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AGU journal highlights -- June 23, 2011

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL); Geochemistry, Geophysics, Geosystems (G-Cubed); and Paleoceanography (PA).

In this release:

  1. Estimating climate effects of contrails
  2. Did Aboriginal forest burning affect Australian summer monsoon?
  3. Determining the trigger of East Asian dust storms
  4. El NioSouthern Oscillation variability persisted in warmer world
  5. Constraining the trigger for ancient warming episode
  6. Next generation atmospheric model improves hurricane forecasting
  7. Theorized magnetic plasma behavior demonstrated in lab

Anyone may read the scientific abstract for any already-published paper 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. 10.1029/2011GL047629. 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. Estimating climate effects of contrails

Condensation trails, or so-called contrails, formed by freezing of ice crystals in the exhaust from aircraft jet engines could affect climate. Like natural cirrus clouds, contrails change atmospheric temperatures not only by blocking sunlight from reaching the Earth's surface but also by preventing terrestrial radiation from escaping the Earth's atmosphere. However, contrails' effects on climate are not well constrained because only few studies of contrail properties exist, and hence, their microphysical properties are poorly known.

In a new study, Voigt et al. directly measured ice particle sizes and numbers in 14 contrails from 9 different aircraft of the present-day commercial fleet, including the largest operating passenger aircraft. They obtained an extensive data set of contrails from which they determined the contrail optical depth, a measure of how much light is attenuated by these man-made clouds.

They use their measurements to estimate that the radiative forcing of line-shaped contrails is about 15.9 milliwatts per square meter, which represents a small positive contribution to the anthropogenic global warming. Yet an expected doubling of aircraft passenger transport within the coming two decades will enhance contrail effects on the atmosphere. The detailed contrail measurements will help modelers working to assess the actual and future impact of aviation on climate.

Source: Geophysical Research Letters, doi:10.1029/2011GL047189, 2011

Title: Extinction and optical depth of contrails

Authors: C. Voigt: Institut fr Physik der Atmosphre, Deutsches Zentrum fr Luft- und Raumfahrt, Oberpfaffenhofen, Germany; and Institut fr Physik der Atmosphre, Johannes-Gutenberg University, Mainz, Germany;

U. Schumann, P. Jessberger, T. Jurkat, and A. Petzold: Institut fr Physik der Atmosphre, Deutsches Zentrum fr Luft- und Raumfahrt, Oberpfaffenhofen, Germany;

J.-F. Gayet: LaMP, University Blaise Pascal, Clermont-Ferrand, France;

M. Krmer: IEK-7, Institute for Energy and Climate Research, Forschungszentrum Jlich, Jlich, Germany; T. Thornberry and D. W. Fahey; Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA.

2. Did Aboriginal forest burning affect Australian summer monsoon?

For thousands of years, Aboriginal Australians burned forests, creating grasslands. Some studies have suggested that in addition to changing the landscape, these burning practices also affected the timing and intensity of the Australian summer monsoon. Different vegetation types can alter evaporation, roughness, and surface reflectivity, leading to changes in the weather and climate.

On the basis of an ensemble of experiments with a global climate model, Notaro et al. conduct a comprehensive evaluation of the effects of decreased vegetation cover on the summer monsoon in northern Australia. They find that although decreased vegetation cover would have had only minor effects during the height of the monsoon season, during the premonsoon season, burning-induced vegetation loss would have caused significant decreases in precipitation and increases in temperature. Thus, by burning forests, Aboriginals altered the local climate, effectively extending the dry season and delaying the start of the monsoon season.

Source: Geophysical Research Letters, doi:10.1029/2011GL047774, 2011

Title: Did aboriginal vegetation burning impact on the Australian summer monsoon?

Authors: Michael Notaro: Center for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin, USA;

Karl-Heinz Wyrwoll: School of Earth and Environment, University of Western Australia, Crawley, Western Australia, Australia;

Guangshan Chen: Center for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin, USA.

