AGU journal highlights -- Dec. 31, 2009

December 30, 2009

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL).

In this release:

  1. Indian Ocean climate event recurs quicker with global warming
  2. Natural variability led to extra-cold 2008
  3. Loss of sea ice stirs up Arctic waters
  4. How much ice needed to create Martian land formations?
  5. Major offshore quake could surge inland to Seattle area
  6. Permafrost thaw may accelerate Arctic groundwater runoff
  7. Major Australian droughts traced to different causes
  8. How the Moon gets its exosphere
  9. Saturn's auroral hiss is asymmetrical
  10. Northern South America rainier during Little Ice Age
  11. Continental roots stress Earth's surface
  12. Window opens into Moon's past volcanism
  13. Explaining plasma motion around Saturn
  14. Is the airborne fraction of anthropogenic carbon dioxide increasing?

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. 10.1029/2009GL040613. 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. Indian Ocean climate event recurs quicker with global warming

The Indian Ocean Dipole (IOD), an oscillation of sea surface temperatures in the Indian Ocean, has become a major influence on the weather variations in the Indian Ocean region. During positive IOD events, abnormally warm sea surface temperatures in the western Indian Ocean are accompanied by severe droughts over the Indonesian region and heavy rainfall over east Africa. To learn more about IOD patterns, Nakamura et al. study a 115-year coral record from Kenya. They analyze coral oxygen isotope ratios, which trace rainfall anomalies, to reconstruct IOD variability. The results add to evidence that the IOD has been occurring more frequently in recent decades. They find that before 1924, the IOD occurred approximately every 10 years, but since 1960, IOD events have been occurring approximately 18 months to 3 years apart. The authors suggest that global warming effects on the western Indian Ocean have driven the observed shift in IOD variability and note that the IOD has replaced the El Niño-Southern Oscillation as the major driver of climate patterns over the Indian Ocean region.

Title: Mode shift in the Indian Ocean climate under global warming stress

Authors: Nobuko Nakamura, Hajime Kayanne, Hiroko Iijima, and Toshio Yamagata: Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan;

Timothy R. McClanahan: Marine Programs, Wildlife Conservation Society, New York, New York, USA;

Swadhin K. Behera: Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040590, 2009

2. Natural variability led to extra-cold 2008

An especially cold year in North America in 2008 led some members of the public and the media to question the scientific consensus on human-induced global warming. In addition, the cool global temperatures during the past decade may appear to contrast with the warming expected due to human influence. To clarify the roles of human influence and natural climate variability, Perlwitz et al. use observed temperature data and a suite of climate model simulations to analyze factors contributing to the 2008 North American temperature conditions. They find that the anthropogenic forcing in 2008 did contribute to temperatures warmer than would otherwise have occurred but that those human-induced effects were overwhelmed by a particularly strong bout of natural cooling. The authors determine that the North American cooling likely resulted from a widespread natural coolness in the tropical and northeastern Pacific Ocean. The study implies that the abnormally cool 2008 is not likely part of a prolonged cooling trend and that general warming trends are likely to continue.

Title: A strong bout of natural cooling in 2008

Authors: Judith Perlwitz, Jon Eischeid, and Taiyi Xu: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA, and Earth System Research Laboratory, NOAA, Boulder, Colorado, USA;

Martin Hoerling: Earth System Research Laboratory, NOAA, Boulder, Colorado, USA;

Arun Kumar: Climate Prediction Center, NOAA, Camp Springs, Maryland, USA

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041188, 2009

3. Loss of sea ice stirs up Arctic waters

The Arctic Ocean is generally considered a remarkably quiet ocean, with very little mixing, because a cover of sea ice prevents wind from driving the formation of internal waves. To study this effect and investigate how melting sea ice might affect ocean mixing in the Arctic, Rainville and Woodgate analyze data from moorings in the northern Chukchi Sea. They find that when the ocean was mostly covered with ice, even strong winds did not generate much response in it. On the other hand, during the summers when less sea ice was present, wind generated large internal oscillations and increased turbulence. The extent of Arctic sea ice in the summer has been declining significantly in recent years, likely resulting in increased internal wave generation, the authors note. Because internal waves bring deeper waters closer to the surface, the results have important implications for Arctic Ocean ecosystems and ocean dynamics.

