since 1998. The authors used their decomposition to predict future surface temperature. The resulting forecast indicates that from 2009 to 2014, global temperature will increase by 0.15 degrees Centigrade, as both anthropogenic forcing and solar irradiance increase. However, from 2014 to 2019, decreasing solar irradiance will likely cancel much anthropogenic warming, so temperatures will increase only slightly during those years. Because their projections are based on recent past observations rather than on numerical models, the authors believe their forecasts are the most reliable available.

Title: How will Earth's surface temperature change in future decades?

Authors: Judith L. Lean: Space Science Division, Naval Research Laboratory, Washington, D. C., USA;

David H. Rind: NASA Goddard Institute for Space Studies, New York, New York, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038932, 2009;

2. Terrain suggests recent ice age in Mars

Two hypotheses have been suggested to explain the distribution of ice in the near subsurface at middle to high latitudes on Mars: diffusion of water vapor into porous regolith; and atmospheric deposition of ice, snow, and dust during recent ice ages. To determine which of these hypotheses is correct, Schon et al. use data from the High Resolution Imaging Science Experiment (HiRISE) to examine the structure of exposed subsurface mid-latitude Martian terrain. The authors observe that the terrain is characterized by layered deposits multiple meters thick that stretch over many hundreds of meters. They suggest that climate variations are most likely the source of this stratification. The layers probably formed as dust, ice, and snow were deposited on the ground during recent ice ages, which occurred during periods when the tilt of Mars's axis of rotation was higher than usual. Vapor diffusion would be unlikely to result in the observed layered structure, the authors point out. They note that the observations also suggest that significant subsurface ice may remain in the 30 - 50 degrees mid-latitude regions.

Title: A recent ice age on Mars: Evidence for climate oscillations from regional layering in mid-latitude mantling deposits

Authors: Samuel C. Schon and James W. Head: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA;

Ralph E. Milliken: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038554, 2009

3. Understanding the human role in global dimming

Aerosols, which are released into the atmosphere through fossil fuel combustion, forest fires, dust storms, and volcanoes, reduce air quality and scatter incoming solar radiation, limiting the transmission of light to the Earth's surface. This phenomenon is known as "global dimming." Understanding the exact roles of human and natural sources in global dimming is critical to developing strategies to improve air quality and mitigate climate change. To learn more, Streets et al. focus on aerosol optical depth (AOD), an indicator of how much light penetrates the atmosphere. The authors test the hypothesis that changes in AOD over time are caused by the changing patterns of anthropogenic emissions. Using an aerosol radiation and transport model, they find that between 1980 and 2006, AOD trends generally agree well with observed trends in surface solar radiation in most regions. Trends in human-made contributions to AOD in general seem to follow the changing patterns of industrial and economic activity, and the contributions of human activities to total aerosol concentrations in each world region are estimated.

Title: Anthropogenic and natural contributions to regional trends in aerosol optical depth, 1980-2006

Authors: David G. Streets and Carolyne Yu: Argonne National Laboratory, Argonne, Illinois, USA;

Fang Yan: Argonne National Laboratory, Argonne, Illinois, USA; also at Department of Civil and Environmental Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois, USA;

Mian Chin and Thomas Diehl: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;

Natalie Mahowald: Department of earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA;

Martin Schultz: Forschungzentrum Juelich, Juelich, Germany;

Martin Wild: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland;

Ye Wu: Department of Environmental Science and Engineering, Tsinghua University, Beijing, China.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011624, 2009;

4. Satellite promises to improve air quality forecasts

The 2007 U.S. National Research Council Decadal Survey recommended that NASA launch a new satellite, called the Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission. GEO-CAPE's high spatial and temporal resolution will identify sources of aerosols and ozone precursors and track the transport of air pollutants, including the carbon monoxide that arises from combustion processes. Edwards et al. use observing system simulation experiments (OSSEs) to simulate exactly how GEO-CAPE would improve forecasts of carbon monoxide concentrations. Though OSSEs are routinely used to test whether incorporating new measurements improves weather forecasts, this study marks the first to apply the concept of OSSEs to predicting pollution events in the context of the atmosphere's variable chemical composition. Through studying wildfires in Alaska and Canada in 2004, which led to significant carbon monoxide pollution over the United States, the authors find that GEO-CAPE data would greatly improve surface air quality forecasts compared with current satellite observations alone. The authors hope that the tools and methodologies developed not only will help answer important science questions but also will help NASA select priorities for future missions.