3. Determining the trigger of East Asian dust storms

In the past two decades, there has been a dramatic increase in the occurrence of dust storms over East Asia. The trigger for this increase has been elusive because the ability of gusting wind to whip up a dust storm depends on a large number of factors, ranging from the level of snow and vegetation cover to differences in soil moisture and salt levels. Scientists note that these factors fall into two broad categories: Either the wind has increased its ability to wear away at the earth (increased erosivity), or the soil is more susceptible to the wind's assault (increased erodibility).

Using a database of wind speed, weather, and dust storm observations stretching back to 1970, Kurosaki et al. seek to determine whether the East Asian increase was caused by changing erosivity or erodibility. The authors find that the rise in dust storms in desert regions can be attributed largely to an increase in the frequency of strong winds. For crops and grasslands, however, the researchers tie the increase in storms to a change in erodibility, indicating that the soil had somehow changed. They propose that changes in the ground cover provided by dead leaves in the spring could be the driving factor. If so, then observations of plant growth and precipitation during the summer could provide a platform on which to base forecasts of the frequency of dust storms the following year.

Source: Geophysical Research Letters, doi:10.1029/2011GL047494, 2011

Title: What caused a recent increase in dust outbreaks over East Asia?

Authors: Yasunori Kurosaki and Masato Shinoda: Arid Land Research Center, Tottori University, Tottori, Japan;

Masao Mikami: Meteorological Research Institute, Tsukuba, Japan.

4. El NioSouthern Oscillation variability persisted in warmer world

Changes in the distribution of sea surface temperature associated with the El NioSouthern Oscillation (ENSO) cause significant changes in weather. In the past 40 years it has been observed that the frequency and intensity of El Nio events have been increasing. Scientists would like to know what will happen to ENSO variability as the world's climate warms. To find out, some have looked to the mid-Piacenzian Warm Period (mPWP), a period about 3.26 to 3.03 million years ago that was about 3 Celsius (5.4 Fahrenheit) warmer than present day and that may be analogous to what can be expected in the future if climate continues to warm. Some studies have suggested that during the mPWP, there was actually no ENSO variability but rather a permanent El Nio state.

To learn more about ENSO variability during the mPWP, Scroxton et al. analyze the isotopic composition of planktonic foraminifera from the eastern equatorial Pacific, as well as ENSO simulations conducted with a coupled ocean atmosphere climate model. Their proxy and model data suggest that interannual ENSO variability did persist during the mPWP, with a mean state similar to a modern El Nio event. Furthermore, they found that during the mPWP, ENSO events may have been more regular and more intense.

Source: Paleoceanography, doi:10.1029/2010PA002097, 2011

Title: Persistent El NioSouthern Oscillation variation during the Pliocene Epoch

Authors: N. Scroxton: Department of Earth Sciences, University of Oxford, Oxford, UK; Now at Research School of Earth Sciences, Australian National University, Acton, ACT, Australia;

S. G. Bonham: School of Earth and Environment, University of Leeds, Leeds, UK;

R. E. M. Rickaby, S. H. F. Lawrence, and M. Hermoso: Department of Earth Sciences, University of Oxford, Oxford, UK;

A. M. Haywood: School of Earth and Environment, University of Leeds, Leeds, UK.

5. Constraining the trigger for ancient warming episode

The Paleocene epoch (approximately 66 to 56 million years ago) was sandwiched between sudden climate shifts and mass extinctions. The boundary between the end of the Paleocene and the beginning of the Eocene (P-E boundary) saw the global average temperature soar by 5Celsius (9 Fahrenheit) over a few thousand years, leading to a pronounced reorganization of both terrestrial and oceanic plant and animal communities. The P-E boundary warming was triggered by an influx of atmospheric carbon dioxide, but the influx's ultimate trigger is still being debated. Other prominent warming events within the Paleogene (~66 to 23 million years ago), the broad time span that encompasses the Paleocene and Eocene, have been linked to regularly recurring changes in the eccentricity of the Earth's orbit that take place on 100,000- and 405,000-year cycles. Proponents of this view suggest that an alignment of the two cycles could lead to the warming of deep ocean waters, melting frozen methane and triggering an increase in atmospheric carbon dioxide. However, some studies have suggested that the P-E boundary warming was instead the product of geological processes, where carbon-rich rocks were baked by injected magma, which eventually liberated the carbon to the atmosphere. Deciding between proposed explanations for the cause of the P-E warming, whether they are astronomical or geological, depends on accurately pinning the event in time.