Title: Observations of internal wave generation in the seasonally ice-free Arctic

Authors: Luc Rainville and Rebecca A. Woodgate: Applied Physics Laboratory, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041291, 2009

4. How much ice needed to create Martian land formations?

Some land formations on Mars suggest the presence of water ice. These features could have been created by viscous creep of ice below the surface in Martian permafrost. To determine how much ice would be needed to form the observed topography on Mars, Durham et al. conducted laboratory experiments to simulate the frozen Martian sand. They tested frozen sand mixtures with different types of sand and with various amounts of ice. Surprisingly, the authors find that the most important factor affecting whether the mixture was mobile or jammed was not the amount of ice, or the temperature or stress on the mixture, but the dry packing density of the sand. They find that the minimum amount of ice needed to mobilize the sand is the amount that fills the pore space at the dry packing density of the sand. The results help explain some of the land formations seen on Mars and provide a new way to estimate Martian water content.

Title: Mobility of icy sand packs, with application to Martian permafrost

Authors: William B. Durham and Hendrik J Lenferink: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;

Asmin V. Pathare: Planetary Science Institute, Pasadena, California, USA;

Laura A. Stern: U.S. Geological Survey, Menlo Park, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040392, 2009

5. Major offshore quake could surge inland to Seattle area

Large megathrust earthquakes occur on average every 550 years on the northern Cascadia subduction zone, where the Juan de Fuca tectonic plate subducts underneath the North American plate off the coast of Washington State. Scientists had thought that major earthquakes were likely to occur only offshore, but a new analysis by Chapman and Melbourne shows that a giant earthquake could extend deeper along the subduction zone, and thus farther inland. The authors analyze 15 small, nonearthquake periodic slip events known as episodic tremor and slip (ETS) detected between 1997 and 2008 to develop a model of seismogenic coupling between the two tectonic plates. Their model accurately predicts the deformation of the overlying North American plate as measured by Global Positioning System stations and shows that seismic stress accumulation reaches to about 25 kilometers (15.5 miles) depth, or about 60 km (37.3 mi) inland, rather than stopping offshore at about 15 km (9.3 mi) depth, as had been thought. This indicates that the Seattle metropolitan areas could be subject to a magnitude 8.9 earthquake (from just the Washington State segment of Cascadia). The authors conclude that results point to a need to reassess the Cascadia megathrust seismic hazard.

Title: Future Cascadia megathrust rupture delineated by episodic tremor and slip

Authors: James S. Chapman and Timothy I. Melbourne: Department of Geological Sciences, Central Washington University, Ellensburg, Washington, USA

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040465, 2009

6. Permafrost thaw may accelerate Arctic groundwater runoff

As the Arctic warms, permafrost will degrade, potentially resulting in increased groundwater runoff as frozen ground that had blocked the flow of water melts. To investigate how groundwater systems will evolve as surface temperatures rise, Bense et al. develop a model to simulate an idealized aquifer covered by a layer of permafrost. They ran the simulation under three scenarios, starting with three initial surface temperatures (-2, -1.5, and -1 degrees Celsius, or 28.4, 29.3 and 30.2 degrees Fahrenheit), corresponding to different permafrost thicknesses. In each case, they increased the average seasonal surface temperature by 3 degrees C (5.4 degrees F) over 100 years, an average of model predictions for temperature increase in the Arctic over the next century. After the warming period, in each scenario the temperature was then held constant for the next 1100 years. The authors find that although the initial distribution of ice influences the response, in all cases groundwater flow to streams and rivers accelerates over time. In fact, the results indicate that substantial increases in groundwater flow are likely over the next few centuries even if surface air temperatures stabilize at current levels.