Title: A satellite observation system simulation experiment for carbon monoxide in the lowermost troposphere

Authors: David P. Edwards, Avelino F. Arellano Jr., and Merritt N. Deeter: Program for Atmospheric Composition Remote Sensing and Prediction, National Center for Atmospheric Research, Boulder, Colorado, USA.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011375, 2009;

5. Method may glimpse tropical storm traits 20 days ahead

The Madden-Julian Oscillation (MJO), a predictable, planetary-scale pattern of atmospheric circulation, has been observed to influence tropical storms. If weather models can simulate the effects of the MJO on tropical storm intensity, location, and risk of landfall, they could be used to improve tropical storm predictions. To demonstrate this possibility, Vitart uses a numerical weather prediction model to create a series of 46-day hindcasts covering the period from 1989 to 2008. The author finds that the model simulations show that tropical storm characteristics and risk of landfall vary significantly with the phase of the MJO, consistent with observations. Because the phase of the MJO can be predicted up to several weeks in advance, it should be possible to use numerical models to predict tropical storm activity up to about 20 days out, the author notes. This would fill a gap between short-range predictions of storm tracks and longer-range seasonal forecasts, and could lead to reductions in loss of life and property.

Title: Impact of the Madden Julian Oscillation on tropical storms and risk of landfall in the ECMWF forecast system

Author: F. Vitart: European Centre for Medium-Range Weather Forecasts, Reading, UK.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039089, 2009;

6. Surface heat patterns influence outermost atmosphere

Can processes occurring near Earth's surface, such as evaporation and condensation, affect conditions in the exosphere, the outermost portion of the atmosphere, which reaches thousands of kilometers above the surface? To explore the issue, Forbes et al. use measurements from the Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites to determine the temperature of the exosphere as a function of local time, longitude, and latitude. The authors find that exosphere temperature varies significantly with longitude, and they show that this variability in the exosphere is linked to several diurnal and semidiurnal thermal tides that begin near Earth's surface as heat is released by evaporation and condensation. These tides, which reflect variations at Earth's surface, such as the distribution of land and sea, had previously been shown to influence the thermosphere at about 110 kilometers (68 miles) in altitude; the new results are the first to demonstrate that these tides extend all the way to the exosphere. The authors also suggest that similar effects could be occurring on other planets, including Mars.

Title: Surface-exosphere coupling due to thermal tides

Authors: Jeffrey M. Forbes and Xiaoli Zhang: Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA;

Sean L. Bruinsma: Department of Terrestrial and Planetary Geodesy, Centre Nationale D'Etudes Spatiales, Toulouse, France;

Jens Oberheide: Physics Department, University of Wuppertal, Wuppertal, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038748, 2009;

7. Oscillation modes identified in Earth's core

The Earth's fluid outer core motion cannot be directly observed, but it can be inferred from magnetic observations. To better understand core dynamics, Dickey and de Viron represent the core as a set of 20 coaxial cylinders rotating about Earth's axis. The authors calculate the core angular momentum with a complex model that uses surface magnetic data as an input, and analyze the resultant data to capture the common variability among the different cylinders. They find that the first four robust modes, corresponding to torsional oscillations, propagate from the core-mantle boundary inward to the inner core with diminishing amplitude with periods of 85, 50, 35, and 28 years. The results are in excellent agreement with modes found in two previous studies. All three analyses (one based on theory and two based on geomagnetic observations) agree on the periods of the isolated modes; the authors note that this concurrence is exceptional and provides strong evidence of the existence of these modes.

Title: Leading modes of torsional oscillations within the Earth's core

Authors: Jean O. Dickey: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;

Olivier de Viron: Institut de Physique du Globe de Paris, University Paris Diderot, CNRS, Paris, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038386, 2009;



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