One of the most accurate techniques available, uranium-lead (U-Pb) radioactive isotope dating, relies on finding the right kind of mineralsin this case, zircon encased in ancient volcanic ashin the right sedimentary layer. Drawing from an outcrop in Spitsbergen, the western island of the Arctic Svalbard archipelago, Charles et al. performed U-Pb dating of zircon tied to the P-E boundary. The analysis puts the date of the powerful warming between 55.728 and 55.964 million years ago. According to the authors' chronology, the warming took place during a downward swing from a period of astronomically induced warming, suggesting that the P-E boundary heat anomaly was triggered by a geological mechanism.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2010GC003426, 2011

Title: Constraints on the numerical age of the Paleocene-Eocene boundary

Authors: Adam J. Charles, Ian C. Harding, Heiko Plike, John E. A. Marshall and Ian W. Croudace: School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, UK;

Daniel J. Condon: NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, UK;

Ying Cui and Lee Kump: Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA.

6. Next generation atmospheric model improves hurricane forecasting

Accurately predicting hurricane development from one season to the next is a problem that crosses multiple temporal and spatial scales. The driving force behind the overall activity level for a year depends on broad climate dynamics, such as the El NioSouthern Oscillation or the Madden-Julian Oscillation (MJO), while the generation of an individual tropical cyclone is a product of small-scale fluctuations in atmospheric moisture or sea surface temperature. Classically, these broad- or small-scale events have fallen within the realm of climate or weather forecasting models, respectively. However, researchers have recently developed the High-Resolution Atmospheric Model (HiRAM), which can flip between weather forecasting and climate modeling. HiRAM has 25-kilometer (15.5-mile) horizontal resolution and can properly represent clouds, both important improvements over current models.

Relying on HiRAM's ability to cross spatiotemporal scales, Chen and Lin retroactively predict the 2000 to 2010 hurricane seasons using only information that would have been available to forecasters at the time. The authors' predictions for North Atlantic hurricane activity are able to account for 92 percent of the observed variability between seasons. Further, they are able to predict the onset of the MJO, an intraseasonal surge in rainfall that spawned in the Indian Ocean. Perhaps most important, the researchers accurately predict both a prominent decrease in hurricane activity between the 2005 and 2006 seasons and a sharp spike between 2009 and 2010. The researchers note that HiRAM is currently limited only by the strength of modeling computers, suggesting that the predictions will continue to improve as computer processors grow in power.

Source: Geophysical Research Letters, doi:10.1029/2011GL047629, 2011

Title: The remarkable predictability of inter-annual variability of Atlantic hurricanes during the past decade

Authors: Jan-Huey Chen: Program in Atmospheric and Oceanic Science, Princeton University, Princeton, New Jersey, USA; and Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, USA;

Shian-Jiann Lin: Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, USA.

7. Theorized magnetic plasma behavior demonstrated in lab

Theoretical calculations have indicated that in magnetized plasmas with two ion species, Alfvn waves, oscillations of the magnetic field due to plasma currents, can become trapped in a resonator. Now such a resonator has been demonstrated for the first time in a laboratory. Vincena et al. created the resonator in a magnetized plasma with hydrogen and helium ions. Their observations agree well with theoretical predictions. Similar resonators could occur in the magnetosphere around Earth and other planets, where the resonant modes can interact with energetic particles.

Source: Geophysical Research Letters, doi:10.1029/2011GL047399, 2011

Title: Laboratory realization of an ion-ion hybrid Alfvn wave resonator

Authors: S. T. Vincena, W. A. Farmer, J. E. Maggs, and G. J. Morales: Department of Physics and Astronomy, University of California, Los Angeles, California, USA.


Contact: Peter Weiss
American Geophysical Union

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