Title: Evolution of shallow groundwater flow systems in areas of degrading permafrost

Authors: V. F. Bense: School of Environmental Sciences, University of East Anglia, Norwich, UK; G. Ferguson: Department of Earth Sciences, Saint Francis Xavier University, Antigonish, Nova Scotia, Canada; H. Kooi: Faculty of Earth and Life Sciences, VU University, Amsterdam, Netherlands.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039225, 2009

7. Major Australian droughts traced to different causes

Southeastern Australia has been subject to several severe, long-term droughts during the past century, including the "Federation" drought (1895-1900), the "World War II" drought (1937-1945), and the "Big Dry" (1997 to present). All three droughts were widespread and devastating, but until now the causes and nature of these three droughts had not been compared. Verdon-Kidd and Kiem highlight the differences in the nature and causes of these three droughts. They find that the droughts exhibited different severity, spatial extent, and seasonality. In addition, the three droughts resulted from different climate modes: The El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO) were the primary drivers of the Federation drought; the Southern Annular Mode (SAM) and ENSO were major causes of the Big Dry; and a combination of Indian Ocean, ENSO, and SAM was a causal factor of the World War II drought. The authors note that most attempts at forecasting droughts have focused on ENSO as a primary driver; the new results indicate that planners and drought managers should take into account other climate modes and their interactions when predicting drought conditions.

Title: Nature and causes of protracted droughts in Southeast Australia - Comparison between the Federation, WWII and Big Dry droughts

Authors: Danielle C. Verdon-Kidd and Anthony S. Kiem: School of Environmental and Life Sciences, University of Newcastle, Callaghan, New South Wales, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041067, 2009

8. How the Moon gets its exosphere

Several decades ago scientists discovered that the Moon, long thought to have no atmosphere, actually does have an extremely thin exosphere. Scientists generally believe that the ions that make up the lunar exosphere are generated at the Moon's surface by interaction with solar photons, plasma in the Earth's magnetosphere, or micrometeorites. However, scientists have been uncertain about which processes are the main contributors of lunar exosphere ions. Using instruments aboard the Japanese lunar orbiter SELENE (also known as Kaguya), Tanaka et al. made the first spacecraft-based observations of the lunar exosphere when the Moon was inside Earth's magnetosphere. They detect ions of several elements at 100-kilometer (62-mile) altitude above the lunar surface. Previous studies have detected Moon-originating ions when the Moon was in the solar wind; this new study is the first to detect such ions when the Moon was not affected by solar wind particles or the Earth's magnetotail plasma. The results, which provide new evidence about the origin of the lunar exosphere, are consistent with the idea that solar photon-driven processes dominate in supplying exosphere components.

Title: First in situ observation of the Moon-originating ions in the Earth's Magnetosphere by MAP-PACE on SELENE (KAGUYA)

Authors: Takaaki Tanaka, Yoshifumi Saito, Shoichiro Yokota, Kazushi Asamura, Masaki N. Nishino, Hideo Tsunakawa, Masaki Matsushima, and Futoshi Takahashi: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan;

Hidetoshi Shibuya: Department of Earth Science, Kumamoto University, Kumamoto, Japan

Hisayoshi Shimizu: Earthquake Research Institute, University of Tokyo, Tokyo, Japan;

Masaki Fujimoto and Toshifumi Mukai: Institute of Space and Astronautical Science, Japan; Aerospace Exploration Agency, Sagamihara, Japan;

Toshio Terasawa: Department of Physics, Tokyo Institute of Technology, Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040682, 2009

9. Saturn's auroral hiss is asymmetrical

Saturn emits "auroral hiss," a whistler-mode electromagnetic emission observed in the magnetosphere at high latitudes. This emission is similar to auroral hiss emitted by Earth. However, unlike Earth's auroral hiss, Gurnett et al. find that Saturn rotates in a beam-like matter around the planet. Using data taken by the Cassini spacecraft, the authors observe that the auroral hiss emitted by Saturn has a different rotation rate in the northern and southern hemispheres; the period is about 10.6 hours in the northern hemisphere and about 10.8 hours in the southern hemisphere. They note that the rotation periods match the modulation periods of another type of radio emission, Saturn kilometric radiation, which was also recently found to rotate at different rates in the two hemispheres. This new observation confirms a fundamental north-south asymmetry in the rotation rates of high-latitude plasma phenomena in the two hemispheres. The authors suggest that the results also have implications for understanding how the planet's rotation is transferred to the magnetosphere plasma.

Title: A north-south difference in the rotation rate of auroral hiss at Saturn: Comparison to Saturn's kilometric radio emission

Authors: D. A. Gurnett, A. M. Persoon, J. B. Groene, A. J. Kopf, G. B. Hospodarsky, W. S. Kurth: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040774, 2009

10. Northern South America rainier during Little Ice Age

During the Little Ice Age (LIA; covering approximately the fifteenth through the eighteenth centuries), northern South America experienced about 10 percent more rainfall than during the twentieth century, according to Reuter et al. The authors analyze two new records of oxygen isotopes (which track precipitation levels) from cave formations in northeastern Peru. They attribute the higher rainfall in northern South America during the LIA to cooler spring sea surface temperatures in the tropical North Atlantic. Furthermore, the authors note that some studies have shown that during the twentieth century, a significant amount of rainfall variability in northern South America was related to the El Niño-Southern Oscillation (ENSO), with higher rainfall occurring during La Niña conditions. However, the authors determine that the ENSO was probably not as significant an influence on rainfall during that time period as it is now. The authors suggest that the results point to a need to reevaluate some ideas about hydroclimate change over South America during the past millennia.

Title: A new perspective on the hydroclimate variability in northern South America during the Little Ice Age

Authors: Justin Reuter, Lowell Stott, and Deborah Khider: Department of Earth Sciences, University of Southern California, Los Angeles, California, USA;

Ashish Sinha: Department of Earth Sciences, California State University, Carson, California, USA;

Hai Cheng and R. Lawrence Edwards: Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041051, 2009

11. Continental roots stress Earth's surface

The Earth's rigid lithosphere varies laterally in thickness and strength. Areas of thicker, older lithosphere known as continental roots penetrate deeper into the mantle in some places under continents. Because these continental roots are in contact with deeper, more viscous mantle, the shear traction at the base of the lithosphere in those areas is increased by up to a factor of 4 compared with a model lithosphere without continental roots. To study how those areas of increased traction affect patterns of lithospheric stress above, Naliboff et al. examine a model of mantle flow coupled to a model of the elastic lithosphere. The authors find that greater traction at the bottom of thicker areas of continental lithosphere raises elastic stress in the lithosphere above by at most a factor of 1.5. Furthermore, greater lithospheric stress is not located simply in small areas directly above deep continental roots; instead, increased stress is spread out over a larger regional area. The study highlights the need to incorporate variations in lithosphere thickness and strength into models of both mantle flow and lithospheric deformation.

Title: Modification of the lithospheric stress field by lateral variations in plate-mantle coupling

Authors: J. B. Naliboff: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA

C. P. Conrad: Department of Geology and Geophysics, SOEST, University of Hawaii, Honolulu, Hawaii, USA

C. Lithgow-Bertelloni: Department of Earth Sciences, University College London, London, United Kingdom

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040484, 2009

12. Window opens into Moon's past volcanism

Lava tubes, underground cave-like channels through which lava once flowed, are commonly found on Earth. Scientists have debated whether these tubes could form on the Moon as well, but no studies have yet conclusively identified features that indicate the presence of lunar lava tubes. Using images from the SELENE (also known as Kaguya) spacecraft's high-resolution cameras, Haruyama et al. identify a vertical hole that they believe is a skylight in an intact lava tube. The hole is located in the Marius Hills region, a volcanic area on the Moon's nearside. The authors find that the nearly circular hole is about 65 meters (213 feet) in diameter and about 80-88 m (262-289 ft) deep. They consider possible formation mechanisms and conclude that the skylight most likely formed when part of the lava tube roof collapsed. The authors believe that the discovery could have implications for studies of lunar volcanism. In addition, because lava tubes are sheltered from the harsh environment on the Moon's surface, such tubes could one day be useful for lunar bases.

Title: Possible lunar lava tube skylight observed by SELENE cameras

Authors: Junichi Haruyama, Tomokatsu Morota, Yasuhiro Yokota, and Makiko Ohtake: ISAS, JAXA, Sagamihara, Japan;

Kazuyuki Hioki and Seiichi Hara: NTT DATA CCS Corporation, Tokyo, Japan; Motomaro Shirao: Tokyo, Japan;

Harald Hiesinger and Carolyn H. van der Bogert: Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany;

Hideaki Miyamoto: University Museum, University of Tokyo, Tokyo, Japan; Akira Iwasaki: Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan;

Tsuneo Matsunaga: Center for Global Environmental Research, NIES, Tsukuba, Japan; Shunsuke Nakanotani: Mitsubishi Space Software Co., Ltd., Tsukuba, Japan;

Carle M. Pieters: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040635, 2009

13. Explaining plasma motion around Saturn

Understanding the motion and source of the plasma around Saturn is important for understanding the dynamics of the magnetosphere. Pontius and Hill present a theory that describes plasma transport in Saturn's magnetosphere, including processes that add new mass to the plasma and those that remove momentum from the plasma without changing plasma mass. Using observational data from the Cassini spacecraft on the angular velocity of plasma around Saturn along with chemistry models of Saturn's magnetosphere, the authors calculate the distribution of new mass entering the magnetosphere. They confirm that most of the plasma comes from a neutral gas region near the orbit of Saturn's moon Enceladus and quantify the rate at which plasma mass is added to the magnetosphere from this region. The distribution and source of mass addition is important because it affects the rotation rate of the magnetosphere. The work provides a new method of analysis that could be useful for future studies.

Title: Plasma mass loading from the extended neutral gas torus of Enceladus as inferred from the observed plasma corotation lag

Authors: D. H. Pontius Jr.: Department of Physics, Birmingham-Southern College, Birmingham, Alabama, USA;

T. W. Hill: Department of Physics and Astronomy, Rice University, Houston, Texas, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041030, 2009

14. Is the airborne fraction of anthropogenic carbon dioxide increasing?

Most of the carbon dioxide emitted by human activity does not remain in the atmosphere, but is instead absorbed by the oceans and terrestrial ecosystems. In fact, only about 45 percent of emitted carbon dioxide stays in the atmosphere. However, some studies have suggested that the ability of oceans and plants to absorb carbon dioxide recently may have begun to decline and that the airborne fraction of anthropogenic carbon dioxide emissions is therefore beginning to increase. Many climate models also assume that the airborne fraction will increase. Because understanding of the airborne fraction of carbon dioxide is important for predicting future climate change, it is essential to have accurate knowledge of whether that fraction is changing or will change as emissions increase. To assess whether the airborne fraction is indeed increasing, Knorr reanalyzes available atmospheric carbon dioxide and emissions data since 1850 and considers the uncertainties in the data. In contradiction to some recent studies, he finds that the airborne fraction of carbon dioxide has not increased either during the past 150 years or during the most recent five decades.

Title: Is the airborne fraction of the anthropogenic carbon dioxide emissions increasing?

Authors: Wolfgang Knorr: Department of Earth Sciences, University of Bristol, Bristol, UK.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040613, 2009

American Geophysical Union

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